"scratchy" radio transmission

[[email protected]]

I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim

[Don W]

Jim,

Could be lots of things, but I'd be willing to bet a cold beer that
its in the audio input path to your radio, and not between the radio
and the antenna given that you receive fine.

Here's a few thoughts that _might_ help:

Loose/broken wire in mic - Try a different microphone
Oxidation in mic jack - Clean both the male and female jacks
Loose/broken wire between mic jack and radio
Cold solder joint in either mic, jack, etc
PTT switch starting to oxidize internally

Good luck finding the problem.

Don W.

wrote:
> I am looking for ideas here; ATC says my radio transmissions sound
> 'scratchy' or 'staticy' as little as 15 miles out on callup. I have
> been going nuts trying to find the cause. The aircraft has 2
> independant radios, 2 antenna systems (one on top of the fuselage, one
> underneath). One radio replaced as an update recently. Both radios have
> been checked out by the factories recently and pronounced fine (one
> Narco Mk12D and one Icom). Been through the wiring and grounds several
> times and found nothing. Tried different headsets (note: the voice
> sideband through the headset sounds perfect). ATC said no difference
> with alternator shut off. Reception is always perfect. Complaints seem
> to pick up more in the cold weather. Don't know if its important but
> maybe once or twice each summer I get alot of static in the headset
> that seems to disapate in a minute or two, but never in the winter.
> If I were generating a large static charge in the dry winter air
> wouldn't it effect my reception? Some days I get complaints even when
> taxiing for takeoff. Any ideas what to try next without going on an
> expensive fishing expedition?
>
> Jim
>

[RST Engineering]

Knowing what kind of aircraft it is would help a bit.

Jim




> wrote in message
ups.com...
>I am looking for ideas here; ATC says my radio transmissions sound
> 'scratchy' or 'staticy' as little as 15 miles out on callup. I have
> been going nuts trying to find the cause. The aircraft has 2
> independant radios, 2 antenna systems (one on top of the fuselage, one
> underneath). One radio replaced as an update recently. Both radios have
> been checked out by the factories recently and pronounced fine (one
> Narco Mk12D and one Icom). Been through the wiring and grounds several
> times and found nothing. Tried different headsets (note: the voice
> sideband through the headset sounds perfect). ATC said no difference
> with alternator shut off. Reception is always perfect. Complaints seem
> to pick up more in the cold weather. Don't know if its important but
> maybe once or twice each summer I get alot of static in the headset
> that seems to disapate in a minute or two, but never in the winter.
> If I were generating a large static charge in the dry winter air
> wouldn't it effect my reception? Some days I get complaints even when
> taxiing for takeoff. Any ideas what to try next without going on an
> expensive fishing expedition?
>
> Jim
>

[[email protected]]

Full metal jacketed RV-4

[[email protected]]

If it was a broken/loose wire in the mic circuit I would think I would
not get a good sidetone in the headset. Have seriously cleaned both of
the mic jacks but now that you mention it I'll bet it could be the PTT
button (duh!). I probably need to make a debounce circuit or something
if possible. At least I could try taking a passenger to try the rear
xmit button. Thanks for the "wake up call".

Jim

[RST Engineering]

If, as you have told us, the sidetone is clear and loud, then the odds of it
being in the microphone/jacks/PTT switch are nearly zero. My bet is an RF
problem of some sort.

And why would you consider some sort of "debounce" circuit? DO you have a
clue what a debounce circuit does?

Jim




> wrote in message
oups.com...
> If it was a broken/loose wire in the mic circuit I would think I would
> not get a good sidetone in the headset. Have seriously cleaned both of
> the mic jacks but now that you mention it I'll bet it could be the PTT
> button (duh!). I probably need to make a debounce circuit or something
> if possible. At least I could try taking a passenger to try the rear
> xmit button. Thanks for the "wake up call".
>
> Jim
>

[Cy Galley]

I would try replacing the coax from Radio to antenna in ONE piece. You
might first try just loosening and tightening the connections. If you have
your coax all tie-rapped down, Just leave it and see if a new coax loosely
installed does the job. Tie-rapping too tightly can damage the coax. Many
times it is just bad ends.

--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
> wrote in message
ups.com...
>I am looking for ideas here; ATC says my radio transmissions sound
> 'scratchy' or 'staticy' as little as 15 miles out on callup. I have
> been going nuts trying to find the cause. The aircraft has 2
> independant radios, 2 antenna systems (one on top of the fuselage, one
> underneath). One radio replaced as an update recently. Both radios have
> been checked out by the factories recently and pronounced fine (one
> Narco Mk12D and one Icom). Been through the wiring and grounds several
> times and found nothing. Tried different headsets (note: the voice
> sideband through the headset sounds perfect). ATC said no difference
> with alternator shut off. Reception is always perfect. Complaints seem
> to pick up more in the cold weather. Don't know if its important but
> maybe once or twice each summer I get alot of static in the headset
> that seems to disapate in a minute or two, but never in the winter.
> If I were generating a large static charge in the dry winter air
> wouldn't it effect my reception? Some days I get complaints even when
> taxiing for takeoff. Any ideas what to try next without going on an
> expensive fishing expedition?
>
> Jim
>

[Don W]

It is odd that he has a clear "sidetone", receives fine, and yet the
transmission is scratchy (apparently on both radios?). Also that both
radios apparently benchcheck fine.

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?

Don W.

RST Engineering wrote:

> If, as you have told us, the sidetone is clear and loud, then the odds of it
> being in the microphone/jacks/PTT switch are nearly zero. My bet is an RF
> problem of some sort.
>
> And why would you consider some sort of "debounce" circuit? DO you have a
> clue what a debounce circuit does?
>
> Jim
>
>
>
>
> > wrote in message
> oups.com...
>
>>If it was a broken/loose wire in the mic circuit I would think I would
>>not get a good sidetone in the headset. Have seriously cleaned both of
>>the mic jacks but now that you mention it I'll bet it could be the PTT
>>button (duh!). I probably need to make a debounce circuit or something
>>if possible. At least I could try taking a passenger to try the rear
>>xmit button. Thanks for the "wake up call".
>>
>>Jim
>>
>
>
>

[Ron Webb]

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.

Even a fancy Anritsu test set I borrowed from work showed no great problem.

But when I removed those drill patterns, things started working fine.

[[email protected]]

>And why would you consider some sort of "debounce" circuit? DO you have a
clue what a debounce circuit does?


Jim <

A friend at work told me it is like an electronic switch that "locks on
and off" to prevent a poor or intermittant connection. Maybe I don't
have a correct clue about it but I have the utmost confidence someone
will straighten me out on the subject. Thats why I came here; to learn.

Jim

[[email protected]]

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: Re: "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

[[email protected]]

>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

[[email protected]]

>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

[Ron Webb]

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
>

[RST Engineering]

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.

[[email protected]]

>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

[RST Engineering]

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
oups.com...


> 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.

[[email protected]]

>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

[Cy Galley]

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


> wrote in message
oups.com...
> 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: Re: "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
>