Dave,
One quick question:
*Any modern solid state transmitter will produce broadband noise. The
broadband noise is generally worse at less than full output power. I'm
guessings that's where your BB noise is originating.*
Can you recommend a test procedure I could use to determine how much of
this broadband noise is originating in my transmitter (so that I can
determine whether it is within spec)?
73,
Matt NQ6N
On Mon, Feb 24, 2020 at 2:00 PM David Eckhardt <davearea51a@gmail.com>
wrote:
> Just a few thoughts, here:
>
> Any modern solid state transmitter will produce broadband noise. The
> broadband noise is generally worse at less than full output power. I'm
> guessings that's where your BB noise is originating. It could also be made
> worse by the 'thing' that is producing the multiplication and mixing of
> your 7.058 kHz signal.
>
> Through the directional coupler, you comment the second harmonic of your
> 7058 frequency is extremely low. That's good and evidence that the filters
> are working as designed. Sample the whole enchilada from the air by
> hanging a shorted o'scope probe ('ground' lead attached to the probe
> center) on the input of the spectrum analyzer. This forms a
> quick-and-dirty current probe with the advantage that there will be no
> 60-Hz and harmonics present, unlike a clip lead attached to the input of
> the spectrum analyzer. Key your transmitter and see if the resulting
> second harmonic and the mixing product are any stronger than when connected
> through the directional coupler. The coupler shows you the output of the
> transmitter - clean. The quick-and-dirty current probe will display what
> is 'in the air' due to anything that is locally producing the
> multiplication and mixing.
>
> I'll also bet that the source of the mixing is also producing the 2X your
> 7058 kHz frequency. It, by itself, could be emitting the 'unpleasant'
> signature you found due to one of your wall warts.
>
> Connect a small wire (very short as a function of the wavelength at 14
> MHz) to a battery powered radio and walk around the property with the radio
> tuned to 14.786 MHz. Your transmitter must be keyed for this to work.
> Maybe, just maybe, this might reveal the source of the multiplication and
> mixing.
>
> I had a problem a bit like this long....long ago (a half-century ago!!)).
> We had a "budget" AM BC station down the hill and about 0.75 miles
> line-of-site distant from my location. When I ran CW at 100 watts on 80,
> 40, and 20 meters (no amp at the time) to my wire antennas, all kinds of
> mixing products and noise reared their ugly heads. Problem turned out to
> be neglected and improperly dressed and tightened guy wires at the AM
> transmitter location (FCC got involved).
>
> Dave - WØLEV
>
> On Mon, Feb 24, 2020 at 7:26 PM Matt NQ6N <matt@nq6n.com> wrote:
>
>> Hello --
>>
>> I'm not sure how much background to offer about my shack, so I'll skip to
>> the specific question I have and then below that I will describe in a bit
>> more detail the steps I've been using to figure out the source of RFI.
>>
>> When I transmit at 7058 KHz at low power (testing today transmitting 6W)
>> there is a very strong mixing product of AM 670 that appears at 14.786
>> MHz. It seems to be 7058 * 2 + 670 = 14786.
>>
>> There are MANY such mixing products all over the bands when I am
>> transmitting at 7058, including a loud mixing product of Radio Martí at
>> 296
>> KHz.
>>
>> Question 1: Is this normal, something that always happens and likely has
>> nothing to do with my interstation interference?
>>
>> Question 2: If it is not normal, then would the source of radiation of the
>> mixing products be a likely culprit for the interstation interference?
>>
>> The mixing product at 14.786 is audible on a portable short wave receiver
>> as I walk around the property.
>>
>> Next I'll briefly describe the station configuration:
>>
>> The rig is a Flex 6600. At first I thought my interstation issues had to
>> do
>> with the typical scenario such as the second harmonic on 40m overwhelming
>> the receiver on 20m. However, in my setup the interference seems to be
>> much more severe.
>>
>> Transmitting 6 watts on 40m, most of 20m sees a noise floor increase of
>> over 10 dB. This is transmitting on the Flex 6600 which has additional
>> BPF
>> filtering on contest bands AND a VA6AM high power filter.
>>
>> When I connect a spectrum analyzer to the output of the VA6AM filter via a
>> directional coupler, the second harmonic is incredibly weak to the point
>> where visualizing it is difficult on my Siglent 3021x spectrum analyzer.
>> It is hard for me to believe that this tiny signal is strong enough to
>> cause a 10 dB noise floor increase on 20m.
>>
>> The signal path includes the Flex 6600, a 403A 8x2 switch, and wire
>> antennas. Interconnections betweenn the rig, VA6AM BPFs, and switch box
>> all use RG-400 jumpers, and all antenna feed lines are LMR-400.
>>
>> The antennas are suspended in the trees above the house. The minimum
>> spacing between the 40m Vee and 20m dipole is approximately 4 feet.
>>
>> Does it make sense that transmitting *6W* with all this filtering there
>> would be enough signal getting into the 20m antenna to cause a 10 dB noise
>> floor increase?
>>
>> I have tested using a different transmitter AND a different receiver and
>> the 20m noise floor increase while transmitting QRP appears to be present
>> in both cases.
>>
>> In my last RFI search I discovered a few wall warts whose power cords
>> seemed to be radiating some kind of mixing products, and so I added
>> ferrite
>> or disconnected each of the ones that did this. Unplugging them
>> completely
>> does not reduce the harmonic.
>>
>> Question 3: What should I be RDFing to find the source?
>>
>> My hypothesis is that the mixing products described above are NOT normal
>> and thus I have been under the impression that RDFing them would help
>> locate whatever is doing the mixing. Is this correct?
>>
>> One other data point:
>>
>> I found one wall wart and mains circuit that appeared to be adding "junk"
>> onto the second harmonic of the transmitted signal. The second harmonic
>> does not sound like a pure sine wave. Instead it is wide and has a very
>> unpleasant sound. I found that removing that one wall wart improved the
>> purity of the second harmonic.
>>
>> Question 4: Would it make sense to try to RDF the source of the "junk"
>> sound on the second harmonic?
>>
>> I have tried turning off each of the breakers in the breaker panel (except
>> the one powering the TX), and none have any impact on the tone of the
>> second harmonic OR the BCB station mixing. I will repeat this test today
>> and also try transmitting on the KX2 to verify that the mixing products
>> are
>> still there.
>>
>> I'd also welcome ideas for things to test/try that might shed some light
>> on
>> what is going on.
>>
>> 73 and thanks much for any advice. I have temporarily removed my amp from
>> the system and have installed the 403A switch and RG400 in an attempt to
>> make it easier to deduce useful things from various tests.
>> _______________________________________________
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>> RFI@contesting.com
>> http://lists.contesting.com/mailman/listinfo/rfi
>>
>
>
> --
>
> *Dave - WØLEV*
> *Just Let Darwin Work*
> *Just Think*
>
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