[Elecraft] Snap-On Ferrites and RF Noise On 160M

[Jim Brown]

On Sun, 18 Apr 2010 19:00:19 -0400, Bill Coleman wrote:

>I found that putting a snap-on ferrite core around the power cord reduced 
>the spur to an S5 indication. A similar reduction could be had by placing 

>snap-on ferrite around the power output leads.

Study my tutorial on RFI and ferrites. 

http://audiosystemsgroup.com/RFI-Ham.pdf 

Most snap-on ferrites are designed for maximum effectiveness in the 150 MHz 
range. ANY "snap-on" ferrite is next to useless on 160M unless multiple 
turns are wound around it. If the wiring in question is radiating the trash 
as a common mode signal, you need enough turns around a ferrite core to 
place the very low-Q resonance of the ferrite choke near the frequency of 
the noise. This takes AT LEAST 10 turns through a relatively large #31 
ferrite core. 

I've successfully used some moderately horrific cheap switchers in my ham 
shack by putting good chokes on both input and output cables, and by adding 
RF capacitors across both the AC line and the DC line. You can also 
suppress differential noise from SMALL levels of DC current with a choke on 
only one conductor. High levels of DC current will saturate a choke on only 
one conductor, but most of us don't worry a lot about RX noise when we're 
transmitting. :)  

Another point. It has LONG been known that twisted pair wiring minimizes 
the radiation and pickup of noise, yet we use parallel wire cable ("zip 
cord") for power wiring in our ham shacks. That's plain stupid if we care 
about noise and RF pickup! The problem is finding twisted pair cables of 
suitable size.  

73,

Jim K9YC