antenna in composite airframe

All this talk about antennas got me thinking about my install. I have
a Glasair with the antenna in the aft fuse. mounted on an aluminum
ground plane. Should this be connected to the a/c ground?
Ed

On Sat, 24 Nov 2007 08:37:54 -0800 (PST), et
wrote:

All this talk about antennas got me thinking about my install. I have
a Glasair with the antenna in the aft fuse. mounted on an aluminum
ground plane. Should this be connected to the a/c ground?
Ed

I take it that you have it mounted as a 1/4 wave vertical over a metal
plate (or wire radials) for a ground plane? If so is it mounded with
the metal plate on the bottom or top. I'd mount it inverted with the
plate in the top of the fuselage. However I used half wave dipoles and
a ferrite core around the coax to reduce currents flowing on the
outside of the coax.

The problem with he ground plane in the G-III or any plastic airplane
is all the cables and wiring running around with no wide ground. That
sometimes requires "playing" with the coax to eliminate feedback and
interaction.

In your case the aircraft ground is the shield on the coax. Any other
ground is going to be another wire run back there. A vertical mounted
on a metal plate for a ground will likely have an impedance much lower
than the 50 ohms you want. The dipole will likely be higher but it is
affected by nearby wires and control cables as well. However it is
balanced and the ferrite core makes things a bit simpler.

Check with RST engineering. I believe Jim used to make dipoles out of
copper ribbon you'd just stick up in the vertical stab (keep away from
the rudder hinge) or on the inside of the fuselage and glass over
them. Very nice construction and an easy install without the worries
of a ground plane. As I recall they weren't expensive either. But if
you have a G-III don't stick one inside the carbon fiber horizontal
stab.

Roger (K8RI)