TI Technology Days 2010 Range Considerations for RF Networks Richard Wallace Abstract The antenna can be one of the most daunting components of wireless designs. Most information available relates to large antenna’s related to Amateur Radio (HAM), Cellular Applications and expensive Whip Antennas that can never be used in a low cost application. This session covers the basics that most engineers would need to know to predict the expected Range Distance between wireless devices and design parameters to achieve an optimum Range Distance. At the end of this session, the attendees should be aware of antenna requirements, the Antenna Documentation Support available and be able to select an Antenna Reference Design for their application. Agenda • Antenna Theory • Antenna Measurements • Range Estimation – Friis Basic Formula – Improved Range Estimation – Best Accuracy Estimation • Antenna Reference Designs – 2.4 GHz – 2.4 GHz & 868 MHz Dual Band – 868 / 915 / 955 MHz – 433 / 315 / 169 MHz – CC-Antenna-DK • Antenna Support Documentation – Antenna Selection Quick Guide (DN035) – Comprehensive Antenna Selection Guide (AN058) Antenna Theory – Basic Function of an Antenna • Transmit mode: Transform RF signals into electromagnetic waves, propagating into free space TX • Receive mode: Transform electromagnetic waves into RF signals RX Range OBS: The antenna is a key component for the successful design of a wireless communication system. Antenna Theory – All Monopole Antennas are Derivatives of Dipole AC current through *1: /2 Dipole an inductor lags the produces the most voltage by 90 power at the ends of degrees the antenna with little power at the feedline. Current Voltage Power /4 /4 *1 *1 All monopole antennas are derivatives of a simple dipole where one ¼ wavelength radiator is in air and one ¼ wavelength radiator is imaged into the GND and serves as the second radiator. Antenna Theory – Wavelength Calculations in Free Space Wavelength is dependent on frequency which is referenced to the speed of light (299 792 458 m / s): meters = 2.99792458E8 m/sec f (GHz) where GHz = 1E9 Wavelength for several frequency ranges, all units are in cm: λ / 4 λ / 4 λ λ Frequency air FR4 air FR4 2.4 GHz 3.1 1.5 12.5 5.9 915 MHz 8.2 3.9 32.8 15.5 868 MHz 8.6 4.1 34.6 16.3 433 MHz 17.3 8.2 69.3 32.7 315 MHz 23.8 11.2 95.2 44.9 27 MHz 277.8 130.9 1111.1 523.8 Antenna Theory – Antenna Considerations • Numerous issues to consider when selecting the antenna: – Antenna placement – Board size available for antenna layout – Operating frequency – Ground plane for ¼ wavelength antennas – Antenna mismatch (VSWR) – Objects that alter or disrupt Line of Sight (LOS) – Antenna gain characteristics – Antenna bandwidth – Antenna Radiation Efficiency Antenna Theory – Antenna Radiation Patterns – Traditional Coordinate System An Isotropic Antenna is a theoretical antenna spec in dBi and radiates equally in all directions of a sphere. Antenna Measurement Coordinate System • x-y plane (θ = 90 deg) is the azimuth plane (horizontal plane) • y-z plane (φ = 90 deg) is the elevation plane (vertical plane) Antenna Theory – Antenna Radiation Patterns – 3D OTA CTIA Measurements Example 1: 2.4 GHz Yagi Directional Antenna (DN034) Example 2: 868 MHz Meandering Monopole Antenna (Dual Band Option, 2.4 GHz, DN024, Rev E) Antenna Theory – Antenna Parameters Important parameters • WaveLength, . Antenna size for dipole relative to the c wavelength of transmission. f • Polarization the direction of the electric field to the electromagntic wave. • Impedance, Z. A measure of the total opposition to current flow in an alternating current circuit, made up of Z = R + iX two components, ohmic resistance and reactance, and usually represented in complex notation. • Bandwidth is the range of frequencies where the return BW = 100 ( (FH – FL) / FC) loss is below VSWR of 2 • Efficiency () is the ratio of power in watts actually P radiated to the power into the antenna terminals rad %100* P in Antenna Theory – Antenna Parameters Important parameters • Gain, G. is the maximum radiation beam of the highest beam. This parameter takes into account VSWR mismatch and energy losses. – Important to remember that antennas do not amplify RF. Since antennas cannot create energy, the total power radiated is the same as an isotropic antenna. Any additional energy radiated in the directions it favors is offset by equally less energy radiated in all other directions. – IEEE Gain Definition: GIEEE = Radiated Power / Delivered Power = D • Directivity, D. Antenna directivity is usually measured in dBi, or decibels above isotropic sphere antenna. – The directional antenna has a maximum directivity greater than 0dB. • Resonance Frequency is the electrical resonance is related to the electrical length of the antenna. Antenna Theory – Antenna Q and Bandwidth Properly designed antenna’s should cover the range of frequencies over which the antenna can operate correctly with sufficient bandwidth. TI defines the antenna’s bandwidth in Hertz when VSWR less than 2:1, or return loss of greater than -9.5dB • Bandwidth (BW) can be defined in percentage of the operating frequency: BW = 100 ( (FH – FL) / FC) • There is a direct relationship between Q and bandwidth f f Q HL fc Antenna Theory – Antenna Categories • Two fundamental types of antennas – Single ended antennas • Usually matched to 50 ohm • Needs a balun if the chip has a differential output • Easy to characterize with a network analyzer – Differential antennas • Can be matched directly to the impedance of the radio • Can be used to reduce the number of external components • Complicated to make good design • Difficult to measure the impedance and to characterize. Antenna Theory – Resonant Antennas • Resonant antennas are often used – Monopole: λ/4 – Bent Monopole: λ/4 – Inverted F: λ/4 – Dipole: λ/2 – Folded dipole: λ/2 Antenna Theory – Frequency “v” Size • Lower frequency increases the range – Theoretically, reducing the frequency by a factor of two doubles the range (line of sight) • Lower frequency requires a larger antenna – λ/4 at 433 MHz is 17.3 cm (6.77 in) – λ/4 at 915 MHz is 8.2 cm (3.22 in) – λ/4 at 2.4 GHz is 3.1 cm (1.20 in) • A meandered structure is a compact dipole with inductive loading - λ/4 at 2.4 GHz feedline Antenna Theory – Max. Power Transfer (VSWR) Moritz Von Jacobi’s maximum power theory states that maximum power transfer happens when the source resistance equals the load resistance. As impedances are mis-matched, part of the transmitted signal is reflected back into the source which is the Voltage Standing Wave Ratio (VSWR); the ratio of the reflected waveform to the transmitted waveform. With antenna design: VSWR is a measure of how well the input impedance of the antenna matches the characteristic impedance of the output from the RF network. Impedance mismatch will reduce performance ! Antenna Theory – Antenna Types - Commonly Used antennas • Commonly used antennas – PCB antennas • No extra cost • Size demanding at sub 433 MHz • Good performance at > 868 MHz – Whip antennas • Expensive solutions for high volume • Good performance • Hard to fit in many applications – Chip antennas • Medium cost • Good performance at 2.4 GHz • OK performance at 868-955 MHz • Poor performance at 433-136 MHz – Wire / Helical antennas • Low cost • Ideal at sub 433 MHz Antenna Measurements - Done in Lab • S-parameter measurements – Q and Bandwidth – Matching - Mismatch – Return Loss • Spectrum Analyzer – Power Delivered to Antenna – Bandwidth – Relative Measurements of Radiated Power • TI Tools – Receive Signal Strength Indicator (RSSI) SmartRF studio – Frequency Sweep Function (Ideal for bandwidth measurements) Antenna Measurements - Impedance Magnitude of mismatch in dB with respect to frequency. The smith chart shows how the impedance varies with No exact impedance can be obtained from this format. frequency. Useful tool to find the values of the antenna matching component values. VSWR circles can be used to see how well the antenna is matched. Question: The dashed red line is shown at 14dB. What is the VSWR ? S11 S11 Antenna Measurements - Impedance - Smith Chart Short circuit Match Open circuit parallel L series L Inductive parallel C series C Capacitive Antenna Measurements - Impedance Matching Mismatch between the antenna and the feed line results in losses and will reduce the radiated power from the antenna. Inductors and capacitors in shunt and series can be used to achieve the desired impedance matching. Antenna Measurements - Characterization How to characterize antennas: 1. Measure the reflected power at the feed point of the antenna 2. Measure the radiated power across the bandwidth of interest 3. Measure the radiation pattern in an Anechoic Chamber 1. 2. 3. Antenna Measurements - Anechoic Chamber z + = y x • Patterns give directivity and gain of the antenna. • Radiated power is also measured. • Done in an anechoic chamber to eliminate multi-paths for accurate plots. • Done through contracted 3rd party companies. Antenna Measurements - andwidthB With a Spectrum Analyzer With a Network Analyzer BWVSWR2 fHL f Antenna Measurements - Reflection ZZ 1 l 0 VSWR ZZl 0 1 VSWR 1 20S11 log( ) 20 log dB VSWR 1 VSWR S11dB Reflected Delivered power % power % 1 -∞ 0 100 1.1 -27 0.2 99.8 1.2 -21 0.8 99.2 1.5 -14 4 96 2 -9.5 11.1 89.9 3 -6 25 75 4 -4.4 36 64 5 -3.5 44.4 55.6 5.8 -3 50 50 10 -1.7 66.9 33.1 Antenna Measurements - Mounting of Cable for S11 Measurements Tip: It is invaluable to have semi-rigid cables in the lab for debugging RF. Solder first shielding onto an earth plane and then solder the 50ohm connection. Minimize risk for riping off tracks when connecting to the semi- rigid cable. Supplier: AMSKA (www.amska.se) Ready made semi-rigid cables are A50451229, ASC047-PSMAf-0,3-200, Cable Assembly, with SMA-f and quite expensive but can be re-used 3mm stripped, 20cm again.