[RFI] Common Mode Choke Materials

[Jim Brown]

On 4/6/2021 7:15 PM, Tony wrote:
> I was surprised to see that the choke was much more effective when 
> placed at the SDR Play receiver than it was at the antenna (active 
> loop). The noise floor dropped 15db with this configuration and only a 
> few db with the choke placed at the antenna.
> 
> My guess is that the bulk of the noise is coming from the PC connected 
> to the SDR since the only things on in the shack were the PC and 
> monitor. 

It certainly does suggest that the computer/SDR are a strong noise 
source. Now that you've knocked it down a lot, you'll start hearing 
other noise sources. And I agree with Lloyd that you should try a choke 
at the antenna.

I placed another choke on the USB cable that connects the two
> devices, but that had a minimal effect.
> 
> I'm seeing -120 dbm noise floor on 160 and -130 dbm on 80 with the 
> simple mix31 choke and the active loop so I'm happy about that.

These numbers are not meaningful, because you don't know the gain of 
antenna and its preamp. What matters is signal to noise ratio on weak 
signals.

  It
> doesn't do much for bands below 160, but I guess that's to be expected.

Right. 160 is the lowest ham band on which #31 is useful. #75 is the 
weapon of choice for 650m. I've not looked at 2200m, but the dimensional 
resonance of 2.4-in o.d. #75 cores kicks in just below the 630m band, so 
all you have is inductance, and it will take a LOT of turns to get much 
choking impedance that low in frequency.

A quick look at page 34 of the pdf catalog suggests that #76 might be 
useful on 2200m. But while some toroids large enough to wind chokes are 
in the catalog, the only parts stocked by distribution are much too 
small. The largest in stock anywhere is 0.383" o.d., 0.18" i.d., 0.131" 
thick. All of which means that if you want any of the larger ones, 
you've probably got to order 500 pieces and be prepared to wait for them 
to be manufactured. :)

73, Jim K9YC