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School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio Links The Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001 Antenna Fundamentals (4) R. Struzak [email protected] 15 Feb 2001 Property of R. Struzak 1 • Note: These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Ryszard Struzak. These materials may not be published, copied to or issued from another Web server without the author's express permission. Copyright © 2001 Ryszard Struzak. All commercial rights are reserved. If you have comments or suggestions, please contact the author at [email protected]. 15 Feb 2001 Property of R. Struzak 2 Summary Slide • • • • • • Antenna Measurements Antenna Calibration Anechoic Chamber TEM Cells 3D Radiation Pattern Measurements Conclusion 15 Feb 2001 Property of R. Struzak 3 Transmission vs. Reception 15 Feb 2001 Property of R. Struzak 4 Transmit/ Receive Equivalence • Reciprocity theorem. – The radiation pattern and radiation resistance of an antenna is the same when it transmits and when it receives, if no non-reciprocal devices are used. • Does not apply to active antennas – 15 Feb 2001 Property of R. Struzak 5 Antenna Measurements 15 Feb 2001 Property of R. Struzak 6 Radiation Pattern Measurements (1) • Measured antenna in receiving mode – The antenna is rotated (or the radiowave source is moved around) – The power received (output voltage) is registered vs. the direction angle (azimuth, elevation) 15 Feb 2001 • Measured antenna in transmitting mode – The antenna is rotated (or the field-strength meter is moved around) – The field-strength is registered vs. the direction angle (azimuth, elevation) Property of R. Struzak 7 Radiation Pattern Measurements (2) • Laboratory • Field – Special test site – In-situ measurements • Open field • Anechoic chamber • Near-field / Far field calculation • Scaling 15 Feb 2001 Property of R. Struzak • Measuring instruments in car, balloon, aeroplane, or helicopter • Actual distance / standard distance problem • Environmental effects 8 Electric Field Measurement • Dipole antenna • Balance matching • Impedance matching 15 Feb 2001 Property of R. Struzak 9 Wideband Antennas • Impedance and radiation pattern of antenna are frequency dependent • Wideband antennas – – – – Conical antennas Equi-angular antennas Log-spiral antennas Log-periodic antennas 15 Feb 2001 Property of R. Struzak 10 Impedance Matching • For maximum power transfer the load impedance must match the source impedance: RLOAD = RSOURCE XLOAD = -XSOURCE • Transmission line must terminate in its characteristic impedance • The balanced/ unbalanced mode-continuity must be assured or a transformer (balun) must be used BALANCED /2 UNBALANCED 15 Feb 2001 Property of R. Struzak 11 Magnetic Field Measurement Slot • Loop antenna • Screen against electric component 15 Feb 2001 Property of R. Struzak 12 Calibration 15 Feb 2001 Property of R. Struzak 13 Gain Measurements: 2 Antennas • Reciprocity method – 2 identical antennas are used: one as the transmitting antenna and another as receiving antenna – The ratio of the power received to power transmitted is measured 15 Feb 2001 PR PT GT GR 4r GT GR G G Property of R. Struzak PR 4r PT 14 2 Gain Measurements: 3 Antennas • The 3-antenna method can be used to calibrate 3 arbitrary antennas. • 3 measurements are made, giving 3 equations with 3 unknown gains • It is the only method applicable to active antennas that cannot be used in transmit mode. 15 Feb 2001 P12 PT G1G2 4r 2 P23 PT G2G3 4r P13 PT G1G3 4r Property of R. Struzak 2 2 15 Calibrating Test Antennas (1) • Simulation of freespace conditions – Removing the reflected ray by using absorbers – Exploiting directivity (radiation nulls) – Practical with vertical polarization – Does not require anechoic chamber 15 Feb 2001 Property of R. Struzak 16 Calibrating Test Antennas (2) • Exploiting reflection – Using conducting surface – Adjusting antenna height to receive maximum – Practical with horizontal polarization – Does not require anechoic chamber 15 Feb 2001 Property of R. Struzak 17 Measurements in the Field • Relative (comparative) measurements using an auxiliary antenna of known radiation pattern eliminate the distance dependence 15 Feb 2001 Property of R. Struzak Auxiliary antenna Antenna under test 18 Far-Field Conditions L 1. R >> (/2) 2. R >> 2L2 / R R+ R O 15 Feb 2001 << / 16 (22,5 deg. instead of 0 deg.) (R+)2 = R2 +(L/2)2 R2 + 2R + 2 = R2 +(L/2)2 2R + 2 = (L/2)2 R ~ (L/2)2 /(2) = L2 /8 R >> (L2 /8) * (16/ ) Important when dealing with radiation nulls Property of R. Struzak 19 Example • Antenna diameter = 2 m 1. 300 MHz ( = 1 m) • • /2 = 1/ 6.28 ~ 0.16 m 2L2/ = 8 / 1 = 8 m 2. 3000 MHz ( = 0.1 m) • • 15 Feb 2001 /2 = 0.1/6.28 ~ 0.016 m 2L2/ = 8 / 0.1 = 80 m Property of R. Struzak 20 Anechoic Chamber • • • • Needs Benefits Problems Practical examples and applications • Photographs 15 Feb 2001 Property of R. Struzak 21 TEM Cells • • • • • Needs Benefits Problems Practical examples and applications Double-polarization TEM Cell at the author’s institute (photographs) 15 Feb 2001 Property of R. Struzak 22 3D–Radiation Pattern Measurements • • • • Needs Benefits Problems Flying radio-measurement laboratory of the author’s institute (photographs) 15 Feb 2001 Property of R. Struzak 23 Conclusion • Antenna: substantial element of radio link • We have just reviewed – – – – – Basic concepts Radio wave radiation physics Elementary radiators Antenna systems Antenna measurements 15 Feb 2001 Property of R. Struzak 24 For Further Readings • Kraus JD: “Antennas” (1998) • Stutzman WL et al.: “Antenna Theory and Design” (1981) • Johnson RC: “Antenna Engineering Handbook” (1993) 15 Feb 2001 Property of R. Struzak 25 Computer Programs • Pozar D. “Antenna Design Using Personal Computers” • Li et al., “Microcomputer Tools for Communication Engineering” 15 Feb 2001 • NEC (Numerical Electromagnetics Code) • Free software: – NEC Archives: (www . gsl . net / wb6tpu /swindex . html). More than 300 NEC matches. Property of R. Struzak 26 Thank you for your attention 15 Feb 2001 Property of R. Struzak 27