[TowerTalk] UHF aircraft antenna

[Jim Lux]

On 12/31/14 8:34 PM, n0tt1 at juno.com wrote:
> Guys, just curious....what's "inside" one of these aircraft
> glide slope antennas and how are they matched
> to say, a 50-ohm coax.

Basically half of a folded dipole: a folded monopole. Narrow band 
matching: Glide slope is a bit above 300 MHz with a span of about 5 MHz, 
so about 1-2% BW

http://en.wikipedia.org/wiki/Instrument_landing_system

  One ham has one of these
> mounted on his car and he uses it for RFI
> location....the one with the "tap" on the upside-down U.
> Kinda looks like a dual loop antenna.
>
> There is another antenna with a single loop in
> the upper right of the screen...looks like an insulator
> in the middle of the loop.

Aerodynamic design, basic conservativeness in the aviation biz, avoiding 
patents from other companies. There's only so many ways to make an omni 
300 MHz antenna.
Actually, you might want a pattern that's sort of fore/aft, since you 
generally don't start looking at the GS until you're on final and lined 
up with the localizer.