[TowerTalk] Ferrite beads for common mode current

[Jim Lux]

At 06:48 PM 7/25/2007, jeremy-ca wrote:
>On higher HF and 6M simply winding all the turns in one direction adds a lot
>of capacitance which defeats the effectiveness.


I wonder if this can be quantified..

Say you have two pieces of coax side by side that are 1 meter long 
(about 1 foot loop diameter). Say the spacing is twice the thickness 
of the jacket.. call it 2 mm total?

Further, let's be pessimistic, and say it's a 1 cm wide plate 
separated by 2mm.  So we have

C = 8.85E-12 * 1 meter * 0.01 meter/0.002 meter  = 8.85E-12 * 5 = 
about 45 pF... that's pretty big at 50 MHz..  about 70 ohms impedance.

String 4 of those in series (as if you had 5 parallel coils) and 
you're up to about 300 ohms.


Interesting...