Rectification can take place between any 2 pieces of metal. Non metallic
guys use metal at their ends to connect to the tower and ground anchors.
Nearby metal buildings, fences, flagpole, farm equipment, well pump,
irrigation system, etc are all to be considered.
Do you have this problem on any otherband?
Does it change with the direction of the antenna?
What happens when you configure the 20M station to just the basics of
transceiver and amp? No filters, stubs, computers or cables.
Make a checklist of what has been checked and eliminated; then go to
untested items.
Carl
KM1H
----- Original Message -----
From: "Nils Petter Pedersen" <la7sl@online.no>
To: <amps@contesting.com>
Sent: Wednesday, September 05, 2007 2:55 PM
Subject: Re: [Amps] crossmodulation in PA ? Thanks for all good advice
> 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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