Topband: Ferrites and verticals

[Jim Brown]

On 8/11/2014 12:34 PM, jcjacobsen at q.com wrote:
> Is a choke needed on the feed line

The function of a common mode choke at that point is to prevent the coax 
from becoming a radial.  Your radial system, whatever it is, also does 
that, to the extent that it's a good radial system.  32 radials 32 ft 
long is not a great radial system for 160M, but it isn't terrible 
either. :)  The extent to which a choke might help depends on the length 
of the feedline and what problems if any you are experiencing.  Common 
mode current on the coax can contribute to RX noise, and can put RF in 
the shack.  Since simple verticals aren't usually very quiet on RX, a 
choke isn't likely to help with the noise.

>   and if so, how many #31 snap on ferrites are needed for proper function.

A single turn through  #31 and #43 ferrite cores is useless on 160M 
because it provides only a small value of inductance. The problem with 
inductance at that point is that a feedline shorter than a quarter wave 
(treating it as a long wire antenna, so Vf = 0.98) is capacitive, so 
inductance simply cancels some of the inductive reactance and the 
feedline current increases.

A good choke for 160M needs a lot of turns through a #31 core. That 
means either small coax through a medium size core, or multiple big 
cores with big (RG8/213) coax.

See specific winding recommendations in k9yc.com/RFI-Ham.pdf Chapter 8 - 
Choke Cookbook, and study the preceding material to understand how it 
all works.

73, Jim K9YC