[RFI] just what everyone needs!

[Cortland Richmond]

Andy,

The way I unravel this is to ask how a ferrite BEAD works. It 
concentrates magnetic fields along a wire -- adding inductance --and in 
a lossy medium too, where the changing field is partly transformed into 
heat.  This works even on a single wire.

Now imagine the ferrite has very low cross section, but extends along 
the wire for some considerable distance.  It will still work, but the 
volume of ferrite per unit length will be much less, and it will 
concentrate the field in each unit length much less (= lower inductance) 
as well. Given inductive reactance rises with frequency, the higher 
frequencies will be where a thin covering of ferrite material is more 
effective.

Will it filter in a coax? Maybe -- if permeability,  loss and thickness 
are right, skin effect isolates the center and and outer parts of the 
ferrite from each other, so they act like separate DM chokes. Otherwise 
the fields from shield and center conductor will pass through ferrite 
(with some loss) and cancel (or partly cancel) like a CM choke on a 
differential pair. Nothing is 100 percent and the change occurs at what, 
6 dB/octave?

On a ferrite covered single wire in an RF field... If the skin depth is 
small enough, fields won't much penetrate the ferrite, some being 
dissipated, some flowing lengthwise (being dissipated)  and the rest 
will be reflected -- shielding the wire without being a return circuit.

Oversimplified, perhaps, but it's just a 1st approximation.  And it's 
2348 EDT!

Cortland Richmond
KA5S

On 9/10/2012 2245, Andy wrote:
> This is where I don't follow, especially the "how". One way to get 
> attenuation is if it is like a coaxial xmsn-line and the Filter Layer 
> is in the signal's return path, and imparts a frequency dependent 
> loss. But that requires "grounding" (let's call it "terminating") the 
> Filter Layer on both ends. Or perhaps the Filter Layer acts similarly 
> to a ferrite bead (like the distributed equivalent of a bead) which is 
> not in direct electrical contact, but its presence around the wire 
> alters the magnetic field and changes the impedance. I am guessing by 
> your reply that perhaps this is closer to what is really going on here?