Topband: Shortend Vertical model questions

[Bob Tellefsen]

Tony
I'll coat-tail on Tom's comments.

I've gone through somewhat the same exercise you are.
I now use a vertical 40 ft tall with a cage structure.  That is, the
vertical section
is three #17 wires in parallel, with a triangular cross section.  All wires
are 1 ft
apart and held there with stiff wire spacers.  I use a top loading coil of
1/8" diameter copper tubing, as it is lighter and cheaper than copper wire
of the same size.  For the top loading wires, I use five 30-ft long #24
stranded wires.  This is my Mark III antenna, a definite improvement over
previous efforts.  The cage structure is my attempt to achieve a useful
bandwith in a heavily loaded antenna.  I cover the bottom 50 kHz of the band
within a 1.8:1 swr.

You asked why more inductance is required for top loading.  Consider.  If
you used a less-than full length vertical element with a base loading coil,
you have the capacity of the entire vertical section to ground resonating
the base coil.  As you move the coil up the vertical section, there is less
antenna above it, thus less capacity to ground.  Resonance demands more
inductance then.  At the top, there is almost no capacity to ground other
than from the coil itself.  Hence, very large inductance needed.  Now, add
the capacity hat.  With more capacity, you need less inductance.  Carried to
its limit, with the right amount of top-loading capacity, no inductance at
all will be needed.  This is the ideal, but not all of us have the
real-estate to achieve it.  Top wire loading with some top inductor is
efficient, easy to do, and works very well.

As a point of reference, I'm a QRPer, and run 5 watts all the time.  I have
worked a number of east coast stations, including Tom, in the past several
160m contests.

Hope this helps a bit.

73, Bob N6WG
Newark, CA on SF Bay