> Screwdriver antennas present an interesting matching
problem. When fully
> retracted, typically 10 meters, the antenna is close to a
> vertical and has an impedance of about 35 Ohms. However
when extended to
> 80 meters it has an impedance of about 1 Ohm radiation and
say 5 Ohms
> coil. That is close to a 6:1 difference in the radiating
> this let's assume a constant ground from 80 to 10 meters.
It isn't that bad. The ground losses in my large F-250 Super
Cab long bed truck with the antenna mounted in a good clear
spot on the upper rail of the bed were around a dozen ohms
on 80 meters, and only a couple ohms on ten meters.
The inductor loss also is higher than you estimated. The
base impedance excursion is more like a 2:1 ratio or so. Of
course that will vary with vehicle.
I use a shunt coil for matching. The trend of the change
from band to band is such that the impedance step-up is less
on higher bands.
Capacitors move the opposite way, and require changing with
Just use a shunt inductor. The most you would need is two
values as a general rule.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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