Jim,
No question that a separate filter for the green wire can make a difference
when there's RF current on that lead. But I'd wait to add the extra filter
until I was sure it was needed. The case yesterday is an example of one in
which it wasn't. We weren't surprised, because we'd already pre-tested the
fix with another filter, checked for currents on the ground lead and
satisfied ourselves that a properly-selected 2-circuit filter would cure
the problem.
I suspect that cases in which green-wire filtering is needed could be cured
with a 3-phase line filter. Something like Corcom's 20AYP6C. This would
be easier to install, but the cost would be 4 to 6 times that of the filter
we used yesterday. I've found them at hamfests, though.
I've never looked closely at the problem, but I wonder how the 31-mix cores
behave in the presence of high-current, 20 kHz fundamental frequency pulses
that drive the PWM motor in our treadmill. That mix would suppress the
higher-frequency harmonics, but would the core eventually change its
properties because of the transients it has to live with. (The load
current rating of our treadmill was 15 Amps.) Is it possible the core
could begin to generate more noise at Topband frequencies if it became
easier to saturate?
Brad KV5V
On Tue, Jan 27, 2015 at 10:13 PM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
> 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
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|