BTW. Somewhere I saw a Comm antenna diplexer that works both
ways - quite expensive (about $500). You can run two transreceivers
using one antenna.

Did you see one of them - I forgot where I saw it.
Can somebody give me the URL?

If you have used or tested them do they work OK?
Signal loss, problems... etc....?
Never mind the innards circuit theory...


Jim Weir wrote in message . ..
-Does anybody know if those things work?

Yes, I know.


They split a nav antenna
-signal into a VOR and glide slope unit?

That's the definition.


Is significant signal lost?

Not if you do it right.


Here's the deal, and a way to make one for less than $5.

The VOR "catswhiskers" antenna on the top of the vertical fin is cut for
quarter-wave at 113 MHz. Fortunately for us, that is a good threequarter-wave
antenna at the glideslope frequencies, at least close enough for government
work. If you have either a ferrite broadband balun or a quarter-wave coax
balun, those will also work at the glideslope band.

What you've got to do is to couple off the glideslope signal without affecting
the VOR signal. I thought about this for a long time before I came to the
conclusion that a series-resonant circuit at the glideslope band, if done
properly, would introduce minimal loss at the VOR frequency. How, I thought, to
make an inexpensive series resonant circuit?

Hm. How about we take advantage of the fact that a round straight wire has
inductance associated with it. Somewhere around 20 nanohenries per inch for #22
wire. Disk ceramic capacitors have #22 wire leads. How much inductance does a
10 picofarad capacitor need to resonate at 320 MHz? Wow. 20 nanohenries. So,
a 10 pf capacitor with half-inch leads on both sides will be a series resonant
circuit (dead AC short) at the glideslope frequencies.

Get yourself a small metal "hobby box" that will have room for 3 BNC connectors.
Drill 3 holes for the connectors, two very close to one another and another one
exactly one inch away from either one of the other two. Connect the two close
ones with a short piece of heavy solid uninsulated wire (#16 or so -- stripped
Romex will do well). Call the one that is exactly 1" away from the remaining
connector "Antenna In" and the other one "VOR Out". Now connect that 10 pf disk
ceramic capacitor from the "Antenna In" point to the remaining connector. Call
that remaining connector "Glideslope Out".

Why doesn't it affect the VOR? Because that 10 pf capacitor has an impedance of
somewhere around 150 ohms down at the VOR frequencies. As somebody else noted,
shorting the glideslope connector to ground or leaving it open will not cause
more than 1 dB of mismatch to occur.

Sweet, huh? Hm. I smell a Kitplanes column coming up... {;-)


Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com