Topband: Short, Fat Vertical

[Tom Rauch]

> I'm pretty much restricted to a maximum height of 40' for 
> any antenna I
> install on my covenant-restricted lot.  I have thought 
> about an inverted L,  but I
> also wonder if I can achieve a greater effective height by 
> using  a fatter
> radiating element, primarily using wires that are spread 
> apart.

The tallest effective height occurs with uniform current 
over the vertical length. A really fat radiator has high 
distributed capacitance and tends to increase current taper, 
which is not what we want.

> I've seen antennas that use a sharp V configuration or an 
> apex-down  triangle
> to apparently achieve greater bandwidth and perhaps more 
> electrical  height.
> I would imagine that increasing the angle of the apex 
> would  eventually
> create diminishing returns, as ground losses would 
> eventually  increase.

The area to concentrate on is the ground system. Once you do 
that, concentrate on the flattest capacitance hat you can at 
the top. There isn't any good reason to use a really thick 
radiator or a tapered radiator unless you need extremely 
wide bandwidth. Many of the things that increase bandwidth 
will also increase ground losses by increasing ground 
current.

> Also, what about multiple wires in a tall rectangle shape? 
> I've seen  some
> references to a three wire folded element in which the 
> center conductor is
> broken and fed, and the spacing is 1 foot between wires.

Contrary to folklore, that does nothing but change the 
feedpoint impedance. It won't reduce ground losses even the 
smallest amount and it won't be any broader in bandwidth 
than  feeding the same effective diameter cage as a single 
element.

> Any thoughts here?  I was thinking of combining this fat 
> vertical  element
> with some type of top loading wire (either an L or a T), 
> that could also be fat
> if that would be advantageous.  I've got a  few trees that 
> would enable me to
> hide this structure, but I can't get too  fancy.

More capacitance at the top is a good idea, so long as you 
don't move the current maxima past the center of the 
radiator.

> I've also thought of a sloper, as I have a mast on the 
> roof peak that is
> about 45' at the top.  The disadvantage here is that the 
> high voltage end  would
> be close to the ground and could cause some losses.

The disadvantages here are you can't make a good ground 
system on the roof and it is very difficult to feed without 
exciting the feedline shield.

I'm afraid there isn't any magic or free lunch. A good 
ground system is almost the entire problem, and anything 
else you do adds a lot of complication for no real advantage 
over a simple L or T made out of a reasonable gauge wire. 
The bulk of losses will always be in the ground system, and 
once you maximize the value of current distributed over the 
entire vertical area by moving the current maximum to the 
wire middle you will have done all you can ever do for that 
height radiator so far as radiation resistance goes. The 
thicker you make the vertical wire the more end capacitance 
you will need to place the current maxima at the center- and 
you won't decrease antenna losses while you will increase 
ground currents slightly.

You'd never see the difference between a fancy radiator and 
an Inverted L made from #14 wire the same height.

73 Tom