A groundplane should have a radius at least 5% longer than the driven antenna. That means that a true groundplane for 121.5 megahertz would need to be about 52 inches in diameter.
For our small KR's, the best alternative is to use a counterpoise made up of the braid of the coaxial cable that is about 5% longer than the driven element. I put my antenna about two feet behind the seat on the sidewall with the feedpoint about midway up the sidewall. The driven element was floxed into place vertically, with the top curving toward the center of the turtledeck. The braid counterpoise was floxed down the sidewall, then across the floor of the fuselage. I have used this type of antenna in my Renegade with excellent results. Jim Vance va...@claflinwildcats.com
