emc Zhong Chen ETS-Lindgren

EMC Antenna Fundamentals

electing the right antenna for the example, the gain of an antenna may direction than an isotropic source. job can be a difficult task. In not be of any concern if it is used to Higher directivity is associated with Smany cases, manufacturer excite a reverberation chamber. narrower beamwidth. terminologies and specifications are so varied that it is difficult to compare Directivity And Gain Gain, by IEEE definition, is the product them. A firm grasp of the basic Passive devices, such as most of the of the directivity and the ohmic terminologies and their limitations is antennas used in EMC, cannot amplify efficiency (sometimes called ohmic loss essential. The right antenna is an signals they receive, or radiate more factor). Most EMC antennas are made essential part of a test system. energy than provided. Gain and of aluminum or other highly conductive Depending on the specific application directivity specify an antenna’s ability metals. In these antennas, the ohmic and the equipment used, one type of to concentrate a transmitted signal in a loss is insignificant; therefore, gain is antenna may be better suited than desired direction, or receive a signal the same as directivity. This is where another. from that direction. confusion arises, since such a gain definition is rarely the one you The discussion in this article is focused Directivity describes how well an encounter in an EMC application. on antennas, which by definition are antenna concentrates radiation intensity devices that convert time-varying in a certain direction, or receives a Gain in EMC applications typically voltages into a radiated electromagnetic signal from this direction. This is in includes an additional mismatch factor. field. The keyword here is “radiated.” comparison to an omni-directional To illustrate this, let us assume the Many field generating devices, such as (isotropic) antenna. Note that an antenna is in transmitting mode (the TEM (or Crawford) cells, GTEM, isotropic antenna is simply a theoretical argument applies to receiving antennas parallel plates, or tri-plates (widely used model, and is not possible to construct as well). Note the IEEE definitions in automotive component testing), are one physically. A theoretical isotropic above are based on the net power strictly speaking not antennas. Energies antenna has a directivity 0 dBi (“dBi” delivered to the antenna. In reality, stay within the devices and are not means dB over an isotropic source). A antennas are never perfectly matched to radiated into space. half wave dipole has a directivity of the source, and energies are reflected at 2.14 dBi. This means a half wave the antenna port. The net power is the In EMC applications, antennas are dipole can concentrate 2.14 dB more subtraction of the forward power and primarily used for radiated emissions energy in its maximum radiation the reverse power (in dB terms). measurements, radiated immunity testing, site qualification testing (normalized site attenuation), or other applications such as exciting a reverberation chamber. In a specific application, one set of parameters may be more important than another. For Figure 1

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Although it is different from the IEEE where is the magnitude of the Beamwidth is typically measured when definition, it is a practical one (this is reflection coefficient, and RL is the power received has fallen half (3 dB sometimes referred to as apparent gain). return loss is dB. In terms of power down) of the boresight direction. This is For example, an antenna can have a relationship: called half-power beamwidth or 3 dB very directive pattern but, if it is not beamwidth. Beamwidth can be used to close to 50 Ω in characteristic roughly estimate the size of a uniform impedance, very small electric field field area in an immunity test. However, levels will result when connected to a where Pnet is the net power, and Pfwd is many other factors come into play for 50 Ω source (used for most the forward power. For example, if an establishing uniform field, such as instruments). In most EMC antenna has a VSWR=2:1, the reflection from the ground or walls of a applications, gains published in the magnitude of the reflection coefficient chamber. Moreover, patterns could be antenna catalogs include this mismatch is 1/3, the return loss is 9.5 dB, and measured at a different distance than factor. 11% of the power is reflected (or 89% the one used in the immunity setup, net power is delivered to the antenna). thus the near field effect is different. An analogy: water (RF signal) flows Beamwidth is not a magic number that through pipes with unequal diameters. Antenna Factor establishes the size of the uniform field plane. Some of the water goes through and Antenna factor (AF) is another practical some is reflected (Figure 1). term for EMC engineers, and seldom Phase Center used outside EMC applications. It Reflection Coefficient/VSWR/Return provides a receiving antenna with the A radiated wavefront has a curvature Loss relationship between the incident when in near field. In far field, the curvature is so large that it can be Reflection coefficient, VSWR and electromagnetic field and the voltage on regarded as a plane wave. The apparent return loss all describe the same a 50 Ω load connected to the antenna. center of the curvature is the phase physical phenomenon as discussed in In equation form: center. For many EMC antennas, such the last paragraph, that is, the mismatch as biconical antennas or dipoles, the factor. If a mismatch occurs, there is a phase centers are quite obvious. For log standing wave established in a periodic antennas, the phase center transmission line, the voltage ratio of where E is the incident electric field, Ω moves from the back to the front as the maximum to the minimum is called and V is the voltage on the 50 load. frequency goes up. The measurement -1 voltage standing wave ratio (VSWR). AF has a unit of 1/m, or dB m . distance from the antenna to the device The closer the VSWR is to unity, the Antennas with smaller AFs are more under test is unclear. In practice, a better the match is. Reflection sensitive to the incident field. It is compromise has to be made for the coefficient is the ratio of the reflected interesting to note that AFs generally distance. voltage to the forward voltage. increase with frequency. It comes as no Reflection coefficient can be a complex surprise then that to measure the same Polarization number, as the reflected voltage does field level, more sensitive receivers are The polarization of a radiated wave has not always line up in phase with the needed for higher frequencies. AFs are to do with the radiated vector field forward voltage. The smaller the normally provided by antenna traced out as a function of time. It is magnitude of the reflection coefficient, manufactures or calibration labs. The beyond the scope of this article to the better the match is. Return loss is accuracy of AFs directly affects explain the full meaning of polarization simply the magnitude of logarithmic radiated emissions measurement. It is in all its detail. Fortunately, many EMC recommended antennas be calibrated form of the reflection coefficient. antennas are linearly polarized, such as annually to minimize the measurement dipoles, biconical antennas, log periodic Since all these terms describe the same uncertainties. dipole arrays, and horns. Linearly physical property, there is a one to one polarized antennas radiate vector field relationship among them. The simple Antenna Pattern And Beamwidth in a single direction which is less equations are, Antenna pattern, in simple terms, is the complicated than other polarizations. response of an antenna as a function of Any imperfections are measured by the viewing angle. In a strict sense, antenna cross-polarization ratio (the ratio of the pattern is a descriptor for the far field field level in the intended direction to response. In practice, EMC that of its orthogonal direction). Some measurements are often performed in antennas are circularly polarized for the near field, and antenna pattern is special applications, such as conical log taken quite liberally as well to include spiral antennas as required by near field responses. MIL-STD 461.

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Balance in the near field region, electric fields telecommunications equipment (where Coaxial cables attached to the antennas do not relate to the magnetic field by radiated emission and immunity tests Ω are inherently imbalanced, because they 377 . Magnetic field cannot be for electrical field at 10 kHz are are asymmetrical with respect to derived simply from the electric field. required). ground. In other words, the cable shield The designs of these antennas ensure to the ground is different from the they predominantly produce or respond Dipole Antennas center pin to the ground. Some to magnetic fields. Dipoles are tuned to specific antennas, such as biconical antennas, frequencies, from approximately 30 employ balun (short for Balanced-to- MHz to a few GHz. They are narrow Unbalanced transformer) to overcome band. To cover a wide bandwidth, they the imbalance. Basically, a balun need to be tuned manually. Dipoles are provides low impedance or an easy pass often used as reference antennas through to the differential current, and because the dipole elements high impedance to the common mode performance can be theoretically current. An unbalanced antenna has calculated. Interestingly, Roberts’ different responses depending on which dipoles, which are specified in the side is up when polarized vertically. A ANSI C63.5, have balun designs that large amount of common mode current are difficult to characterize, so exists between the shield of the feed performance of the Roberts’ dipoles is cable and the ground plane. This causes hardly calculable. Dipoles are seldom large measurement uncertainties. used in everyday measurements, due to the need for individual tuning at each Bandwidth frequency. Bandwidth is “the range of frequencies within which the performance of the Rod Or Monopole Antennas antenna, with respect to some Rod antennas are the counterparts of characteristic, conforms to a specific loop antennas. They are designed to standard” [1]. The definition is quite respond to electric fields from 30 Hz to broad. It does not explicitly specify 50 MHz. Since rod antennas are so what the “characteristics” or “specific small compared to the wavelengths (at Biconical Antennas standard” are, so the term bandwidth is 30 Hz, the wavelength is 10,000 km), subjective. Depending on the Biconical antennas typically cover the amplifiers within the antennas are frequency range from 20 MHz to 300 application, the typical characteristics sometimes necessary for small signals. can include all or some of the terms MHz. All wire-cage biconical antennas Rod antennas are typically required for on the market have similar size and discussed previously. Engineering the GR-1089-core standard for network judgments are needed to determine shape (approximately 1.36 m wide). what is acceptable for your application. This is because they are based on MIL- STD-461 specifications from the 1960s, Typical EMC Antennas which has become the de facto standard. Due to the small electrical Several types of antennas can size at below 50 MHz, they have very sometimes all satisfy the basic high input impedance (high VSWR). requirements of a measurement. The Balun performance is crucial for following list provides brief features, biconical antennas. Common mode application notes and possible current can be easily induced on the drawbacks of the typical EMC antennas feed cable (common mode impedance to hopefully aid readers in selecting the is no longer large compared to the input best fit. impedance of the antenna). Ferrite Loop And Magnetic Field Coil Typically used in the frequency range from 20 Hz to 30 MHz for measuring magnetic fields. At these frequencies, measurements are in effect within the near field region. Unlike in the far field,

106 CONFORMITY¨ 2004: THE ANNUAL GUIDE emc beads are often used on the feed cable frequency is increased. In ANSI or polarized field eliminates the need for to suppress the common mode. In CISPR standards, emissions horizontal and vertical measurements addition, feed cables should be measurements are performed from the separately. It is mostly used for MIL extended out a meter or more center of the log antenna boom. For standard measurements. The frequency horizontally before dropping vertically immunity tests, EN61000-4-3 requires range is typically from 100 MHz to to the ground to reduce possible measurements be made from the tip of 1000 MHz. Note that when measuring interference. the log antenna. The gain of a LPDA is the gain of a circularly polarized typically around 5 dBi, which provides antenna with a linearly polarized one, Calculable Biconical Antenna a good compromise between the gain appears 3 dB lower because of Calculable biconicals combine the best beamwidth and sensitivity (or power the polarization mismatch. elements of a biconical antenna and a and field strength requirement). dipole antenna in that they are theoretically calculable and broadband. Bicon/Log Hybrid They look very much like regular Bicon/log hybrid antennas are biconical antennas. The main sometimes referred to by their trade differences are that the baluns can be names, Biconilog or Bilog. The hybrid entirely characterized with a network combines the frequency range of a analyzer, the elements are precisely biconical antenna and a log antenna, Broadband Ridged Waveguide Horn constructed, and their responses are which is approximately 20 MHz to These versatile and broadband ridged numerically computed. These result in several gigahertz. They have become waveguide horns can cover 200 MHz to theoretically computed antenna factors increasingly popular, as there is no 40 GHz. The horns for low frequencies that can be used for site validation band break during a test. can be physically large. For example, testing or free space factors for radiated the horn that covers 200 MHz to 2 GHz emissions testing. The accuracy of a is approximately 37x39x29 inches. calculable biconical antenna is actually Since gain for these antennas are better than those of a Roberts’ dipole, generally larger (around 10 dBi), the because the balun performance is beamwidth is narrower. One should individually calibrated. The uncertainty make sure that the beamwidth meets the is better than 0.25 dB for its antenna measurement requirement. factor [2].

Just like biconical antennas at 20-50 MHz, hybrid antennas are electrically small. To increase the transmit efficiency, some hybrids employ end Standard Gain Horn Antenna loading techniques to compensate for These are very similar to dipoles or Log Periodic Dipole Arrays the size. They typically have T-shaped calculable bicons in that the gains can Log periodic dipole arrays (LPDA) or L-shaped bowtie elements. These be theoretically computed. Compared to typically cover the frequency range of antennas should only be used for the ridged waveguide horn, they are 80 MHz to a few gigahertz. As immunity testing. The coupling narrow band. Many horns are needed to discussed previously, the phase center between the loading elements and their cover a broad frequency range. of a LPDA moves from the back of the surroundings are very strong, and they antenna boom to the front as the introduce large measurement uncertainties for emissions testing [3]. If these antennas are to be used for both emissions and immunity testing, the end plates should be removable for emissions testing configurations.

Conical Log-Spiral Antenna The distinctive difference between the conical log spiral antenna and most other antennas is that the electric field is circularly polarized. The circularly

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Antenna Calibrations free-space AF by theoretically baluns are assumed to be either 50 Ω or Ω There are several methods to calibrate removing the ground plane effect. The 200 [4]. Note that this is not the an antenna. Much confusion exists for conducting ground plane is there to perfect solution either, since some free-space antenna factors and antenna establish a repeatable calibration commercial antennas have balun factors obtained in a specific setup environment. impedances that vary drastically with (sometimes called geometry-specific frequency. Even a well-made one does antenna factors). Understanding how There are some important assumptions not have a perfect match. The balun AFs are arrived is important in made in the standard site method. First, impedances have significant influences choosing the right antennas and their the calculation to remove the ground on the correction factor. associated AFs. plane assumes the antennas under test have radiation patterns of a point dipole The best approach is probably to use Standard Site Method (i.e. a donut shape pattern - uniform in the calculable biconical antennas. The H-plane and figure “∞” in E-plane). baluns are individually calibrated, and The standard site method is specified in Second, no coupling exists among no approximations are made regarding ANSI C63.5. This method is best suited antennas and the metal ground plane. their electrical performance. for “dipole-like” antennas, such as And third, antennas are in the far field dipole, bicon, log, and hybrid antennas. so that the physical size of an antenna For log antennas, there is currently no The site attenuation, or the insertion has no effect, i.e. the antennas are correction table provided. These major losses between the transmit and receive immersed in a uniform field. factors contribute to errors: antennas, are measured. The basic setup for a standard site method includes a However, these simple assumptions are ● Non-stationary phase center with large, flat, and unobstructed conducting not always acceptable. The errors for a respect to frequency (distance ground plane (made of metal). One single bicon antenna factor derived between antennas is vague) antenna is set to be at a fixed height, from the standard site method can be as while the other one is scanned from 1 large as 2 dB [4]. This means if these to 4 m in height. The maximum ● Large deviation of the antenna pattern AFs were used for normalized site response between the two antennas is to that of a dipole (note that the gains attenuation test, the total error would be recorded. Typically, three antennas are are approximately 5 dBi or more) 4 dB (because an antenna pair is used needed to perform such a calibration, for the site attenuation). A new ANSI and they are measured in three pairings. It is not as straightforward to derive C63.5-2000 draft standard addresses Calculations are then performed to correction factors for log antennas as this limitation by providing correction derive the antenna factors. Although a for bicons. The biconical antennas are factors. The correction factors are based ground plane is used, the aim of the similar in mechanical design while log on numerical simulations, and the C63.5 standard site method is to obtain antennas vary by make and model. Research is in progress to develop a new method, which is based on a complex fit normalized site attenuation scheme. Interested readers can refer to [5] for more details.

Reference Antenna Method This is another method specified in ANSI C63.5. It is basically a substitution method. The responses between two known antennas are measured (specifically two Roberts’ dipoles), and then one is replaced by the antenna under test. The antenna factor is derived from the difference. Mutual coupling between the two standard antennas and the antenna under test, as well as ground plane effects, can be significantly different, leading to significant errors.

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Other Methods distance. In that case, the calibration is free space environment. The true There are other calibration methods, in free-space condition. antenna factors in the specific test such as those used for loop antennas environment are height dependent. The and rod antennas. Some labs use Use Of Antennas For Radiated Emissions free space AF provides a good variations of the ANSI C63.5 method to Testing compromise. calibrate log, dipole and bicon Radiated emissions tests are defined in antennas, such as standard field method, ANSI C63.4 in the US or the equivalent Use Of Antennas For Radiated Immunity or standard antenna method with EN standards in Europe. The test setup Testing precision dipoles etc. One important is very similar to the antenna calibration Most immunity tests are performed per note is that, even if a perfect free-space setup. A large flat, unobstructed metal European standard EN 61000-4-3. It antenna factor is obtained, one should ground plane is used. The equipment requires the establishment of a uniform still apply the correction factors under test is set on a low dielectric field plane where the EUT would be. provided by ANSI C63.5 for table, which is 80 cm high. The table is The typical setup is illustrated in normalized site attenuation test. This is placed on a turntable, which can Figure 2. because normalized site attenuation provide a full 360° scan. The receive (NSA) tests are not in free space. The antenna is scanned from 1 to 4 m for The antenna power handling capability correction factors are used to correct the both horizontal and vertical is an important parameter in such a test. influences from the test setup, i.e. the polarization, and the maximum Many antennas are designed to have differences between free space and the readings are recorded and compared to superior performance in balance and specific geometry of an NSA setup. the standards. impedance matching, and not designed to handle high powers required in an Calibrations For High Gain Antennas For emissions measurements, it is immunity test. For an immunity High gains, such as horn antennas, have recommended to use free-space antenna antenna, the balance is not as important, narrow beamwidth. They do not see factor. This is despite the fact that the since the purpose is to establish a ground plane when placed in a close measurements are not performed in a known electromagnetic field. In the

Figure 2

112 CONFORMITY¨ 2004: THE ANNUAL GUIDE setup shown, the isotropic field probe is found in other antenna papers in an the key in setting up a calibrated field. attempt to make the points easier to understand. This is not meant to A field probe is a special kind of trivialize these topics. The reader can antenna. It consists of three independent refer to [1,6] for more in-depth broadband antennas, which are oriented explanations on many important orthogonally. The field levels are antenna topics. measured and reported digitally through a fiber optical link to a readout unit or a References computer. The total field is summed as [1] C. A. Balanis, “Antenna Theory RMS values of the three axes: Analysis and Design”, Second Edition, John Wiley & Sons, Inc., A field probe is a broadband New York, 1997. instrument. If more than one frequency [2] Z Chen and A Cook, “Low component exist, a field probe responds Uncertainty Broadband EMC to all of them. This is in contrast to Measurement Using Calculable antennas connected to a spectrum Precision Biconical Antennas,” 2000 analyzer, where the analyzer IEEE International Symposium on discriminates between frequencies. It is Electromagnetic Compatibility, thus critical to ensure the purity of the Washington, DC, 2000. signal in an immunity testing setup, [3] Zhong Chen, “Understanding the especially the harmonic field generated measurement uncertainties of the by a power amplifier. bicon/log hybrid antennas”, ITEM 1999. Just as antenna factors are extremely [4] Z Chen and M Windler, “Systematic important for emissions measurement, Errors in Normalized Site the calibrations of probe factors are Attenuation Testing,” Compliance vital for immunity tests. Calibration Engineering 17, labs typically provide a frequency no. 1 (2000): 38Ð48. correction table for each probe. This [5] Z Chen and MD Foegelle, “An corrects the reading for the specific Improved Method for Determining frequencies. Another important factor is Normalized Site Attenuation Using the linearity of a probe. This is a Log Periodic Dipole Arrays,” , 2000 parameter that measures how faithful a IEEE International Symposium on probe measures at different field levels. Electromagnetic Compatibility, Washington, DC, 2000. Modern probes have internal adjustments for linearity, making sure [6] J.D. Kraus, “Antennas”, Second probe readings are correct not only at Edition, McGraw-Hill, 1988. the calibration field levels (e.g. 20 V/m). Not all probe calibrations are About The Author equal. When performing a probe Zhong Chen is a principal design calibration, a simple frequency response engineer at ETS-Lindgren, and can be calibration without re-adjusting reached by e-mail at Zhong.Chen@ets- linearity is insufficient in most cases. lindgren.com.

Conclusion This article is based on “Antenna Fundamentals” tutorial presented at A broad range of topics have been the IEEE International Symposium on discussed in this article, including the Electromagnetic Compatibilities in basic parameters of antennas, the Minneapolis, 2002. The author wishes common types of antennas used in to thank Karen Phillips of EMC, their calibrations and ETS-Lindgren for her help composing applications in radiated emissions and and designing the presentation and this immunity tests. Intentionally left out of article, and ETS-Lindgren for their this article are many detail theoretical total support. discussions, equations and formulas

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