Hi everybody !
At our small contest station LN8W / LA8W we have a strange problem..
It appears that we have crossmodulation taking place in the power amplifier.
When running 1 KW to a 20m yagi at 90 feet, I detect cross modulation
products from BC-stations appearing on a range of frequencies. I have
excluded
unwanted mixing in poor connections in the antenna,feedline or the immediate
surroundings like tower and nearby antennas or metal constructions. Field
measure-
ments have been done using a Hewlett Packard rf analyzer - spectrum
analyzer.
In line measurements have also been done using same equipment with
coupler/attenuator.
The mixing products are not strong, we are talking millivolts of RF in the
line, but that is enough to
QRM receivers on site by reradiating products that can not be removed by
filters or stubs, as they
appear in wanted bands. Please note that receiver overloading is not an
issue here, neither is analyzer
overloading as high perfomance filters are used in the measurements.
>From my time as a communications engineer I am well aware of such mixing
taking place in VHF power
amplifiers in dense repeater sites, and then usually in transistor stages.
On a few occasions I have seen
it happen in Class C tube amps, but never in linear tube amps. The
following amps have been tested, all at
the 1KW level : Acom 2000A , Ameritron AL1200, Tokyo HL2K and Commander
1200. Results
vary by 4 to 6 db . My questions to you all will be this : Have you seen
this happen ? If so, what is the cure ?
I have a strange idea that this might occur in the power output detectors in
the amps as these are using diodes
to detect current, usually via a pickup coil. Unwanted signals could
possibly be reradiated from there...
This is a far shot, and I would like other ideas here before going to
drastic measures. My dream would be to
put high power bandpass filters on all feedlines, but that would be a very
expensive project.
Any ideas please ? ! I am receiving mail from group as digest, but I will
reply as soon as digest arrives here.
best 73's to all from
LA7SL Nils ( Peter)
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