nuckolls.bob(at)aeroelect
Guest
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 Posted: Tue Feb 07, 2012 9:15 am Post subject: Com and Nav Radio and Antenna Questions |
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At 09:56 AM 2/7/2012, you wrote:
| Quote: | Bob, I have a question. How much performance degradation, TLAR
estimate, is there from placing a com antenna in a horizontal or
near horizontal position? Are not com radio signals vertically
polarized? Doesn't mounting them horizontally increase there
directional selectivity, for lack of knowing the proper term?
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The ONLY time that an antenna delivers on it's theoretical
performance (radiation efficiency and patters) is on the
antenna test range under controlled conditions.
Many antennas can approach theoretical performance in service
when situated in free space and "ground" configurations
that are large compared to the frequency of interest.
In other words, antennas for GPS and transponders at
1000+ MHz do pretty well.
As you go down in frequency, it becomes increasingly
difficult to configure an antenna installation that
even approaches theoretical performance. On small structures
like airplanes with lots of 'sticky-outs' (wings, stabilizers,
landing gear, whirring props, other antennas, etc) both
radiation pattern and polarity are distorted. Out
in the vacuum of space, the theoretical loss for
a vertical antenna talking to a horizontal antenna is
huge . . . in practice here on the ground antenna range,
it's on the order of 10-20 dB. But these will be "centered
up" measurements where the two antennas are looking right
at each other.
However, as soon as you bolt the antenna to an airplane,
one's ability to predict performance variables goes down.
Someone who has measured the effects of vertical vs. horizontal
polarization on airplanes like yours might offer some
advice based on experience, but I've never met an RF guru
who would attempt to put a quantitative answer to your
question without going out to measure it - a difficult
task at best. RF squirted from (or at) your airplane's
compromised antenna is subject to MANY unpredictable
variables.
Once you've relocated your antenna, YOU are going to
become the AeroElectric-List guru for having carried out
the experiment. Put the second antenna on along with
a switch that will let you switch quickly between the
'compromised' and 'optimized' antennas. Do some listening
tests with weak stations as you fly a 360 pattern listening
for 'hot' and 'dead' spots while making not of the relative
signal strength of 'compromised' vs. 'optimized' antennas.
We conducted similar tests on our production aircraft
using a ground station at the field for a signal source.
Automated acquisition systems would gather data on
received signal strengths as the airplane maneuvers.
But attempting to predict relative antenna performance
without conducting real tests is sorta like hypothesizing
about "The theory of everything".
Here's some further reading on the topic.
http://www.astronwireless.com/topic-archives-selecting-antennas.asp
http://www.astronwireless.com/topic-archives-antennas-polarization.asp
http://home.comcast.net/~n9rla/Antenna_Polarization.PDF
Bob . . .
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