On Wed, 25 Apr 2007 12:11:13 -0400, Dan Zimmerman N3OX wrote:
>Any loop response to common mode currents on the feedline will ruin
>the pattern. Preserving the balance of the loop with respect to
>ground helps common mode rejection, and helps preserve the nulls.
This is an ideal application for a common mode choke on the feedline
snug up to the antenna feedpoint. As a starting point, follow the
guidelines in the Choke Cookbook below.
http://audiosystemsgroup.com/RFI-Ham.pdf
BTW -- I am in the middle of measuring some chokes wound with coax
(the data is for chokes wound with small diameter wire). When I'm
finished with that, I'll post that data and make some revisions to
the "cookbook." For now, it is sufficient to say that if you want a
choke to work above 80 meters, you need to wind it with all the
cores at one point in the coil of coax, and with the turns spread
wide outside the toroid. If you wind the turns very tightly together
(as coax baluns have traditionally been wound), the stray
capacitance will lower the resonant frequency and begin shunting the
choke above about 5 MHz.
73,
Jim Brown
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