Ok, here is some info....

An antenna for 122.8 or thereabouts is about 2.45 meters full wave. A
vertical quarter wave is typically 50 ohms with a ground plane. A
quarter wave is .58 meters or 22.9 inches. If you use the airplane as
the ground and place a 22.9 inch antenna that is somewhat vertical from
the fuselage you might have it. Use 50 ohm coax to the point and ground
the shield to aircraft chassis at that point. Test the SWR (standing
wave ratio). It should be 1.5 to 1 or less. An 'L network' may be
needed to tune out any reactance. This is an experimental antenna ONLY.
A local ham club could really help out in testing the antenna. THeir is
something called an MFJ Antenna Analyzer that will tell you the
resonant frequency of the antenna and its impedance.

A well matched antenna should do fine with just 2 watts of output.
There are other types of antennas that could be used but the 1/4 wave
vertical is easy.
You will probably want to slant the antenna back at 30 degrees or so.

The above information should not be used for aviation communications
and is for experimental purposes only.

Rob
NV7F