[Elecraft] Coiled Coax Choke

[Fred Jensen]

On 10/20/2014 7:28 PM, Dauer, Edward wrote:

> I am thinking of adding a coiled coax common-mode choke to my system.
> Presently it goes K3-KPA500-KAT500 then 100 ft coax to the base of an
> inverted Vee¹s center support pole (collapsible fiberglass), then 30 feet
> up the pole to a 1:1 balun feeding a fanned 10-15-20 Vee.  The idea, for a
> couple of reasons, is to put the choke at the base of the pole, something
> I haven¹t seen discussed in the literature.

Are you having common-mode RF problems in your shack?

Two questions I am wondering
> about:  1. The choke would be 100 feet from the tuner and about 25 feet
> from the antenna/balun.  Probably not as good as having it right at the
> Vee, but is that compromise worth while?

Maybe, every installation is unique.

2.  For appearance sake I am
> thinking about winding the coax around the base of the support pole.  It
> is 2 inches in diameter but I could slip a 3 inch PVC over that section to
> increase the diameter.

If you're having RFITS problems ["RF In The Shack"], there are better 
and far less visually obvious ways to suppress that using common mode 
ferrite chokes than winding up your coax around anything.  If you don't 
have RFITS problems, "nothing" might be the best course of action

> Anyone know an algorithm for determining the
> number of turns in the winding if I do that?

Air inductor equations are in the ARRL Handbook and ARRL Antenna Book. 
If you really need to do this, I'll suggest you look at the publications 
from Jim, K9YC, at audiosystems.com first ... he pretty well has this 
subject knocked.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 2015 Cal QSO Party 3-4 Oct 2015
- www.cqp.org