Cecil Moore wrote:
At whatever
frequency causes the stinger to be 1/8WL, measure the
impedance. That will be fairly close to the characteristic
impedance of the stinger at the measurement point.

As a data point, using EZNEC's VERT1.EZ 40m vertical,
the feedpoint impedance at 3.6 MHz is 6 - j356 ohms.
That would make the Z0 at the feedpoint around 360
ohms and Z0 no doubt increases with distance above
ground.
--
73, Cecil http://www.w5dxp.com