Thanks Jeff. The AM station has the same power both day and night, just
goes from 2 towers to 4 to change the pattern.
Rusty bolt or fence line etc seems the most likely. The problem does
seem to disappear when its raining out, which helps verify this theory.
It might be a needle in a haystack trying to find this, so maybe
remoting the receiver might be the easiest.
Thanks and I'll continue to investigate. I can try temporarily moving
the frequencies apart about 100Khz and see if my 5Mhz theory holds water.
Tony
Jeff DePolo wrote:
>
>
> > So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I
> > realize that
> > the 4th harmonic of a 5KW broadcast station isn't very powerful
>
> Well...it *shouldn't* be very strong. It has to be attenuated 43 + 10
> * log
> (Pwatts) as measured in the field (not at the transmitter output
> terminals).
> If you have access to a field intensity meter that covers up to 5 MHz,
> or a
> spectrum analyzer and a calibrated antenna, you can measure it yourself.
>
> AM stations that change power and/or pattern at night sometimes use a
> different transmitter between day and night depending on the power levels.
> Some stations also have pre-sunrise, post-sunset, or critical hours
> authorizations that are intermediate power levels between day and night
> power levels, or as an adjunct to daytime-only authorization. Bottom
> line -
> the 4th harmonic content may vary due to a combination of pattern,
> transmitter power output, or even different transmitters.
>
> > but
> > being in its nearfield might be enough to cause a mix with the UHF
> > transmit output.
>
> Well, 1 or 2 miles isn't really near-field, but in any case, the field
> intensity may be relatively high depending on all of the other variables
> (power, pattern, etc.).
>
> Usually interference to VHF/UHF involving mixes with AM broadcast occur
> somewhere at or near the VHF/UHF site, not at the AM site. In some cases,
> the problem can actually be caused within the equipment on the ground
> rather
> than externally at the antenna or on the tower. If it's an in-the-cabinet
> mix, it could be caused by inadequate RF shielding. Before going on a wild
> good chase, I'd ensure that everything is properly RF-shielded, shielded
> cables are used for interconnects, grounding is good, all shields are in
> place, all mechanical connections (e.g. screws) are tight, no oxidizes or
> corroded connectors, etc.
>
> To rule out a lot of AM coming down the coax (which is fairly unlikely for
> most VHF/UHF antenna designs), install a high-pass filter. Even a shorted
> quarter-wave stub should give a fair amount of attenuation down in the MW
> range. If you have any in-line lightning protection (Polyphasers, et al),
> try removing them.
>
> But, more than likely, if in fact the AM station is the cause (either its
> fundamental or a harmonic), you have a passive intermodulation mix,
> the old
> "rusty bolt" problem. It could be in your antenna, on your tower, in your
> duplexer, in a corroded connector, who knows where. Divide and conquer is
> the only way to try to isolate it.
>
> --- Jeff WN3A
>
>
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