Posted by
Jim Brown-10 on
Apr 13, 2005; 9:03pm
URL: http://elecraft.85.s1.nabble.com/Dishwasher-QRN-to-my-K2-sn-1031-tp377012p377013.html
On Wed, 13 Apr 2005 14:29:46 -0500, Kenneth A. Christiansen wrote:
>is a broadband noise. I have a portable radio and find noise all the
way
>from 2 mHz to 20 mHz.
Some thoughts. First, this issue has been discussed on the RFI list, and
there are some RFI specialists who hang out there. A study of their
archive is in order.
Second, there are two basic ways the RF can be getting out of the washer
-- radiated directly from an unshielded cabinet, and conducted on the
power wiring. I suspect the power wiring.
The first solution I would try with that is to form a common mode low
pass filter on the power line. The easiest way to do that is wind as
many turns of the power cord as you can around a 2.4" #43 toroid
(commonly called an FT240-43), AND add bypass capacitors (around 0.05uF)
from both sides of the AC line to the ground, and make sure the chassis
of the machine is tied to this same ground.
So, coming out of the machine, there's the choke, followed by caps to
ground. That is, the caps are on the LINE side of the choke. AND there's
a bond that does NOT go through the choke between the washer and the
equipment ground at the outlet. This bond should be as short as
possible, because it can be carrying some of the trash, and that trash
can radiate.
As to the number of turns: you probably can't put too many on it. I
would aim for at least 7 turns, and try for a few more. What happens
when you do this is that the ferrite choke develops a broad peak in its
impedance that is actually a resonance between the inductance and the
stray capacitance, and with the resistance of both the wire and the
ferrite thrown in. With more turns, that resonance will move down in
frequency. I've been working with Mike, W4EF, on a research project on
these toroidal chokes, and will publish the work this fall. Mike's data
shows that anything from about 7 to about 10 turns is ideal for 3.5 - 20
MHz, and the greater number of turns will help things on 80 and 160.
If this solution doesn't make a dent, the trash is getting radiated. In
that case, I would call the mfr and tell them to fix it.
Jim Brown K9YC
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