I'm a bit more optimistic than some of the other based on years of
experience on a similar but less extreme site.
We operate VHF/UHF/Microwave contest from Mount Graylock in West Mass.
That site has a UHF TV Station, FM station, A few commercial services,
and several ham repeaters. We put cavity filters in the Rx path on
every band but 6 meters and as a result have _no_ problem with
interference from the high power transmitters or other electronics.
For a while, we did suffer some interference on VHF, about 10dB above
the noise, when we pointed to the TV station building. We got tired of
that and one of the guys traced it to a unused, unpowered TV preamp
connected to an unused TV Rx antenna. The thing acted like a diode and
caused mixing between the Nth harmonic of the station and the Mth
harmonic of the TV aural carrier and the result happened to fall on 2M.
On 2M EME, the site is _much_ cleaner and quieter than my home site with
very few spurious. (no plasma TV, no Tivo, not that many PCs, etc)
On most bands the noise rise several dB when the antennas point towards
the town south of us at a distance of about 10 miles. But nothing from
the big guns on the mountain.
If we take off the cavity filters, the Rx chain immediately overloads
from the massive out of band signals and noise floor rises to S9. Don't
TV and FM stations tend to have high quality design and relatively good
spurious and noise performance. The commercial two-way radio is maybe a
There is probably a strong, even exponential, relationship between the
number of transmitters and the chance of spurious. If there's diode
junction somewhere, and even a rusty joint can be nonlinear enough to
create mixing, then more transmitters mean many more combinations that
may land on your frequency.
I'd try it. Get a cavity filter for 144.2 MHz and your other bands of
choice, a good preamp, a short yagi, and take it up there. You might be
On 07/13/2010 11:03 PM, Grant Saviers wrote:
> We all dream of that perfect site - 1000' above all else for 360
> degrees. I found such a site, but so did 6 or so commercial
> tower/transmitter operators before me. The towers range from 100 to
> 250', from guyed to freestanding, from VHF to microwave stuck all over
> the height with sidearms. Three are high power FM/TV, multi-tenant
> master antennas that have stations with ERPs of several 100 kw, maybe
> one at 1000kw. These are about 200' away and start at 150' up and my
> quick research indicates at least 6 FM broadcast transmitters. It
> doesn't appear that there are any AM/HF transmitters from the antenna
> So is this worth taking a harder look, or is it hopeless for ham HF
> operation? Some of the questions that come to mind are:
> 1. Can I put up a guyed tower (e.g. 90') with a swing arm stack? (county
> codes are ok) Can the other tower owners/licensees stop me? What if my
> tower affects their permitted pattern?
> 2. How much filtering will I need? I can imagine building a Faraday
> cage ham shack for about the cost of a top of the line rig and filtering
> everything in/out.
> 3. If I filter enough so my receiver isn't totally overloaded in the HF
> bands, will the transmitter to transmitter IMD products get me? Or misc
> rectification or phase noise?
> 4. There are other residences within a few hundred feet. Where can I
> find the field exposure calculations for these transmitters? 5. Is there
> an index to licensees if I know the tower lat/lon?
> Grant KZ1W
> TowerTalk mailing list
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