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 15 Feb 2001

Antenna Fundamentals (4)

R. Struzak [email protected]

Property of R. Struzak 1

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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

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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

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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 – 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 15 Feb 2001 Property of R. Struzak 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

BALANCED • 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  /2 UNBALANCED Property of R. Struzak 15 Feb 2001 11

Magnetic Field Measurement

Slot • Loop antenna • Screen against electric component 15 Feb 2001 Property of R. Struzak 12

Calibration

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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

P R



P T G T G R G T G

 

G R



G P R P T

4 

r

  4 

r

2 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.

P

12 

P T G

1

G

2

P

23 

P T G

2

G

3

P

13 

P T G

1

G

3  4 

r

 4 

r

 4 

r

2 2 2 15 Feb 2001 Property of R. Struzak 15

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 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 Property of R. Struzak 17 15 Feb 2001

Measurements in the Field

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

 R 15 Feb 2001 L

Far-Field Conditions

R R+  O 1.

2.

R >> (  /2  ) R >> 2L 2 /   <<  / 16 (22,5 deg. instead of 0 deg.) (R+ R 2  ) 2 + 2R = R 2 +(L/2) 2  +  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 Property of R. Struzak 19

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 15 Feb 2001 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” • NEC (Numerical Electromagnetics Code) • Free software: – NEC Archives: (www . gsl . net / wb6tpu /swindex . html). More than 300 NEC matches.

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Thank you for your attention

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