Topband: Coax Choke Balun

[K4SAV]

Bill Gillenwater wrote:
> I need some suggestions, or the "go-ahead". I am getting ready to wrap 30 turns of RG213 around a 14" piece of 8" round PVC, for use as a choke on my inverted L.  Is there a better way? 
The choke balun will work (depending on how much impedance you really 
need).   There are some advantages to using a ferrite core, but 
obtaining the core(s) is sometimes a problem. 

30 turns of RG213 should take up about 12.7 inches of length (since 
RG213 is 0.41 inches overall diameter).  That should leave you 0.65 
inches on each end for anchoring the coax.  That's a little close so you 
may end up with only 29 turns or a little longer form. 

Assuming your PVC form is really 8 inches in diameter (an 8 inch PVC 
pipe should be close to 8.8 inches OD), with 30 turns that should give a 
low frequency inductance of 99.1 uH, a distributed capacitance of 20.2 
pf, and a series resistance of 5.3 ohms at 1.8 MHz.  Impedance at 1.8 
MHz should be about 1500 ohms.  The coil resonant frequency should be 
about 3.56 MHz.  Note: Operating close to the resonant frequency of the 
choke is OK.  It will be a very high impedance at its resonant frequency.

A choke may, or may not, solve your problem.  Usually when you have RF 
on the feedline from an inverted L it's because there are not enough 
radials.  It could also be due to a resonant length of feedline and 
breaking that up with a choke can help.  Although if you have a few 
elevated radials, then RF on the feedline should be expected and a choke 
should help.

Jerry, K4SAV