Topband: short vertical

[Tom Rauch]

> Herb's comment regarding cancellation is real.
> You could raise the base impedance, and wind up
> with a nicely matched ineffective antenna.
>
> I would shorten the wires up to maybe 15', and run
> the perimeter wire between them.

Remember too that the real advantage of top loading is not
replacing the loading coil. The real advantage is current
distribution becomes more uniform. That is accomplished when
capacitance at the top dominates distributed capacitance in
the vertical section below the hat.

When a short antenna has uniform thin cross section, it has
nearly triangular current distribution. When the top
capacitance dominates the system, it has nearly uniform
(constant) current in the vertical section. Radiation
resistance (which is not the same as the feed impedance) can
quadruple when the antenna changes from triangular to
uniform current.

Making the sloping top wires too long or slope too much can
actually hurt the system. That's why adding a perimeter wire
like Herb suggested is a good idea, as is making the hat
wires as horizontal as possible. You could make the antenna
resonant on the low end of 80 meters and switch in a loading
coil at the base for 160. If I was tight on space that's
what I would do. There would be very little change in
efficiency if you used a good coil at the bottom as opposed
to  a coil at the top, provided the hat dominates the
distributed capacitance of the antenna.

73 Tom