[50mhz] Birdie on .125

[Bill VanAlstyne, W5WVO]

Hi John,

It's probably coming from outside your house -- though in-house RF sources will
usually show beam directionality also, even with the beam mounted on the roof.
Since it's new, my guess is that it's a mix between two (or more) outside
VHF/UHF sources, one of which is either new or on a new frequency. Could be TV,
FM BC, FM two-way, or practically any other thing. Since you live in a
relatively rural area, sorting it out might not be out of the question if you
persist at it. Good luck!

Here in Albuquerque (as in most other large cities), we just have to live with
this stuff. With a mountaintop full of commercial broadcasting and two-way
transmitters, visible line-of-site from anywhere in the upper Rio Grande valley,
passive mixing of any of the scores of powerful signals coming from up there is
something we can't avoid and really can't trace down. Most of the passive
mixing, as evidenced by our beam headings, is happening right up there on the
mountaintop, where the RF is so thick you could cut it with a knife. Even
oxidized guy wires, corroded door hinges, you name it, can turn into a passive
mixer. You put enough RF voltage across an oxidized contact point -- a
"non-linear junction" in technical terms -- and some tiny part of it may well
mix down to a heterodyne frequency and reradiate. All it takes is a few
milliwatts (or even microwatts) line-of-site to your beam, and you have a
"birdie".

Even more common than these so-called "rusty bolt" passive mixes are those that
actually take place in the output stage of one of the involved transmitters. A
nearby high-level signal gets picked up by the transmitter's antenna and coupled
back (usually common-mode) into the final amplifier stage, where it is mixed
with the transmitter's fundamental signal, and a heterodyne (plus its harmonics)
is reradiated by the transmitter antenna. Even if drastically attenuated by
filtering, all it takes is a few radiated microwatts to be detectable to (and
cursed by) us hams.

The problem is that these spurious emissions are typically well below the FCC's
requirements in terms of amplitude attenuation. In other words, there's "nothing
wrong" from the perspective of the FCC and the transmitter operator; the
transmitter's operation is meeting legal specs. The fact that the emission is
coming from a line-of-sight mountaintop, you have a high-gain directional
antenna pointed at it, and you're listening to it with a modern pre-amped
receiver so sensitive it can hear a gnat blink in Siberia, is really not their
problem. And in most cases there is nothing we hams can do about it.

These mixes are oftentimes very unstable (thankfully!) because they can involve
multiple harmonics, passive remixes of other mixes, etc. And in some cases, any
change in the weather can affect the intensity or even the presence of one of
these nasty little buggers. So my guess is that it will just go away before you
ever find where it's coming from. Hope so...

In any case, you should immediately get hold of a copy of the ARRL RFI Book by
Ed Hare, if you haven't already. Worth its weight in 3rd-order intermodulation
products.  ;-)

Bill / W5WVO
Rio Rancho, NM


John Geiger (NE0P) wrote:
> Recently a kind of nasty birdie has shown up on 50.125, which makes
> listening to the calling frequency frustrating.  The Autonotch and
> beat cancel on the TS2000 will handle it pretty well, but still not
> as well as if it wasn't there.  I have tried turning off/unplugging
> several things in the home (computers, VCR, DVD players) with no luck
> so far.  One ham had a problem with his internet router, so I tried
> unplugging that also, along with the cable modem.  No luck there.
> Has anyone on the list have this problem before, and if so, what was
> the final cause of it?  It peaks about S3 from the NE direction, and
> will get weaker when I move the miniquad to the direct south or NW.
> That suggests that it could be outside the house, and might go drive
> around the neighborhood a little this weekend to try and localize it,
> but would like to eliminate things in the house first.
>
> 73s John NE0P
> EM04
>
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