Nav antenna diplexers

I have no idea if that will work or for that matter what the hell
nanohenries (and neither does my spell checker)are but it does sound like
fun. Now my question, can someone that has no clue as to what you wrote do
much, all or any of the electronics work on their kit built IF they are
going with an all electric IFR set-up?

If so PLEASE tell me you have a book. I'm still a ways off on my plane from
having to do this stuff but I am thinking about it.


Gig Giacona
www.peoamerica.net/N601WR

"Jim Weir" wrote in message
...


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