[TowerTalk] Ferrite beads for common mode current

[Rick Karlquist]

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.  If you are planning to do this, you
should measure the impedance of the choke at 50 MHz before installing
it.

Rick N6RK