[Elecraft] Artificial RF Ground

[Jim Brown]

On Thu, 02 Apr 2009 17:49:26 -0700, David Gilbert wrote:

>I agree with K9YC that a coil of coax will normally do the job just 
>fine,

Note that I am NOT recommending "a coil of coax," I am recommending 
a coil of coax through a suitable ferrite core. BIG difference. A 
coil of coax is inductive (and not a big inductance). A feedline 
less than a quarter wave looks capacitive (as a radial or common 
mode circuit element), so a simple inductor (the coil of coax) can 
resonate with the line and INCREASE the current. Not a good thing. A 
feedline between 3/4 and 1 1/4 wavelength is also inductive. Also, 
velocity factor does NOT apply to the common mode circuit, but Rudy 
Severns (N6LF) has shown that elevated radials DO exhibit a velocity 
factor as they interact with the earth. Google to find his website 
for really excellent work on this. 

A ferrite choke is a low Q parallel resonant circuit. When wound to 
put its resonance at the transmitting frequency, it looks like a BIG 
resistor in series with the common mode circuit, and it blocks the 
common mode current, no matter what the length of the coax. 

As to WHERE the choke should go -- if you're short of radials and 
WANT the coax to be another radial, then put the choke at the shack. 
I have a lot of radials and the coax run to my 160 vertical is short 
(about 35 ft), so my choke is at the feedpoint. BTW -- think about 
it -- I'm running legal power to that vertical, which is only 25 ft 
from my operating position, and I've NEVER had even the slightest 
symptom of RFI in my shack on 160M.  

I do NOT see coax laying on the ground picking up any more induced 
current than if it were up in the air. Remember that the EARTH is a 
LOUSY CONDUCTOR, and the object of radials is to put that current in 
low loss wire rather than lossy earth. 

73,

Jim Brown K9YC