Vigilant1 wrote:

Greetings,
I'm not a ham, but I am hoping a radio pro here can offer some advice.
I am a private pilot and the VHF radio in my plane (118-137 MHz) is not
behaving well. Reportedly my transmissions are weak and only partially
intelligible. I sent the unit back to the factory a year ago, they
replaced some bad capacitors and adjusted the TX frequencies, but now
the problem is back.

I bought a handheld unit (Yaesu FTA-550) and am considering just using
that in flight--it is legal, and performance is supposed to be adequate
if used with an external antenna. But, my installed radio (a Val 760
model) is a little more powerful and it is a less "kludgy" fit in my
aircraft's cramped interior.

I'd like to do some testing to find out if my existing radio really has
a problem, or if I've got a problem with my cable or connectors to the
external antenna.
Ideally I'd like to be able to see field strength and look at the
spectrum of the TX. Is there an inexpensive meter for doing this? I've
seen some very impressive add-ons for Android devices or USB devices for
laptops that do analysis of various kinds, and was hoping something
already existed for at least VHF field strength measurement. I won't
use it often, so I don't want to spend a lot of money, but an avionics
shop will probably charge me $100 just for a one-time diagnosis--I'd
rather have the tools to do it myself.

Thanks in advance for any assistance, and accept my apologies if I've
used incorrect terminology, etc.

Mark

An external instrument will tell you very little and the cheapest
inline instrument that would be useful (a SWR meter) is going to
cost at least $50 to buy, which is getting close to the $100 shop
fee without having done anything.

If you know any HAM's you can probably borrow one.

Things to try that are free:

Pull the radio from the tray and slide it in and out a few times. If
the contacts for the antenna have gotten "dirty" this should improve
things. If it does, go to Radio Shack and get a can of spray contact
cleaner that leaves no residue, pull the radio, spray the contacts
and seat and unseat the radio a few times.

Check the entire run of the coax back to the antenna to make sure
nothing has damaged it and there are no sharp kinks in the coax.
Sharp kinks will eventually cause problems inside the cable.

Check the connection to the antenna. Unplug and replace the cable
a few times. If that helps, use contact cleaner.

Inspect the antenna itself. If it is just a metal rod, check the
insulators between the antenna and the airframe for corrosion of
any kind and tightness.

If the antenna is the kind enclosed in plastic/fiberglass, check
for cracks where water could be getting in.

FYI, I used to work in an avionics shop in another life.

What your shop will do is put an SWR meter between the radio and cable,
then the cable and antenna. If it is bad at the back of the radio and
good at the antenna, then the cable will be inspected and replaced
if there is something "funny" in the run, and if not, replace the
connectors at both ends.

If it is bad at the antenna, the antenna gets replaced.


--
Jim Pennino

Jim,
Thanks. So, do what I can to improve the integrity of the most obvious/easily fixed failure points external to the radio. An SWR can be used to see if what I'm doing is improving things, at least as far as the antenna, then it's just a check of the physical condition of that piece. Seems simple enough.

Of course the SWR won't tel me anything about the integrity of the signal itself (am on on frequency, is it spilling over, am I overmodulating, etc).

Can I learn anything by checking the cable's (cable and connector's) resistance? It's 50 ohm cable and about 12 feet long.

Again, I appreciate your patience and support.

Mark