[RFI] advice wanted: finding the source of mixing products

Mon Feb 24 15:00:09 EST 2020

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 at 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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-- 

*Dave - WØLEV*
*Just Let Darwin Work*
*Just Think*