----- Original Message -----
From: Tom Rauch<mailto:w8ji@contesting.com>
To: john battin<mailto:jbattin@msn.com> ;
Topband@contesting.com<mailto:Topband@contesting.com>
Sent: Thursday, May 12, 2005 9:54 PM
Subject: Re: Topband: Cone of silence
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.>>
When the antenna wire is just hanging there it is so
reactive it could have a very wide range of matching loss
due to the complex interaction of the reactance and
transfomer reactance. When the T is added with the resistor,
we now have added 450 ohms of intentional loss resistance at
the same time we eliminate most reactance. I wouldn't even
try to guess what happens to mismatch loss without looking
at the impedance back into the transformer (the transfomer
might not be bilaterial, especially when misterminated
severely) and receiver and knowing the antenna impedance. We
can be pretty sure the radiation resistance of the vertical
wire increased by a factor of three or four due to current
distribution changing from triangular to nearly uniform, but
then we added losses with the resistor.
THESE issues are exactly why I took two months in the summer of 2003 to make
careful measurements before re-doing my beverages. The impedance into the
transformer was exactly 400 ohms ... a W8JI transformer with a pad between it
and the receiver to stabilize the input impedance of the receiver. The point
here is that if the signal goes down by 20 db. when the beverage wire is
disconnected, then that level represents the minimum level of signal that will
be picked up by the vertical feed. So if someone is trying to null a signal or
noise by more than 20 db. this is a major factor.
As we add wires to the top of it, we increase the current in the vertical
wire and increase its' receiving capability. I tried a 1/4 wave single wire
running toward and away from the source and the signal went up 15 db. (looks
like short unterminated beverage). When I ran the wire at right angles to the
source, the signal went down about 11db. to where the level was about 4 db
higher than the vertical alone. The signal was not stable, and varied a lot as
I walked around. When I added the other 1/4 wire to make a T, the signal
stabilized at about 3db. over the vertical feed or about 17 db down from the
beverage starting point. At this point I was convinced the signal was not being
picked up by the T. These measurements used the 400 ohm resistor between the
vertical and the T to simulate the impedance of the beverage when it is
properly terminated. Without this resistor the low impedance of the T would
cause more top loading of the feed wire.
But even if you disregard these T measurements, the fact that the vertical
feed is only 20 db down even without top loading makes it a major contributor
the the pattern of a beverage and getting back to the original question of COS,
a major factor in that as well.
OH, by the way, after all these measurements I converted some of my beverages
to elevated T terminations. The F/B ratios were much better, but they did not
receive one tiny bit better. In my location, the noise is usually
omni-directional, and hence the width of the front lobe dominates performance.
John K9DX
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