Don-
I can't comment on the three antenna substitution method, because I don't
know the exact technique (though I have a vague understanding about how it
would work).
But I do have a comment about the use of circular and linear polarization
antennas, and making comparisons. I went though this exercise a few months
ago while comparing some (circularly polarized) GSM antennas, using a
linearly polarized signal from a horn antenna. Your circularly polarized
antenna is certain to have some non-circularity. This causes what is
referred to as polarization loss. The get the correct polarization loss,
and thus the correct gain, for the circularly polarized antenna when using
linearly polarized signals, the correct model is important.
Start with a linearly polarized field. Measure the received signal with the
real circularly polarized antenna axially rotated to the position of
maximum received signal. Then rotate the antenna 90° about its axis to get
the minimum received field. The ratio between the two received powers is
the axial ratio.
Here is where things get complex, especially since I need to think about it
this way to understand it. Assume you are using crossed dipoles to generate
a perfect circularly polarized field. Assume there are two generators, one
at the center of each dipole. One is set at 1 volt, 0° phase, and the other
is set at 1 volt, -90° phase. Assume an impedance of 73 ohms for all
dipoles (transmit and receive). The total power transmitted is 2×(1²/73) =
27.4 mW. Now assume a perfect circularly polarized antenna of crossed
dipoles receiving the transmitted field. Let's say we receive 0.1 V (with
73 ohm load) at the terminals of each dipole. Via a lossless phasing
section, the two received voltages add in series and in phase giving 0.2
volts. The source impedance is 2×73 = 146 ohms. This gives a received power
of 0.2²/146 = 274 µW.
Let's say the axial ratio was of our real antenna is 3 dB. In this model,
we need to once again generate a field of same TOTAL power to the transmit
dipoles, but with one power 3 dB lower than the other. To accomplish this,
the voltages to the two dipoles are now 1.1547 V and 0.81649 V. The total
power is still 27.4 mW. The perfect circularly polarized antenna now
receives 0.11547 V on one dipole and 0.081649 V on the other dipole. After
the lossless phasing section, the total voltage adds up to 0.1971 V with
the same 146 ohms source impedance. The received power is then 0.1971²/146
= 266 mW. Note that this is only 0.126 dB lower than with a perfect
circularly polarized field.
The behavior of a real circularly polarized antenna with an axial ratio of
3 dB, immersed in a perfect circularly polarized field, is exactly the same
as the model above.
I have to thank the folks at Wi-Sys for helping me to understand this.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
"UMBDENSTOCK,
DON"
<djumbdenstock@ty To
coint.com> [email protected]
Sent by: cc
[email protected]
Subject
Antenna Gain
04/17/2006 01:46
PM
Dear Forum Colleagues,
I have been doing inductive loop systems for awhile and find myself a bit
rusty on antenna theory.
Given that an antenna of interest is a 900 MHz, circular polarized patch
antenna, we need to determine the gain of that antenna for FCC purposes.
If we use 2 antennas of our design, and the other an arbitrary antenna,
will the 3 antenna substitution method give a reasonably accurate
indication of gain? The arbitrary antenna could be a circular polarized
antenna, a horn antenna, a coat hanger or a dog ( J )....does the type of
antenna used for the 3rd antenna matter for the 3 antenna method to
establish the gain of the test item antennas??
Don Umbdenstock
Manager Compliance Engineering
Tyco Safety Products / Sensormatic
6600 Congress Avenue
Boca Raton, FL 33487 USA
Phone: 561.912.6440
[email protected]
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