[RFI] rf current meter and ferrites

Jim Brown jim at audiosystemsgroup.com
Tue Sep 22 13:52:07 PDT 2009

On Tue, 22 Sep 2009 23:06:40 +0300, Kostas Stamatis wrote:

> I thought 
>tha the difference is between rig and ferrites.

It's a bit more complicated than that. Study my tutorial on ferrites 
and RFI.  


The short answer is that the cable you're putting the ferrite choke 
onto is an antenna, and the ferrite choke is a very low Q parallel 
resonant circuit that modifies the behaviour of that antenna. The 
characteristics of the resonant circuit, including its resonant 
frequency, are set by mainly by varying the number of turns and the 
material, and secondarily by varying the winding style. Near 
resonance, the choke looks like a high resistive impedance, so it 
divides the wire into two pieces. The part of the wire between the 
noise source and the choke radiates, the rest of the wire does not. 
Far from resonance (more than an octave or two), the choke looks 
either inductive (below resonance) or capacitive (above resonance), 
and can resonate with the cable to make it a better antenna. 


Jim Brown K9YC

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