Hello everybody !
I have been reading all replies and thank everybody for all advice so far.
I agree that crossmodulation in power meter diodes is highly unlikely
due to the reasons explained to me. I also thought that a linear amp would
be a poor mixer as one of you pointed out. Because of the low "injection
signals"
involved ,-rectifying "diode" joints somewhere in or near the antenna
structure
seems to be the most likely cause. I would like to add the following to my
previous
message :
We are using coax filter stubs as adviced by K2TR and K1TTT . These are on
the amp outputs.
We are also using Dunestar bandpass filters (series 600 switchable) on all
stations. These are
between transceiver and amp. We are using phillystrand or similar non
conductive guy systems
that was given to us from a commercial site taken off air. All tower joints
are jumpered by heavy copper
wire, mainly for lightning safety. We have an extensive underground rf-and
lightning ground system.
We serviced the 20M beam a couple of years ago. And I am using a pad
(attenuator) on the analyzer
input :-) by the way my analyzer tells me if I am out of dynamic range and
it does so loudly :-)
I usually go for 20 to 30 dB below that warning, at least...
So.. I think I just have to accept the fact that we have to order a mobile
crane again. At this qth
in the middle of nowhere they usually charge 1000 USD just to get here and
set up equipment !
That was my reason for the desperate search for another explanation.
But thank you everybody. I will read all mail from this group. Seems to be
an active group with
well qualified people. I would like to get info on any company that can
supply high power bandpass
filters at affordable prices. Bandpass on every antenna line would be a very
attractive solution for
us regardless of this specific problem. They would need to handle US max
legal pwr + safety
margin. Single band units preferred. I know of just one company capable of
delivering. They usually
deliver military stuff and would charge us an amount that could buy me a
nice new car... any info anybody ?
Nils LA7SL
----- Original Message -----
From: "Nils Petter Pedersen" <la7sl@online.no>
To: <amps@contesting.com>
Sent: Wednesday, September 05, 2007 2:41 PM
Subject: 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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