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[Ron Youvan]

Hi:
  Tom said:
snip
A J-pole is not as ground independent as one might think or read.  It is an
end-fed 1/2 wl antenna , fed by a 1/4 wl stub. Because the end impedance of
a 1/2 wl is not infinite, but rather some finite impedance between a few
hundred and a few thousand ohms (depending on conductor diameter), the
terminal current of the balanced line driving the actual radiator are not
balanced. This causes the balanced feedline section of the J-pole to have
common mode current adding destructive radiation to the pattern that raises
the wave angle and lowers gain.
snip
  The impedance transforming stub can be bent at any angle to the radatior, it seems to make no difference to me.
  True the feedline does "adding destructive radiation to the pattern ", but if you put a 1/4 wave sleeve balun over the feedline, (ASSUMING COAX) connecting it to the bottom of the "J" with the open end facing the shack, there is no longer a problem.  Works for me!
  If, as you sweep the antenna with a return loss bridge, you move the feedline around (with your hand) and it effects the responce (antenna tuning) you have feedline radation.  Adding the sleeve balun reduces this effect to NIL.
  Adding a ground screen can alter the antenna's launch angle.  An elevated end feed vertical antenna has a very desirable `to the horizon' launch angle.
  I will be constructing one 40 feet up, for 40 meters as soon as I can. (100' tower + 4 feet of a 10 foot pipe)

     73  de Ron ka4inm@qsl.net    Best Regards  HAPPY '99
  VISIT my HAM WEB SITE at: http://www.qsl.net/ka4inm


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