[Elecraft] OT 20M Moving QRM (Video)

[Jim Brown]

On Thu,3/3/2016 4:13 PM, Fred Jensen wrote:
> Weigh it.  Most small wall warts are also very light.  I've got a 
> couple of heavy larger ones in the junk box and the difference is hard 
> to miss.  The cable from it is the radiator of course, a BC band radio 
> tuned to a clear spot [preferably at the high end] will usually show a 
> distinct increase in noise as you move it up the cable.
>
> I was choking a 4 A feed to the wireless equipment and used a couple 
> of 3 1/2" OD [or so] toroids that N6XI gave me.  Consulting the K9YC 
> recipe, I wound 12 turns of Radio Shack red-black pair on one,

For the record, ALL zip cord, including the red/black stuff and the 
glorified zip cord sold as super exotic speaker wire, is TERRIBLE for 
RFI. Twisted pair is FAR superior, and should always be used in any 
situation where there is RFI. This has been well known for at least a 
century -- in the early days of telephony when the only noise source was 
50/60 Hz power and telephone wiring often ran on the same poles as power 
wiring, the only crosstalk prevention was a crossover of the telephone 
pair at every other pole.

> and as many turns as I could get in one layer on the other, wired them 
> in series and then stacked them using zip ties.  My theory was that 
> the one with lots of turns would get the 160-80 m stuff and the 12 
> turns would get 40-30-20. 

Max possible turns is about right for 160M on #31, 8 turns is about 
right for 40-30-20. This is for a 2.4-in o.d. toroid.

> They went right at the Chinese RPOJ power supply in the 24 VDC cable. 
> I don't know the mix but I think probably 34 or 43, I think they were 
> part of a group buy by the contest club.

All our group buys have been #31.

>
> My experience with the small clamp-ons has been poor, unless you get 
> several turns of the cable through it, and even then I don't think 
> there's enough permeability in the magnetic circuit to do much.  It's 
> pretty hard to find linear wall warts these days.

Right. A choke without enough wound turns is a waste of time and money. 
BUT inductance is proportional to the length of the cable path through 
the ferrite, and the clamp-ons are much longer than the toroid, so fewer 
turns are "right" with clamp-ons as compared to toroids. Roughly half 
the number of turns is a good rule of thumb. 5 turns is a  good starting 
point for a #31 clamp that's about 1-inch long for 40M - 10M.  2 turns 
is about right for 6M, with several of these 2-turn cores in series 
along the coax.

I'm currently working on a step-by-step tutorial on chasing noise, and 
with some detail about different noise sources. First publication will 
be in the next issue of NCJ.

73, Jim K9YC