Believe it or not (I checked with trusty wattmeter), I get good SWR
across the entire Comm band using a cheap ELT antenna I bought from
Chief Aircraft for about $35 several years ago (the one that has a very
flexible whip). I have one mounted on the top of the (leading edge)
fairing that covers the wing joint on my Corben Junior Ace The fairing
basically is the leading edge, (filling the gap between wing panels) and
goes ffrom front spar top to front spar bottom. The metal is about 6"
wide, so it would seem a bit small for a ground plane, but it works. I
can routinely talk air to air over about 70 miles with both planes at
pattern altitude. Air to ground (from 1000 feet) is 20-30 miles or so.

Scott


wrote:

Orval Fairbairn wrote:

In article ,
"ccwillwerth" wrote:


Hi, I am about ready to cover my Cub type airframe, but need a place to
attach a com antenna. I was considering brazing a plate to the airframe so
that it would be just under the fabric. The antenna is the type that has a
ceramic insulator on the bottom of the antenna that insulates the stainless
steel antenna from the airframe.


Charlie, you will get several opinions, but here is mine.

If at all possible, get a "broadband" VHF Com antenna, not a wire whip.
The "broadband" fiberglass antennas have a VSWR of less than 2:1 across
the range of 118 to 136MHz, while the metallic whip will have an
bandwidth of only about 5Mhz where the VSWR is below 2:1. At the band
edges, the VSWR will be high enough to cause the VSWR protection
circuitry in transistorized transmitter to shut the output power down
to nearly zero. Although the wire-whip can be cut&tuned to just Unicom
frequencies (122.7 to 123.6 Mhz), it could be marginal for transmission
at some ATC frequencies. The whip will work ok for receiving even at
the band-edges, because the receiver doesn't care about the VSWR.

If your fuselage is anything like my Piper PA20, there are enough metal
tubes to act as a ground plane without adding any additional conductive
material, other than a mounting plate. I would put the antenna base on
a metal plate which is just below the plane formed by the fabric.
Radius the edges of the plate so that the fabric doesn't ride on a
sharp edge. The plate could be long enough to bridge between two
fuselage cross-brace tubes, but only about 4 to 6" wide. It must be
electrically "bonded" to the cross-braces, so to avoid drilling holes
in the cross-braces, your idea of welding some attach "ears" to the
cross-braces to mount the plate is good.