[TowerTalk] Ferrite beads for common mode current

[Jim Lux]

At 06:04 PM 7/25/2007, Rick Karlquist wrote:
>Jim Lux wrote:
> > You can also use a larger diameter toroid and run multiple turns of
> > your coax through it. The inductance of the choke goes as the square
> > of the number of turns, so 5 turns through one core is like 25 cores
> > in a row (in an ideal world... there's capacitance and leakage L...
> > but still... )
> >
> >
> > Jim Brown's website http://www.audiosystemsgroup.com/ has a paper on
> > various ways to make RF chokes for feedlines (among other things)
> > with measured data over a wide range of frequencies. It's that
> > RFI-Ham pdf file referenced in recent posts.  5 turns on a #31 2.4
> > inch core looks like around 1Kohm Z.. (see page 12)  page 28 has a
> > photo of a mondo choke.  I use one very similar to this for
> > connecting a network analyzer to antennas (except I use smaller coax).
> >
> > Jim, W6RMK
>
>This approach is good on HF, but on 6 meters, with RG213, the trouble
>is that the minimum bend radius of the RG213 coax forces rather large
>loops.  When the total length of coax is too long in terms of wavelengths,
>the current is no longer of uniform phase, and the inductance no
>longer goes up as N squared.

Good point, and that's probably why the measured data shows less 
improvement for the higher frequencies (as well as the other 
issues).  That's the great thing about (the other) Jim's paper, he 
has all the measurement data for typical arrangements.




Jim, W6RMK