Topband: Broadband vertical

[Tom Rauch]

> Barrie, W7ALW wrote:
>
> "A question occurs about how much of the 160 meter band
can be covered
> when shunt-feeding."
> ==========
> The bandwidth of a shunt-fed vertical is about the same as
a series-fed
> vertical.

Actually not unless the shunt system is a VERY thick or wide
conductor.

A shunt fed vertical has an additional resonant circuit
formed by the series capacitor and the reactance of the
shunt conductor. Say you tap at a 50 ohm resistive point.
Unless the tower is resonant above 160 meters by just the
right amount, you'll need a series C to cancel reactance of
the shunt conductor.

Very roughly the Q of the shunt system is a bit higher than
the reactance of the capacitor over the resistance. If you
had a 150pF series cap  reactance would be about 580 ohms.
Assuming 50 ohms resistance Q would be a tad higher than
580/50 or 11.6

That additional Q would cause a substantial reduction in
bandwidth, as well as large voltages across the capacitor at
higher power. With a thin drop wire the Q of the gamma
almost always dominates the overall Q (and bandwidth) of the
system, making the thickness of the tower much less
important in determining BW.

The bandwidth actually becomes greater when the tower is too
short, and the reactance of the tower allows a larger C (or
maybe no C at all) in the drop wire.

To improve bandwidth, you can take steps that increase the
amount of capacitance used in the shunt feed capacitor. That
would be a larger diameter drop wire or a cage of drop
wires. Rather than messing around switching components, I'd
use a multiple conductor drop system with wires spaced as
far apart as possible.

73 Tom