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transponder gets into comm transceiver

 
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nuckollsr(at)cox.net
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PostPosted: Mon Jan 30, 2006 5:59 am    Post subject: transponder gets into comm transceiver Reply with quote

Quote:
Comments/Questions: Bob, the reception on my KX-155 transceiver is garbled
by the interrogation responses from my KX-76 transponder. The correlation
is unambiguous... the interference is only noted when the reception is
unsquelched by a transmission from another station and the transponder
interrogation response light is lit; additionally the problem disappears
if I put the transponder in STBY. I checked grounds and they seem
good. I've already had the transponder serviced once, and replaced it
once. Seems like anything I do to it corrects the problem for a short
time, then it comes back again. Any thoughts? I've got a Velleman
oscilloscope and an EE background, so I should be able to do a "deep dive"
on this and get the dang thing fixed. Thanks, Dave D

This is most likely an RFI problem. The transponder talks
to the ground in a stream of short (.5 uS) pulses that
encode your squawk and altitude. While average power from the
transponder is low (well under 1W) the peak power can be 100W
or more.

I'd look for bad connector in transponder coax that may be
radiating the interior of the aircraft. Is this a new condition
or has it existed since day-one for the installation? You
might need to reposition the transponder antenna. Check it's
installation too. Dismount, clean the bonding areas under the
mounting hardware and re-install.
Bob . . .
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >


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nuckollsr(at)cox.net
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PostPosted: Mon Jan 30, 2006 7:26 pm    Post subject: transponder gets into comm transceiver Reply with quote

At 07:58 AM 1/30/2006 -0600, you wrote:

Quote:

<nuckollsr(at)cox.net>
>Comments/Questions: Bob, the reception on my KX-155 transceiver is garbled
>by the interrogation responses from my KX-76 transponder. The correlation
>is unambiguous... the interference is only noted when the reception is
>unsquelched by a transmission from another station and the transponder
>interrogation response light is lit; additionally the problem disappears
>if I put the transponder in STBY. I checked grounds and they seem
>good. I've already had the transponder serviced once, and replaced it
>once. Seems like anything I do to it corrects the problem for a short
>time, then it comes back again. Any thoughts? I've got a Velleman
>oscilloscope and an EE background, so I should be able to do a "deep dive"
>on this and get the dang thing fixed. Thanks, Dave D

This is most likely an RFI problem. The transponder talks
to the ground in a stream of short (.5 uS) pulses that
encode your squawk and altitude. While average power from the
transponder is low (well under 1W) the peak power can be 100W
or more.

I'd look for bad connector in transponder coax that may be
radiating the interior of the aircraft. Is this a new condition
or has it existed since day-one for the installation? You
might need to reposition the transponder antenna. Check it's
installation too. Dismount, clean the bonding areas under the
mounting hardware and re-install.


Bob,

Thanks so much for making a personal response!

The condition did not seem to be a problem early in the airplane's
life, but got progressively worse, and then I had the transponder
checked and replaced. Since the cavity was bad, I convinced myself
that maybe the old cavity was spewing electromagnetic radiation
across the spectrum, thereby clobbering my receiver. However,
the problem occasionally returned, and this last weekend was
particularly bad.

I took a look at all the connectors and mounts before, but will do
so again. Actually the first time I had this problem I thought that
the colocated antenna cable runs of the VHF and transponder might
be the problem, . . .

I've never seen two coaxes "talk" to each other if they were
properly installed . . .

. . . and I "re-plumbed" quite a bit of the system to move them apart.
This effort included replacing most of the connectors, none of which had
any effect. Is there an easy way to check connectors or mounts with
a scope?

No, a pretty sophisticated thing called a network analyzer is needed.
If you've recently refurbished the connectors, then this is a very
low order probability. Let's call the coaxes and connectors 'good'.
I've had others tell me that they do not think this is RFI, but rather
the result of the transponder transient injecting some bad stuff onto
either the +14 vdc or the ground circuits of either the radio or the
audio panel. One gent told me he had a similar problem which he believed
was the result of his switching regulator putting trash on the power
line as it tried to deal with the transponder power surge. He said he
put a choke on his audio panel power supply to solve the problem. I'm
now thinking that one of the things I should have checked was whether
the interference only occurs during VHF transmissions, or alternately
clobbers my intercom as well. Maybe any audio would be getting clobbered,
not just VHF, and I only correlated it with VHF reception because
that's the only time my audio was keyed/de-squelched. Might be getting in
through the microphone circuit?

Get a couple of 6v lantern batteries and rig yourself a 12v "test"
battery. Try running the transponder from this battery by disconnecting
the power wire at the bus breaker and clip-leading the battery into the
system. If the noise goes away, then it's conducted onto the 14V line.

If this is the case, filtering at the source is better than filtering
at the victim.

Noted today that The Technician's Notebook by Jerry Gordon lists a service
bulletin KT-76-1 that talks about replacing the power supply inverter
transformer in order to reduce the electromagnetic radiation from the
transponder power supply. I would think that any service bulletin would
have been completed on the rebuilt transponder that I purchased just last
year, but this might be a clue.

Hmmmmm . . . I'm having trouble figuring out how this transformer
would go bad and get worse over time.
Thanks again for all the great advice! Just ordered a new copy of the
"Connection".... the one I have now is #6 or so, and I'm sure there's
a lot more good info in it by now!

Be sure to keep it up to date by downloading revisions from
the website as they are published. You only need to buy the
book once that way!

Check out chapter 16 on noise. I speak of the test battery
technique for powering either victims or antagonists from
the battery to see if observed effects change or go away.

The same chapter speaks of the value of isolating headset
and microphone jacks from airframe ground and carrying their
respective ground leads all the way back to the intercom on
shields or conductors in the bundles.

Be sure to let us know what you find!

Bob . . .


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