[TowerTalk] Bazooka 1/4 wave balun

[Tom Rauch]

> One way is if you have an air core inductor that you can
measure the loss
> of (say you've got it resonated with a capacitor at 14
MHz). Shove the
> suspect hunk of plastic in the middle and see if the Q of
the coil
> drops.  If not, you're good to go.

Space winding an inductor reduces turn-to-turn capacitance
and increases inductor Q. Fill the area between turns in
with a dielectric and Q decreases.   That's because
capacitance and circulating currents increase. Knowing that,
how do we quantify how much Q loss is caused by an increase
in turn-to-turn capacitance as a dielectric is brought near
or against the turns and how much is lost through power
dissipation in the dielectric? How many people can measure
Q?

How does placing the PVC in a strong magnetic field test
characteristics as a dielectric in a strong electric field?

I think the only valid test would be to make a stub from the
PVC exactly as planned and measure the impedance of the
stub. Since the choking ability of the "Bazooka" is
determined by the operating frequency  impedance of the open
end of stub, and since anything that lowers the impedance of
the stub or increases losses in the stub formed between the
outside of the coax shield and the inside of the sleeve also
reduces choking impedance, even the best dielectric would
reduce effectiveness of the sleeve choke.

The sleeve needs to have largely air dielectric between it
and the coax shield. The coax shield has to be small outside
diameter compared to the inside diameter of the sleeve.

That's why the Isopole vertical with large open sleeve ends
and air dielectric works so well for reducing feedline
shield currents, and why folding the braid back for 1/4 wl
over a PVC coax jacket makes a very poor balun.

73 Tom