[Antennas] measuring SWR

[Pat W_]

A Jpole should require a Balun at the feedpoint due to the unbalanced nature 
of the feed.  Adding feedline to any unbalalnced system will show a change 
in resonance because the feedline has become part of the antenna!.  In fact, 
a  change in resonance when adding feedline is a test for the need for a 
balun.  (Jpoles also show a change in impedance due to the height, but, that 
will change the SWR when changing the height, not Resonance).

Add a current Balun(1:1).  Test for resonance at ground level, add some 
multiple of 1/4 wave of feedline and test for resonance again.  If resonance 
doesn't change then the Balun is working and it's safe to put the antenna up 
in the air.

Pat W0OPW

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