[600MRG] Vertical and loading questions.

[Kenneth G. Gordon]

First of all, my only present antenna is a 55' tall all-band single-trap vertical. It amounts to 1/2 
of a W3DZZ all-band dipole set on end. It exhibits a 2:1 or less SWR over most non-WARC 
bands, 80 through 10 meters. I have installed 20 radials under it, each is 32 feet long.

Some time aqo, I had an occasion to want to use it on 160 meters. I wound a coil out of #12 
solid copper wire 24 turns on a 4" diameter ceramic form, connected one end of the coil to 
the base of the vertical, and connected the coax center conductor to a tap on the coil.

The shield of the coax is connected at the antenna to the radial field.

I started out with the entire coil, but soon discovered by trial and error that I needed only 
some 18 turns, or about 28 uH, to get a 1:1 SWR on 1815 kHz.

This combination worked fine, but had a very narrow bandwidth. I would have to re-adjust the 
taps on the coil even when I moved only as much as 500 Hz.

Now I am trying to figure out why this worked this way, and I have come to the conclusion 
that the capacitance in the coax cable, 100 feet of RG-8, in some combination with the 
components in the antenna coupler, have something to do with it.

100 feet of RG-8 results in something like 2490 pfd.

Has anyone here run into anything like this?

Attempting to model this setup seems to show that if I were to use this antenna/feedline 
combination at 630 meters, I would only need something on the order of 50 uH of inductance 
at the bottom of the antenna.

When I calculated the necessary inductance to make my antenna alone resonant at 630 
meters, I came up with a value of 680 uH.

Anyone have a clue as to what might be going on here?

Ken W7EKB