[600MRG] RFI identified

[Dave Riley]

Getting the noise out has been the primary effort around here.
1. Using SDRdx as a spectrum analyzer helps much.
2. Using twin lead from antenna to receiver is another good clue from Dallas Lankford.
3. Coupling transformers at each end of the feed is a MUST.
4. A center tap at the loop antenna winding to a local ground rod helps.
5. If I'm lucky then the direction of most noise is different from the signal direction. Adjust location.
6. Knowing the total local noise compared to a short at the antenna termonal .
     At 630m I get between 9 and 12 db which tells me that a low noise preamp is not necessary at the antenna.


On 1/4/2025 7:29:32 AM, Paul N1BUG FN55mf <paul at n1bug.com> wrote:
Hi Tom,

Thanks for sharing your knowledge and experience.

I am going to try a commercial filter. I spent some time looking and
found very few in stock anywhere that offer more than a couple dB at 137
kHz which is also affected and of interest. I ended up ordering one that
claims 39 dB common mode, 70 dB differential mode at 150 kHz (in a 50
ohm circuit), 42/70 at 500 kHz. I will give that a try.

The LF/MF receive antenna is about 200 feet from the refrigerator but
there is electrical wiring in a detached shed about 120 feet from the
antenna. I wish I had the space to move all antennas further away.

Paul, N1BUG



On 1/3/2025 9:41 PM, Tom W8JI wrote:
> Having had to clean up multiple SMPS for commercial applications and fix
> RFI around here from multiple things, my view is the following.
>
> The enclosure and wiring of the refrigerator is far too small and
> confined to be much of a radiator. The refrigerator is also a poor
> "counterpoise" for injecting a whole lot of common mode out the cord,
> although very close that could be an issue.
>
> Usually the problem will be differential mode excitation of power cord
> wires. The wires then act like two or three wire leaky transmission line
> and carry or distribute that noise as it gradually radiates.
>
> Almost every SMPS I have cleaned up has been from making every conductor
> leave at the same noise potential. The bulk of the filtering is with
> bypass capacitors that hold the exiting leads all at the same noise
> potential. This is usually much more effective than series chokes,
> although individual chokes in each line (which could be beads) can
> "help" the capacitors make everything the same potential.
>
> The problem on 630M is having low enough capacitive reactance to hold
> the exiting conductors all near the same potential for 630M noise
> without having enough current through capacitors to nip you or cause
> other problems on 60 Hz, even though that is about an 8000:1 difference.
> Any capacitor also should be VDE/UL/CSA rated for line bypass.
>
> With a three wire cord I would bypass the hot to neutral, then the
> neutral to safety ground. This is the safest way.
>
> I think the suggestion of a good commercial line filter is a good one,
> they usually tie all the conductors together for differential noise.
> Otherwise use a few bypass capacitors and see what happens.
>
> 73 Tom
>
>
>
>>> On 1/3/2025 11:47 AM, Paul N1BUG FN55mf wrote:
>>>>
>>>> Early this morning I found it the culprit. It is my refrigerator!
>>>> Perhaps a line filter will help, although my luck is rarely that good.
>>>>
>>>> I'm still chasing one, possibly two serious RFI sources affecting 2200m
>>>> but won't go into that here.
>>>>
>>>> 73,
>>>> Paul N1BUG
>

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