Topband: Elimination of Treadmill RFI on 160 meters

Jim Brown jim at
Tue Jan 27 23:13:10 EST 2015

On Tue,1/27/2015 3:19 PM, Brad Rehm wrote:
> You make a good point about the difference between our definition of CM
> noise and the definition implied in Part 15.  If the external torroid made
> a difference for you, then it's important and worth having.

Exactly. The commercial filter addresses only differential mode. The 
ferrite choke is necessary to suppress what we (and Part 15) call common 

Several years ago, I put some good quality line filters in electrical 
boxes for use on Field Day and California QSO Party county expeditions 
with our Yamaha generators. They didn't do much -- to kill the moderate 
trash, I had to to wind multiple turns of the line cord  through a big 
ferrite core. I used a big 1-in i.d. Fair-Rite #31 "clamp-on" that's a 
couple of inches long (the biggest they make). Depending on the diameter 
of the line cord I had used, I was able to get either three or four 
turns through it. That was enough to kill 20-10M, which is what we heard 
in the tri-banders that were close to the generator, and for the 80/40 
dipoles that were much further away. If the 80/40 antennas had been 
closer we would likely have needed more turns.

Did the commercial line filters do any good?  I don't know -- but they 
certainly didn't hurt, and the boxes I put them in have a bunch of 
outlets on them for power distro. :)  OTOH, one of the guys used to 
bring his big RV with a noisy generator on-board. All I used there was 
as many turns as I could get through a small stack of 1.4-in i.d. #31 
toroids as close as I could get them to the generator, and it did the job.

73, Jim K9YC

More information about the Topband mailing list