Topband: Modelling/calculating the radiation efficiency of short verticals with a capacity hat plus whip?

[George Stewart, VK5ALS]

Hi Top Banders,

Can anyone give me some help in arriving at the change in radiation
resistance (efficiency) for a 40 foot vertical on 1.8MHz with a top capacity
hat 20 feet in diameter with 8 radial arms that then has a 30 foot "whip"
placed above the capacity hat?

I do not know how to start to work out a problem like this one!

Common sense would indicate that the shielding effect of such a capacity hat
would largely negate the effect of the "whip".

(The "whip" I have in mind is a fibre glass fishing rod blank with a copper
wire run up the inside of the rod).
 
I cannot help but think that going from say a 40 foot vertical to a 70 foot
vertical must make quite a difference especially as the Achilles heel of the
whole system is the inability to put in anything like a decent ground for it
to work against.

No doubt there is a combination of top hat diameter and 30 foot "whip" which
would theoretically outperform the 40 foot vertical with a 20 foot diameter
capacity hat?

73

George, VK5ALS