Topband: Cone of silence

[john battin]

  ----- Original Message ----- 
  From: Tom Rauch<mailto:w8ji at contesting.com> 
  To: Topband at contesting.com<mailto:Topband at contesting.com> 
  Sent: Thursday, May 12, 2005 1:27 PM
  Subject: Topband: Cone of silence


  Just one quick comment.

  I can't think of an easy accurate way to measure radiation
  from a vertical antenna end. If the antenna is terminated in
  a center tapped "T", the T wire used as a "non-radiating"
  counterpoise wire picks up nearly as much signal as the
  vertical section!

  The only way I can think of to measure end effect would be
  to use perhaps a dozen elevated radials in the termination.

  Maybe someone can model the antenna both ways and compare
  the patterns.

  73 Tom

  I believe a good start is to leave the feed system with the matching transformer in place and just cut the beverage off, leaving only the vertical piece. In my case the signal dropped by about 20 db. I tried terminating the top of the vertical wire through a 450 ohm resistor to a T of 1/4 wave wires, and the signal increased  3-4 db. I believe the increase was due to the increase in current caused by the top loading. I should have tried a bunch of wires to make an elevated ground plane, but observing the insensitivity to the placement of the T wires made me believe they were acting as a counterpoise and not an antenna. Using only one wire instead of the two involved in the T however caused  high levels of pick-up by the reference that was very sensitive to placement.
  John 
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