On Wed, 29 Nov 2006 21:20:16 GMT, Owen Duffy wrote:

On Wed, 29 Nov 2006 15:24:37 -0500, John Ferrell
wrote:


For ease in modeling I have a 2*28 foot (56 ft overall) free space
dipole: This frees me from ground effects. I used #12 wire to simplify
wire intersections. I have NOT experimented with spacing and wire
diameters yet.

Keep in mind that the less your model resembles the real antenna, then
the less the results are applicable.

I am not sure that your dipole in free space is better or even as good
as modelling the vertical over perfect ground as a first step.

If you are using a 28' #12 wire vertical, then it makes sense to model
that, but if you are using a self supporting tube or mast of much
larger diameter, then you are modelling something else.

Gamma matches become impractical when they call for extremely small
tuning capacitors, so your challenge is to find a solution that uses a
practical tuning capacitor.

Start the search with a low tap point, equal diameter conductors, and
wider spacing. Move the tap point up to increase R. Check the tuning
capacitor required. Explore the sensitivity to changes in all three +
driven element length.

Owen

Here is a better set of numbers. The antenna(s) involved are strictly
lab experiments. I have not worked out the details of putting the
source where I want it with a vertical, it would be in the ground
plane. The objective was to get a "feel" for the shunt match problem.
With your guidance I think I am on the right path.

My early conclusions (subject to change!) is that I don't want to put
too many resources into a grounded vertical antenna. It will be easier
in the future to match an insulated antenna. If one already has a
grounded tower available it might make good sense to shunt feed it.
John Ferrell W8CCW