Topband: Maximum size of capacitance top hat?

[Tom Rauch]

> "More capacitance at the top is a good idea, SO LONG AS 
> YOU DON'T MOVE THE
> CURRENT MAXIMA PAST THE CENTRE OF THE RADIATOR".
>
> Does this apply more to the situation where the antenna is 
> in a "T" or
> inverted "L" configuration or can one visualize a capacity 
> top hat in the
> same way with regard to the current distribution?

Someone will have to model your vertical George, but if you 
don't move the antenna much below resonance on the operating 
frequency you are safe. The 20 foot hat should be no problem 
at all. Are you planning a coil right below the hat?

What I was speaking of is maximum radiation resistance. 
Maximum radiation resistance for a given height *vertical* 
occurs when the current is distributed over the length of 
the vertical as uniformly as possible. Since we can't 
generally control current taper over length, this means 
placing the current maximum in the center will maximize 
current over the entire height.

We apply a given amount of power and what isn't turned to 
heat will radiate. In fractional wavelength antennas a 
longer spatial area of unopposed current means less current 
is needed to radiate the very same power. Bunch the current 
up at one end or make the "in a straight line" distance 
smaller for a given applied power, and current goes up. 
(That means radiation resistance goes down.)  The same thing 
happens when something cancels or fights the radiation with 
opposing radiation--current goes up for the same radiated 
power.

Another factor enters when the ground system provides a very 
lossy terminal for the feed system. In a case like that we 
might want to move current up higher than the center to 
reduce current flowing in the lossy counterpoise connection. 
This could be true even if we pay a penalty in radiation 
resistance, so long as the increased feedpoint impedance 
offsets the increase in overall current up higher in the 
antenna.

The entire thing is complex but with a short radiator like 
the one you propose performance differences caused by moving 
current maximum around in the vertical are small. The real 
key is getting the best possible ground system installed and 
keeping losses in the loading coil down. When the antenna is 
very short there isn't much current taper to deal with.

There isn't much magic with any of this. It all comes down 
to a good ground system and then not doing anything grossly 
wrong with the radiator.  Like making it too thick or using 
a poor loading design. There isn't any point in 
over-thinking the problem, just do the best ground you can 
and don't use a really bad loading scheme.

73 Tom