Topband: Simple motor driven telescoping aluminum vertical for

[Tom Rauch]

low bands?
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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 
another.

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
W8JI@contesting.com 


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