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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
