RF interference issue again (esp. for E Drucker and Jim Weir and other RF wizards)

Snowbird wrote:

Mark Mallory wrote in message ...
Isn't troubleshooting fun? ;^)

Just Ducky (pun intended).

So what do you suggest to distinguish?

My ideas a
1) return to antenna farm w/ handheld and try to stay long enough
to get a station ID, also confirm lat long coordinates
if interference received:

2) fly w/out handheld in the plane
if interference received:
3) remove #2 nav com and ELT from plane
fly again
if no interference:
4) put one back


Sounds like a good plan.

I would try disconnecting *all* the antennas on the plane except for the *good*
com's (and the transponder's, which you need under the clsB and which IMO is
unlikely to be the problem.) Note the results on the good com, then hook them
back up one by one...


I'm also thinking it might be worth a call to the TV station,
to see if they have someone who cares because maybe they have
better equipment and could check this out. (or maybe I can get
the FAA guy to call...)


Yes, I mentioned this in my first post... get in contact with their Engineering
Department and explain the problem. They'd be *more* than interested, they're
technically qualified guys, and they have the necesssary equipment.

As a ham radio guy and former Broadcast Engineer myself (1970s), I know I'd be
chomping at the bit to track it down!


You mentioned the problem was considerably worse on *one* com than the other...
if the problem IS in the plane, that's my guess as to the source (see my other
post). Can you disconnect the antenna from *that* com, and fly with just the
other one?


No problem, but it would be easier (as well as owner-allowed maintenance)
to just remove the radio -- if that would suffice? Or are you thinking
it's the antenna/cable?


Unless the problem is a corroded connector or such, probably not. I like the
idea of disconnecting the BNC at the antenna because it eliminates as much as
posible.




IF it's the radio -- unfortunately it's a TKM. Are there bench
checks our local avionics guy should be able to run to see if it's
up to spec, before we ship it out to them?


Possibly... if they're clever and have the equipment, they might be able to rig
up two signal generators with a 3db hybrid combiner to simulate the two suspect
interfering signals.

Another idea that doesn't require any test equipment is as follows: go flying
and check the frequency 119.5 (also 119.475 & 119.525) on all your radios, and
see if you receive distorted TV audio. This is the second harmonic of ch. 2's
aural carrier, and if something on the plane is generating second-order intermod
(at 127.0), the same mechanism should generate the second harmonic as well.
(Think of this as the sound carrier mixing with *itself*) It's likely that ch.2
has gone to pains to ensure their radiated 2nd harmonic is quite clean... so you
DON'T hear it, it *might* mean that the problem is outside the airplane (or the
interference is caused by some different mechanism.)

If you drive back to the farm, you can check these frequencies with your
handheld as well.


Thanks!
Sydney


Good luck! Keep us posted...