That’s easy. RF is picked up on the AC lines then enters any nonlinear device
plugged into those lines, such as a walk wart. There, RF is rectified,
harmonics are generated, and that’s mixed with all the garbage on the power
line, then re-radiated via conducted emissions. Many harmonics are normally
expected. This is the classic problem for any SO2R or M/M station.
Case study: At 8P5A, Tom has very little ancillary stuff plugged in and yet he
had your problem. He traced it to a single LED light. Once removed, the
harmonic garbage disappeared.
Gary Johnson NA6O
gwj@wb9jps.com
> On Dec 21, 2020, at 4:06 PM, rfi-request@contesting.com wrote:
>
> From: David Eckhardt <davearea51a@gmail.com>
> Subject: [RFI] QUESTION
>
> I'm out of ideas on this one! Where does the raspy modulation come from
> on the harmonics which changes with position in my radio room on a battery
> operated receiver?
>
> Situation:
>
> transmit on 7.010 MHz
> receive on second harmonic, 14.020
>
> transmitter: IC-7300
> receiver: IC-7610
> and reversed
> Demod: CW or SSB (or AM, for that matter)
>
> Fundamental sounds clean. Second harmonic sounds modulated by 120-Hz (and
> a few harmonics of the line) and quite raspy - pretty awful.
>
> Receiver: portable battery operated receiver (Grundig G3) with same
> transmitters: I can walk around the room tuned to the second harmonic and
> find places in the room where things are clean and other positions where
> the second harmonic (and third) sound awful with 120 Hz raspy
> 'modulation'. Any idea how I can account for this localized behavior? I'm
> out of theories.
>
> Dave - W?LEV
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