Matt Yes, cheap unfiltered wall warts can act as mixersAnd produce mix products
depending on their Switching frequencies Multiple turns 6 or more on the
input and output type of the wall warts with 31 Cores like FT120-31 andFT240-31
Round Cores should fix the problem If the Wall wart emits HF noise it is a
good Mixerto receive your HF signal and spew out mix products MikeWB6DJI
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On Monday, February 24, 2020, 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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