Hi Nils,
For the bandpass filters you can check with Ranko 4O3A (ex-YT6A)
as I've just returned from vacation in 4O land and saw these filters
myself during
my visit to 4O3A. I think he sells them in 2 versions 1.5kW and 5kW.
You can see them at http://www.yt6a.com/en/hi-power-filters/index.php
Regards,
Sasha PC4X
P.S. I am not affiliated with 4O3A's business in any way ...
Nils Petter Pedersen wrote:
> 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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