Limit of performance at 15 miles is equivalent to, if all other variables ideal,
either you or ATC transmitting about 50 milliwatts! Verses 7-8 watts on your
end, or up to 50 watts per FCC rules on ATC's end, except for ARTCC Centers.
An 8" x 20" ground plane might be very directional (bet Ferg would know!) and if
the pattern is weak in fore/aft direction, that's where you want it stronger
for usual VFR comm at least. 20" diam round metal anything, or 20" center to
the corners of a square, or 4 legs of copper wire from 'Romex' house wire will
be the same as copper mesh at VHF.
If your avionics guy checked the cable for DC continuity and no short, that's
valid
90% of the time -- the other 10% is how RF behaves - doesen't see just DC,
so a loose swap with an RG-58 patch cable will check that.
I also can't see how mounting an antenna which requires a ground plane inside
the
fuse can assure there's no interference from the pitch push tube. That could
have been the problem with the Bob Archer. To the extent others have found
success, luck can control, so different temporary mounting locations is one
attempt
at a fix. It will affect radiating pattern, and you can check this by
flying at a distance from an ATIS broadcast to produce very weak reception and
fly gentle 360's. Once when I botched the dimensions of an idiot-simple "cable
balun" to fix a nav antenna problem, them 360's at just 50 nm from a
high-altitude
(mucho watts) VOR showed how badly I done! She's perfect now.
Sounds like sumpin's radically wrong, if your tech guy checked xmit power and
receive
sensitivity, as amateur antenna installs can be very forgiving. Today
I helped a buddy out when I told him he sounded weaker than usual, but still
clear.
Seems he had fixed his headliner and had to renew a whip antenna connection
which he broke. He connected both repaired center and shield ring connectors
(no BNC, original antenna on a '46 Luscombe) to the one threaded stud of
the antenna, thus shorting the two wires together. Worse yet, he uses only a
dinky 2-watt handheld connected to said antenna. Still worked...RF doesn't
fully
see the DC short.
Regards,
Fred F.
> Hi guys,
>
> I'm hoping some of our very talented team of builders can give me some
> advice on a unique Com radio antenna setup I am planning. I've been
> flying my trigear now for 4 years and have never been happy with the
> transmit and receive distances I'm getting from my radio......a Garmin
> 250 XL GPS/Com unit. Initially I installed the Bob Archer antenna, but
> after a year or so I wasn't happy with the performance and switched to a
> Comat "general aviation" type antenna. I built an aluminum ground plane
> about 8" wide and 20" long which I installed inside the fuselage behind
> the bulkhead. The Comat antenna was mated to that ground plane. I got
> better performance than with the Bob Archer, but still I think I should
> be doing better than a 15 mile maximum range. I had the radio bench
> checked, and the coax cable checked by an avionics shop, and they
> pronounced everything in good working order. Their suggestion was to
> glass in a copper wire mesh ground plane to the bottom inside of the
> fuselage, and then to mount the antenna EXTERNALLY under the belly of
> the plane. A couple of questions arise. Has anyone else ever tried
> this? What were the results? and more importantly, what should the
> dimensions be for the copper wire mesh (screen)? I was thinking that
> the larger the better, but there may be some technical calculation that
> I'm unaware of that would dictate a particular size. Any and all
> comments and advice will be appreciated. Thanks!
>
> Regards,
>
> Garry V. Stout
> N4220S Tri A060
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