REFLECTOR: radio problems

Dave Black dvblack at comcast.net
Tue Aug 23 11:25:16 CDT 2005


Tim,

> I have  Garmin CNX-80  and SL-30 radios.   I am
> able to transmit fine but both radios will only recieve if you
> are within several hundred feet. Beyond that no reception.  I
> have dipole  antennas,  nav and comm in each wing that were
> prebuilt so I have no acess to the antennas Each wing has one
> comm and one nav antenna.  

The first thing I'd want to check is your antennas. 

I had a great deal of trouble with the antennas in my prebuilt wings, so I
know first hand some of the antennas are not wired correctly. In my case, the
co-ax had broken during the wing building process right where it attaches to
the antenna. The effective result was no antenna at all. Cheap co-ax with a
single center conductor had been used by the wing builder. All wiring and
cables within aircraft are supposed to be stranded, rather than single
conductor in order to minimize the fatigue effects of vibration or bending. 

After fixing that, I found the antenna's proximity to carbon layups within the
winglet made it a very ineffective antenna anyway. In the end, I opened the
winglets, removed the prebuilt antennas, and installed new antennas. 

But before you rip things apart, TEST your antenna with an antenna tester.
It's a quick test involving a variable-frequency transmitter and SWR meter in
one box. You can sweep across all the frequencies in the aviation band and
draw a Standing Wave Ratio curve. That curve should be below 2:1 across the
COMM band, and the lowest part of the curve should center on the band. By the
way, if you're wondering which antenna is where, you can run your hand along
the wing or winglet during the SWR test. When you approach the antenna, the
SWR goes way up. 

>From your description, though, I would anticipate a SWR curve of 10:1 or
higher -- which means your antenna is useless or disconnected.

If you know someone who is into Ham Radio, he may have an antenna tester and
would probably be more than happy to check your antennas for you. Failing
that, visit your local avionics shop. They will be able to easily perform this
test in just a few minutes. 

Dave Black
Shortwing RG


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