Com Standing Wave Ratio?

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

Roy Smith wrote:
Tauno Voipio wrote:

The ratio of voltage and current of a RF signal in a transmission
line (called characteristic impedance) is fixed and determined by
the dimensions and materials of the transmission line.


Considering how much abuse the typical piece of coax absorbs over GOK how
many years of being scrunched into the rats nest of wiring behind most GA
panels, I suspect the characteristic impedance is anything but fixed.

As long as the scrunch is short compared to the wavelength
(in the cable, for coax about 2/3 of free-space wavelength),
the signal could not care less, and, for practical purposes,
the characteristic impedance is constant. If we have to take
the attenuation into account, the impedance is not even real,
but complex, containing capacitive or inductive components.

Not to mention that a lot of radio guys barely know which end of a
soldering iron to pick up. SWR Meter? That's the box which tells you how
****ty a job you did of putting the connectors on before you button up the
panel and write out the customer's bill.

You can also crash a new car before delivering it to
the customer ...

--

Tauno Voipio
tauno voipio (at) iki fi

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