> 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
