Com Standing Wave Ratio?

On Feb 26, 9:38 am, "Jim B" wrote:
Can someone explain a com radio's Standing Wave Ratio?

During our trip to Key West, our #1 KX155 got damp. I removed it, dried it
out, and just to be safe, swapped places with our #2 KX155. After several
hours of flying and just after take off from Champaign IL, we noticed that
when in altitude hold mode our Stec 60-2 would enter a slight dive when
transmitting on the #2 com (now in the top slot). This happened when in
either Nav or HDG mode and when coupled to either the VOR or the GPS. I
found the following on Stec's web site and am looking for a little
education.

Q: Where do I start when the aircraft either climbs or dives when the mike
button is pressed to transmit?
A: Over the years it has become customary to place the autopilot at the
bottom of the radio stack. Check the standing wave ratio on the comm.
radios, a high S.W.R will affect the altitude transducer. Make sure the coax
cables are routed away from autopilot wiring, and check for proper antenna
bonding.

Thanks as always,

Jim

The guys responding are very helpful and this list is good for that. I
have a couple of questions, How did the radio get wet and how wet???
Just curious,,,,

Cabin leaks slightly during only the heaviest rain storms. We haven't been
able to find the leak. Not very wet, could have just been high humidity,
not sure.

Full story if nothing more than for the archives and to jog my memory:

Airplane sat on the ramp at Key West a couple weeks ago while they got over
3" of rain during some pretty heavy storms. Islands to the north got up to
10" during the same period. After the rain stopped I went out to the
airport to check the plane, noticed several drops on one of the vertical
cabin support posts ahead of the pilots window. No wet carpets, no dampness
or free moisture on the glare shield, no other water noted anywhere, but
when I checked the avionics the #1 nav lit up with 188.88 on active and
standby. When I turned the speaker on, nothing but static coming from the
nav side of #1, same thing through headsets.

Pulled both #1 and #2, no free moisture noted in the trays or on the radios
or connections but it felt like 110% humidity in the cabin. Took #1 back to
the hotel and placed it in front of a fan, no heat, for approximately 10
hours. Reinstalled it in the #2 slot before we departed, no static, nav now
displayed normal, operated normal. Flew home with #2 in the #1 slot driving
the autopilot. Refueled at Eufaula, AL after 3+ hours. Everything normal.

Refueled again in Champaign, IL after another 3+ hours, everything normal
until we leveled off after climb out. Then #2 in #1 slot started the
pitchdown when transmitting with alt hold thing. Not severe, just a mild
pressure on the yoke, but noticeable. Checked transmit on the #1 com (the
wet one) in the #2 slot, no problems. Checked Nav vs Hdg mode on the
autopilot, pitch down in either mode. Checked GPS vs VOR, pitch down when
coupled to either source.

Swapped them back to original locations after we got home. Next day flew
some approaches with a friend, local tower complained of heavy static during
transmissions from the #1 (wet) com. No pitchdown with this radio noted.
No complaints of static prior to Key West flight.

So I've got static on #1, and #2 causes pitch down, but no static, when
installed in the #1 slot.

Jim

Jim Burns wrote:
Cabin leaks slightly during only the heaviest rain storms. We haven't been
able to find the leak. Not very wet, could have just been high humidity,
not sure.

Full story if nothing more than for the archives and to jog my memory:

Airplane sat on the ramp at Key West a couple weeks ago while they got over
3" of rain during some pretty heavy storms. Islands to the north got up to
10" during the same period. After the rain stopped I went out to the
airport to check the plane, noticed several drops on one of the vertical
cabin support posts ahead of the pilots window. No wet carpets, no dampness
or free moisture on the glare shield, no other water noted anywhere, but
when I checked the avionics the #1 nav lit up with 188.88 on active and
standby. When I turned the speaker on, nothing but static coming from the
nav side of #1, same thing through headsets.

Pulled both #1 and #2, no free moisture noted in the trays or on the radios
or connections but it felt like 110% humidity in the cabin. Took #1 back to
the hotel and placed it in front of a fan, no heat, for approximately 10
hours. Reinstalled it in the #2 slot before we departed, no static, nav now
displayed normal, operated normal. Flew home with #2 in the #1 slot driving
the autopilot. Refueled at Eufaula, AL after 3+ hours. Everything normal.

Refueled again in Champaign, IL after another 3+ hours, everything normal
until we leveled off after climb out. Then #2 in #1 slot started the
pitchdown when transmitting with alt hold thing. Not severe, just a mild
pressure on the yoke, but noticeable. Checked transmit on the #1 com (the
wet one) in the #2 slot, no problems. Checked Nav vs Hdg mode on the
autopilot, pitch down in either mode. Checked GPS vs VOR, pitch down when
coupled to either source.

Swapped them back to original locations after we got home. Next day flew
some approaches with a friend, local tower complained of heavy static during
transmissions from the #1 (wet) com. No pitchdown with this radio noted.
No complaints of static prior to Key West flight.

So I've got static on #1, and #2 causes pitch down, but no static, when
installed in the #1 slot.

Jim



The symptoms point to a bad contact somewhere. If there are
socketed circuits on the boards, re-socketing them may be in
order.

A worse possibility with paper-based circuit board is that the
base expands when wet, and this can break feed-through connections
through the board (called vias in the jargon).

I'd take the unit to a shop with measuring instruments to see
if the 'static' follows the radio (and not the installation place).

--

Tauno Voipio
tauno voipio (at) iki fi