The International Cessna^® 170 Association

A quarter wave at 110MHz is 27" in air, but only 17" in coaxial cable because the waves travel slower in the coax.

This kind of balun is called a Roberts Balun, or a folded balun. There's not a lot of literature describing how it works, and there's some question about whether the stub should be 17" or 27", but it's normally made 17. And that works. It even passes enough GS signal, and that's about 330MHz.

New VOR antennas have a transformer-type balun molded into the base.

DaveF wrote:A quarter wave at 110MHz is 27" in air, but only 17" in coaxial cable because the waves travel slower in the coax.

This kind of balun is called a Roberts Balun, or a folded balun. There's not a lot of literature describing how it works, and there's some question about whether the stub should be 17" or 27", but it's normally made 17. And that works. It even passes enough GS signal, and that's about 330MHz.

New VOR antennas have a transformer-type balun molded into the base.

Isn't it wonderful, the amazing amount of shared knowlege which exists in our group?
I love it!

(Now if someone could only explain why the metallic taste in my mouth after I said that antennas are not "dark majic".)

Yes, I made a 26" one. Yes, it lays on the headliner behind me.
I bet the short one would work just fine. If I ever have need to take the antenna off, I'll put on a short one and check it out.

I asked about this because of erroneous CDI indications. I had the nav radio and VOR indicator removed and bench tested first and both tested good. All wiring and connections are just a few years old and were not suspected bad. The only piece of the puzzle left was the balun. I made a new one with RG400 and simply duplicated the 17 inch loop that was removed. A ground test yesterday and flight test today revealed a successful repair. Sure was cheaper than a new GI106 indicator!!