Chris,
Diameter of whip or loading rod is not critical. Enter average diameter
over its length.


If not stated, bandwidth is between 3dB points when transmitting. When
receiving, bandwidth is broader, say the 2 dB points, because the receiver
input resistance damps down the very high antenna Q.


Earth electrode resistance depends on size of vehicle and type of soil under
it. For a mobile home enter 3 ohms. For a volkswagon beetle enter 14 ohms.


A larger coil form will give a narrower bandwidth. Double length and height
and for the same inductance (fewer turns), coil Q will also be doubled. You
can't have high efficiency without high Q and narrow bandwidth.


If a flexible whip is used, to reduce overall height it can be curved
backwards in parallel with the vehicle's roof. This also allows fewer turns
of thicker wire on the helix and a higher coil Q with greater efficiency.
There will be an overall gain but not so great as if the whip is vertical
throughout its length.


Leave a gap equal to helix diameter between the bottom of the helix and a
metal roof.


The helix is in fact the L&C Z-matching coil. On 160m, in order to resonate
with its own self-capacitance, it needs a large inductance. There will be
several hundred turns. Therefore, to have a loss as low as possible it has
to be physically large. The only space available is in the antenna - and
that's why it's there.


Ignore silly irrelevant done-to-death arguments about whether the current at
one end of the helix is or is not the same as the other.


The most important dimension of short verticals is overall height. Next
comes physical size of helix. The fatter it is the better. Obesity is not a
problem. Only the wind.
----
Reg, G4FGQ