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

[Jim Brown]

On 2/24/2020 2:03 PM, Matt NQ6N wrote:
> Jim,
> 
>  > As noted above, that is NOT a meaningful test.
> 
> Interesting. I just did that test (kx2 tx, mains power off, portable 
> battery powered RX) and the mixing products are still there.
> 
> I realize based on your comment that I have a large number of 
> decommissioned wall warts around the shack which may be acting as 
> passive mixers.

They must have antennas attached to function as mixers. An SMPS laying 
in a drawer doesn't have much of antenna, so is unlikely to cause a 
problem.

BUT -- if those decommissioned wall warts are LINEAR (they will be 
larger and heavier than SMPS with the same power capability), you can 
use them to replace SMPS. See that chasing noise tutorial.
> 
> Can I eliminate a wall wart from suspicion for causing this particular 
> mixing product if I hold the RX antenna near it and the signal level 
> does not increase?  

Yes

I realize that the wall wart could be causing other
> problems, but just trying to figure out a strategy for attacking what 
> appears to be the most significant source of mixing.
> 
> I have a lot of decommissioned wall warts around the shack that could be 
> causing problems as passive mixers, so I will gather those up and 
> remove them.
> 
> Two other observations:
> 
> 1) Holding the portable RX antenna near certain mains outlets and light 
> switches seems to increase the signal strength of the mixing product at 
> 14.786 MHz.  Is there a next diagnostic step that makes sense to test 
> these?

The wiring to those outlets could be simply coupling to your radio's 
antenna and extending it.
> 
> 2) At the edge of the property are two boxes that appear to be owned by 
> AT&T. They are adjacent to the one that is owned by Comcast that my CATV 
> coax plugs into.  The portable receiver seemed to peak near the AT&T 
> boxes. However they are also near the folded counterpoise I use on my 
> 160m antenna. 

I wouldn't expect an FCP to be part of a 20M problem generated by 
something close to it.

I'm going to try moving the FCP wires and testing whether
> the signal still increases near those boxes.

If the IM signal is stronger here than other places, it's possible that 
it's the source. Or it may be the antenna coupling thing again. Both 
boxes have multiple long antennas connected to them.

73, Jim
> 
> 
> Thanks again for all the help, Jim, Dave and others.
> 
> 73
> Matt NQ6N
> 
> On Mon, Feb 24, 2020 at 3:40 PM Jim Brown <jim at audiosystemsgroup.com 
> <mailto:jim at audiosystemsgroup.com>> wrote:
> 
>     On 2/24/2020 1:03 PM, Matt NQ6N wrote:
>      > Thanks much for the tips, Jim.  Do you know if the wall warts can
>     cause
>      > mixing when they are not plugged into the wall or if the circuit
>     breaker
>      > is turned off?
> 
>     They can cause mixing if connected to wires that can function as RX and
>     TX antennas. And when operating, they produce noise which gets radiated
>     and received by your antennas. SMPS are noise generators, which the
>     major reason for a search out and destroy mission.
> 
>      > In my testing the other day I did not see any difference in the
>     14.786
>      > MHz mixing product when turning off all of the breakers in the
>     breaker
>      > box. Does that test eliminate any warts on those circuits as
>     culprits
>      > for the specific mixing that remains present at 14.786?
> 
>     No, power is not required for them to cause mixing -- it's passive
>     device in that regard.
> 
>      > My next test will be to power down the entire house and transmit
>     with
>      > the KX2 on battery, connected directly to the antenna, and
>     confirm that
>      > the mixing product is still there (eliminating anything in the shack
>      > itself).
> 
>     As noted above, that is NOT a meaningful test.
> 
>     73, Jim K9YC
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