Try it. Yes,its not perfect, but its as close as computing the C of the
short antenna and computing the inductance required, and then the coil
dimensions and the computation took much less time. If the mutual
coupling between the turns shows up as too much inductance then its a
lot easier to take off turns than to add turns. Strange thing is that a
coil alone will resonate in quarter wave or halve wave mode at very
close to the wire length's wavelength.
If wound on a 6" diameter form, that would take up about 20 inches per
turn, 77 feet is 924 inches, or 46 turns. The coil loss will be low if
the turns are spaced about one wire diameter.
The trouble with computing the antenna C is that the computation is
likely to neglect that the antenna is a fraction of a wavelength, e.g.
will compute for 1 KHz, but the length of the antenna makes that wrong
at 1.8 MHz. At that length, the length of the antenna has a bigger
effect on C than the diameter. And if the coil is too much a simple
series capacitor in the tuner will bring it up in frequency and the
capacitor is likely more robust at handling current than the coil of the
tuner.
73, Jerry, K0CQ
On 8/15/2010 9:54 PM, Ken Brown wrote:
Fundamentally the 43 foot vertical is way too short for 160 and needs
a BIG loading coil containing about 77 feet of wire, and forcing that
to be done in the tuner will tend to smoke the tuner.
You are correct that a 43 foot radiator is way to short to be resonant
on 160 meters, and either an added length of wire or an inductance can
compensate for the capacitive reactance to make it a non-reactive,
resonant load. I do not think that you are correct to say that a loading
coil would require 77 feet of wire to provide the necessary inductive
reactance. Because of the coupling (mutual inductance) of the multiple
turns of the coil, the necessary inductive reactance will be had with
less than 77 feet of wire. If it is not wrapped into a coil, then 77
feet would be about what you would need.
DE N6KB
|