>With all the recent discussion about antenna efficency and interference
---From other sources I have a question on my groundplane for the transponder
antenna. It is .016 alum x 18" dia and is located under the LH seat pan and
trapped by the cage/shell bolts. Should it be insulated from the cage or
does it not matter? Presently the bolts make it elect bonded to the cage.
I'm curious about the 18" diameter . . . it's not optimum for
a dedicated ground plane. On plastic aircraft, the ground
plane should have a RADIUS equal to the seated height
of the antenna. In the case of transponder antennas,
this would be 2.6" which translates to 5.2" in diameter.
The rational for an optomized ground plane is that it takes
place of what might otherwise be a relatively large expanse
of aluminum skin found on the heavy-iron. By making it
physically matched to the operating frequency of the
antenna, performance is optomized.
Having said that, would I recommend any changes to the
system you've described? . . . . No. If one were to
set off at a distance with a signal strength measuring
device and compare your installation with an optomized
installation and an identical installation on an all
metal airplane, the differences would be very hard to
measure.
The concept of an optomized ground plane provides a
minimum size for a system not yet installed. It's
not worth your trouble to tear out and replace what
you already have.
Bob . . .
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