Electrical Help? Shielded speaker wires, but still static... [Archive]

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noel.wade

March 2nd 10, 08:23 AM

Any electric gurus out there willing to help?

Ever since re-doing the wiring in my DG-300 last year, I get
intermittent loud static on my radio. It seems to be connected to
whether or not I have ship's power going to my EW MicroRecorder. The
odd thing is that I followed the Becker wiring instructions and made
sure to use a shielded wire for the connection from the Becker to the
remote speaker/PCB assembly. I would love ideas on how to help stop
this! Details below - any thoughts, see anything obvious that I'm
failing to do?

Powered Equipment in the panel:
-----
Becker Radio AR3201
Cambridge S-Nav (LCD and quick-response vario)
Tasman V1000 Vario
B400 (backup vario)
EW Avionics MicroRecorder
iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
connectors with ferrite rings at each end)

Wiring description:
-----
All connections are done with Anderson Powerpole connectors.
The battery (mounted behind the seat and appropriately fused) provides
power to a central block of connections under the instrument pedestal
- via an unshielded ~6-foot run of 18-gauge Tefzel wire. The central
block consists of a common power bar and a common grounding bar, both
covered with plastic and with Powerpole connectors soldered to them
(done by a commercial outfit, not me).
From the central block, power is routed through 3 different Klixon
Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
the B400 & SNAV).
From the circuit breakers, the wires route to a block of Powerpole
connectors that are joined together to make a "quick disconnect" block
so that the instrument pedestal can be easily removed.
(NOTE: Anderson powerpole connectors have plastic housings that slide
together to form these blocks, but all electrical connections are
still distinct and physically isolated from each other).
The power wire for the EW route from this quick-disconnect block
through a SPST switch (so I can force the EW & PDA to run off their
internal batteries if I have a concern about ship's power levels).
The B400 also has its power wire run through a 3-position switch to
control whether it is off, running on ship's power, or running on its
backup battery.
The Becker has a simple run of wire from the circuit breaker to the
main harness; and from the harness (attached at the back of the
transceiver), it runs as a big bundle back to an area behind the seat
where the remote speaker & circuit-board are located (this runs
through the same flex conduit as the ship's power line - my options
for routing are limited). As stated before, the speaker wire in this
harness is a single-conductor shielded wire, with the shield being
used as the ground.

I thought that shielding the speaker wire was all that I needed to do,
to isolate it from noise generated by the other systems; but
apparently that's not enough.

A little help, anyone?

Thanks, take care,

--Noel

noel.wade

March 2nd 10, 08:58 AM

Quick correction:

I used 14 gauge Tefzel for the main power & ground wire (from battery
to distribution block/bus). 18 gauge everywhere else.

--Noel

Peter Wyld[_2_]

March 2nd 10, 11:16 AM

Do you really mean intermittent (occasional) or do you really mean regular
(not continuous, but in bursts every second or so)?

Data bursts from GPS engines can be received by radios etc. E.g. a Garmin
GPS 90 in our Tigermoth generated bursts of noise on the Dittel FSG50 in the
aircraft, they wer not electically connected in any way.
The interference was most prevalent around 130MHz, but disappeared away from
that freq. (+/- 1 or 2 MHz).

Keep the wiring as far apart as practical, screen it if possible.

"noel.wade" > wrote in message
...
> Any electric gurus out there willing to help?
>
> Ever since re-doing the wiring in my DG-300 last year, I get
> intermittent loud static on my radio. It seems to be connected to
> whether or not I have ship's power going to my EW MicroRecorder. The
> odd thing is that I followed the Becker wiring instructions and made
> sure to use a shielded wire for the connection from the Becker to the
> remote speaker/PCB assembly. I would love ideas on how to help stop
> this! Details below - any thoughts, see anything obvious that I'm
> failing to do?
>
> Powered Equipment in the panel:
> -----
> Becker Radio AR3201
> Cambridge S-Nav (LCD and quick-response vario)
> Tasman V1000 Vario
> B400 (backup vario)
> EW Avionics MicroRecorder
> iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
> connectors with ferrite rings at each end)
>
> Wiring description:
> -----
> All connections are done with Anderson Powerpole connectors.
> The battery (mounted behind the seat and appropriately fused) provides
> power to a central block of connections under the instrument pedestal
> - via an unshielded ~6-foot run of 18-gauge Tefzel wire. The central
> block consists of a common power bar and a common grounding bar, both
> covered with plastic and with Powerpole connectors soldered to them
> (done by a commercial outfit, not me).
> From the central block, power is routed through 3 different Klixon
> Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
> the B400 & SNAV).
> From the circuit breakers, the wires route to a block of Powerpole
> connectors that are joined together to make a "quick disconnect" block
> so that the instrument pedestal can be easily removed.
> (NOTE: Anderson powerpole connectors have plastic housings that slide
> together to form these blocks, but all electrical connections are
> still distinct and physically isolated from each other).
> The power wire for the EW route from this quick-disconnect block
> through a SPST switch (so I can force the EW & PDA to run off their
> internal batteries if I have a concern about ship's power levels).
> The B400 also has its power wire run through a 3-position switch to
> control whether it is off, running on ship's power, or running on its
> backup battery.
> The Becker has a simple run of wire from the circuit breaker to the
> main harness; and from the harness (attached at the back of the
> transceiver), it runs as a big bundle back to an area behind the seat
> where the remote speaker & circuit-board are located (this runs
> through the same flex conduit as the ship's power line - my options
> for routing are limited). As stated before, the speaker wire in this
> harness is a single-conductor shielded wire, with the shield being
> used as the ground.
>
> I thought that shielding the speaker wire was all that I needed to do,
> to isolate it from noise generated by the other systems; but
> apparently that's not enough.
>
> A little help, anyone?
>
> Thanks, take care,
>
> --Noel
>

qflyer1

March 2nd 10, 12:31 PM

On Mar 2, 6:16*am, "Peter Wyld" > wrote:
> Do you really mean intermittent (occasional) or do you really mean regular
> (not continuous, but in bursts every second or so)?
>
> Data bursts from GPS engines can be received by radios etc. *E.g. a Garmin
> GPS 90 in our Tigermoth generated bursts of noise on the Dittel FSG50 in the
> aircraft, they wer not electically connected in any way.
> The interference was most prevalent around 130MHz, but disappeared away from
> that freq. (+/- 1 or 2 MHz).
>
> Keep the wiring as far apart as practical, screen it if possible.
>
> "noel.wade" > wrote in message
>
> ...
>
> > Any electric gurus out there willing to help?
>
> > Ever since re-doing the wiring in my DG-300 last year, I get
> > intermittent loud static on my radio. *It seems to be connected to
> > whether or not I have ship's power going to my EW MicroRecorder. *The
> > odd thing is that I followed the Becker wiring instructions and made
> > sure to use a shielded wire for the connection from the Becker to the
> > remote speaker/PCB assembly. *I would love ideas on how to help stop
> > this! *Details below - any thoughts, see anything obvious that I'm
> > failing to do?
>
> > Powered Equipment in the panel:
> > -----
> > Becker Radio AR3201
> > Cambridge S-Nav (LCD and quick-response vario)
> > Tasman V1000 Vario
> > B400 (backup vario)
> > EW Avionics MicroRecorder
> > iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
> > connectors with ferrite rings at each end)
>
> > Wiring description:
> > -----
> > All connections are done with Anderson Powerpole connectors.
> > The battery (mounted behind the seat and appropriately fused) provides
> > power to a central block of connections under the instrument pedestal
> > - via an unshielded ~6-foot run of 18-gauge Tefzel wire. *The central
> > block consists of a common power bar and a common grounding bar, both
> > covered with plastic and with Powerpole connectors soldered to them
> > (done by a commercial outfit, not me).
> > From the central block, power is routed through 3 different Klixon
> > Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
> > the B400 & SNAV).
> > From the circuit breakers, the wires route to a block of Powerpole
> > connectors that are joined together to make a "quick disconnect" block
> > so that the instrument pedestal can be easily removed.
> > (NOTE: Anderson powerpole connectors have plastic housings that slide
> > together to form these blocks, but all electrical connections are
> > still distinct and physically isolated from each other).
> > The power wire for the EW route from this quick-disconnect block
> > through a SPST switch (so I can force the EW & PDA to run off their
> > internal batteries if I have a concern about ship's power levels).
> > The B400 also has its power wire run through a 3-position switch to
> > control whether it is off, running on ship's power, or running on its
> > backup battery.
> > The Becker has a simple run of wire from the circuit breaker to the
> > main harness; and from the harness (attached at the back of the
> > transceiver), it runs as a big bundle back to an area behind the seat
> > where the remote speaker & circuit-board are located (this runs
> > through the same flex conduit as the ship's power line - my options
> > for routing are limited). *As stated before, the speaker wire in this
> > harness is a single-conductor shielded wire, with the shield being
> > used as the ground.
>
> > I thought that shielding the speaker wire was all that I needed to do,
> > to isolate it from noise generated by the other systems; but
> > apparently that's not enough.
>
> > A little help, anyone?
>
> > Thanks, take care,
>
> > --Noel

By any chance, do you have a ZAON PCAS? They are known for breaking
squelch.

Tim

lanebush

March 2nd 10, 01:08 PM

I installed a microair comm last week in my Cirrus. I to am getting
static that sounds like morse code. Like you I noticed that when the
EW microrecorder is turned off the static goes away. Evidently the
GPS receiver emits electrical impulses that are received by the comm
radio. I believe I can adjust my squelch so that it is not too
annoying. I don't believe the speaker wire is the culprit.

[email protected]

March 2nd 10, 01:53 PM

On Mar 2, 3:23*am, "noel.wade" > wrote:
> Any electric gurus out there willing to help?
>
> Ever since re-doing the wiring in my DG-300 last year, I get
> intermittent loud static on my radio. *It seems to be connected to
> whether or not I have ship's power going to my EW MicroRecorder. *The
> odd thing is that I followed the Becker wiring instructions and made
> sure to use a shielded wire for the connection from the Becker to the
> remote speaker/PCB assembly. *I would love ideas on how to help stop
> this! *Details below - any thoughts, see anything obvious that I'm
> failing to do?
>
> Powered Equipment in the panel:
> -----
> Becker Radio AR3201
> Cambridge S-Nav (LCD and quick-response vario)
> Tasman V1000 Vario
> B400 (backup vario)
> EW Avionics MicroRecorder
> iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
> connectors with ferrite rings at each end)
>
> Wiring description:
> -----
> All connections are done with Anderson Powerpole connectors.
> The battery (mounted behind the seat and appropriately fused) provides
> power to a central block of connections under the instrument pedestal
> - via an unshielded ~6-foot run of 18-gauge Tefzel wire. *The central
> block consists of a common power bar and a common grounding bar, both
> covered with plastic and with Powerpole connectors soldered to them
> (done by a commercial outfit, not me).
> From the central block, power is routed through 3 different Klixon
> Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
> the B400 & SNAV).
> From the circuit breakers, the wires route to a block of Powerpole
> connectors that are joined together to make a "quick disconnect" block
> so that the instrument pedestal can be easily removed.
> (NOTE: Anderson powerpole connectors have plastic housings that slide
> together to form these blocks, but all electrical connections are
> still distinct and physically isolated from each other).
> The power wire for the EW route from this quick-disconnect block
> through a SPST switch (so I can force the EW & PDA to run off their
> internal batteries if I have a concern about ship's power levels).
> The B400 also has its power wire run through a 3-position switch to
> control whether it is off, running on ship's power, or running on its
> backup battery.
> The Becker has a simple run of wire from the circuit breaker to the
> main harness; and from the harness (attached at the back of the
> transceiver), it runs as a big bundle back to an area behind the seat
> where the remote speaker & circuit-board are located (this runs
> through the same flex conduit as the ship's power line - my options
> for routing are limited). *As stated before, the speaker wire in this
> harness is a single-conductor shielded wire, with the shield being
> used as the ground.
>
> I thought that shielding the speaker wire was all that I needed to do,
> to isolate it from noise generated by the other systems; but
> apparently that's not enough.
>
> A little help, anyone?
>
> Thanks, take care,
>
> --Noel

Obviously, first thing is turn all stuff off and see what makes
problem go away. Ensure squelch is functional.
Second- Yes this is not elegant- start wrapping everything in foil
with foil gounded. Make sure all grounds come to a common point.
See what happens.
Also check that you don't have side tone from mike circuit enabled.
Good luck.
UH
Moving antennas and wires is a common solution.

Andy[_1_]

March 2nd 10, 02:36 PM

On Mar 2, 1:58*am, "noel.wade" > wrote:
> Quick correction:
>
> I used 14 gauge Tefzel for the main power & ground wire (from battery
> to distribution block/bus). *18 gauge everywhere else.
>
> --Noel

Based on the information you have given the cure may be to put an RF
filter in the EW power supply wiring as close to the EW as possible.
Start with a ferrite ring.

It is also possible you will reduce the problem my moving the EW power
lead as far as possible from any other wiring.

Andy

Darryl Ramm

March 2nd 10, 06:02 PM

Noel

Two usual suspects and how I would normally diagnose them are...

Power supply coupling (or at least the power supply lead of the EW
acting as an antenna). This is the most likely. Turn on stuff. Hear
the interference? Unplug the power cable from the MicroRecorder but
leave it it running on batteries. Ideally do this don't just switch
off the external power, I want the cable hanging out the EW removed.
Interference gone away? It was coupling via the power supply cables.
Install a choke/ferrite toroid as close to the power connector on the
EW Microrecorder as you can. See past discussions on r.a.s. for
different choke circuits for the Zaon MRX (if it does not go away with
a simple toroid you may want to build a better choke with capacitors
as well). If the interference did not go away when you disconnected
the EW then the problem is ....

RF leakage coupling (i.e. your radio picking up radio interference
leaking out of the EW). This could be via the radio antenna in the
tail or via faults in the radio coax cable near the device causing the
interference. Are there any other problems with the radio
transmissions/reception? Does the coax cable connector look and feel
firmly attached, coax in perfect condition? Does wiggling things back
there make any noises? Try undoing the coax connector from the radio
and reconnecting it. Homemade coax connections are often problems,
either breakage of the center wire or the braid coming loose from the
connector. Crimp type with a long ferrule that support the coax well
are best. If that does not fix things you can just try moving the EW
or wrapping parts in foil, but if that is needed then you should send
the unit back to be checked. I'd also check out in that case if it
interferes with other radios, to try to work out if it is a problem in
the EW or your radio. But the problem is much more likely to be just
plain everyday power supply coupling described above.

BTW shielded or unshielded wiring to the speaker is not near the top
things I would think about. The speaker circuit is low impedance (i.e.
takes a lot of interference to affect it) and likely won't pick up RF
energy in a way you can hear it. And many glider manufactures do just
fine with cheap zip/cheapo rubber speaker cable to the speakers, even
though they should use something better.

And since you already know the problem goes away when you disconnect
the external power this shows that the problem is likely power supply
coupling. So next step is to try a choke or ferrite toroid on the EW,
close as possible to the EW. Yes as others already mentioned, I just
wanted to emphasize that is the right next step, not looking for other
RF leakage. Tinfoil hats can come later if this does not work :-)

Darryl

On Mar 2, 12:23*am, "noel.wade" > wrote:
> Any electric gurus out there willing to help?
>
> Ever since re-doing the wiring in my DG-300 last year, I get
> intermittent loud static on my radio. *It seems to be connected to
> whether or not I have ship's power going to my EW MicroRecorder. *The
> odd thing is that I followed the Becker wiring instructions and made
> sure to use a shielded wire for the connection from the Becker to the
> remote speaker/PCB assembly. *I would love ideas on how to help stop
> this! *Details below - any thoughts, see anything obvious that I'm
> failing to do?
>
> Powered Equipment in the panel:
> -----
> Becker Radio AR3201
> Cambridge S-Nav (LCD and quick-response vario)
> Tasman V1000 Vario
> B400 (backup vario)
> EW Avionics MicroRecorder
> iPAQ 4700 (powered through the MicroRecorder via Goddard DB9f & I38C-1
> connectors with ferrite rings at each end)
>
> Wiring description:
> -----
> All connections are done with Anderson Powerpole connectors.
> The battery (mounted behind the seat and appropriately fused) provides
> power to a central block of connections under the instrument pedestal
> - via an unshielded ~6-foot run of 18-gauge Tefzel wire. *The central
> block consists of a common power bar and a common grounding bar, both
> covered with plastic and with Powerpole connectors soldered to them
> (done by a commercial outfit, not me).
> From the central block, power is routed through 3 different Klixon
> Circuit Breakers (one for the Radio, one for the Tasman & EW, one for
> the B400 & SNAV).
> From the circuit breakers, the wires route to a block of Powerpole
> connectors that are joined together to make a "quick disconnect" block
> so that the instrument pedestal can be easily removed.
> (NOTE: Anderson powerpole connectors have plastic housings that slide
> together to form these blocks, but all electrical connections are
> still distinct and physically isolated from each other).
> The power wire for the EW route from this quick-disconnect block
> through a SPST switch (so I can force the EW & PDA to run off their
> internal batteries if I have a concern about ship's power levels).
> The B400 also has its power wire run through a 3-position switch to
> control whether it is off, running on ship's power, or running on its
> backup battery.
> The Becker has a simple run of wire from the circuit breaker to the
> main harness; and from the harness (attached at the back of the
> transceiver), it runs as a big bundle back to an area behind the seat
> where the remote speaker & circuit-board are located (this runs
> through the same flex conduit as the ship's power line - my options
> for routing are limited). *As stated before, the speaker wire in this
> harness is a single-conductor shielded wire, with the shield being
> used as the ground.
>
> I thought that shielding the speaker wire was all that I needed to do,
> to isolate it from noise generated by the other systems; but
> apparently that's not enough.
>
> A little help, anyone?
>
> Thanks, take care,
>
> --Noel

Frank[_12_]

March 2nd 10, 06:53 PM

Noel,

One quick test that I didn't see mentioned - does the interference go
away when you disconnect the antenna connector to the radio? If it
does, then the interference is coupled in through an air path to the
radio antenna or antenna coax at RF frequencies. If it doesn't, then
the interference is probably coupled in from one set of power leads to
another. It is also possible to have power lead coupling at RF freqs,
and this is what RF chokes (beads) are intended to address.

Another very effective method of suppressing interference via power
leads is to use a separate power & ground pair from each instrument,
and twist each pair for most of its length. A properly done 'twisted
pair' is quite effective at suppressing 'common-mode' interference at
all frequencies, while a ferrite RF choke is only effective above
100MHz or so. I use a cordless drill to form twisted pairs as this
forms a more uniform twist. A well formed twisted pair will shorten
the normal wire length by a factor of 1.5 to 2. In other words, if
you form a twisted pair from two individual 5' long wires, you will
wind up with a twisted-pair cable about 2.5 - 3.3' long. If possible,
the untwisted portions at each end should be less than 6" (i.e. less
than about 0.1 wavelengths at 150MHz).

Hope this helps,

Frank TA

Doug Hoffman[_3_]

March 2nd 10, 07:34 PM

Darryl Ramm

March 2nd 10, 07:48 PM

On Mar 2, 11:34*am, Doug Hoffman > wrote:
> On Mar 2, 3:23*am, "noel.wade" > wrote:
>
> > Any electric gurus out there willing to help?
>
> > Ever since re-doing the wiring in my DG-300 last year, I get
> > intermittent loud static on my radio. *It seems to be connected to
> > whether or not I have ship's power going to my EW MicroRecorder. *The
> > odd thing is that I followed the Becker wiring instructions and made
> > sure to use a shielded wire for the connection from the Becker to the
> > remote speaker/PCB assembly. *I would love ideas on how to help stop
> > this! *Details below - any thoughts, see anything obvious that I'm
> > failing to do?
>
> I've a Becker panel radio. *Last year it suddenly became unusable due
> to loud intermittent "static" noise. *I finally tried turning the
> squelch screw on the side of the case, you may have to remove your
> radio to get at it. *That fixed it. *Have no idea what changed. *Has
> worked flawlessly ever since.
>
> Regards,
>
> -Doug

That may make the symptom go away but the risks now is not being able
to hear weak radio transmissions.
The extra concern here was you described it as a "loud noise". If its
something that is breaking squelch with a large signal turning up
squelch to silence this is a bad idea.

If this suddenly started happening it could be something as simple as
a wire behind the panel moved closer to another, a poor connector or
break developing in a coaxial cable (cable flexed differently than
previously), etc. It it may be worth trying to diagnose the root
cause. Start with identifying what device is causing the periodic
interference. Start by turning down the squelch so that you get fairly
often occurring interference then poke around to make that better/
worse.

Darryl

Doug Hoffman[_3_]

March 2nd 10, 07:59 PM

On Mar 2, 2:48*pm, Darryl Ramm > wrote:

> That may make the symptom go away but the risks now is not being able
> to hear weak radio transmissions.

> It it may be worth trying to diagnose the root
> cause. Start with identifying what device is causing the periodic
> interference.

Thanks Darryl,

I'll check it out.

Regards,

-Doug

noel.wade

March 3rd 10, 02:45 AM

Thanks for the tips so far, all.

One note: There is a ferrite toroid on the Goddard EW DB9f connector
within about 3 inches of the EW MicroRecorder, and close to the PDA
hookup as well. My PDA and antenna are slightly different; but I use
basically this exact setup:
http://www.cumulus-soaring.com/goddard/EWmicroRec-DB9f-0-3.jpg
(Note that the actual red-black power leads connect up at the DB-9
connector and have no ferrite toroids close-by. Not sure if that
matters)

Maybe I ought to try the Radio-shack noise filter that's discussed
over at the AeroElectric Connection site (http://www.markdownalley.com/
products/270-051.jpg), and wire it just up-stream from the radio?

I'll also play with the Coax antenna connector and let you all know
what I find!

Take care,

--Noel

Darryl Ramm

March 3rd 10, 03:15 AM

On Mar 2, 6:45*pm, "noel.wade" > wrote:
> Thanks for the tips so far, all.
>
> One note: *There is a ferrite toroid on the Goddard EW DB9f connector
> within about 3 inches of the EW MicroRecorder, and close to the PDA
> hookup as well. *My PDA and antenna are slightly different; but I use
> basically this exact setup:http://www.cumulus-soaring.com/goddard/EWmicroRec-DB9f-0-3.jpg
> (Note that the actual red-black power leads connect up at the DB-9
> connector and have no ferrite toroids close-by. *Not sure if that
> matters)
>
> Maybe I ought to try the Radio-shack noise filter that's discussed
> over at the AeroElectric Connection site (http://www.markdownalley.com/
> products/270-051.jpg), and wire it just up-stream from the radio?
>
> I'll also play with the Coax antenna connector and let you all know
> what I find!
>
> Take care,
>
> --Noel

Have you tried combinations of the PDA powered off while the EW is on
to see if it is the PDA or EW that is causing interference?

I would do the easiest change possible first. Run each of the red and
black wired though a ferrite toroid (one large enough to wrap the wire
through a few times. Try it as far up the wire as you can. Keep those
red/black wires as short as possible.

Change one thing at once. Start by trying to fix interference at its
source not the receiver unless you have to.

Radio Shack junk: not in my glider!

Darryl

noel.wade

March 7th 10, 05:39 PM

So in true troubleshooting/gremlin fashion, I spent over an hour
trying to get the radio to make static yesterday, and couldn't. :-P
Argh!

If this is a problem that only appears when I'm airborne, it's going
to be a real pain to track down!
*sigh*

--Noel

Darryl Ramm

March 7th 10, 05:59 PM

On Mar 7, 9:39*am, "noel.wade" > wrote:
> So in true troubleshooting/gremlin fashion, I spent over an hour
> trying to get the radio to make static yesterday, and couldn't. :-P
> Argh!
>
> If this is a problem that only appears when I'm airborne, it's going
> to be a real pain to track down!
> *sigh*
>
> --Noel

Noel

Do you have a transponder? If not ignore the rest of this post.

If you do this could be related to transponder interrogation. Possibly
a combination of noise from two sources breaking squelch.

With a transponder the same basic comments apply, with coupling either
through the power supplies or RF. If you switch briefly to standby in
flight does the noise go away? If so start with the power supplies and
try putting a filter/choke on the transponder power supply leads close
to the unit. Also look at the transponder coax cable for problems.

Ground radars will interrogate your transponder typically at 5 second
(approach radar) or 12 second (area radar) intervals, of course you
may be interrogated by multiple radars and also TCAS and similar
systems (that can be at much higher rates (~1/sec). On the ground you
may be outside of radar interrogation and if your transponder antenna
is under the glider (where it usually should be) may not see TCAS
interrogations on the ground. TCAS interrogations will come and go but
radar will be consistent. Repeatable noise bursts at these kind of
rates can be a clue its transponder related.

Darryl