Topband: Tophat? Does it have to be at the top?

[George Stewart (VK5ALS)]

Hi Topbanders,

While we are milking this thread for what it is worth I would like to
throw in a closely related question I have been sitting on for a while.

Given that in a simple situation where one is constructing a dedicated
short antenna for 1.8Mhz that the best place for the top hat is right at
the top of the vertical section, how do you then calculate what is the
optimum radius/length of a top hat that declines from the horizontal? I
suppose another way to view it is what percentage of the vertical can
one afford to cover with the top hat.

>From memory I believe Tom, W8JI mentioned a situation where some MF
beacons had the top set of guy wires doubling as the top hat and these
nearly reached the ground. Someone else recently mentioned 20 to 25 feet
on a 60 foot vertical as a good compromise between cancellation effects
and added capacitance.

My plan is to erect a 40-foot vertical using 1 1/2 inch diameter
aluminium masting then to use the top set of four guy-wires declined at
45 degrees with a skirt wire connecting the bottoms together.

Is there a way of calculating the optimum length for this situation?  

At the risk of showing my ignorance - how do you arrive at the total
capacitance of this arrangement?

73

George, VK5ALS