nuckolls.bob(at)aeroelect
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 Posted: Mon Jun 07, 2010 12:08 pm Post subject: Strange radio transmit problem - any suggestions? |
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At 12:25 PM 6/7/2010, you wrote:
Guys,
Thanks for your suggestions regarding this problem.
It turned out the radio was transmitting carrier wave through the frequency range, but only transmitting voice below about 120MHz, and above 130MHz. When no voice was going out there was no side tone either. Becker said if its transmitting on any frequency then there's nothing wrong with the radio as there is only one transmitter, etc.
I believe I have solved it by replacing the antenna feeder. I'm not sure if the problem was a degraded piece of coax or a poor termination. The previous feeder was a piece of RG58 of unknown pedigree and is now a piece of RG400 that happened to be in my spares box and was 6" too long  (Makes a change - everything is usually 6" too short). Crimp on terminations from Stein using Bob's method note.
I still don't really understand what was happening - I guess the radio was smart enough to only provide a side tone when it was actually radiating speech on the carrier wave, and it realized that no speech was going out on the frequencies in question.
From your description it now seems likely that
you had a bad shield connection at one or both
ends of the original coax. When deprived of good
shield connection, a feed line that NORMALLY
conducts most energy of interest between antenna
and radio becomes a radiator. Further, the
most efficient frequency of radiation becomes
a function of length and proximity to other
conductors in the airplane. Depending on a
host of variables, the "problem" can manifest
over a narrow range of frequencies. Finally,
it may be a strong source of potentially interfering
energy where the interference can even manifest
in the transceiver itself!
Your particular "rogue antenna" seems to have been
particularly efficient in the band where transmission
was degraded. In this case, the low level audio
stages of your modulator were probably saturated and
prevented from passing the small microphone signal
to the right places in the radio.
I can tell you that the majority of antenna feed line
problems have root cause in poorly installed or
damaged connectors. Further, EITHER connector
being damaged can produce mind-boggling behaviors
in ship's systems.
I'm pleased that you found the problem and have
shared your findings with us.
Bob . . .
Bob . . . [quote][b]
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