Topband: Mysterious Noise - Final

Jim Brown jim at
Sat Nov 11 17:07:15 EST 2006

On Fri, 10 Nov 2006 13:58:19 -0700, Tod-ID wrote:

>I certainly hope that what I have written will make noise location and
>resolution easier for another reader.

Heres a solid step to FIXING the problem once you find it. It works for 
most 120VAC noise sources that are carried on or radiated by the AC line. 
This fix is specifically tailored to 160-40 meters, but will also help on 
the higher bands. 

1) Find a length of 2-conductor twisted pair rated for power line voltage 
long enough to wind 14 turns around a #31 FT-240 size ferrite toroid. The 
size (gauge) of the conductors only needs to be big enough to carry the 
load current of the noise source. For example, must professional IEC 
power cords are #18. If you cant find a #31 toroid, #43 will work almost 
as well on 40, but will have about 6 dB less noise suppression on 160. 

2) You have just built a common mode choke. Put that in series with the 
power line that feeds the noise source. 

3) Add a capacitor of 0.33 - 0.47 uF across the line (that is, from hot to 
neutral) on the side of the choke that is closest to the breaker box. Do 
NOT add capacitors to ground (the green wire). Doing so would cause the 
noise to flow on the green wire, which would radiate it just like any other 
antenna and defeat your filter. 

4) Carry the safety ground past the filter youve just made. 

5) To make the noise suppression more effective above 10 MHz, add a second 
choke in series with the first  one, as well as a second capacitor of at 
least 0.1 uF. The second choke should have 6-7 turns. #43 is just as 
effective as #31 above 7 MHz. 

6) Install this choke as close to the noise source as possible. The AC 
wiring between the choke and the noise source can radiate the noise. 

The capacitors you use MUST be rated for AC line voltage. I use caps rated 
for 250VAC / 600 VDC. 


Jim Brown K9YC

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