1. 15 Feb 2001Property of R. Struzak1 Antenna Fundamentals (4) R. Struzak ryszard.struzak@ties.itu.int 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

2. 15 Feb 2001Property of R. Struzak2 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 These materials may not be published, copied to or issued from another Web server without the author's express permission.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 ryszard.struzak@ties.itu.int.

3. 15 Feb 2001Property of R. Struzak3 Summary Slide Antenna Measurements Antenna Calibration Anechoic Chamber TEM Cells 3D Radiation Pattern Measurements Conclusion

4. 15 Feb 2001Property of R. Struzak4 Transmission vs. Reception

5. 15 Feb 2001Property of R. Struzak5 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 –

6. 15 Feb 2001Property of R. Struzak6 Antenna Measurements

7. 15 Feb 2001Property of R. Struzak7 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) 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)

8. 15 Feb 2001Property of R. Struzak8 Radiation Pattern Measurements (2) Laboratory –Special test site Open field Anechoic chamber Near-field / Far field calculation Scaling Field –In-situ measurements Measuring instruments in car, balloon, aeroplane, or helicopter Actual distance / standard distance problem Environmental effects

9. 15 Feb 2001Property of R. Struzak9 Electric Field Measurement Dipole antenna Balance matching Impedance matching

10. 15 Feb 2001Property of R. Struzak10 Wideband Antennas Impedance and radiation pattern of antenna are frequency dependent Wideband antennas –Conical antennas –Equi-angular antennas –Log-spiral antennas –Log-periodic antennas

11. 15 Feb 2001Property of R. Struzak11 Impedance Matching For maximum power transfer the load impedance must match the source impedance:  R LOAD = R SOURCE  X LOAD = -X SOURCE Transmission line must terminate in its characteristic impedance The balanced/ unbalanced mode-continuity must be assured or a transformer (balun) must be used BALANCED UNBALANCED /2

12. 15 Feb 2001Property of R. Struzak12 Magnetic Field Measurement Loop antenna Screen against electric component Slot

13. 15 Feb 2001Property of R. Struzak13 Calibration

14. 15 Feb 2001Property of R. Struzak14 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. 15 Feb 2001Property of R. Struzak15 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.

16. 15 Feb 2001Property of R. Struzak16 Calibrating Test Antennas (1) Simulation of free- space conditions –Removing the reflected ray by using absorbers –Exploiting directivity (radiation nulls) –Practical with vertical polarization –Does not require anechoic chamber

17. 15 Feb 2001Property of R. Struzak17 Calibrating Test Antennas (2) Exploiting reflection –Using conducting surface –Adjusting antenna height to receive maximum –Practical with horizontal polarization –Does not require anechoic chamber

18. 15 Feb 2001Property of R. Struzak18 Measurements in the Field Relative (comparative) measurements using an auxiliary antenna of known radiation pattern eliminate the distance dependence Auxiliary antenna Antenna under test

19. 15 Feb 2001Property of R. Struzak19 Far-Field Conditions 1.R >> ( /2  ) 2.R >> 2L 2 / O R R+   L  << / 16 (22,5 deg. instead of 0 deg.) (R+  ) 2 = R 2 +(L/2) 2 R 2 + 2R  +  2 = R 2 +(L/2) 2 2R  +  2 = (L/2) 2 R ~ (L/2) 2 /(2  ) = L 2 /8  R >> (L 2 /8) * (16/ ) Important when dealing with radiation nulls R

20. 15 Feb 2001Property of R. Struzak20 Example Antenna diameter = 2 m 1.300 MHz ( = 1 m) /2  = 1/ 6.28 ~ 0.16 m 2L 2 / = 8 / 1 = 8 m 2.3000 MHz ( = 0.1 m) /2  = 0.1/6.28 ~ 0.016 m 2L 2 / = 8 / 0.1 = 80 m

21. 15 Feb 2001Property of R. Struzak21 Anechoic Chamber Needs Benefits Problems Practical examples and applications Photographs

22. 15 Feb 2001Property of R. Struzak22 TEM Cells Needs Benefits Problems Practical examples and applications Double-polarization TEM Cell at the author’s institute (photographs)

23. 15 Feb 2001Property of R. Struzak23 3D–Radiation Pattern Measurements Needs Benefits Problems Flying radio-measurement laboratory of the author’s institute (photographs)

24. 15 Feb 2001Property of R. Struzak24 Conclusion Antenna: substantial element of radio link We have just reviewed –Basic concepts –Radio wave radiation physics –Elementary radiators –Antenna systems –Antenna measurements

25. 15 Feb 2001Property of R. Struzak25 For Further Readings Kraus JD: “Antennas” (1998) Stutzman WL et al.: “Antenna Theory and Design” (1981) Johnson RC: “Antenna Engineering Handbook” (1993)

26. 15 Feb 2001Property of R. Struzak26 Computer Programs NEC (Numerical Electromagnetics Code) Free software: –NEC Archives: (www. gsl. net / wb6tpu /swindex. html). More than 300 NEC matches. Pozar D. “Antenna Design Using Personal Computers” Li et al., “Microcomputer Tools for Communication Engineering”

27. 15 Feb 2001Property of R. Struzak27 Thank you for your attention