"scratchy" radio transmission

Cy Galley
Dec 31 2005, 10:30 pm show options

Newsgroups: rec.aviation.homebuilt
From: "Cy Galley" - Find messages by this author
Date: Sun, 01 Jan 2006 03:30:55 GMT
Local: Sat, Dec 31 2005 10:30 pm
Subject: "scratchy" radio transmission
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I would try replacing the coax from Radio to antenna in ONE piece

Both coaxes from both radios to both antennas? Odds seem kinda high
that both are bad, don't you think?

Jim

Jim, do you have an audio mix panel, and if so does the mic input go
to it before splitting to the radios? Also, I read your original post
pretty quickly and didn't notice the part where you said you were
hearing the sidetone okay. That eliminates a lot of potential failure
points.


Another thought: Do you have an "intercom" that both mics and the PTTs

connect to before the mic audio gets to the radios? If so, the
intercom
could be the culprit. There are so many ways that your aircraft could
be wired up that its hard for me to think of all of them. You could
probably get to the bottom of it all pretty quickly with an
oscilloscope
if you have one and know how to use it.


RST, if both radios have the same static problem (with their different
RF XMIT sections, T/R switches, and antennas), it is not very likely an

RF problem.


OTOH, it could be a DC voltage problem, as in the DC voltage supply to
the radios is too low, causing the transmit power to be less than it
should.


Jim, have you tried both radios and verified that ATC hears them both
as
scratchy?

No audio mix panel. Mic goes to the intercom then goes through a DPTT
switch (to deliver Mic to either radio but not both at the same time
and same with the PTT and audio output). When the intercom is turned
off the mic input bypasses it straight to the radios and it is the
'radio' sidetone I hear clearly from both radios, not the intercom's.
ATC verifies both radios sound the same. I used to suspect the landing
gear getting in the way but as I said I have one antenna on the bottom
and another on top. Since I'm always headed to the facility on callup I
even had the whacky idea that the metal prop was getting in the way so
I would turn the plane 90 degrees and call again. Needless to say, no
difference.
I am also leaning to checking the DC supply circuits again. Thanks.

Jim

Bought a homebuilt once. Both radio antennas had been installed the same
way - with the cardboard drilling pattern glued to the aluminum - then
left
there, and the antenna installed over the top of it. No ground plane.


The SWR was OK (1.8:1), believe it or not. It worked out to 10 miles
or
so - then things started getting weak and scratchy.

Both radios are mounted to aluminum rails bolted to fuselage longerons
however each radio has an aditional grounding wire (from it's main
plug) to fuselage as a precaution. The SWRs also check out the same as
you discribe as does the performance. Power output does not seem to be
the issue (again I never checked power output in the winter), just
audio quality.

Jim

It is not the RADIO ground that was needed, it was the GROUND PLANE that is
needed on a 1/4 wave antenna.

http://www.eecs.wsu.edu/~pedrow/classes/ee416/Spring_2004/share_pdfs/teals/April_4/Quarterwavemonopoles1.pdf#search='quarter%20dipole '

You must have the outer conductor of the coaxial cable connected either to a
lot of metal at more or less 90 degrees to the vertical, or you have to get
scientific with 3 or more tuned stubs. This has little to do with the
radio's electrical ground. The link explains what's going on pretty well.

You should be ok with an aluminum RV as long as the coax outer conductor to
the skin reads a short on an ohmmeter.



Both radios are mounted to aluminum rails bolted to fuselage longerons
however each radio has an aditional grounding wire (from it's main
plug) to fuselage as a precaution. The SWRs also check out the same as
you discribe as does the performance.

Jim

That's not a good way to measure whether or not the coax is terminated
properly. I just inspected an antique where the antenna was properly
mounted on a sufficiently big ground plane, but the coax braid wasn't even
connected to the ground plane at the antenna end. You might just a well
have run Romex as coax if that's the case. And, of course, there WAS
continuity to ground as the coax WAS terminated at the radio end to the
airframe.

THe question was whether BOTH antennas or coax could be "bad". I can argue
that if the fellow who made the plane didn't know how to assemble a
connector properly the odds of him screwing up BOTH the coax cables in the
same way are quite good.

THere is no substitute for a good mechanical inspection of the coax
connections at both the radio AND antenna ends.

I didn't notice whether or not the OP said that he borrowed a known good
headset, since that's the one thing that is common to both rigs.

Jim





You should be ok with an aluminum RV as long as the coax outer conductor
to the skin reads a short on an ohmmeter.

I didn't notice whether or not the OP said that he borrowed a known good
headset, since that's the one thing that is common to both rigs.

Several headsets were tried with same results. Both these headsets
worked fine in wife's C-172 (ATC reports crystal clear)
One of the radios has a new store bought antenna on the belly with new
BNC connectors that came on a "factory built" length of coax to the
radio which came with an antenna "pigtail" already on the tray. This
complete replacement setup seems to xmit no better than the "cheesey"
antenna and antique radio it replaced and is reportedly no different
than the 2nd radio/antenna setup.

Jim

Try this description one more time, please. I"m not understanding why a
factory built length of coax with BNC connectors can have pigtails.

Jim


wrote in message
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One of the radios has a new store bought antenna on the belly with new
BNC connectors that came on a "factory built" length of coax to the
radio which came with an antenna "pigtail" already on the tray.

Try this description one more time, please. I"m not understanding why a
factory built length of coax with BNC connectors can have pigtails.


Jim

(...radio which came with an antenna "pigtail" already on the tray)

RADIO which came with an antenna "pigtail" already on the TRAY

Thats "pigtail" on TRAY not on coax from antenna. And before I get
chastised again I realize that the term "pigtail" may not be
technically correct but I think most will get the picture here.

Jim

Not if the same person made the ends and ty-rapped them too tight.


wrote in message
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Cy Galley
Dec 31 2005, 10:30 pm show options

Newsgroups: rec.aviation.homebuilt
From: "Cy Galley" - Find messages by this author
Date: Sun, 01 Jan 2006 03:30:55 GMT
Local: Sat, Dec 31 2005 10:30 pm
Subject: "scratchy" radio transmission
Reply | Reply to Author | Forward | Print | Individual Message | Show
original | Report Abuse

I would try replacing the coax from Radio to antenna in ONE piece

Both coaxes from both radios to both antennas? Odds seem kinda high
that both are bad, don't you think?

Jim