And another one showed up here - My top tower bearing is made of oak so the
mast out of the rotor is only "grounded " thru the rotator. I chased BC
intermod for a few weeks until one day I happened to notice that if I cycled
the rotor brake (HAM IV) -lo and behold the imod would disappear some times.
I had done what you suggested and probed around all the guy wire joints
etc - and did find some very weak rectifcation from my transmitter at a
couple thimbles - but nothing from BC. (I was also looking for external
generation of TVI on CH 3 when I was on 15 meters !)
But bottom line the big BC imod culprit was rectification in the rotor -
probably breaking down a thin film of grease in the bearings. A jumper from
top bell housing to bottom cured that ! (Also explained why my "top loading
" was variable ) When I do my next maintenance and re-install - I will do
the jumper at the top of the tower.
73 de Hank K7HP
----- Original Message -----
From: "jeremy-ca" <km1h@jeremy.mv.com>
To: "Nils Petter Pedersen" <la7sl@online.no>; <amps@contesting.com>
Sent: Wednesday, September 05, 2007 9:36 AM
Subject: Re: [Amps] crossmodulation in PA ?
> 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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>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
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>
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