On 11/14/2010 9:48 AM, Dr. Gerald N. Johnson wrote:
> Sometimes we want to FORCE that voltage balance, sometimes we just want
> to limit feed line shield current on the outside of the line.
It has long been W7EL's contention that you CANNOT force balance in an
antenna system by forcing voltage balance on the line, and I firmly
agree with him. Balance in ANY circuit is defined by the balance of
IMPEDANCE, and the balance of the antenna is usually the most
significant part of that.
The primary concern with balance in an antenna system, INCLUDING THE
LINE, is minimizing common mode current on the line. We care about this
because we do not want the line to radiate to our neighbor's living room
or receive noise from his computers, and because we do not want that
common mode current in our shack. Indded, the primary function of a GOOD
choke at the feedpoint (or down the line below a matching section) is
to disconnect the feedline from the antenna from a common mode point of
view.
For a tutorial discussion of these issues, see the Power Point on
Coaxial Chokes that's on my website, and also W7EL's writings, which
preceded my work by nearly three decades.
http://audiosystemsgroup.com/publish.htm
73, Jim K9YC
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