Jimmie D wrote:

...
I thought the base coil would be for impedance matching, maybe he is tapping
up on the coil to find a 50 ohm point.

That is exactly the way I interpreted his description/pics; and, it's
exactly the way I implemented it (a modified gamma match--implemented in
helix form?) The wire-length/inductance is a 1:1.4 ratio between bottom
helix and upper loading coil--with the 1.4 of the length in the upper
loading coil. An adjustable 20" length of conductor is used between
lower and upper coils. Top whip is a 20" length also. (10 meter design)

From what I estimate, it ended up using, VERY CLOSE! TO, a computed
half-wavelength of wire at 28.050! And, I mean within' 1-3 inches! I
honestly did not expect that ...

Tap ended up almost dead center in the bottom helix turns for a 50 ohm
match; a variable capacitor is in series with this tap point on the
bottom helix and used to tune out the inductance of the tap wire.

I kludged a var/cap together using two small sheets of light aluminum.
These sheets are rolled into tubes and made so one is a smaller dia and
slips very loosely into the one of a larger dia. I then cut some clear
plastic from a drink container, rolled it and use it as the dielectric
(withstands the 1-100 watts test signal) between the inner/outer tubes.
This makes a serviceable/usable var. concentric cap. Inner tube is
roughly the dia of a bic pen (~1/4 inch.)

#8 copper is used, except for the upper loading coil which is #10 copper.

Try one; prove me wrong; I dare ya!

Regards,
JS