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15 Feb 2001 Property of R. Struzak 1
Antenna Fundamentals (4)
School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio LinksThe Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001
15 Feb 2001 Property of R. Struzak 2
• 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 These materials may not be published, copied to or issued from another Web server without the author's express permission.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 3
Summary Slide
• Antenna Measurements
• Antenna Calibration
• Anechoic Chamber
• TEM Cells
• 3D Radiation Pattern Measurements
• Conclusion
15 Feb 2001 Property of R. Struzak 4
Transmission vs. Reception
15 Feb 2001 Property of R. Struzak 5
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 6
Antenna Measurements
15 Feb 2001 Property of R. Struzak 7
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)
15 Feb 2001 Property of R. Struzak 8
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 9
Electric Field Measurement
• Dipole antenna
• Balance matching
• Impedance matching
15 Feb 2001 Property of R. Struzak 10
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 11
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
UNBALANCED
/2
15 Feb 2001 Property of R. Struzak 12
Magnetic Field Measurement
• Loop antenna
• Screen against electric component
Slot
15 Feb 2001 Property of R. Struzak 13
Calibration
15 Feb 2001 Property of R. Struzak 14
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
r
P
PG
GGG
rGGPP
T
R
RT
RTTR
4
4
2
15 Feb 2001 Property of R. Struzak 15
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.
2
3113
2
3223
2
2112
4
4
4
rGGPP
rGGPP
rGGPP
T
T
T
15 Feb 2001 Property of R. Struzak 16
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 17
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 18
Measurements in the Field
• Relative (comparative)measurements usingan auxiliary antenna of
known radiation pattern eliminate the distance dependence
Auxiliary antenna
Antennaunder test
15 Feb 2001 Property of R. Struzak 19
Far-Field Conditions
1. R >> (/2) 2. R >> 2L2 /
O
RR+
L
<< / 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
R
15 Feb 2001 Property of R. Struzak 20
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) /2 = 0.1/6.28 ~ 0.016 m• 2L2/ = 8 / 0.1 = 80 m
15 Feb 2001 Property of R. Struzak 21
Anechoic Chamber
• Needs
• Benefits
• Problems
• Practical examples and applications
• Photographs
15 Feb 2001 Property of R. Struzak 22
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 23
3D–Radiation Pattern Measurements
• Needs
• Benefits
• Problems
• Flying radio-measurement laboratory of the author’s institute (photographs)
15 Feb 2001 Property of R. Struzak 24
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 25
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 26
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”
15 Feb 2001 Property of R. Struzak 27
Thank you for your attention