nuckolls.bob(at)cox.net
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 Posted: Tue Jul 29, 2008 5:36 pm Post subject: Grounding of com antenna on Tube and Fabric airframe |
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At 11:16 AM 7/28/2008 -0700, you wrote:
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I have a Kitfox 7 that I just had inspected (I have 1 hour on it so far)
and am having issues with my transmission and reception quality and
effectiveness. Here is a brief description of my problem:
When at the airport (taxing, and in the pattern) my radio seems to work
fine with the exception of an intermittent static blip every two seconds
or so. As I get farther out, the intermittent blip becomes more
pronounced to the point that my transmission and reception is horrible
(transmission is worse than reception though by a long shot). Basically,
my communications become unintelligible around 4-5 miles out. I
originally didn't have a ground plane other than the mounting plate for
the antenna (which the manual says is fine) so I added one on the advice
of a couple people and the problem has not changed a bit.
Here is a question: Does the antenna itself have to be electrically
bonded to the airframe? I think the answer is yes and according to the
maker of my antenna, mine is through the mounting hardware. However, the
builder's manual says to remove any fabric and grind down the powder
coating under the antenna to get a good bond. Would this be my
problem? Would the issues I have described be symptomatic of a poorly
grounded antenna? I know just enough about electronics to be dangerous so
any help and advice would be greatly appreciated.
I guess I should state that I have checked my antenna cable for continuity
and it all checks out and the shielding is completely isolated. Also, my
radio is clear of all other noise such as engine, alternator, etc.
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The first thing to do is get an antenna analyzer of some
type hooked to your feedline at the transceiver end
and see what it says. This is not an absolute test of
performance but it IS an absolute test of non-performance.
High SWR says something is seriously wrong without telling
you exactly what it is.
Another experiment you can try is substitute a test
antenna. Get some brass shim stock from machine shop
or sheet from Hobby Lobby/local hardware store. Thinner
the better. Wrap a piece around the strut of your airplane
1/2 way out to the wing attach. Secure with rubber banding
or wraps of plastic tape around the brass . . . don't
get the tape goo on your paint.
Take a piece of coax (RG58 will do) and remove outer jacket
and shield to expose 20" of inner insulation and conductor.
This is the "antenna". Do this . . .
http://aeroelectric.com/articles/shldwire/shldwire.html
Cut shield pigtail to about 1/2" and tack solder to
brass. Tape coax down strut and bring into cabin
at trailing edge of door or through window. Install
coax connector and attach to transceiver.
I've built dozens of these antennas to make temporary
installation on ferry aircraft and they've performed
well without having to drill holes and/or make more
complex installations on the a/c.
If THIS antenna works "better", then you'll need
to investigate the mounted antenna more closely.
If not, then the problem is in the radio. That "every
two seconds" thing is a bit buggy . . . antennas
don't tell time worth a hoot but electronics is
much better at it.
Bob . . .
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