Ham Radio Help Desk

An airplane mechanic friend, who also happens to own a 35 year old single engine Cessna airplane, was working on his tail light when he saw broken and aging components in his tail mounted navigation antenna. While working to fix the problems he discovered an interesting balun technique.

These antennas are usually just two 1/4 wave dipole elements mounted horizontally on the top of the rudder.

This seems the obvious place to use a balun to take the unbalanced coax and connect it to the balanced antenna.

Indeed, this appears to be what Cessna has done and this fellow’s airplane receives good comments about its receiving capability from the pilots who fly his airplane.

So here is how the antenna coax is arranged to connect the dipole to the coax…

Balun used in Cessa aircraft for VHF Navigation Receiver

Note how the center conductor never ever electrically connects to anything. It would appear there has to be some capacitive coupling in the 1/4wave – 20% section.

As this has been operating in the many hundreds of Cessna aircraft built over the decades, this, quite obviously, works. Can any one of you viewers of this web site explain why?

Thanks.

Tags: Airplane, Aviation, balun, Cessna