My experience with commercial VHF/UHF sites when everything else has been
tried has always been a non linearity diode joint in the guy system. Always
at a cable clamp or guy grip with the guy anchor area being the most likely.
The real cure is to replace the hardware but that is often not possible.
What does work is placing a few 77 mix large ferrite beads over the guy
wire at the anchor. The diode joint will still be there but the beads
suppress the radiation.
Full and split beads are available thru Amidon; I dont know about European
sources. If the non linearity occurs at high HF or VHF/UHF frequencies a 43
mix is better.
Carl
KM1H
----- Original Message -----
From: "Nils Petter Pedersen" <la7sl@online.no>
To: <amps@contesting.com>
Sent: Wednesday, September 05, 2007 8:41 AM
Subject: [Amps] crossmodulation in PA ?
> 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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>
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