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…
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?
Further research indicates the above topology is a form of “Roberts’ Balun” initially designed many decades ago in 1957 for FCC field work by Willmar Roberts.
This is a somewhat like a Folded Balun with the addition of feeding the center wire of the coaxial line back into the stub.
The result is two contra-frequency-dependent effects which broaden the usable frequency range of this assembly.
The Roberts’ Balun examples I found usually have the looped center conductor terminate inside the stub rather than go all the way through it. This may be why the above example doesn’t seem to function well in the lab.
- Proceedings of the IRE, “A New Wide-Band Balun,” Proceedings of the IRE, Volume 45, Issue 12, pp 1628-1631; December 1957.