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

: Sydney, have a look at the fcc link I posted a couple messages ago. It lets
: you look up towers' owners by lat/long. Your tower is owned by KTVI chan 2.

: Thanks.

Cool, I thought that you might have kill-filtered me for some reason, because
no one other than me seemed to be getting my messages!

: So it *is* something specific to our plane I guess.
: Although it's an intermittant problem for us, too.

Aha, I think we're on to something.

: : We didn't have this problem before last spring.
:
: Did channel 2 recently add a digital TV transmitter? Like, last spring?

: I believe so, yes.

On further reflection, this may be a red herring. Digital TV is in the
220+ MHz region.

: So here's what I'm thinking.

: That tower is TV Channel 2 (60-65 MHz I think?)
: Channel 5 which is nearby would be 79-84 MHz.

: This makes me think that marker beacons, at 75 MHz,
: are the logical suspect for causing a problem.

Channel 2 is 54 to 60 MHz, the 2nd harmonics of are 108 to 120 MHz.

This leads me to suspect one of the *NAV* radios. Can you physically
remove them from your plane, one at a time, and leave them in your car?
This would take their front end circuitry out of the area. Then try the
other one. The COM radios would also be out of the picture. Perhaps
you've already done this?

The reason that I'm harping on radios is that intermodulation
distortion needs a detector or a modulator to occur, such as in the
RF front end of a radio. I don't think that an antenna by itself is
sufficient to cause it.

Also, you might try taking out the nav antenna splitter. I'm not sure
that this should make any change, but if we're using buckshot methods...

Since I changed jobs I don't have my trusty HP 8591E spectrum analyzer
anymore, if I did I'd consider a trip to St Louis!

: But can the marker beacon antenna, by itself, be somehow
: bringing signals into the plane to be received by the
: rubber whip antenna of our handheld?

If you disconnect the MB antenna from the MB receiver, it is unlikely
that the end of the coax could act as much of a radiating element. I have
made a passive radiator before, but that's 2 antennas connected to each
other.

: If this is a possibility, how do we safely remove the
: marker beacon antenna for testing purposes? Do we need

If you've got the bent metal rod kind, disconnecting the little floating
wire will disconnect the MB antenna from the in-plane electronics, though
I'm inclined to dismiss the MB system.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place, you can get a "terminator" cap from most electronics
stores that cater to the ham radio crowd. I'm not sure Radio Shack sells them.
You'd want a 50-ohm terminator, and whatever adapters are needed to connect
it to the end of the antenna coax. If I were doing this, I'd probably
terminate the RF input to the radio, as well.

Please keep us (me) informed, we're trying to help the best we can!
--
Aaron Coolidge (N9376J)