----- Original Message -----
From: <k6se@juno.com>
To: <W8JI@contesting.com>; <topband@contesting.com>
Sent: Tuesday, May 28, 2002 10:52 AM
Subject: Re: Topband: Shunt feeding a tower with side mounted yagi
> Tom, W8JI wrote:
> 'While "shield" may be a poor choice of words, a large yagi or yagis
> mounted low on a shunt-fed tower can have a detrimental effect on system
> efficiency if they are large enough or enough of them. I prefer to think
> of it as "robbing" the upper part of the antenna of current.'
> ==========
> Let me restate my previous statement -- "Contrary to the belief of many,
> a beam located at a lower level on the tower does not shield any top
> loading in the form of other beams which are above that level." -- I
> indeed did mean "shield", because that is the term used by many of the
> "many".
> I also agree with Tom that 160m current distribution could be altered
> such by the low-mounted beam to be detrimental.
> 73, de Earl, K6SE
_______________________________________________
Shielding & robbing both indicate what is happening. The RF current is just
looking for the path of least resistance. So the impedance of each element
dictates what percentage of the available current that element is going to
rob, or shield from the other element(s).
An example is my 60 ft tower with beam as top load. This combo resonated as
a vertical at 2.4 mhz. I added a 20 ft whip of aluminum tubing above the
beam for a total of 80 ft height, resulting in very little lowering of
resonant frequency. The whip impedance was very high as compared to the
beam impedance.
When I rigged the same 20ft whip with 80+ ft of #12 wire attached to the top
and zig-zagged up and down 4 times, 3 ft out from the whip, the impedance
dropped and the linear z-z topload soaked up the current. The tower then
resonated at a center freq of 1.850 mhz, with a wide 2:1 bandwidth.
That beam is still robbing some of the vertical's current & producing very
little desired radiation, so the next step may be to totally isolate the
beam from the tower & further increase the size of the topload, to maximize
that current curve all the way to the top.
73/dx, om Doug / NX4D
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