----- Original Message -----
From: Tom Rauch<mailto:w8ji@contesting.com>
To: Topband@contesting.com<mailto:Topband@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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