Common EMC Measurement Terms

According to Krause1, “A radio antenna may be defined as the cal, though the AF and TAF can be computed from each structure associated with the region of transition between a other. The expressions for these computations may be found guided wave and free space, or vice versa”. The EMC test in the section “Antenna Terms and Calculations”. The TAF is community uses specialized versions of radio antennas for not a direct function of frequency, but it is a function of measuring electric and magnetic field amplitudes over a very distance. broad frequency spectrum. Antenna Pattern EMC test engineers must perform very precise measurements. Sometimes the terms used in describing these measurements The antenna pattern is typically can be confusing. This section describes the meaning of some of a polar plot of the relative these terms. response of an antenna as a function of viewing angle. The Antenna Factor “on-axis” viewing angle where the response of the antenna is a This is the parameter of an maximum is called “the EMC antenna that is used in boresight axis”. The pattern the calculations of field shows the response of the antenna as the viewing angle is strength during radiated varied. Typically, simple antennas exhibit a “dipole response” emissions measurement. It where the pattern of the antenna is donut shaped with the relates the voltage output of dipole on the common axis of the donut. More complex a measurement antenna to antenna patterns are approximately pear shape with the the value of the incident field producing that voltage. The units bottom of the pear facing away from the antenna. are volts output per volt/meter incident field or reciprocal meters. As can be seen in the derivations, the analytical Gain expression for Antenna Factor (AF) has the equivalent of frequency in the numerator, thus AF will typically increase with The gain of the antenna is a increasing frequency. EMCO antennas used for radiated parameter that describes the emissions testing are individually calibrated (the AF is directly directional response of the antenna measured) at all appropriate measurement distances. The compared to an isotropic source, a calibrations produce values that are defined as the “equivalent theoretical antenna that radiates free space antenna factor”. This antenna factor is measured the same amount in all directions. by EMCO using the three antenna measurement method over The higher the gain, the better the a ground plane. The calibration procedure corrects for the antenna concentrates its beam in a specific direction. The presence of the reflection of the antenna in the ground plane, simple example is a light bulb compared to a flashlight. The giving the value that would be measured if the antenna were in flashlight, with its reflector, concentrates the light in one “free space”. The typical antennas used for measurements are direction, where the light bulb produces light in all directions. broadband antennas such as BiConiLogTM and log periodic antennas. In case of any disagreement, a “reference antenna”— Bandwidth a tuned resonant dipole—is considered to be the arbiter for The bandwidth of an antenna measurement purposes. is the operating frequency range of the antenna and is Transmit Antenna Factor expressed in MHz. The Transmit Antenna Factor Typical EMC antennas will (TAF) is similar to the AF in have a ratio of upper useful that it describes the perfor- frequency to lowest useful frequency on the order of 5 to 1. mance of an EMC antenna over Some unique designs provide ratios of as much as 25 to 1. This its frequency range of ratio is a dimensionless ratio, i.e., it has no units. operation. This parameter relates the E-field produced by an antenna, at a given distance, to the input voltage at the input terminals of the antenna. The units are volts/meter produced per volt of input, so the final units are reciprocal meters just as the AF. The AF and TAF are not the same nor are they recipro-

USA: FINLAND: UK: SINGAPORE: ONLINE: 68 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Entdecken Sie weitere interessante Artikel und News zum Thema auf all-electronics.de!

Hier klicken & informieren! Beamwidth RF Power Terms As Applied To Antennas This parameter is the There are descriptor of the viewing several ways angle of the antenna, in of discussing the plane of measure- RF power as ment, typically where the used to excite response has fallen to one antennas for half the power received the generation when the antenna is perfectly aligned. EMC antennas are of electromagnetic fields. This set of related definitions is normally designed to provide a viewing angle in the primary provided to understand the definitions as used in this plane, approaching 60° between the half power points where catalog. the response of the incident is 3 dB down, if at all possible. Forward Power — The output from an amplifier The only specific requirements are found in IEC 801 - 3 and that is applied to an antenna input to generate an IEC 1000 - 4 - 3, which, if the analysis of the requirement is electromagnetic field. performed, translates into a minimum viewing angle of 28 ° (± 14 °) at the 1 dB down points. The 1 dB down response is Reflected Power — When mismatch exists at the usually compatible with the 3 dB down value of 60 °. antenna port, a fraction of the power applied (the forward power), is reflected from the antenna back Reflection Coefficient toward the amplifier. This is termed the reflected power. The voltage reflection coefficient, typically ρ, is the ratio of the voltage reflected from the load of a transmission line to the Net Power — The power applied to the antenna that voltage imposed on the load of the same transmission line. Its is actually radiated is called the net power or radiated Terms EMC Measurement Common – values vary from zero to one. When the load impedance is power. It is the difference between the forward power “matched” to the source impedance (has the same value), and the reflected power. Usually, this value cannot be and the characteristic impedance of the transmission line, directly measured, but is computed from the direct then the reflection coefficient is quite small (no reflection), measurement of forward and reflected power by approaching zero. When there is “mismatch” (the load taking the difference: impedance differs from the characteristic impedance) the reflection coefficient can approach one (almost all incident Pnet (W ) = Pforward (W ) – P reflected (W ) power is reflected). Reference In this catalog the Net Power is the power value that is required The reflection coefficient is usually determined by a measure- as an input to an antenna to generate a specific E-field level. ment of the Voltage Standing Wave Ratio. It is then computed from:

(VSWR–1 ) ρ = | | (VSWR+1 )

VSWR The Voltage Standing Wave Ratio (VSWR) is a measure of the Polarization mismatch between the source and load impedances. Numeri- cally, it is the ratio of the maximum value of the voltage The orientation of the measurement axis of a linearly measured on a transmission line divided by the minimum polarized antenna with respect to the local ground plane. value. Vertical polarization occurs when the measurement axis of When the value is high, most of the power delivered from a the antenna is perpendicular to the local ground plane. generator is reflected from the antenna (load) and returned Horizontal polarization occurs when the measurement axis is to the generator. The amount of power not reflected is parallel to the local ground plane. Most EMC test specifica- radiated from the antenna. This means that the generator tions require measurements in both vertical and horizontal rating for an antenna with a high VSWR can be quite high. polarizations of the measurement antenna. Users should choose an antenna with a low VSWR when 1 John D. Krause, Ph.D., Antennas, McGraw-Hill, New York, 1950, p. 1. possible. As a practical matter, this may not always be possible, particularly at lower frequencies.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 69 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Antenna Calculations 1 Definition of Antenna Factor 2 Conversion of Signal Levels from mW to µV This term is traditionally tied to the recieve antenna factor, in a 50Ω System the ratio of field strength at the location of the antenna to the Voltage and power are equivalent methods of stating a signal output voltage across the load connected to the antenna. level in a system where there is a constant impedance. Thus:

2 E V .➊ AF = .➊ P= V R where: where: –1 P = Power in Watts AF = Antenna Factor, meters E = Field Strength, V/m or µV/m V = Voltage Level in Volts V = Load Voltage, V or µV R = Resistance Ω

–3 Converting to dB (decibel) notation gives: For power in milliwatts (10 W), and voltage in microvolts (10–6 V),

E ➋ ➋ . V µ =P + 107 . AFdB(m –1) = 20 log dB( V) dBm V or: 3 Power Density to Field Strength ➌ . AFdB(m–1) = EdB(V/m) – VdB(V) An alternative measure of field strength to electric field is power density: The antenna factor is directly computed from: E 2 9.73 .➊ P = 0 .➍ AF = (m –1 ) d 120 π λ √ g

where: where: λ = Wavelength (meters) E = Field Strength (V/m) 2 g = Numeric Gain P = Power Density (W/m )

In the same sense, for magnetic fields, as seen by Common Values:

loop antennas: EPD 200 V/m 10.60 mW/cm2 .➎ AF = H – V HdB(S/m) dB(A/m) dB(V) 100 V/m 2.65 mW/cm2 10 V/m 26.50 µW/cm2 In terms of flux density (B-Field): 1 V/m 0.265 µW/cm2 ➏ µ . AFB =AFH + 20 log ( ) ➐ 4 Power Density at a Point . AFB =AFH – 118, T / V P G .➊ P = t t0 Loop antennas are sometimes calibrated in terms of equivalent d 4π r2 electric field, where:

In the far field, where the electric and magnetic fields are ➑ η . AFEdB(m–1)= AFHdB(S/m) + 20 log related by the impedance of free space: ➒ π . AFEdB(m–1)= AFHdB(S/m) + 20 log (120 ), or 2 P d = Power Density (W / m ) P = Power Transmitted (W) = AFHdB(S/m) + 51.5 dB t where: G = Gain of Transmitting Antenna η = the Impedance of Free Space t r = Distance from Antenna (meters) = 120 π Ω

USA: FINLAND: UK: SINGAPORE: ONLINE: 70 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com 5 Friis Transmission Formula 7 The Friis Transmission formula describes Power received by an .➊ G = 20 log f dB ( MHz) antenna in terms of power transmitted by another antenna:

–AFdB(m –1) – 29.79 λ 2 ➊ PtGtGr . Pr = (4 π r) 2 Power Required to Generate a Desired Field Strength at a Given Distance when Antenna Factors are Known where: 8 P = Power Received (W) r .➊ P =20log E dB(W ) 10 ( desired (V/m)) Pt = Power Transmitted (W) + 20 log d G = Numeric Gain of Receiving 10 ( m)

r Calculations Antenna Antenna –

– 20 log f 10 ( MHz) Gt = Numeric Gain of Transmitting Antenna +AFdB(m –1) +15 r = Separation Between Antennas (meters) Relationship Between Frequency and Wavelength λ = Wavelength (meters). in Free Space

9 Reference .➊ f λ =c 6 Electric Field vs Power Transmitted (Far Field) The electric field strength at a distance from a transmitting where: antenna such that the electric and magnetic field values are f = Frequency (Hz) related by the impedance of free space is: λ = Wavelength (meters) √ c = Velocity of Light ➊ 30PtG t . E V/m = 8 r =3x10m/s

where the terms are as defined above. a simpler relationship is:

300 For simple radiating devices having low gain, far field .➋ λ = conditions exist when: f MHz

λ ➋ ≥ 0 . r Decibel Formulas π 2 10 A decibel is one tenth of a Bel, and is a ratio measure of relative amplitude. In terms of power, the number of where: decibels is ten times the logarithm to the base 10 of the λ = Wavelength (meters) ratio.

For more complex antennas having higher gain values, In terms of power: far field conditions exist when: .➊ dB = 10 log P /P 2 10 ( 1 2) 2D .➌ r ≥ 0 λ where P1 and P2 are in watts.

where: D = Maximum Dimension of the Antenna (m)

Relationship of Antenna Factor and Gain in a 50 Ω System

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 71 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com In a constant impedance system, power references can be made 11 Transmit Antenna Factor between different measurement points. They can also be related to The transmit antenna factor of an antenna is computed from voltage or current measurements: the gain or receive antenna factor, and is a measure of the transmitting capabilities of that antenna. It is valid under the .➋ conditions of measurement of the receive antenna factor, in Power Ratio = 10 log P /P a 50 Ω system. 10 ( 1 2)

2 ➊ TAF =G – 2.22 – 20 log d . dB(m –1) dB 10 m V1 /R 10 ( ) = 10 log10 V 2/R 2 2 Where:

TAF = Transmit Antenna Factor dB(m –1) Antenna Calculations Antenna V10 – = 20 log10 G = Antenna Gain of V dB 2 Transmitting Antenna

d m = distanceDistance (m) (m) R 10 – 10 log10

R 2 Alternatively: Reference .➋ TAF = 20 log f –AF –1 for a constant impedance system. dB ( MHz ) dBm – 32.0 – 20 log r ➌ m . R 1 =R2 ( ) and: 12 Computing Power Required for a Specific Field Inten- .➍ sity Given Power Required to Generate 1 Volt/meter VV00 Antenna transmitting capabilities are often given in terms of Pwr RatioRatio (dB)= (dB)= 20 20 log log 1 1 1010 the input power to an antenna to generate 1V/m at one or

V22 more distances. The input power required to develop a different electric field level value is found by: Also: ➊P =P + 20 log E .➎ G =10log g . dB(W) dB(W)(1 V/m) 10 desired V/m dB ( ) ( )

.➏ V = 20 log V/V dB(reference) ( reference)

.➐ P = 10 log P/P dB(reference) ( reference)

where a typical reference for voltage is microvolts and a typical reference for power is milliwatts.

The reverse relationships are:

.➑ g=10GdB /10

.➒ V=10VdB (reference) /20

.➓ P=10PdB (reference) /10

USA: FINLAND: UK: SINGAPORE: ONLINE: 72 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com SI Prefixes, Multipliers, & Abbreviations SI Derived Units

PREFIX MULTIPLIER SYMBOL QUANTITY NAME SYMBOL pico 10-12 p area square meter m2 nano 10-9 n volume cubic meter m3 micro 10-6 µ frequency hertz Hz s-1 milli 10 -3 m mass density kilogram per cubic meter kg/m3 kilo 103 k speed, velocity meter per second m/s Mega 106 M angular velocity radian per second rad/s Giga 10 9 G acceleration meter per second squared m/s2 angular acceleration radian per second squared rad/s2 force newton N kg•m/s2 2 SI Base Units pressure pascal Pa N/m work, quantity of heat joule J N•m Antenna Calculations Antenna QUANTITY NAME SYMBOL power watt W J/s –

length meter m quantity of electricity coulomb C A•s mass kilogram kg potential difference volt V W/A time second s electric field strength volt per meter V/m electric current ampere A electric resistance ohm Ω V/A capaticitance farad F A•s/V magnetic flux weber Wb V•s SI Prefixes, Multipliers, & Abbreviations inductance henry H V•s/A magnetic flux density tesla T Wb/m2 Reference CONSTANT COMPUTATIONAL VALUE magnetic field strength ampere per meter A/m Ω Speed of light c = 2.99792458 x 108 m/s admittance siemens S 1/ in a vacuum: ≈3.00 x 108 m/s electricantenna factor per meter 1/m ȏ µ 2 magnetic antenna factor siemens per meter s/m Permittivity 0 = 1/( c ) F/m constant: ≈ 8.85 x10-12 F/m µ π -7 Permeability 0 =4 x 10 H/m constant: ≈ 1.26 x 10-6 H/m

Conversion Table for Magnetic Units TO CONVERT FROM: Unit System: SI (MKS) CGS Magnetic Qty.: BHBHB Units: tesla amp-turn/m gauss oersted gamma tesla 1 4π x 10-7† 10-4 10-4† 10-9 amp-turn/m 7.96 x 105‡ 1 79.57747‡ 79.57747 7.96 x 10-4‡ gauss 104 4π x 10-3† 11† 10-5 4‡ -3 ‡ -5‡ TO: oersted 10 4π x 10 1 110 gamma 109 4π x 102† 105 105† 1 MULTIPLY BY ABOVE VALUE

† Assumes µ = 1; if µ ≠1, multiply by value of µ to convert from H to B. ‡ Assumes µ = 1; if µ ≠1, divide by value of µ to convert from B to H. 1 tesla ≡ 1 weber/m2.

For example, 1 tesla = 104 gauss. 1 gauss = 79.6 ampere-turns/m in an unloaded coil (µ = 1). If µ = 2.50, 1 tesla = 7.96 x 105 / 2.50 = 3.18 x 105 amp-turns/m.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 73 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Understanding Radiated Emissions Testing

In a radiated emissions test, electromagnetic emissions The Radio Noise Meter emanating from the equipment under test (EUT) are measured. Typically, the radio noise meter is either a receiver or a The purpose of the test is to verify the EUT’s ability to remain spectrum analyzer. Either is essentially a 120 kHz bandwidth, below specified electromagnetic emissions levels during tunable, RF microVolt meter calibrated in dB µV. A signal operation. A receive antenna is located either 3 or 10 meters response is shown in Figure E. from the EUT. In accordance with ANSI C63.4, the receive antenna must scan from 1 to 4 meters in height. The scanning The calculation of the measured E-field signal level helps to locate the EUT’s worst-case emissions level. is then given by: µ µ Figure 1 shows a block diagram of an emissions test system such E(dB V/m) = V(dB V) + CL1(dB) – PAG(dB) + CL2(dB) + as might be used for ANSI C63.4 testing. The test set-up is AF(dBm-1) composed of a receive antenna, a first interconnecting cable, a where: preamplifier, a second interconnecting cable, and a radio noise E(dB µ V/m) = Measured E-field meter (receiver or spectrum analyzer). V(dB µ V) = Radio Noise Meter Value The purpose of each of the components of the CL (dB) = Loss in Cable 1 radiated emissions test setup are: 1 The Receive Antenna PAG(dB) = Preamplifier gain

The performance measure of this antenna in relating the value CL2(dB) = Loss in Cable 2 of the incident E-field to the voltage output of the antenna is the AF(dBm-1) = Antenna Factor Antenna Factor. This is usually provided by the manufacturer in dB with units of inverse meters. A variety of antennas can be used for these measurements. Typically, a combination of two This computed value can be compared to the published antennas is used to cover the frequency range from 30 MHz to specification limit for determination of whether the measured 1000 MHz—a biconical covering the frequency range of 30 to value is less than the specification limit, thus showing compli- 200 MHz and a log periodic covering the frequency range of 200 ance with the requirement. to 1000 MHz. New antenna technology, such as EMCO’s BiConiLogTM antenna, can cover the complete frequency range. This Antenna Factor is shown at A. The First Interconnecting Cable This cable connects the antenna output to the preamplifier input. There is a reduction in measured signal amplitude due to losses in the cable. To increase accuracy, these losses need to be added to the measured value of the voltage out of the antenna to compensate for the losses. Cable loss is shown at B. The Preamplifier The preamplifier is typically used with spectrum analyzers to compensate for the high input noise figure typical of such devices. Receivers may not need this device. The amplifier makes the measured signal larger, thus the final answer must be corrected by subtracting the gain of the preamplifier. Preampli- fier gain is shown at C. The Second Interconnecting Cable This cable connects the output of the preamplifier to the radio noise meter. There is a reduction in measured signal amplitude due to losses in the cable. To increase accuracy, these losses need to be added to the measured value of the voltage out of the antenna to compensate for the losses. Cable loss is shown at D.

USA: FINLAND: UK: SINGAPORE: ONLINE: 74 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Understanding Radiated Emissions Testing Emissions Radiated Understanding

Figure 1. Radiated Emissions Test –

µ µ –1 E(dB V/m) = V(dB V) + CL1(dB) – PAG(dB) + CL2(dB) + AF(dBm )

Figure 1. Radiated Emissions Test Notes on Figure 1 A is the Antenna Factor (AF) versus frequency. The units are meters–1. The AF relates the RF voltage output of an Reference antenna to the E-Field causing the voltage to appear. B, D are the reduction in signal that is caused by losses in the interconnecting coaxial cables. C is the low noise figure preamplifier gain, which can vary with frequency. E is the value measured at some frequency by the 120 kHz bandwidth radio noise meter.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 75 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Understanding Radiated Immunity Testing

In a radiated immunity test, a test signal of RF energy, The Omni-directional Probe System typically three or ten volts per meter, is directed at the The probe system is used to directly measure the value of the equipment under test (EUT), and the EUT’s reaction to this field strength, at E. test signal is analyzed. The purpose of the test is to demon- Figure 2 shows a graphical display of the signals of the immunity strate the ability of the EUT to withstand the excitation of the test system. This figure also includes computations of the signal signal, without showing degraded performance or failure. The levels at a specific frequency of 100 MHz. more immune a product is to this test signal, the better it should operate when other electronic or electrical equipment The output level is given by: is present in its environment. µ µ –1 E(dB V/m) = SGout(dB V) + AG(dB) + TAF(dB) m ) Figure 1 shows a block diagram of an immunity test system such as might be used for IEC 61000-4-3 testing. The test where: µ setup is composed of a signal generator, an amplifier, a E(dB V/m) = The E-field test level forward/reverse power coupler with its associated power µ SGout(dB V) = The signal generator output meter, a radiating antenna, and an omni-directional E-field probe system. AG(dB) = Amplifier gain TAF(dB) m–1) = The transmit antenna factor The purpose of each of the components of the radiated immunity test setup is: The variables and terms in the expression above are used for The Signal Generator calibration test setups. They demonstrate how instrumentation The signal generator is used to provide the test signal. It should and facility factors contribute to meet the typically required have adequate output resolution to allow precise setting of the E-field uniformity value of – 0.0 dB, + 6.0 dB. Remember that reference level of the E-field to within 1 % of the desired level. actual testing to demonstrate that the EUT will not malfunction The signal generator must be capable of providing the desired when exposed to the desired level requires the addition of 80 % 80 % AM with a 1 kHz sine wave for testing. A typical signal amplitude modulation with a 1 kHz sine wave to the test signal. generator output is shown at A. 2 This, in turn, requires an additional 5.1 dB {10 x log10 [(1.8) ]}of The Amplifier linear gain from the amplifier than is found during calibration. The amplifier increases the test signal strength to levels, that when applied to the antenna, will produce the desired E-field levels. Note that EMC test amplifiers are specified with a minimum gain. Due to the extremely wide bandwidth, they can show ripple of several dB in the pass band. The amplifier must be operated in a linear mode to assure repeatability. A typical amplifier response is shown at B. The Forward / Reverse Power Coupler The forward / reverse power coupler is placed in-line with the amplifier output to the antenna input, as near to the antenna as practical. The difference in the forward and reverse power (the net power) is recorded to determine the input level necessary for developing the desired test signal, and to show that this desired input to the antenna is developed during testing. This is shown at C. The Antenna The antenna generates the desired E-field. Its performance in generating the E-field is given by the Transmit Antenna Factor (TAF), as shown at D.

USA: FINLAND: UK: SINGAPORE: ONLINE: 76 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Probe Probe

F/O Link Understanding Radiated Immunity Testing Immunity Radiated Understanding –

Reference Figure 1. Block Diagram of Typical Immunity Test Setup with Signal Levels & Characteristics Added

Figure 1. Radiated Immunity Test Notes on Figure 1 ❶ ❷ For immunity testing, test distance is from tip of antenna.

A is signal generator output. ❸ Immunity amplifiers are typically specified at minimum B is amplfier gain versus frequency. gain. Pass band ripple is result of extra wide bandwidths. C is signal generator output + amplifier gain (= input to antenna. D is transmit antenna factor (TAF). E is E-field level generated (= input to antenna + (TAF).

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 77 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Understanding Radiated Immunity Testing Immunity Radiated Understanding –

Reference

Figure 2. Graphical Representation of Immunity Test System Signal

Figure 2. Radiated Immunity Test Notes on Figure 2 ❶ Required test field is 10 V/m (= 20 dB V/m = 140 dB µ V/m). ❷ At 100 MHz TAF is – 8.66 dB => Vin to Antenna is 140 – (–8.66) = 148.66 dB µV.

❸ At 100 MHz Signal Generator Output is Antenna Input – Amplifier gain (= 148.86 – 47 = 101.86 dB µV = 123 µV).

USA: FINLAND: UK: SINGAPORE: ONLINE: 78 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com EMC Antenna Parameters and Their Relationships Parameters Antenna EMC –

Reference

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 79 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com EMC Antenna Parameters and Their Relationships Their and Parameters Antenna EMC –

Reference

USA: FINLAND: UK: SINGAPORE: ONLINE: 80 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com 5

6 EMC Antenna Parameters and Their Relationships Parameters Antenna EMC –

7 Reference

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 81 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Computing Required Input Power for a Given E-Field Level at a Given Distance

Introduction Table 1. Expressions for Computing G , G (dB), and i i This Application Note explains three separate methods for the m-1 calculation of input power to an antenna to achieve a specific AF(dB ), Given A Value for One Parameter value of E-field for immunity or susceptibility testing, at a specific distance from the antenna. The estimates of required power agree well between the three methods (See Table 2.), so any of the three methods can be used as a function of informa- tion available regarding the antenna.

Calculations This section describes three completely different, independent methods for calculating the input power required for a given E-field value as a function of frequency, at a given distance from the antenna. Inherent assumptions are that bore-sight alignment exists from the antenna to the point where the E-field is evaluated, and that ideal propagation conditions exist. Sample values, at 100 MHz, used in the calculations are: The three methods are: ◗ numerical gain over an isotropic antenna, G , = 2.05, ◗ i The Friis Transmission Formula ◗ gain in dB of the antenna, Gi , (dB) = 3.1 dB, ◗ The Transmit Antenna Factor ◗ Antenna Factor , AF (dB m-1) = 7.1 dB m-1 ◗ Using information from published catalog or data sheet values of E-field for a given reference level of E-field. Figure 1 shows the geometry assumed for the calculations, and some of the important variables. Input power to an antenna to develop specified E-field value is readily accomplished, given some combination of the following information: ◗ numerical gain, Gi ◗ gain in dB, Gi , (dB), and ◗ antenna factor, AF (dB m-1).

The required information is: Figure 1. ◗ distance from the transmitting antenna reference point, Geometry for the Calculation of Input Power ◗ frequency, and for a Given E-Field ◗ required E-field level. Expressions for computing any of these input variables, given a Note that the reference point for calibration is different for value for one, are shown in Table 1. the two standards applied for calibration of E-field generating antennas. The Society of Automotive Engineers Aerospace Recommended Practice 958 is applied for calibration of antennas used in MIL-STD testing, for a spacing of 1 meter, tip-to-tip. The American National Standards Institute C63.5 is applied from

USA: FINLAND: UK: SINGAPORE: ONLINE: 82 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com feed point for biconical dipole antennas, and, as shown, from Thus: the mid-point of all elements of a log periodic antenna’s element array. These measurement reference points on the antennas are important because they define the starting point of the distance to the point in free space, where the value of the E- Remembering that: field is defined. These calculations are valid for estimates of input power when testing will be conducted in an anechoic chamber. The ARP and ANSI calibration methods produce a value for AF that is Then: referred to as the “equivalent free space antenna factor”. This means that the effects of the calibration environment are removed from the antenna factor, and that the numerical values are very close to that which would be measure if the antenna or: had, in fact, been calibrated under true free space conditions.

Method I Using the Relationship for E-field at a and: Distance Derived from Ohm’s Law for Free Space and the Friss Transmission Formula: A variant of the Friss transmission formula is: To compute input power requirements, a linear value for voltage is required:

It relates the E-field [E (V/m)] produced at a distance [r (m)] due to net input RF power [Pt (W)] being applied to an antenna Then the required power input is: with known gain[Gt ], (see Figure 1). Solving for input power gives:

As an example calculation, suppose that a 10 V/m field was required at 3 meters for immunity testing. If the antenna chosen is a log periodic antenna with a gain of 2.05 at 100 MHz, the input power required is: Computing Required Input Power for a Given E-Field Level E-Field a Given for Power Input Required Computing –

Method II Using Transmit Antenna Factor The transmit antenna factor is a measure of the effectiveness of the given antenna in transmitting electromagnetic power. It Reference relates the RF voltage [Vin (Volts)] to the E field [E| d (V/m)] at a distance [d(m)] from the antenna, as determined at the distance of calibration of the antenna. (See Figure 1.) It is given by:

For the example given, Gt = 3.1 dB and d = 3m

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 83 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Method III Calculation of Required Input Power Using Comments Published Graphical Data: Three different computations of the desired input power, as Using the graphical data, it is estimated that the input power shown above, give three similar answers. The results are required for 10 V/m at 3 meters is 0.15 W. A typical plot of summarized in Table 2. Examination of these answers reveals input power for a given E-field level is shown in Figure 2. that the variance of the three answers is just under 2.5%, using the largest of the answers as a reference.

Table 2.

Summary of Results

Method of Computation Answer Friss Transmission Formula 14.63 W Transmit Antenna Factor 14.69 W Using Published Chart Values 15.00 W

The larger value obtained from using the Chart Values is likely to be due to inaccuracy in reading the chart. This value is 4.67 % (about 0.2 dB) larger than the other values, with more precise input. Comparing the two values with numerical input data gives a 0.03 dB difference. Figure 2. Note that the Friss Transmission Formula, used in Method l, Typical Plot of the Input power does not consider the effects of the ground plane. The answer for a Specific E-Field Value derived from this formula agrees with the answer derived using Method ll’s “equivalent free space antenna factor” value As seen in Figure 2, the 100 MHz value for input power for methodology. The results from these first two Methods also 1 V/m is approximately 0.15 W. agree with Method lll’s answer. This is derived using a graphical portrayal of the transmit antenna factor as computed from the Computing the input power in dB: measured antenna factor.

Cautions These computed values are based on the nominal conditions of

Computing Required Input Power for a Given E-Field Level E-Field Given a for Power Input Required Computing “free space” testing, i.e., testing in a Anechoic Chamber, to The desired E-field strength is 10 V/m: – contain the fields generated. They are useful estimates for other conditions, but engineering judgment must be applied to the selection of amplifier in all cases. Then power for 10 V/m is Other adjustments for sizing the input amplifier include the antenna input VSWR. At the upper and lower ends of their bandwidth, some antennas will have a VSWR that far exceeds the nominal value. In this case a correction, as shown in Reference Or, substituting the actual values: Figure 3, should be applied.

The linear value of the input power is:

USA: FINLAND: UK: SINGAPORE: ONLINE: 84 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com With respect to sizing the amplifier for use with a given antenna, remember that most calibration measurements are conducted with continuous wave (CW) excitation of the antenna. When actual testing is accomplished, it is usually accomplished with amplitude (AM) modulation. The most recently published specification requires 80 % AM with a 1 kHz sine wave. This requires an amplifier with 1.8 times the linear voltage output range of the CW signal. This, in turn, requires the amplifier output to be (1.8)2 larger than for the CW case, since power input increases as the square of the input voltage. This means that the power amplifier gain will need to be 10 x log [(1.8)2] = 5.1 dB greater than needed for the CW case. A summary of these factors is shown in Table 3. Figure 3. Table 3. Correction for Input Power Required, Summary of Input Power Allowances when the Antenna Input VSWR Exceeds 2:1. for Sizing an Amplifier If a numerical result is desired, the VSWR correction can be computed from: Factor Allowance (dB) True Linear Operation 1.0 Calibration Distance 2.5 Modulation Allowance 5.1 TOTAL 8.6

Remember also that the amplifier must be operated in its linear operating range, at least 1 dB below the 1 dB compression Thus, any amplifier input computed for use may actually point. need a 8.6 dB higher rating for proper continuous operation. In addition, no amplifier is as reliable running close to or at In addition, if the antenna input VSWR is more than 2:1, maximum rated power, thus an allowance for rating to assure additional compensation for the reflected power from the that the amplifier is running at about 90 % of rated power will antenna port is required. produce almost indefinite operation. This adds another NOTE: Cable loss is not considered in this allowance table since it approximately 1 dB to the required output power. is a factor of cable length.

Note that the ANSI calibration distance for measurement of the Level E-Field a Given for Power Input Required Computing antenna factor (AF) of log periodic antennas (at the center of –

DERIVATION AND USE OF FORWARD POWER GRAPHS the element array), is different than the distance where the Forward Power graphs in this catalog are derived from several immunity test calibration is performed (measured from the tip methods and each chart indicates which method was used. Since of the antenna). The ratio of these distances, in dB, should be SAE and ANSI antenna factors are typically within a few dB of free- added to the power amplifier rating to more closely estimate space antenna factors, graphs indicating “Forward Power Derived From AF” are valid for free-space environments, such as a fully the required power input level. For a typical EMCO large log anechoic chamber or an absorber-treated ground plane. Graphs periodic antenna, this difference in distance is just under indicating “Forward Power Measured Over Conducting Ground” are

1 meter. If the calibration distance is 3 meters from the tip of valid only for the specific geometry listed (antenna/field probe height Reference ÷ and separation) on an OATS or in a large high quality semi-anechoic the antenna, the correction would be 20 x log [ 4m 3m] = chamber. Graphs indicating “Forward Power Measured Over Ferrite 2.5 dB. Smaller antennas will need a lesser allowance. Ground” generally fall between the ground plane and free-space values. As with the ground plane numbers, these results are strictly valid for the same geometry on an OATS or a semi-anechoic chamber with comparable ferrite panels on the floor. Small chamber power requirements depend on the chamber’s dimensions and antenna location. Typically, small chamber power requirements will fall in the vicinity of the conducting and ferrite-ground results.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 85 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com REFERENCEAntenna Selection by Test Type

FCC 15 RADIATED EMISSIONS IEC/CISPR/EN RADIATED EMISSIONS SAE J1113 RADIATED EMISSIONS

20 MHz - 200 MHz 3104C Biconical 2 0 MHz -200 MHz 3104C Biconical 200 MHz - 1 GHz 3101 Conical Log Spiral 30 MHz - 300 MHz 3110B Biconical 30 MHz - 300 MHz 3110B Biconical 1 GHz - 10 GHz 3102 Conical Log Spiral 30 MHz - 300 MHz 3124 Calculable Biconical 30 MHz - 300 MHz 3124 Calculable Biconical 100 MHz - 1 GHz 3103 Conical Log Spiral

2 0 0 MHz - 2 GHz 3106 Dbl RdgWaveguide 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 20 MHz - 200 MHz 3104C Biconical

1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 30 MHz - 300 MHz 3124 Calculable Biconical

1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide 30 MHz - 1 GHz 3121C Dipole 30 MHz - 300 MHz 3110B Biconical 30 MHz - 1 GHz 3121C Dipole 26 MHz - 2 GHz 3142B BiConiLogTM 200 MHz - 2 GHz 3106 Dbl Rdg Waveguide 26 MHz - 2 GHz 3142B BiConiLogTM 80 MHz - 2 GHz 3144 Log Periodic 1 GHz - 18 GHz 3115 Dbl Rdg Waveguide 80 MHz - 2 GHz 3144 Log Periodic 200 MHz - 5 GHz 3147 Log Periodic 30 MHz - 1 GHz 3121C Dipole 200 MHz - 5 GHz 3147 Log Periodic 200 MHz - 2 GHz 3148 Log Periodic 26 MHz - 2 GHz 3142B BiConiLogTM 200 MHz - 2 GHz 3148 Log Periodic 960 MHz - 40 GHz 3160 Std Gain Horns 80 MHz - 2 GHz 3144 Log Periodic 960 MHz - 40 GHz 3160 Std Gain Horns 960 MHz - 40 GHz 3161 Std Gain Horns 200 MHz - 5 GHz 3147 Log Periodic 960 MHz - 40 GHz 3161 Std Gain Horns 10 kHz - 30 MHz 6502 Loop - Active 200 MHz - 2 GHz 3148 Log Periodic 1 kHz - 30 MHz 6507 Loop - Active 10 kHz - 30 MHz 6502 Loop - Active RADIATED EMISSIONS FCC 18 20 Hz - 5 MHz 6511 Loop - Passive 1 kHz - 30 MHz 6507 Loop - Active 20 MHz - 200 MHz 3104C Biconical 10 kHz - 30 MHz 6512 Loop - Passive SAE J1113 RADIATED IMMUNITY 30 MHz - 300 MHz 3110B Biconical IEC/CISPR/EN RADIATED IMMUNITY 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 200 MHz - 1 GHz 3101 Conical Log Spiral

1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 2 0 MHz - 3 0 0 MHz 3109 Dbl Rdg Waveguide 1 GHz - 10 GHz 3102 Conical Log Spiral

1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 100 MHz - 1 GHz 3103 Conical Log Spiral

30 MHz - 1 GHz 3121C Dipole 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 2 0 MHz - 3 0 0 MHz 3109 Dbl Rdg Waveguide TM 30 MHz - 300 MHz 3124 Calculable Biconical 26 MHz - 2 GHz 3140 BiConiLog 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide

TM TM 26 MHz - 2 GHz 3142B BiConiLog 26 MHz - 2 GHz 3142B BiConiLog 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 80 MHz - 2 GHz 3144 Log Periodic 80 MHz - 2 GHz 3144 Log Periodic 26 MHz - 2 GHz 3140 BiConiLog 200 MHz - 5 GHz 3147 Log Periodic 200 MHz - 5 GHz 3147 Log Periodic 26 MHz - 2 GHz 3142B BiConiLogTM 200 MHz - 2 GHz 3148 Log Periodic 200 MHz - 2 GHz 3148 Log Periodic 80 MHz - 2 GHz 3144 Log Periodic 960 MHz - 40 GHz 3160 Std Gain Horns 960 MHz - 40 GHz 3160 Log Periodic 200 MHz - 2 GHz 3148 Log Periodic 960 MHz - 40 GHz 3161 Std Gain Horns 960 MHz - 40 GHz 3161 Log Periodic 960 MHz - 40 GHz 3160 Log Periodic 10 kHz - 30 MHz 6502 Loop - Active 1 kHz - 30 MHz 6509 Loop - Passive 960 MHz - 40 GHz 3161 Log Periodic 1 kHz - 30 MHz 6507 Loop - Active SAE J551 RADIATED EMISSIONS 1 kHz - 30 MHz 6509 Loop - Passive 20 Hz - 5 MHz 6511 Loop - Passive 20 MHz - 200 MHz 3104C Biconical SAE J1507 RADIATED IMMUNITY VCCI RADIATED EMISSIONS 30 MHz - 300 MHz 3124 Calculable Biconical 960 MHz - 40 GHz 3160 Std Gain Horns 30 MHz - 300 MHz 3110B Biconical 20 MHz - 200 MHz 3104C Biconical 960 MHz - 40 GHz 3161 Std Gain Horns 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 30 MHz - 300 MHz 3110B Biconical 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 30 MHz - 300 MHz 3124 Calculable Biconical SAE J1551 RADIATED IMMUNITY 30 MHz - 1 GHz 3121C Dipole 30 MHz - 1 GHz 3121C Dipole 26 MHz - 2 GHz 3142B BiConiLogTM 960 MHz - 40 GHz 3160 Std Gain Horns 26 MHz - 2 GHz 3142B BiConiLogTM 80 MHz - 2 GHz 3144 Log Periodic 960 MHz - 40 GHz 3161 Std Gain Horns 200 MHz - 5 GHz 3147 Log Periodic 200 MHz - 5 GHz 3147 Log Periodic 200 MHz - 2 GHz 3148 Log Periodic RADIATED EMISSIONS 200 MHz - 2 GHz 3148 Log Periodic SAE J1816

VCCI RADIATED IMMUNITY 30 Hz - 50 MHz 3301B Rod - Active 30 Hz - 50 MHz 3301B Rod - Active 10 kHz - 30 MHz 6502 Loop - Active 10 kHz - 30 MHz 6502 Loop - Active 200 MHz - 1 GHz 3101 Conical Log Spiral 1 kHz - 30 MHz 6507 Loop - Active 1 kHz - 30 MHz 6507 Loop - Active 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 10 kHz - 30 MHz 6512 Loop - Passive 10 kHz - 30 MHz 6512 Loop - Passive 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide

30 MHz - 1 GHz 3121C Dipole SAE J551 RADIATED IMMUNITY NSA 65-6 TRANSMIT 26 MHz - 2 GHz 3140 BiConiLogTM 20 MHz - 300 MHz 3109 Biconical 26 MHz - 2 GHz 3142B BiConiLogTM 1 kHz - 30 MHz 3303 Rod - Active 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 80 MHz - 2 GHz 3144 Log Periodic 1 kHz - 30 MHz 6509 Loop - Passive 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 200 MHz - 2 GHz 3148 Log Periodic 26 MHz - 2 GHz 3140 BiConiLogTM 1 kHz - 30 MHz 6509 Loop - Active NSA 65-6 RECEIVE 26 MHz - 2 GHz 3142B BiConiLogTM 30 Hz - 50 MHz 3301B Rod - Active VDE RADIATED EMISSIONS 960 MHz - 40 GHz 3160 Std Gain Horns 1 kHz - 30 MHz 6509 Loop - Active 1 kHz - 30 MHz 6507 Loop - Active 10 kHz - 30 MHz 6502 Loop - Active SAE J1338 RADIATED IMMUNITY CISPR/EC RADIATED EMISSIONS 960 MHz - 40 GHz 3160 Std Gain Horns 30 Hz - 50 MHz 3301B Rod - Active 960 MHz - 40 GHz 3161 Std Gain Horns

USA: FINLAND: UK: SINGAPORE: ONLINE: 86 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com MIL-STD-285 TRANSMIT MIL-STD-461E RADIATED SUSCEPTIBILITY NACSIM RADIATED EMISSIONS

20 MHz - 200 MHz 3104C Biconical 200 MHz - 1 GHz 3101 Conical Log Spiral 200 MHz - 1 GHz 3101 Conical Log Spiral 20 MHz - 300 MHz 3109 Biconical 1 GHz - 10 GHz 3102 Conical Log Spiral 1 GHz - 10 GHz 3102 Conical Log Spiral 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 100 MHz - 1 GHz 3103 Conical Log Spiral 100 MHz - 1 GHz 3103 Conical Log Spiral 1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide 20 MHz - 300 MHz 3109 Biconical 20 MHz - 200 MHz 3104C Biconical

960 MHz - 40 GHz 3160 Std Gain Horns 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 30 MHz - 300 MHz 3110B Biconical

960 MHz - 40 GHz 3161 Std Gain Horns 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 30 MHz - 300 MHz 3124 Calculable Biconical TM 26 MHz - 2 GHz 3140 BiConiLog 1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide 200 MHz - 2 GHz 3106 Dbl Rdg Waveguide TM 26 MHz - 2 GHz 3142B BiConiLog 960 MHz - 40 GHz 3160 Std Gain Horns 1 GHz - 18 GHz 3115 Dbl Rdg Waveguide 200 MHz - 2 GHz 3148 Log Periodic 960 MHz - 40 GHz 3161 Std Gain Horns 18 GHz - 40 GHz 3116 Dbl Rdg Waveguide 1 kHz - 30 MHz 3303 Rod - Passive 26 MHz - 2 GHz 3140 BiConiLogTM 4 0 0 MHz - 6 GHz 3164 Diag Dual Polar Horn 1 kHz - 30 MHz 6509 Loop - Passive 26 MHz - 2 GHz 3142B BiConiLogTM 960 MHz - 40 GHz 3160 Std Gain Horns 80 MHz - 2 GHz 3144 Log Periodic 960 MHz - 40 GHz 3161 Std Gain Horns MIL-STD-285 RECEIVE 200 MHz - 2 GHz 3148 Log Periodic 26 MHz - 2 GHz 3142B BiConiLogTM 20 MHz - 200 MHz 3104C Biconical 2 0 Hz - > 5 0 kHz 7603 Magnetic Field Coil 200 MHz - 5 GHz 3147 Log Periodic 20 MHz - 300 MHz 3109 Biconical 3 0 Hz - > 5 0 kHz 7605 Magnetic Field Coil 200 MHz - 2 GHz 3148 Log Periodic 30 MHz - 300 MHz 3110B Biconical 3 0 Hz - > 5 0 kHz 7606 Magnetic Field Coil 3 0 Hz - 5 0 MHz 3301B Monopole (Rod)

1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 10 kHz - 30 MHz 6502 Loop - Active

1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide MIL-STD-1541 RADIATED EMISSIONS 1 kHz - 30 MHz 6507 Loop - Active 20 Hz - 5 MHz 6511 Loop - Passive 960 MHz - 40 GHz 3160 Std Gain Horns 200 MHz - 1 GHz 3101 Conical Log Spiral

960 MHz - 40 GHz 3161 Std Gain Horns 1 GHz - 10 GHz 3102 Conical Log Spiral Type Selection Antenna by Test 26 MHz - 2 GHz 3142B BiConiLogTM 100 MHz - 1 GHz 3103 Conical Log Spiral –

200 MHz - 2 GHz 3148 Log Periodic 20 MHz - 200 MHz 3104C Biconical 30 Hz - 50 MHz 3301B Rod - Active 30 MHz - 300 MHz 3110B Biconical 1 kHz - 30 MHz 6507 Loop - Active 30 MHz - 300 MHz 3124 Calculable Biconical TELECOM 200 MHz - 2 GHz 3106 Dbl Rdg Waveguide 1 GHz - 18 GHz 3115 Dbl Rdg Waveguide 440 MHz - 460 MHz 3125 Dipole (450) 18 GHz - 40 GHz 3116 Dbl Rdg Waveguide 590 MHz - 610 MHz 3125 Dipole (600) 30 MHz - 1 GHz 3121C Dipole

824 MHz - 915 MHz 3125 Dipole (870) 960 MHz - 40 GHz 3160 Std Gain Horns Reference 9 3 5 MHz - 960 MHz 3125 Dipole (950) 960 MHz - 40 GHz 3161 Std Gain Horns 1600 MHz -1620 MHz 3125 Dipole (1610) 26 MHz - 2 GHz 3142B BiConiLogTM 1710 MHz -1785 MHz 3125 Dipole (1750) 200 MHz - 2 GHz 3148 Log Periodic 1805 MHz -1880 MHz 3125 Dipole (1840) 10 kHz - 30 MHz 6502 Loop - Active 1850 MHz -1910 MHz 3125 Dipole (1880) 1 kHz - 30 MHz 6507 Loop - Active 2440 MHz -2460 MHz 3125 Dipole (2450) 20 Hz - 5 MHz 6511 Loop - Passive 2990 MHz -3010 MHz 3125 Dipole (3000) 400 MHz - 6 GHz 3164 Diag Dual Polar Horn MIL-STD-1541 RADIATED SUSCEPTIBILITY

100 MHz - 1 GHz 3103 Conical Log Spiral MIL-STD-461E RADIATED EMISSIONS 20 MHz - 300 MHz 3109 Biconical 200 MHz - 1 GHz 3101 Conical Log Spiral 200 MHz - 2 GHz 3106 Dbl Rdg Waveguide 1 GHz - 10 GHz 3102 Conical Log Spiral 1 GHz - 18 GHz 3115 Dbl Rdg Waveguide 100 MHz - 1 GHz 3103 Conical Log Spiral 26 MHz - 2 GHz 3140 BiConiLogTM 20 MHz - 200 MHz 3104C Biconical 26 MHz - 2 GHz 3142B BiConiLogTM 30 MHz - 300 MHz 3110B Biconical 80 MHz - 2 GHz 3144 Log Periodic 30 MHz - 300 MHz 3124 Calculable Biconical 200 MHz - 2 GHz 3148 Log Periodic 2 0 0 MHz - 2 GHz 3106 Dbl Rdg Waveguide 960 MHz - 40 GHz 3160 Std Gain Horns 1 GHz - 1 8 GHz 3115 Dbl Rdg Waveguide 1 kHz - 30 MHz 6509 Loop - Passive 1 8 GHz - 4 0 GHz 3116 Dbl Rdg Waveguide 20 Hz - 5 MHz 6511 Loop - Passive 4 0 0 MHz - 6 GHz 3164 Diag. Dual Polar Horn 960 MHz - 40 GHz 3160 Std Gain Horns 960 MHz - 40 GHz 3161 Std Gain Horns 26 MHz - 2 GHz 3142B BiConiLogTM 80 MHz - 2 GHz 3144 Log Periodic 200 MHz - 2 GHz 3148 Log Periodic 30 Hz - 50 MHz 3301B Rod - Active 20 Hz - 500 kHz 7604 Coil - Transducer

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 87 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Antenna Selection by Frequency

USA: FINLAND: UK: SINGAPORE: ONLINE: 88 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 89 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Expanded Uncertainty Values for Antenna Calibrations (95% Confidence)

TYPE MODEL FREQUENCY SAE, ARP 958 ANSI C63.5 ANSI C63.5 IEEE 291, IEEE NUMBER RANGE 1M 3M 10M 1309, ANSI C63.4

Conical Log Spiral 3101 200MHz- 200-300 MHz (L&R) 1GHz +/- 2.8 dB 300-850 MHz +/- 0.8 dB 850-1000 MHz +/- 1.4 dB

Conical Log Spiral 3102 1GHz- 1-10 GHz (L&R) 10GHz +/- 0.8 dB

Conical Log Spiral 3103 100MHz- +/- 2.0 dB Type B (L&R) 1GHz

Biconical 3104C 20MHz- 20-200 MHz 200MHz 20-30 MHz 200MHz +/- 1.2 dB +/- 0.9 dB +/- 1.0 dB 30-200 MHz +/- 0.9 dB

Biconical 3109 20MHz- 20-30 MHz 20-30 MHz 20-30 MHz 300MHz +/- 1.0 dB +/- 0.9 dB +/- 1.0 dB 30-200 MHz 30-300 MHz 30-300 MHz +/- 1.4 dB +/- 0.9 dB +/- 0.9 dB 200-300 MHz +/- 2.0 dB

Biconical 3110B 30MHz- 30-200 MHz 20-200 MHz 20-30 MHz 300MHz +/- 1.2 dB +/- 0.8 dB +/- 1.0 dB 200-300 MHz 30-300 MHz +/- 2.2 dB +/- 0.9 dB

BiCal 3124 30MHz- 300MHz - 300 MHz 300MHz +/-0.50 dB

Double-Ridged 3106 200MHz- 0.2-1.9 GHz Waveguide 2GHz +/- 1.0 dB Horn 1.9-2.0 GHz +/- 1.3 dB

Double-Ridged 3115 1GHz- 1-18 GHz Waveguide 18GHz +/- 0.3 dB Horn

Double-Ridged 3116 18GHz- 18-30 GHz Waveguide 40GHz +/- 0.8 dB Horn 30-40 GHz +/- 1.3 dB

Diagonal Dual 3164 400 MHz - .4-6 GHz Polarized Horn 6 GHz +/- 1.0 dB

Octave Horns 3161-01 1GHz- 1-2 GHz 2GHz +/- 0.9 dB

Octave Horns 3161-02 2GHz- 2-4 GHz 4GHz +/- 0.5 dB

Octave Horns 3161-03 4GHz- 4-4.5 GHz 8GHz +/- 1.0 dB 4.5-7.5 GHz +/- 0.4 dB 7.5-8 GHz +/- 1.3 dB

Dipole 3121C 30MHz- 4 Balun Kit 1GHz

Dipole 3121C 30MHz- 30-60 MHz 30-60 MHz Balun 1 60MHz +/- 0.9 dB +/- 1.0 dB

Dipole 3121C 60MHz- 60-140 MHz 60-140 MHz Balun 2 140MHz +/- 0.7 dB +/- 0.7 dB

USA: FINLAND: UK: SINGAPORE: ONLINE: 90 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Expanded Uncertainty Values for Antenna Calibrations (95% Confidence)

TYPE MODEL FREQUENCY SAE, ARP 958 ANSI C63.5 ANSI C63.5 IEEE 291, IEEE NUMBER RANGE 1M 3M 10M 1309, ANSI C63.4

Dipole 3121C 140MHz- 140-375 MHz 140-375 MHz Balun 3 400MHz +/- 1.0 dB +/- 1.0 dB 375-400 MHz 375-400 MHz +/- 1.4 dB +/- 1.4 dB

Dipole 3121C 400MHz- 400-700 MHz 400-700 MHz Balun 4 1GHz +/- 1.0 dB +/- 1.6 dB 700-1000 MHz 700-1000 MHz +/- 1.4 dB +/- 2.1 dB

DB3&4 Tuned 140-400 MHz 140-400 MHz

+/- 0.8 dB +/- 0.8 dB UncertaintyValues 400-1000 MHz 400-1000 MHz

+/- 1.0 dB +/- 1.0 dB –

BiConiLog 3142B 26MHz- 26-30 MHz 26-30 MHz 26-30 MHz 2GHz/ +/- 1.4 dB +/- 1.5 dB +/- 2.1 dB 26MHz- 30-1000 MHz 30-1000 MHz 30-1000 MHz 1.1GHz +/- 0.8 dB +/- 1.0 dB +/- 1.0 dB 1-2 GHz 1-2 GHz 1-2 GHz +/- 1.2 dB +/- 1.3 dB +/- 1.4 dB

LPA 3144 80MHz- +/- 2.0 dB, Type B 80-2000 MHz 80-2000 MHz 3144 Reference 2GHz +/- 0.9 dB +/- 0.8 dB

LPA 3147 200MHz- 200-2000 MHz 200-1000 MHz 200-1000 MHz 5GHz +/- 0.5 dB +/- 0.8 dB +/- 0.8 dB 2-5 GHz 1-2 GHz 1-2 GHz +/- 1.6 dB +/- 0.9 dB +/- 1.0 dB 2-5 GHz 2-5 GHz +/- 1.5 dB +/- 1.5 dB

LPA 3148 200MHz- 200-1000 MHz 200-1000 MHz 200-1000 MHz 2GHz +/- 0.6 dB +/- 0.9 dB +/- 0.8 dB 1-2 GHz 1-2 GHz 1-2 GHz +/- 1.41 dB +/- 0.9 dB +/- 0.9 dB

Rod 3301B 30Hz- 30Hz-50MHz 41 inch Rod 50MHz +/- 0.3 dB

Rod 3303 1kHz- 1kHz-10kHz 30MHz +/- 1.0 dB 10kHz-30MHz +/- 0.1 dB

Loop 6502 10kHz- (TBD) 30MHz

Loop 6507 1kHz- (TBD) 30MHz

Loop 6509 1kHz- (TBD) 30MHz

Loop 6511 20Hz- (TBD) 5MHz

Loop 6512 10kHz- (TBD) 30MHz

Coil 7603 20Hz- No Cal/VSWR Only 50kHz

Coil 7604 20Hz- No Cal/VSWR Only 500kHz

Coil 7605 30Hz- No Cal Req 50kHz

Coil 7606 30Hz- No Cal Req 50kHz

Data effective 8 June 1998. EMCO Laboratory capabilities include all products produced by EMCO and models of the same technology produced by other manufacturers. Uncertainty values reflect the uncertainty analysis conducted using 1997 data. Values released are valid with a 2 sigma (95%) confidence level and are representative of the measurement quality conducted by the EMCO Laboratory using the industry recognized standards listed above.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 91 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Laboratory Standards Compliance List

SAE, ARP 958 - 1997, Society of Automotive Engineers, Aerospace Recommended Practice 958, Electromagnetic Interference Measurement Antennas; Standard Calibration Method. UncertaintyValues

– ANSI C63.4 - 1992, American National Standard, Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz.

ANSI C63.5 - 1988, American National Standard, Calibration of Antennas Used for Radiated Emission Measurements in Electromagnetic Interference (EMI) Control.

ANSI C63.6 - 1988, American National Standard, Guide for the Computation of Errors in Open Area Test Site Measurements. Reference

ANSI C63.7 - 1992, American National Standard, Guide for Construction of Open-Area Test Sites for Performing Radiated Emission Measurements.

ANSI Z540-1 - 1994, American National Standard, Calibration Laboratories and Measuring and Test Equipment - General Requirements.

ANSI Q91 - 1994, Quality Systems - Model for Quality Assurance in Design, Development, Production, Installation and Servicing.

ISO Guide 25 - 1990, International Standards Organization, General Requirements for the Competence of Calibration and Testing Laboratories.

IEEE 291 - 1991, Institute of Electrical and Electronics Engineers, Standard Methods for Measuring Electromagnetic Field Strengths of Sinusoidal Continuous Waves, 30 Hz to 30 GHz.

IEEE Std 1309 - 1996, Institute of Electrical and Electronics Engineers, Standard for Calibration of Electromagnetic Field Sensors and Probes, Excluding Antennas, from 9 kHz to 40 GHz.

NIST Technical Note 1297, 1994 edition, National Institute of Standards and Technology, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results.

NIS 81, Edition 1, May 1994, NAMAS, The Treatment of Uncertainty in EMC Measurements.

USA: FINLAND: UK: SINGAPORE: ONLINE: 92 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com A Statistical Approach to Measurement Uncertainty

MICHAEL D. FOEGELLE EMC Test Systems, Austin, TX

Despite the fact that most authors have skirted the subject of Type A analyses, the method is not so difficult that it should be avoided altogether.

Introduction The methods for determining a value is irrefutable since it has been Over the past couple of years, the measurement uncertainty have been determined from real world measure- topic of measurement uncertainty has divided into two generic classes: ments. The biggest complaint I hear come to the forefront in the interna- · Type A represents a statistical from engineers being exposed to the tional EMC community. In brief, the uncertainty based on a normal Type B uncertainty budget method intention of measurement uncer- distribution. for the first time is the fact that too tainty is to take the more traditional · Type B represents uncertainties many of the terms are either poorly terms of precision, accuracy, random determined by any other means. defined by equipment manufacturers error, and systematic error used in In last year’s ITEM, Manfred or must simply be estimated. In scientific circles and replace them Stecher wrote an article describing many cases, the chosen values may be with a single term. This term the introduction of uncertainty too stringent in order to provide a represents the total contribution to evaluations into various EMC safety margin. On the other hand, the the expected deviation of a measure- standards and explained the tech- desire for smaller total uncertainties ment from the actual value being nique typically used to determine can lead to using smaller estimations measured1-2. measurement uncertainties for EMC than is realistic for some terms that 4 In general, precision is a measure of measurements . (A similar paper was are hard to determine. random error, or how closely re- also presented at the 1996 IEEE Antenna manufacturers have easy peated attempts hit the same point on International EMC Symposium in access to a vast database of antenna 5 a target, while accuracy is a measure Santa Clara, CA. ) The article gives an calibrations with which to determine of systematic error, or how close adequate introduction to the Type B statistical trends. However, as the those attempts are to the center of the evaluation method, which uses data shown here will demonstrate, it target. It is obvious that both individual measurements, manufac- is not necessary to have an extremely contributions must be accounted for turers specifications, and even large sample to get acceptable results. in order to determine the quality of a educated guesses to determine a The real issue in using a statistical measurement, although the combina- combined uncertainty. However, the approach is in determining where it tion of the two can sometimes be author is a little too quick to discard fails and using a Type B analysis to fill more subtle than might be expected. the statistical Type A uncertainty in the gaps. The document NIS 81, There has been some discussion over measurement as impractical. To be “The Treatment of Uncertainty in whether the term reproducibility is sure, the Type A analysis does suffer EMC Measurements”, released by 1 also replaced by uncertainty since the from the very pitfalls which Mr. NAMAS , recommends this exact concept of reproducibility must Stecher points out. However, with a approach. contain variations in the equipment bit of care it is possible to obtain a Certainly a Type A analysis of a set under test (EUT) and therefore does significant amount of useful informa- of measurements can be expected to not represent the same quantity as tion from the technique. include all random errors of the measurement uncertainty3. The advantage of a Type A entire measurand and none of its uncertainty measurement is that systematic errors. But does that when done correctly, the resulting mean that the Type A uncertainty

Reprinted from ITEM 1998, Courtesy of R&B Enterpises.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 93 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com contains only the random portion of nent of the uncertainty may be radiating source such as a comb the individual terms which might be reduced through repeated measure- generator or amplified noise source used in a Type B analysis? Certainly ments of the EUT. In this case, the be used as the EUT for uncertainty not. It is not possible to completely standard deviation of the mean, sq(), determinations. This provides a separate random and systematic given by: repeatable EUT which, when used in 2 effects into individual categories . sq() conjunction with “round robin” type sq()= k Random effects such as positioning n testing, can allow determination of error[6] or cable length and fre- even the systematic elements of your represents the uncertainty of the quency dependence of standing measurement uncertainty to within resulting mean. This last point has waves can serve to randomize such the uncertainty of the round robin been misinterpreted by some due to a systematic errors as site imperfec- test. This type of EUT also provides a confusing statement in section 3.2.6 tions or mismatch errors. Intention- broad continuous frequency range for of NIS 81, “The standard uncertainty, ally varying the setup between uncertainty determination as op- ux(), of an estimate x of an input repeated measurements can do the i i posed to the random spectrum points quantity q is therefore same. of a typical EUT. If a reference ux()= sq ().” This statement is The discussion presented here will i source is not available, a number of certainly true, but some readers have focus primarily on antenna calibra- the same benefits may be obtained construed it to mean that the stan- tion measurements, which have using a stable signal generator and dard uncertainty of a single measure- many similarities to radiated 7-8 appropriate radiating antenna. ment qk is given by sq(). This is emissions measurements. However, = However, transmit cable placement true if n = 1so that sq() sq (k ). many of the techniques demon- This is explained more clearly in and other effects will add some strated will be applicable to radiated section 3.2.3 of NIS 81 where the additional random error and this susceptibility and conducted tests. concept of predetermination is method does not lend itself to inter- Despite the fact that most authors discussed. site comparisons. have skirted the subject of Type A Time constraints and other analyses, the method is not so practical considerations will often Uncertainty Example: difficult that it should be avoided make it unfeasible to perform more Antenna Calibration altogether. In fact, many of the more than a single measurement on an Antenna calibrations have uncer- difficult terms to determine for a EUT. However, repeat measurements tainty aspects similar to that of both typical Type B analysis are already can be performed on a similar EUT to radiated emissions and susceptibility included in the random error of the tests. However, the one systematic predetermine sq()k as the expected A Statistical Approach to Measurement Uncertainty to Measurement A StatisticalApproach total measurement, thus reducing the standard uncertainty of an individual element missing from the test is the – overall task. measurement. If a smaller uncer- absolute value of the fields (or signal tainty due to random errors is levels) involved. Thus, the test only Type A Evaluation of desired, multiple measurements can depends on the linearity of the Uncertainty be made and the value of sq()re- instrumentation and not its absolute Random effects cause repeated duced accordingly. The value of used calibration. However, this does not measurements to vary in an unpre- in this case is the number of mea- effect the validity of this method for

Reference dictable manner. The associated surements made on the EUT, not the determining Type A uncertainties for uncertainty can be calculated by number of measurements used in the EMC tests since the systematic error applying statistical techniques to the predetermination. in field level cannot be determined by repeated measurements. An estimate It should be noted that variations this method without inter-site of the standard deviation, sq()k , of a in the EUT as a function of time, as comparisons or other tests using series of n readings, qk , is obtained well as that of multiple like EUTs as in multiple methods to determine the from the method demonstrated here, will absolute field level. To obtain the data shown here, a 1 n be included in the uncertainty = − 2 sq()kk∑()qq determined through this method. For sample of 26 different antennas ()n −1 = k 1 this reason, and to provide a means to calibrated over a three-and-a-half where q is the mean value of determine some of the systematic month period was used. The anten- measurements. The random compo- contributions to the uncertainty, it is nas were identical log-periodic recommended that a stable reference antennas with a frequency span of 200

USA: FINLAND: UK: SINGAPORE: ONLINE: 94 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com MHz to 1 GHz. The time period spanned from mid-August through the end of October, providing a significant UNCERTAINTY TERMS FROM NIS-81 variation in temperature and weather conditions. The data were measured at 1601 frequency points using a ◆ Estimated standard deviation from a sample of readings: vector network analyzer, low loss cables, and a positioning 1 n sq()= ∑()qq− 2 tower with 0.1 cm positioning resolution. The test was kk− ()n 1 k =1 performed at a 10-meter separation, 2-meter transmit height, and 1- to 4-meter scan height per ANSI C63.5 on an ◆ Standard deviation of the mean of readings: open area test site (OATS). The vector network analyzer sq()k sq()= used has over 100 dB of available dynamic range and n superior external noise rejection, making it ideal for use ◆ Standard uncertainty (of the mean of readings) resulting from on an OATS where ambients would be a problem for a type A evaluation: = traditional spectrum analyzer/tracking generator combi- ux()i sq () nations. Since the network analyzer does not offer a ◆ Standard uncertainty for contributions with a normal max-hold or other such functionality traditionally seen probability distribution: on a spectrum analyzer, it is necessary to perform the U ux()= max-hold by transferring individual traces to a control- i k ling PC and allowing the test software to perform the max- where U represents the expanded uncertainty of the hold. normal distribution (last item below). To facilitate this method, it is also necessary to step the tower and take frequency sweeps at discrete heights since ◆ Standard uncertainty for contributions with rectangular the network analyzer cannot sweep the entire frequency probability distribution: aa+−− band fast enough to get acceptable height resolution when ux()= ii the tower moves continuously. Tests with tuned dipoles i 23 have shown that the variation in the antenna factor at 1 for an asymmetrical distribution, where ai+ and ai− are GHz for a 1-cm step, a 5-cm step, and a single frequency the bounds of the rectangular region, or point continuous motion max-hold is less than ±0.02 dB. a ux()= i If the tower step size was too large, or the sweep time too i 3 slow in the case of a continuous motion, the measurement for a symmetrical region with bounds ± . could be expected to introduce a systematic error since ai missing a peak signal would always result in a measure- ◆ Standard uncertainty for contributions with U shaped Uncertainty to Measurement A Approach Statistical ment lower than the real value. This error exists for probability distribution: – scanned height radiated emissions tests as well as antenna aa+−− calibrations. For an antenna calibration, this error would ux()= ii i 22 result in a larger antenna factor than is actually the case. The standard deviation of the 26 antenna factors and for an asymmetrical distribution, where ai+ and ai− their maximum deviations from the average are shown as are the bounds of the U shaped region, or a a function of frequency in Figure 1. The negative of the = i ux()i standard deviation is also shown for comparison to the 2 Reference negative deviation. The symmetry of the positive and ± for a symmetrical region with bounds ai or where negative deviations is a first indication of how closely the ai is the larger of ai+ or ai− . sample approximates a normal distribution. An asym- ◆ metrical envelope would be a cause for concern and Standard uncertainty in terms of the measured quantity: uy()=⋅ cux ( ) indicate the need for separate positive and negative iii uncertainty values at the minimum. ◆ Combined standard uncertainty:

Neglecting, for the moment, any additional contribu- N uy()= u2 () y tions to the uncertainty due to systematic errors, these ci∑ i=1 data indicate that the expanded uncertainty ( k = 2 ) of an individual calibration is less than ±0.5 dB at all frequen- ◆ Expanded uncertainty: cies. That means that an uncertainty of ±0.5 dB with =⋅ =⋅ Ukuyc () or Ukuypc() greater than 95% confidence can be claimed for this

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 95 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Antenna Factor Deviation From Average represents calibrations of 26 different antennas performed at different times. About fifty percent of the 0.6 antennas were directly off the production line, but the other half 0.4 were re-calibrations of antennas as many as ten years old. Although one 0.2 might expect+SD a batch of antennas to (dB) be as close-SD to identical as possible, 0 this sampleMax-Avg surely must contain some amount Min-Avgof manufacturing uncer- Deviation -0.2 tainty. If this manufacturing uncer- tainty is non-zero, then it must also -0.4 be contained within the above uncertainty. Since the “perfect” -0.6 antenna, in terms of manufacturing 200 300 400 500 600 700 800 900 1000 quality, is defined by the average of all Frequency (MHz) antennas, this uncertainty must be totally random. Figure 1. The Standard Deviation, Maximum Minus Average, and Minimum Minus Average Although this manufacturing of the Antenna Factors from a Set of Twenty-six Identical Antennas. Neglecting any additional systematic errors, the expanded uncertainty (k=2) for an individual antenna factor would then uncertainty may add to the total be twice the standard deviation for a value of approximately ±0.5 dB with greater than 95% calibration uncertainty measured by confidence at all frequencies. this technique, as long as the resulting uncertainty is within an acceptable calibration. A second verification of which deviated from the average range, it does not matter whether or this claim is the fact that, with the more than ±0.5 dB. This provides an not the contribution is large or small. exception of the negative deviation added level of confidence in the In this case, there is also the added above 950 MHz, none of the antennas quality of this uncertainty value. benefit that the intentional introduc- in the sample had antenna factors It should be noted that this data tion of totally random effects due to A Statistical Approach to Measurement Uncertainty to Measurement A Approach Statistical

– Antenna Factor Deviation From Average

3

2.5

2 +SD' Reference 1.5 -SD' (dB) Max'-Avg'

1

Min'-Avg'

0.5

Max-Avg














































































































































































































































































































































































































































Deviation

Min-Avg

0

-0.5

-1 200 300 400 500 600 700 800 900 1000 Frequency (MHz)

Figure 2. The Effect of EUT Variation on the Standard Deviation. Note the Difference in the Maximum Deviation for the Primed Sample Vs. Un- primed. The effect on the standard deviation is sufficient to equal or surpass the maximum deviation in the un-primed sample.

USA: FINLAND: UK: SINGAPORE: ONLINE: 96 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com the antennas may help to randomize average) to be larger than before, of the typical contributions to the systematic contributions to the even though the absolute value of the uncertainty to determine which terms total measurement uncertainty. For minimum curve is the same. are included in the Type A result. In emissions measurements, this is Note that the standard deviation this case, nearly every effect exerts a equivalent to using multiple identical shown in Figure 2 would still allow the random contribution due to the EUTs to determine the Type A claim of ±0.5 dB for the expanded considerable variation in test setup uncertainty. As long as variations in uncertainty from 200 to 600 MHz. It and conditions over the allotted time the EUTs do not add excessive is perfectly acceptable (and often period. Factors such as temperature contributions to the random error, it necessary due to band breaks in and humidity effects, ground plane is possible to obtain a suitable equipment) to generate frequency quality due to ground moisture, uncertainty value from tests of dependent uncertainties. The ground plane warping with tempera- multiple EUTs. expanded uncertainty from 600 to 800 ture , test cable lengths, cable connec- This is not to say that multiple MHz could then be set at ±0.8 dB and tion quality, cable calibration, etc. all EUTs always provide an acceptable from 800 MHz to 1 GHz at ±1.2 dB. varied significantly over the sample. method for performing this type of Table 1 lists some of the typical uncertainty calculation. One or two Cleanup: entries in a Type B uncertainty EUTs with significant deviations Type B Evaluation budget and suggests which ones are from the norm can introduce a likely to be totally random, system- In order to develop a final value for significant change to the resulting atic, or a combination of the two. the expanded uncertainty, it is uncertainty, as demonstrated in The listed distribution type repre- necessary to make an evaluation of all Figure 2. One additional antenna, a different model which varies signifi- Contribution Distribution Random Systematic AF Cal RE RI cantly from the average at different Type frequencies, was introduced to the Antenna factor Normal X X sample to demonstrate the possible Cable calibration Normal X X X effects. Note that for most of the fre- Coupler calibration Normal X X quency range, the deviation is below Receiver/probe linearity Rectangular X x X X X

1 dB, yet the contribution of one Receiver level detection Rectangular x X X X additional antenna is sufficient to change the standard deviation such Antenna directivity Rectangular X X A Statistical Approach to Measurement Uncertainty to Measurement A Approach Statistical AF variation with height Rectangular X X that it is larger than the original – maximum deviations at most Antenna phase center Rectangular X X frequencies. As this example shows, when working with relatively small Field uniformity Rectangular X X samples, it is important not to Frequency interpolation Rectangular X x X X introduce factors which may exag- Distance measurement Rectangular X x X X X gerate the error in the measurement.

Likewise, it is important to avoid Height measurement Rectangular X x X X Reference arbitrarily discarding data because it Site imperfections Rectangular X x X X makes the result look bad! Figure 2 also demonstrates the Mismatch U-shaped X x X X X difference between systematic errors Temperature effects Rectangular X X X X and random errors. One sample with Setup repeatability Type A X X X X a large systematic error (not present in the other samples) can have a Ambient signals Rectangular X x X X significant effect on the uncertainty, EUT repeatability Type A X X X since it changes not only the standard deviation, but also the mean of the Table 1. Various Possible Contributions to Measurement Uncertainty, along with Typical samples. This causes the negative Accepted Distribution Types and Possible Contributions to Both Random and Systematic maximum deviation (minimum - Errors. (For items which might have both random and systematic contributions, a lowercase X represents the typically smaller or less likely contribution.)

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 97 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com from the correct value can only be theoretical for the site contribution. sents the typical accepted distribution less than the correct value. As However, in the case of antenna type for that entry. Note that most mentioned previously, this error was calibration, this option is circular. calculated values are assumed to be verified to be less than ±0.02 dB. This Since the uncertainty of an NSA rectangular, which may often be a error is a good example of the measurement can be no better than more stringent criteria than is difference between performing a that of the antenna calibration, the necessary. Type B analysis of an entire test setup uncertainty of the resulting antenna or using a mixed Type A and Type B calibration could never be better than For the antenna calibration example analysis. The height step has both the NSA measurement plus its given, only a few possible systematic random and systematic components, uncertainty! errors may need to be accounted for. but the random components would Two options remain for antenna In general, a purely systematic error be measured in a Type A analysis. calibrations. The first is to attempt is likely to have a U-shaped distribu- Thus the contribution to the mixed “round-robin” testing to compare tion since it always represents a Type B analysis is not the same as that one site to others and use the average deviation to one side of the real value. for the Type B-only analysis. value as the perfect site. The second A common example of this type of The final and most troubling method, to be published as an contribution is that of cable mis- contribution is that of site imperfec- amendment to CISPR 16-1 in 1998, match. Since a perfect match is tions. Fortunately several factors involves using calculable dipoles to represented by zero reflection and a mitigate this contribution somewhat. verify that a site matches a perfect mismatch in any direction results in Temperature changes over the test theoretical ground plane through an non-zero reflection, the statistical period caused dimensional changes exhaustive sequence of tests10. probability that there is some in the surface of the metal ground Comparisons between antenna mismatch causes the peaks of the plane which would have randomized measurements made using the same distribution to occur away from the the effects somewhat. Also, similar to test system on the NIST (Boulder, zero (matched) value. The effect of the effects of standing waves, the CO) OATS and the OATS used for the mismatch is to change the detected imperfections in the ground plane antenna calibrations given in the level by reflecting the power back will have different effects at different examples show the variation between towards the source. It can also frequencies. Variations in antenna the sites to be on the order of 0.5 dB. generate standing waves in cables positioning with respect to site This variation is of the same order of which then serve to increase or defects will randomize these effects as magnitude as the random uncertainty decrease the detected signal based on well[6, 9]. Thus there is a high contribution and thus an individual A Statistical Approach to Measurement Uncertainty to Measurement A Approach Statistical the frequency and cable length. A

– probability that there will be “worst test is insufficient to draw a conclu-

standing wave is a systematic error in case” points throughout the fre- sion on the systematic error contri- a fixed system for a given frequency. quency band which will capture the bution of the site. It should be However, over frequency, the effect is site imperfection effects in a Type A reiterated here that the assumption of either constructive or destructive, analysis. However, there is always the a “golden site” for comparison giving a net random contribution. In likelihood of a significant systematic purposes is not recommended. the case of antenna calibrations, the effect which is not easily determined. Instead the use of round-robin testing mismatch at the antenna is a part of Reference It should also be noted that NIS-81 for determination of a statistically the calibration, so standing waves are classifies site imperfections as a perfect site, or the new CISPR method the only contribution of concern. All rectangular distribution. This is for site verification is recommended other cable contributions are in- largely due to the fact that existing for validation of an antenna calibra- cluded in the cable calibration, which site verification techniques do not tion site. is part of the random error contribu- provide for individual determination tion. of the random and systematic Another systematic error contri- Total Uncertainty contributions from the site. bution which generates a U-shaped The combined standard uncertainty, For EMC measurements, a good y distribution is the max-hold height uyc (), of a quantity is computed pair of antennas may be used to step. For continuous height scanning, from the square root of the sum of determine the normalized site this is a function of sweep time versus squares (RSS) of the individual attenuation (NSA) of a test site and tower speed. Since the “correct” contributions ux()i . If an individual use the deviation of that value from value is a maximum, any deviation

USA: FINLAND: UK: SINGAPORE: ONLINE: 98 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com uncertainty component does not Conclusion 6. Heinrich Garn, “Uncertainties in directly correspond to the While there are inherent difficulties the Calibration of Broadband EMI measurand, it must first be converted in performing a Type A analysis of a Test Antennas by Using a Refer- into the proper form by applying the test setup, it is important not to ence Antenna,” 1997 Mode- appropriate conversion factor or dismiss the concept altogether. It is a Stirred Chamber, Anechoic =⋅ function, uyiii() cux ( ). For relatively simple matter to obtain Chamber, and OATS Operators example, a positioning uncertainty in sufficient measurement data to Meeting, April 28-May 1st, 1997. centimeters must be converted into produce an acceptable measure of the 7. Brent Dewitt, Daniel D. Hoolihan, its effect on the field in dB before it total random contribution to the “An Analysis of Measurement can be applied to the antenna factor uncertainty. This has the advantage Uncertainty with Respect to IEC uncertainty. Then, for N uncorrelated of providing a measured value and 1000-4-3,” 1997 International individual components, the com- thus limits the number of assump- Symposium on EMC, Austin, TX, bined standard uncertainty is: tions necessary to arrive at a total August 18-22, 1997, pp. 365-369. N expanded uncertainty value. The 8. Edwin L. Bronaugh, “Uncertainty = 2 uyci()∑ u () y. ability to prove uncertainty claims of the Frequency Spectra Pro- = i 1 with measured data is likely to duced by a Reference Radiator,” The expanded measurement uncer- become more important as new EMC Compliance Engineering, 1997 tainty, U, is then determined by regulations are put into effect. Reference Guide, pp. A101-A108. multiplying the combined standard 9. Peter McNair, “Understanding uncertainty by the desired coverage Open Area Test Site Perfor- factor, k, which determines the level mance,” 1997 Antenna Measure- of confidence in the uncertainty ment Techniques Association =⋅ value. Thus,Ukuyc (). For the Symposium, November 17-21, recommended 95% level of confidence, 1997, Boston Massachusetts, pp. k = 2 . References 25-30. Using 0.5 dB for the expanded 10. Jasper J. Goedbloed, “Progress in uncertainty of the random contribu- 1. NIS 81, Edition 1, The Treatment Standardization of CISPR Antenna tion and 0.75 dB as a rectangular of Uncertainty in EMC Measure- Calibration Procedures,” 1995 distribution for the remaining ments, NAMAS, May 1994. International Zurich Symposium contribution due to site imperfec- 2. Barry N. Taylor and Chris E. on EMC, March 7-9, 1995, pp. 111- tions and any other systematic effects, Kuyatt, NIST Technical Note 1297, 116. 1994 Edition, Guidelines for Uncertainty to Measurement A Approach Statistical

2 2 –  05.. 075 Evaluating and Expressing the uy()=   +=05. Dr. Michael D. Foegelle received his Ph. D. in c  2  3 Uncertainty of NIST Measurement Physics from the University of Texas at Results, NIST. Austin, where he performed theoretical and Using k =2 this results in a total 3. C63 EMC Measurement Uncer- experimental research in both Condensed combined expanded measurement tainty Workshop, April 29-30, Matter Physics and EMC. He performed uncertainty of 1.0 dB. Since the 1996. contract EMC research with Dr. J. D. Gavenda random error contribution is a of UT for IBM and Ray Proof where he

4. Manfred Stecher, “Measurement Reference significant portion of the total Uncertainty in EMC,” ITEM 1997, helped to develop a semi-anechoic chamber (uy()/( uq )< 3) it is necessary modeling system. In 1994 he began working ck pp. 58-64, 246, 1997. to use an adjusted value for the for EMCO in Austin Texas, where he is a 5. Edwin L. Bronaugh and John D. coverage factor, k . In this case, the Physicist and Senior Principle Design p M. Osburn, “A Process for the value is around 2.01, but in the case of Engineer. There he has developed the Analysis of Measurement and only a few samples, this value could advanced calibration software used by the Determination of Measurement be as large as 3 to 14. EMC Test Systems Calibration Laboratory, Uncertainty in EMC Test Proce- and a number of new products, including a dures,” 1996 International high speed isotropic field probe and a virtual Symposium on EMC, Santa Clara, device controller. He has authored or CA, August 19-23, 1996, pp. 245- coauthored a dozen papers in the areas of 249. EMC and Condensed Matter Physics. (512) 835-4684, ext. 650.

USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 99 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com Understanding the measurement uncertainties of the bicon/log hybrid antenna

Measurement uncertainty associated with the bicon/log hybrid antenna for radiated emissions and site validation tests relate to many factors, including height dependency, polarization and loading.

ZHONG CHEN ues. A thorough understanding of these EMC Test Systems values is essential. EMI and normalized site Austin, TX attenuation (NSA) tests are performed over a conducting ground plane. Calibration ince their first introduction in 1994 labs may be able to provide very accurate at the Roma International Sympo- calibrations of the free-space AFs, which Ssium on EMC,1 bicon/log hybrid are intrinsic properties of the antennas. antennas have become very popu- Studies have shown that antenna perfor- lar in EMC labs worldwide. Because there mance can change by a few decibels over are no band breaks in frequency sweep, a ground plane, and this effect is antenna test time and effort are reduced. EMC en- type specific. In many cases, the perfor- gineers have assumed that performance of mance of a bicon/log hybrid antenna over these antennas is simply that of a biconical a ground plane is different from that of a antenna at a lower frequency range until bicon or a log antenna. A good free-space it transitions to a regular log periodic di- AF with a low U does not always translate pole array (LPDA) antenna at higher fre- into a low U in the EMI or NSA measure- quencies. Questions have been raised ment due to the influence from the con- about this assumption, and some have ducting ground. suggested that a higher measurement un- This article will address several aspects certainty (U) should be used due to the of measurement uncertainty related to the characterization of the phase center posi- bicon/log hybrid antenna application. They tion and antenna pattern variation from are: the height dependency of the hybrid that of a dipole on which emission and antenna AF above a conducting ground site validation standards are based. Very plane; the geometry and polarization-de- limited research has been conducted on pendent AF and NSA measurement; active the uncertainty evaluation of these hybrid phase center variation with frequency; antennas, despite the fact that more and antenna beam pattern; and the compari- more EMC engineers have come to realize sons of a bicon/log hybrid with separate that predicting and reducing measurement bicon and log antennas. Some manufac- uncertainty has become an important as- turers also apply capacitive loading on the pect of EMC testing. bow tie elements to improve the low fre- Most antenna manufacturers and cali- quency performance of these antennas. bration labs provide individually calibrated This article also explains how this loading antenna factors (AF) with associated U val- impacts the measurement U.

ITEM 1999 Reprinted with permission from ITEM 1999. 1

USA: FINLAND: UK: SINGAPORE: ONLINE: 100 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com HEIGHT/POLARIZATION DEPENDENCY ABOVE A CONDUCTING GROUND PLANE AF is defined as the ratio of the inci- dent electric field over the receive voltage at a 50-ohm load connected to the feed point of the antenna. The free-space AF is obtained when the antenna is in free-space and the in- cident electromagnetic field is a plane wave. The free-space AF is an intrin- sic property of the antenna, just like the physical length of a ruler, and should not vary no matter how the Figure 1. Traditional bicon/log hybrid Figure 2. Enhanced model of the calibration is performed. However, antenna. bicon/log hybrid antenna. just like heat or cold can change the length of a ruler, the environment in height, no matter how accurate the a typical measurement condition is which the antenna is used can also calibration is, there exists an error. not free-space. For total measure- impact the AF. EMI and NSA mea- We may be tempted to say we just ment U, in addition to the antenna surements are performed over a con- need to use a matrix of AFs, so that calibration U obtained from antenna ducting ground plane, and unlike at each different height we could use calibration labs, we must assess ad- temperature, which does not change a different AF. However, for differ- ditional uncertainty values for the a ruler all that much, the ground ent frequencies, AFs are different; for antenna and geometry-dependent plane can change the AF by as much different polarizations, AFs are dif- test setups. as 2 or 3 dB depending on the po- ferent; for different separation dis- larization and height. Different types tances, AFs are also different. It be- STANDARD SITE METHOD of antennas also interact with the comes a practical issue for calibra- CALIBRATION AND ground plane differently, causing the tion in all these different cases and IMPLICATIONS ON NSA effect on the AF to be antenna spe- requires applying a complicated MEASUREMENTS cific. multi-dimensional matrix of AFs dur- ANSI C63.5 calibration calls for a Figure 1 shows a traditional bicon/ ing an EMI test. three-antenna-method calibration log hybrid antenna, while Figure 2 Instead of this complicated web over a ground plane, commonly shows an enhanced model for im- of AFs, is there an acceptable com- known as the standard site method. proved low frequency performance. promise if we are willing to sacrifice In this measurement, the receive an- Let us use the traditional model a little bit of accuracy? It turns out tenna is scanned in heights from

… for different frequencies, AFs are different; for different polarizations, AFs are different; for different separation distances, AFs are also different.

shown in Figure 1 to illustrate the that the free-space AF provides an 1 m to 4 m. NSA measurement as dependency of the AF on height acceptable average. As shown in Fig- defined in ANSI C63.4 is simply the above a conducting ground plane. ure 3, the free-space AF falls right in reverse of the ANSI C63.5 antenna Figures 3 and 4 show numerically- the middle for most of the frequen- calibration procedure. The only sig- calculated AFs at heights of 1 m, 2 cies. This is also why the ANSI, CISPR nificant difference is that for NSA m, 3 m and 4 m for a horizontally or and other international standards measurement, the site is the un- vertically-polarized antenna. Note have moved toward the use of free- known, where for antenna calibra- again that the EMI or NSA measure- space AF for product EMI test in re- tion, the AFs are the unknowns. Understanding the Measurement Uncertainties of the Bicon/log Hybrid Antenna Hybrid of the Bicon/log Uncertainties the Measurement Understanding ments are typically performed for a cent years. However, it is also clear A common question when follow- – height scanning from 1 m to 4 m. If that we may be able to get a near ing the NSA measurement proce- we were to use a free-space AF or perfect free-space AF, but it would dures is “I calibrated my antennas an AF calibrated at a fixed height to not be perfect for our typical EMI or very recently. When I use the AF to do a measurement at a different NSA measurements, simply because do my test, with separate bicons and

2 ITEM 1999 Reference USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 101 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com 20 first look at some antennas calibrated free-spaceFree-space AF AF per the ANSI C63.5 standard site 18 h=1.0mh = 1.0 H-pol m, H polarization method. Figure 5 shows the result- h=2.0m,h = 2.0 H-polm, H polarization ing AFs for a separation distance of 3 16 h=3.0m,h = 3.0 H-polm, H polarization m, with the receive antenna scanned h = 4.0 m, H polarization h=4.0m, H-pol from 1 to 4 m in height. 14 Manufacturer-published data manufacture published data In the standard site method, the 12 discrepancy results not only from the height variation, but also from other

AF (dB/m) 10 factors, such as the non-plane wave Antenna Factor (dB/m) illumination of the receive antenna, 8 mutual coupling between the transmit and receive antennas, and the dipole 6 antenna pattern assumption made in the theoretical model.2 As shown in 4 Figure 5, using a single AF to do an 30 80 130 180 230 280 330 NSA test for all these geometries is a Frequencyfreq (MHz) (MHz) crude approximation. One thing to Figure 3. Numerically-calculated bicon/log hybrid AF at different heights for note is that Figure 5 only shows the horizontal polarization above a conducting ground plane. Manufacturer- difference in the AF for a single an- published data are also shown as triangles. tenna under different geometries. For an NSA measurement, there 20 are two antennas involved, transmit free-spaceFree-space AF AF and receive. The resulting difference 18 h=1.0mh = 1.0 V-pol m, V polarization is the sum of two antennas. For ex- h=2.0m,h = 2.0 V-polm, V polarization ample, at 180 MHz, the free-space 16 h=3.0m,h = 3.0 V-polm, V polarization AF is different from the AF for the h=4.0m,h = 4.0 V-polm, V polarization vertical polarization (h1 = 1.5 m) by 14 Manufacturer-published data manufacture published data 2 dB. If a free-space AF were to be

12 used for an NSA site validation mea- surement, the NSA error just due to

AF (dB/m) 10 the AF difference would be 4 dB (2 Antenna Factor (dB/m) dB from the transmit antenna, and 2 8 dB from the receive antenna). Thus, it is unlikely a site would pass the 6 NSA 4-dB requirement under this condition. 4 This answers the first question of 30 80 130 180 230 280 330 whether the NSA failure is due to the Frequencyfreq (MHz) (MHz) antenna or site: it is probably nei- Figure 4. Numerically-calculated bicon/log hybrid AF at different heights for ther the site or the antenna calibra- vertical polarization above a conducting ground plane. Manufacturer-published tion that is at fault. Perhaps the an- data are also shown as triangles. swer lies in the method being used, and whether the correct AF is ap- log antennas, I can pass the NSA re- and how free-space AF can be used plied. Because the NSA procedure is quirement, but when I use the hy- for product EMI test as an acceptable simply the reverse of the procedure brid antennas, I failed the test. Is that average. To better answer the above for an ANSI antenna calibration, if due to my antennas or my site?” questions, we need to quantify ex- the NSA geometry stays the same as Other questions are “I need to cali- actly how much different geometries the calibration geometry, the errors brate my antennas for site validation; affect the performance of a specific shown in Figure 5 exactly cancel.

Understanding the Measurement Uncertainties of the Bicon/log Hybrid Antenna Hybrid of the Bicon/log Uncertainties the Measurement Understanding which AFs do I need?”, and “Do I antenna. For NSA measurement, This also answers the second need free-space AF, 3-m or 10-m cali- since we are dealing with a tighter question of which antenna calibra- – bration, and how about height and tolerance, we want to decrease our tion is needed for a site validation polarization?” measurement uncertainties. We will test; the geometries for site valida- It was explained above how AFs show that the free-space AF approxi- tion and antenna calibration need to change under different geometries mation becomes inadequate. Let us be identical to get the lowest mea-

ITEM 1999 3 Reference USA: FINLAND: UK: SINGAPORE: ONLINE: 102 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com 20 should not be calculated for a fixed Free-space AF free-space AF distance. For example, when we per- 18 h1h1=1, = 1, H-pol H polarization form a 3-m calibration below 100 h1h1=2, = 2, H-pol H polarization MHz, the bow tie elements are ac- 16 h1h1=1, = 1, V-pol V polarization tive. If the antenna is 1 m long and h1h1=1.5, = 1.5, V-pol V polarization the reference position is the center 14 of the antenna, we are in fact per- forming a 4-m test (0.5 m addition 12 for each antenna). Figure 6 shows

AF (dB/m) d the E max values for a horizontally-po- 10 Antenna Factor (dB/m) larized antenna with the transmit an- tenna at 1 m height. It shows that 8 more than 2 dB of error can be ex- pected just due to the active elements 6 being different from the reference point. 4 30 80 130 180 230 280 330 For antenna calibration, if the ac-

Frequencyfreq (MHz) (MHz) tive center can be accurately char- d Figure 5. Numerically-calculated bicon/log hybrid AF obtained using the 3-m acterized, applying E max for the cor- ANSI C63.5 standard site method. The receive antenna is scanned from 1 to 4 m rect distances will rectify the error. with a step size of 0.05 m. “h1” is transmit height. For a radiated emission test, if the free-space antenna factor is used, surement uncertainty. However, phase center. It appears that the elec- this error cannot be amended, and there is one catch. The antenna cali- tromagnetic fields are radiated from becomes part of the measurement bration site needs to be very good, that center. uncertainty. On the other hand, if a because any errors generated in the Because the phase center moves biconical or dipole antenna is used, antenna calibration will be trans- with frequency, other common ques- this phase center is well-defined, ferred to the site validation test. tions people ask are: “Where do I and a lower measurement uncer- Let us look at the results in Figure measure the distance from the bicon/ tainty is achieved. A log antenna can 5 from another perspective. If we log hybrid antenna for my test? suffer from the same phase center assume that the transmit antenna is Should I measure from the tip of the error, but conceivably, a single log the equipment under test for an emis- antenna or from the center of the antenna is shorter than the hybrid. sion measurement, and we use the antenna?” A typical answer is to mea- The phase center error would be free-space AF to qualify the EMI from sure from the tip for an immunity smaller. For a critical test where low this piece of equipment, the differ- test, and from the center for an emis- measurement uncertainty is desired, ence between the different geom- sion measurement, as specified by a simple dipole, bicon and/or log etries and the free-space AF is the the ANSI, CISPR and IEC standards. antenna are preferred over the hy- error in our measurement. For the It is rather clear that this position is brid antenna. example given in Figure 5, this error just an approximation during the fre- is 2 dB in some cases. The biconical quency sweep. Thus, uncertainties ANTENNA DIRECTIVITY AND antenna was also studied for the are introduced in the measurements BEAM PATTERN same circumstances,2 and the errors by assuming a fixed position. The intent of the ANSI C63.4 NSA were shown to be about 1 dB The question arises for bicon/log and emission measurement is to use smaller. For lowest U, a biconical hybrid antennas from different manu- a field sensor with a dipole pattern antenna is recommended. facturers; these antennas may not have (because the Roberts’ Dipole is the the same design or the same length. undisputed reference). If the antenna ACTIVE PHASE CENTER Are their uncertainties different in an pattern is different from a dipole, it VARIATION WITH EMI test? The answer is absolutely yes. would not be an issue if the mea- FREQUENCY The next question is whether this er- surement were performed in a free- The radiating elements for a bicon/ ror can be estimated. This question space environment as long as we can

log hybrid antenna move from the may be answered by simply looking keep the antenna pointing to the Antenna Hybrid of the Bicon/log Uncertainties the Measurement Understanding

d bigger elements in the back to the at the E max formulation.* equipment under test at all heights – smaller elements in the front as the If we can assume that a bicon/log (boresighting). frequency goes up. The radiating hybrid antenna acts like a series of For a measurement over a con- position for a specific frequency is dipoles radiating in different posi- ducting ground plane, however, d commonly referred to as the active tions at different frequencies, E max there is a signal reflection from the

4 ITEM 1999 Reference USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 103 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com 12 12

11

10 R=3m h=1.0mh = 1.0 m R=4m 10 h=2.0mh = 2.0 m h=3.0mh = 3.0 m h=4.0mh = 4.0 m 9 8 ) 8 ) 6 7 AF (dB/m

max

d edmax (dB uV/m

E (dB uV/m) 6

4 Antenna Factor (dB/m)

5

4 2

3

0 2 30 40 50 60 70 80 90 100 20 40 60 80 100 120 140 160 180 200 220 Frequencyfreq (MHz) (MHz) Frequencyfreq (MHz) (MHz)

d Figure 6. E max for 3-m and 4-m separation distance for a Figure 7. Numerically-modeled AF for a vertically-polarized horizontally-polarized antenna. The transmit antenna height bicon/log hybrid with an L-shaped end-loading. is 1 m, and the receive antenna height is scanned from 1 to 4 m. measurement uncertainty, this value would be unaccept- ably large, as is the case in Figure 7 (on the order of 5 dB). ground plane. The reflected field enters the antenna The solution for making such an antenna suitable for pattern at an angle. The addition of the direct and re- both radiated emission and radiated immunity testing is flected signal will not add the same way if the antenna to make the end-loadings removable. For immunity test- pattern is different. There are certain pattern variations ing, the end-loadings are left on to gain the better match from that of a dipole for the hybrid antenna.1 Any de- (thus requiring a smaller amplifier for a fixed field level). viation due to the antenna pattern needs to be treated Since the purpose of the immunity test is to generate a as a source of measurement uncertainty. Bicon antenna given field level, as long as we can measure the gener- patterns have been illustrated to be close to those of ated field, this coupling is not an issue. In addition, most dipoles,4 so, again, this error is smaller for the biconical immunity tests are performed in a fully-lined anechoic antennas. room, or over a partially absorber-lined ground plane, and this coupling is not as significant. CAPACITIVE-LOADING For an emission measurement, the end-loading should (LOW-FREQUENCY IMPROVEMENTS) FOR be removed. The antenna in that case would simply CERTAIN BICON/LOG HYBRID perform like a traditional bicon/log hybrid antenna. One The VSWR for a hybrid such as the sample shown in thing to note is that the antenna does not need to be Figure 1 is on the order of 20:1 at the 30-MHz range, calibrated for use in immunity mode, thus saving the which means that about 80% of the forward power is cost of calibration for both emission and immunity con- reflected back to the source. To generate a certain field figurations. level for a radiated immunity test, a huge amplifier is sometimes needed. Several manufacturers introduced ca- CONCLUSIONS pacitive-loading to their antennas, such as shown in Fig- This article has presented several issues of measurement ure 2, to improve the mismatch condition. This improve- uncertainties related to the bicon/log hybrid applica- ment is most useful for radiated immunity tests. tion. Many general measurement uncertainty related is- For emission tests, the loading elements can couple sues are not discussed here since they are not particular strongly with the ground when polarized vertically. Fig- to this type of antenna. These include cable mismatch ure 7 is an example of the antenna factors at different uncertainty, site irregularity, site edge diffractions, etc. heights for a vertically-polarized antenna with an L- In an actual measurement, these factors all play impor- shaped enhancement. This L-shaped enhancement is a tant roles in the total measurement uncertainty evalua- variation of the T-shaped bow tie shown in Figure 2. tion. On the other hand, an important issue which tends Even though we could treat such coupling as part of the to be ignored by many EMC engineers is the antenna- Understanding the Measurement Uncertainties of the Bicon/log Hybrid Antenna Hybrid of the Bicon/log Uncertainties the Measurement Understanding specific uncertainties. In most cases, care must be taken

– d 3

* E max is a concept introduced by Smith, German, and Pate and later when using different antennas and their associated an- adopted by the ANSI C63 standard site method and NSA formulation. tenna factors. A compromise must be made between It represents the maximum electric field in a height scan for a tuned the ease of measurement and the accuracy of the mea- dipole with a radiated power of 1 pW. surement.

ITEM 1999 5 Reference USA: FINLAND: UK: SINGAPORE: ONLINE: 104 Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com REFERENCES 1. S. J. Porter, A. C. Marvin, “A New Broadband EMC Antenna for Emissions and Immunity,” Roma International Symposium on EMC; Rome, Italy, September 1994. 2. Z. Chen, M. Foegelle, “A Numerical Investigation of Ground Plane Effects on Biconical Antenna Factor,” IEEE International Sympo- sium on EMC; Denver, Colorado, 1998. 3. A. A. Smith Sr., R. F. German, and J. B. Pate, “Calculation of Site Attenuation from Antenna Factors,” IEEE Transactions on Electro- magnetic Compatibility, Vol. 24, No. 3, August 1982, pp. 301-311. 4. M. J. Alexander, M. H. Lopez, M. Salter, “Getting the Best out of Biconical Antennas for Emission Measurements and Test Site Evalu- ation,” IEEE International Symposium on EMC; Austin, Texas, 1997.

ZHONG CHEN received his M.S.E.E. degree in electromagnetics from the Ohio State University at Columbus. He has been with EMC Test Systems (ETS) since 1996, where he is now a senior electromagnetics and an- tenna engineer. He is a member of the IEEE AP, MTTS, and EMC Societies. (512) 835-4684 ext. 627. E-mail: [email protected]. Understanding the Measurement Uncertainties of the Bicon/log Hybrid Antenna Hybrid of the Bicon/log Uncertainties the Measurement Understanding

–

Reference USA: FINLAND: UK:SINGAPORE: ONLINE: Tel +1.512.531.6400 Tel +358.2.838.3300 Tel +44.(0)1438.730.700 Tel +65.536.7078 [email protected] 105 Fax +1.512.531.6500 Fax +358.2.865.1233 Fax +44.(0)1438.730.750 Fax +65.536.7093 www.ets-lindgren.com