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
mode.
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
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