[TowerTalk] Ferrite beads for common mode current

[Jim Lux]

At 01:58 PM 7/25/2007, n7ka at comcast.net wrote:
>Getting ready to install another 6M yagi here and building another 
>for a friend.  Want to use a choke on the feed line which will be 
>either RG-213 (MIL-C-17 compliant) or on LMR-400UF (super flex).  In 
>all cases I want the capability for 1500W.  I do not like the idea 
>of additional connectors so my plan is to but the beads on the line 
>at feedpoint of the cable that goes down to the hardline just below 
>the top of the tower.  Would Fair-Rite 2643102002 be the correct 
>choice?  The id at 0.505 inch is large enough to fit over the 
>RG-213/LMR-400.  Or do I need another mix and how many beads.


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