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
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• Note: These materials may be used for study,
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Ryszard Struzak. All commercial rights are
reserved. If you have comments or suggestions,
please contact the author at
ryszard.struzak@ties.itu.int.
15 Feb 2001
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Summary Slide
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Antenna Measurements
Antenna Calibration
Anechoic Chamber
TEM Cells
3D Radiation Pattern Measurements
Conclusion
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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
–
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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)
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• 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)
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Radiation Pattern Measurements (2)
• Laboratory
• Field
– Special test site
– In-situ measurements
• Open field
• Anechoic chamber
• Near-field / Far field
calculation
• Scaling
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• Measuring instruments
in car, balloon,
aeroplane, or helicopter
• Actual distance /
standard distance
problem
• Environmental effects
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Electric Field Measurement
• Dipole antenna
• Balance matching
• Impedance matching
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Wideband Antennas
• Impedance and
radiation pattern of
antenna are frequency
dependent
• Wideband antennas
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Conical antennas
Equi-angular antennas
Log-spiral antennas
Log-periodic antennas
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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
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Magnetic Field Measurement
Slot
• Loop antenna
• Screen against
electric component
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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
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  
PR  PT GT GR 

 4r 
GT  GR  G
G
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PR  4r 


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.
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  
P12  PT G1G2 

 4r 
2
  
P23  PT G2G3 

 4r 
  
P13  PT G1G3 

 4r 
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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
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Calibrating Test Antennas (2)
• Exploiting reflection
– Using conducting
surface
– Adjusting antenna
height to receive
maximum
– Practical with
horizontal polarization
– Does not require
anechoic chamber
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Measurements in the Field
• Relative (comparative)
measurements using
an auxiliary antenna of
known radiation
pattern eliminate the
distance dependence
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Auxiliary antenna
Antenna
under test
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Far-Field Conditions
L

1. R >> (/2)
2. R >> 2L2 / 
R
R+
R
O
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 << / 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
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Example
•
Antenna diameter = 2 m
1. 300 MHz ( = 1 m)
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/2 = 1/ 6.28 ~ 0.16 m
2L2/  = 8 / 1 = 8 m
2. 3000 MHz ( = 0.1 m)
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/2 = 0.1/6.28 ~ 0.016 m
2L2/  = 8 / 0.1 = 80 m
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Anechoic Chamber
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Needs
Benefits
Problems
Practical examples
and applications
• Photographs
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TEM Cells
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Needs
Benefits
Problems
Practical examples and applications
Double-polarization TEM Cell at the
author’s institute (photographs)
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3D–Radiation Pattern Measurements
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Needs
Benefits
Problems
Flying radio-measurement laboratory
of the author’s institute (photographs)
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Conclusion
• Antenna: substantial element of radio link
• We have just reviewed
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Basic concepts
Radio wave radiation physics
Elementary radiators
Antenna systems
Antenna measurements
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For Further Readings
• Kraus JD: “Antennas” (1998)
• Stutzman WL et al.: “Antenna Theory and
Design” (1981)
• Johnson RC: “Antenna Engineering
Handbook” (1993)
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Computer Programs
• Pozar D. “Antenna Design
Using Personal Computers”
• Li et al., “Microcomputer
Tools for Communication
Engineering”
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• 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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