[Elecraft] RFI from PC

[Jim Brown]

On Fri, 25 Jun 2004 12:13:54 +0100, Martin.Evans at simoncarves.com wrote:

>The lead has the normal
>split ferrite on it and I had another one in the box so tried that but with
>no difference. Is it worth me buying a large ferrite ring and trying to
>wind a few turns on it. 

YES!  Virtually all of the attention to emissions is centered above 30 MHz, 
because that's where most regulations begin in the US and in Europe. Those 
"usual ferrites" start working at about that frequency -- in fact, they are chosen 
because they do. 

When you wind multiple turns around a ferrite, the frequency of maximum 
effectiveness of that ferrite material will move much lower in frequency. A wire 
passed through a ferrite will look like series R and L, but the values of both R and 
L vary radially with frequency.  At low frequencies, it's pretty much all L, then R 
starts to show up while L drops and eventually it's all R.  Important caveat -- the 
choke only works to suppress COMMON MODE radiation (that is, radiation from 
current flowing along the length of the wiring). It won't help with anything radiated 
by the device itself. 

>From a suppression point of view, R is much better than L, because the L can 
resonate with the C of a short radiator and increase noise current.  Multiple turns 
around the ferrite moves this behavior down in frequency. Depending on the type 
of ferrite material and the number of turns, you can achieve between 1K and 2K 
ohms series impedance at any frequency between 500 kHz and 30 MHz, but also 
depending on the ferrite, the Q of the resulting choke can be fairly high. 

In general, the lower the design frequency of the ferrite, the lower the Q of the 
choking impedance at HF. The #78 material made by Fair-Rite is the lowest 
frequency of their torroids. The #31 material is a bit higher, then #43, and finally #
61 (the highest).  #31 is what you want for HF suppression -- you can get a broad 
enough choke to be effective on several HF bands. #43, the most common 
material, has a bit higher Q, so chokes wound around #43 cores will be effective 
over only one or two HF bands.  8 turns around a 2.4" OD #78 toroid will get you >
1K ohms on 160 meters and the AM broadcast band!  It takes a lot more turns to 
do that with #43 -- 8 turns on #43 will put the peak at about 5 MHz. 

I identified my Ethernet network as the source of some nasty birdies at 3511, 
10106, 10121, 14129, 14130, and 21052 kHz (there are certainly others, but I'm 
a CW guy). I can achieve very good (more than 4 S-units) suppression of these 
birdies with multi-turn chokes wound around #31 and #43 toroids, cylinders, and 
clamp-ons.  

I believe that most of the ferrite toroids sold by Amidon and Palomar are Fair-Rite 
#43. Some are #61. I don't know of any retail vendors for #78 or #31 materials, 
but there are many such products in the Fair-Rite catalog. I've been testing a 
bunch of them for some work I'm doing with RFI to audio systems. 

Jim Brown  K9YC