Bob McGraw - K4TAX wrote:
Why would one feed a dipole, generally considered a balanced antenna, with
an unbalanced coax? Should you do so, then you do need a RF choke at the
feed point to reduce current on the outside of the shield.
Actually, coax is the BEST feedline for a dipole, BECAUSE you can kill
common mode current on it VERY effectively with a ferrite choke, You
cannot do that with twinlead -- way too much leakage flux.
Virtually ALL ham antennas are unbalanced by their surroundings, even
before you put a feedline on them. Trees, buildings, ground slope,
varying conductivity under the antenna, slight asymmetry in the antenna,
etc. Put a parallel wire feedline on a dipole and there will still be
common mode current on the feedline due to those imbalances. That
unbalance creates common mode current on the feedline, the common mode
component of the current radiates, and by reciprocity, couples RX noise
onto the antenna (and fills in the nulls of your beam).
73,
Jim Brown K9YC
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