Topband: shunt fed tower - question?

[Tom Rauch]

> feed is a matter of increased circumference. They used NEC
4.1 and then went
> to field tests which showed that similar improvement in
B/W can also be
> obtained with a series fed  bottom insulated tower having
the same cage
> connected to the top and bottom to the tower.
>

That data has appeared in Ham publications for many years. I
measured a coaxial dipole antenna in the early 70's, when an
employee of mine started telling me how good they were. We
took one of his coaxial dipoles and cut the center conductor
at the middle crossing point. That made each half  just a
hunk of coax, and bandwidth remained the same. Unfortunately
the myth about folded elements has a deeper foothold than
the facts.

What we have to be careful about is the skirt fed antenna. A
larger shunt wire, when the shunt is series tuned, does
increase bandwidth. This is  not just because it makes the
antenna look "fatter". It increases bandwidth because a
wider area shunt wire has less inductance per unit length.
This means we can use more capacitance, and the matching Q
(and matching component voltage) is reduced.

This effect is like the effect of using a wide copper strip
as a ground lead. The wide copper strip has more RF
resistance than a same surface area round conductor, but
because the current-carrying edges are further apart it has
less inductance. Less flux lines enclose the conductor than
would enclose a round conductor of the same area, so
inductance is less.

The same effect will occur in a shunt fed tower's drop lead.
If we add a second conductor spaced well away from the
first, the unit-length inductance is reduced and BW is
increased. At the same time voltage across the capacitor is
reduced, and bandwidth is increased.

73 Tom