[RFI] RFI suppression in alarm system

Jim Brown jim at audiosystemsgroup.com
Thu Oct 22 17:36:26 PDT 2009

On Thu, 22 Oct 2009 15:25:52 EDT, W5OT at aol.com wrote:

>A  G€œrule-of-thumbG€™ says that impedance of 1000 ohms or higher will 
>usually provide  good RFI suppression.  Below is a  table of impedance 
>measurements across the 160, 80, and 40 meter  bands.


Are these common mode chokes or differential mode chokes?  You can 
easily wind your own common mode chokes to cover three harmonically 
related ham bands by winding the right number of turns around a Fair-
Rite #31 2.4-inch o.d. toroid. As you have correctly noted, you can put 
several chokes in series, each tuned to a different part of the 
spectrum. In general, a single choke can cover 160-40M, and you'll need 
a second choke for 30-10M.

See measured data for chokes wound on various Fair-Rite toroids in 

In general, chokes kill common mode current and capacitors across the 
line kill differential mode voltage. Even the best common mode chokes 
have some leakage inductance (because they are not perfectly balanced), 
so a small capacitor across the line works with the remaining inductance 
to form a low pass differential filter. 

Also in general, if the interconnect wiring is longer than about 1/10 
wavelength, you may need chokes and caps on both ends of the line. If 
the line is nearly 1/4 wave, a single choke in the middle and caps on 
both ends my be enough. And remember not to make the cap so large that 
it affects the desired signal. 


Jim Brown K9YC

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