[TowerTalk] Vertcal Dipole

[Tom Rauch]

> wave length of braid can be pulled back over the jacket.
You have to
> strip the jacket back quite a bit more than a quarter wave
as the braid
> gets a lot shorter when expanded.  This also forms an open
1/4 wave stub
> where the coax comes out.  I've never seen any decoupling
used due to
> the configuration.  Think of an Isopole (TM), but used at
20 meters and
> the the shield takes the place of the top cone.

Roger,

The Isopole open sleeve  has a high open-end impedance. That
impedance does the isolation. When the dielectric inside is
mostly air and the ratio of inner conductor OD to outer
sleeve ID is large, the decoupling is very good it is an
excellent design.

Folding the braid back over a poor dielectric isn't good.
The outside can't be 1/4 wl long if the resonance of the
sleeve and highest choking impedance is to be obtained.
That's because VF inside the sleeve is low, and VF outside
is high.

The loss and limited diameter ratio limits choking
impedance.

The folded back braid would hardly have any decoupling.

73 Tom