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)

In article ,
Aaron Coolidge wrote:


: 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.

Doesn't have to be -- _strong_ out-of-band signals can be picked up,
albeit inefficiently, 'mix' at any dissimilar metals contact, genrating
a signal to which the antenna is tuned, and which _will_ then re-radiate.
All in a single-antenna system.

: 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.

Recommendation is to put a terminator on the receiver input, but
simply *short* the antenna cable center-lead to the shield. Putting
a terminator on the antenna cable encourages re-radiation, *if* the
problem is coming from a 'diode-ing' spot somewhere closer to the
antenna. Shorting tends to suppress any passive re-radiation.

We've got one here but it may be a bit far to come.

Paul
Guildford, UK.

"Aaron Coolidge" wrote in message
...
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!

Aaron Coolidge wrote in message ...
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!

No, not at all, though I'm reading news through groups.google.com
which is flaky on propegation

BTW someone said they emailed me. I didn't receive it. This
email address no longer works. If anyone wants to email me
try hoeltzli at swbell dot net. Though this discussion seems
on-topic and of possible general interest so I'd just as soon keep it
on the net.

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

Can this kind of intereference be caused by subtracting frequencies?

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

That's too low for the frequencies where we're getting interference
(124.2 is clean, 126.5 is trash)

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?

Not a problem. Can this really be caused by a radio which is *powered
off*? Because I did try switching the navs to different frequencies
and also turning them off, along with all the electrical power in the
plane.

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!

Oh, bummer! That would have been nice.

: 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

Pretty sure it's a blade, though I admit the MB antenna is "out of
sight out of mind" to me.

If you wish to electrically remove the antenna from the plane while leaving
it physically in place

I'm perfectly happy to take it off and slap some 50 mph duct tape over
the resulting hole, providing it's not going to hurt the MB receiver
to be attached to dangling coax.

If I were doing this, I'd probably
terminate the RF input to the radio, as well.

IIRC this would be a PITA -- the coax goes to the antenna with a
standard BNC connector, but is soldered on to the radio tray.

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

I appreciate this. It's a vexing problem and one which apparently
lies outside our radio guy's experience

Thanks,
Sydney

: 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?

: Not a problem. Can this really be caused by a radio which is *powered
: off*? Because I did try switching the navs to different frequencies
: and also turning them off, along with all the electrical power in the
: plane.

Yes, it can be caused by a radio switched off. With those ACK elt units that
cause problems, they are off when their problems occur!

: I'm perfectly happy to take it off and slap some 50 mph duct tape over
: the resulting hole, providing it's not going to hurt the MB receiver
: to be attached to dangling coax.

It won't.

: If I were doing this, I'd probably
: terminate the RF input to the radio, as well.

: IIRC this would be a PITA -- the coax goes to the antenna with a
: standard BNC connector, but is soldered on to the radio tray.
:

It would be OK to terminate the coax where the antenna was connected.
As someone else pointed out, you can short out the antenna at its
BNC connector to electrically remove it from the plane. You could
probably make a BNC shorting plug from parts found at Radio Shack.

--
Aaron Coolidge (N9376J)