Re: Frequency interferance and TV stations

Doug McLaren wrote:

In article <wqWdnZOh3_pj2UbfRVn-2g@xxxxxxxxxxx>,
Bob Monsen  <rcsurname@xxxxxxxxxxx> wrote:

| >> I am not an expert in this stuff.
| > | > It shows...;-)
|
| Ah, an expert! Perhaps you can explain it then, rather than just | engaging in the noble art of handwaving... ;)

You want TNP to explain why somebody has banned something (but we're
not sure exactly what) at some unnamed fields in some unnamed state?

In any event, you don't always have to be an expert to tell that
somebody else isn't one.

| Actually, the OP was asking for a technical explanation of why ch 20 (or | 21, he can't remember) is prohibited at some fields, and thought that | perhaps it was because of TV ch 4.

| >> On the other hand, I wonder if they are thinking that a TV may | >> retransmit on this frequency? The TV must use an internal oscillator | >> that is .455 away from the carrier to mix the signal to 455kHz. | > | > No. The TV uses typically a 45MHz IF strip,IIRC - and its a LONG time | > ago - with a separte sound IF at a ferquency I forget. Possibly a direct | > take off at 4Mhz.
| | Ok, so that theory is shot.

And any signal that a TV receiver emits on the 72 mHz band should be
_very_ weak anyways, and shouldn't cause problems unless the TV is
defective or really really close to the plane.

| So, we still don't know why is channel 20 (or 21) prohibited at some | fields...

Ultimately, even if we get an `expert' here, we still won't know.

You'll need to ask the people at the fields who actually banned it.
There's a lot of `voodoo' claimed when people talk about interference,
and it may be that the channels are banned for no good reason beyond
that Joe crashed two planes on those frequencies six years ago because
he forgot to charge the batteries (but it was blamed on interference,
of course.)  Or maybe not.

If it's really channels 20 and 21, it could very well be that there's
a pager tower nearby that broadcasts on 72.200 mHz -- right in between
the two channels.  Here in Austin, TX I've detected strong digital
transmissions on 72.660, 72.860 and 72.960 mHz.  None of the local
clubs have banned the channels adjacent to those signals, but maybe
none of the clubs have a pager tower that close to the field.  Or
maybe people just haven't put 2 and 2 together yet and figured out why
they're crashing.  Or maybe it's an unwritten rule that you avoid
those frequencies ...

| Here is an idea... Could it be that, instead of CH 4 affecting the | receiver, the RC radio is somehow making annoying noise on channel 4? | How about the idea that the RC receiver is actually broadcasting a buzz | on the audio frequency?

Seems unlikely.  Any signal emitted by the receiver would be very
weak, and you're not likely to be flying that close to a TV anyways.

However, I've heard that (some?) scanners emit a weak signal 10.7 mHz
above or below what they're listening to -- and our dual conversion
receivers may very well do the same.  But it would be a very weak
signal, and I wouldn't expect it to interfere with TV unless the TV
station was very weak and/or the TV set was very close.

Do we even know if the place where these channels are banned has a TV
station on channel 4?


I think this is as good as any answer I could give. Local oscillator re-transmission is fairly low - and statutorally low in many domestic appliances.

However the mere mechanics of radio design mean that all sorts of remixing, rectification and re-transmission of signals can occur in the presence of nothing more complex than a rusty fence...


In short, if your field has problems on band X, avoid band X...unless you have a few thousand dollars of frequency counters, communication receivers and spectrum analysers that can make a case for why...
.

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