School on Digital and Multimedia Communications Using Terrestrial

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 ryszard. struzak@ties. itu. int 15 Feb 2001 Property of R. Struzak 1

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Summary Slide • • • Antenna Measurements Antenna Calibration Anechoic Chamber TEM Cells 3 D 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 Property of R. Struzak 14

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 Property of R. Struzak 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 >> 2 L 2 / R R+ R O 15 Feb 2001 << / 16 (22, 5 deg. instead of 0 deg. ) (R+ )2 = R 2 +(L/2)2 R 2 + 2 R + 2 = R 2 +(L/2)2 2 R + 2 = (L/2)2 R ~ (L/2)2 /(2 ) = L 2 /8 R >> (L 2 /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 2 L 2/ = 8 / 1 = 8 m 2. 3000 MHz ( = 0. 1 m) • • 15 Feb 2001 /2 = 0. 1/6. 28 ~ 0. 016 m 2 L 2/ = 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

3 D–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 / wb 6 tpu /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