Topband: Coax Choke Balun

[Jim Brown]

On Wed, 11 Nov 2009 09:29:51 -0600, K4SAV wrote:

>The choke balun will work (depending on how much impedance you 
really 
>need).   There are some advantages to using a ferrite core,

There is a MAJOR advantage to using a ferrite core -- it makes the 
choke RESISTIVE rather than inductive. A feedline shorter than a 
quarter wave looks capacitive in the common mode circuit. Adding an 
inductor in series (the coil of coax without the ferrite) tunes out 
that capacitance, which reduces the impedance and INCREASE THE 
CURRENT. 

A ferrite choke with a sufficient number of turns and cores can add 
5,000 ohms of resistive impedance to the common mode circuit that 
can't be cancelled by the length of the line. 

> but obtaining the core(s) is sometimes a problem. 

Naw, you just buy them (but not from the vendors who advertise in QST 
and sell at rip-off prices). See the list of Fair-Rite part numbers 
(the actual manufacturer) and list of vendors on the Fair-Rite 
website and in my tutorial, previously cited. The Cookbook lists the 
number of cores and turns needed for each band. 

Over a period of about five years, I've helped organize group 
purchase of cores for nearly a dozen ham clubs. These parts are very 
reasonably priced if you buy them right. 

73,

Jim Brown K9YC