On Sun, 15 Feb 2009 17:58:56 -0800 (PST), Edward Sylvester wrote:
>I have a LW/Inv L that I will feed thru an auto tuner at the base,
>with radials. Was wondering if I should bother with forming a
>coaxial choke (12-15 turns at 6" diameter) before it enters the
>shack 30 feet away.
I coil of coax with no ferrite is a waste of coax -- it will look
inductive, the rest of the line will look capacitive, and the result
will be lots of feedline current. You DO want a choke on the coax
side of the tuner, but it needs to be 7 turns through five Fair-Rite
#31 1.4-inch i.d. cores (commonly advertised as FT-240-31 by those
mark them up 4X their cost and resell them to hams). If you're
feeding with RG8, you will need to wind the choke before you put the
coax connector on -- you can fit 7 turns through those cores, but
only with no connector attached.
For a tutorial on all the technical background on this see
http://audiosystemsgroup.com/RFI-Ham.pdf
73,
Jim Brown K9YC
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