Topband: Simple motor driven telescoping aluminum vertical for
Wed, 18 Jul 2001 12:34:05 -0400
X-Sponsor: W4AN, KM3T, N5KO & AD1C
Hi Dave and all,
Just to highlight something important...
> bandwidth is 12 kHz. This can be fed with a 1:2 or 4:1 balun. The best
> thing electrically would be to top, or center load, the 40 foot mast with
> a top hat. This is quite a job mechancically though. My last 160m
> antenna was a top loaded 22 foot mast GND mounted. Efficiency was only 10%
> but I had good results on the air.
If the top hat is large enough, there is absolutely no reason to
place the inductor anywhere above the base of the antenna.
Current distribution will be essentially the same, and uniform
throughout the structure, no matter where the loading inductor is
placed if the hat is reasonably large.
The governing "rule" is the hat must have a several times more
capacitance than the distributed capacitance of the structure.
Once that criteria is met, current is essentially uniform and
radiation resistance maximized no matter where the inductor is
placed. This also means you can *decrease* system performance
with a fatter radiator section and a given size top hat!
We also have to be careful estimating efficiency when dealing with
low radiation resistance systems. Losses are never uniformly
distributed in the system because current is not uniformly
distributed, and very subtle changes in current distribution or
resistance in certain areas can make profound differences in
performance. Several ohms of addition loss at one point may not
affect the system at all at one point, yet destroy performance at
If I were building a short transmitting vertical, I'd use a hat large
enough to dominate system capacitance and simply place the
inductor at the bottom, using a series and shunt L to cancel
reactance and match the system. That would offer the best
performance and easiest mechanical construction, and allow the
antenna to be used on multiple bands....with an L network up to
where the antenna is about 5/8th wl tall.
It would work better than taller "whip like" antennas with coils up
high on the antenna, have broader bandwidth, be easier to tune,
more efficient, and handle more power. Again, there is no reason at
all to put the inductor up above ground if there is a properly sized
hat. It is a disadvantage to do so.
73, Tom W8JI
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