 Original Message 
From: "Dan Sawyer" <dansawyer@earthlink.net>
To: <towertalk@contesting.com>
Sent: Wednesday, June 02, 2004 8:52 PM
Subject: [TowerTalk] matching to screwdriver antennas
> All,
>
> Screwdriver antennas present an interesting matching problem. When fully
> retracted, typically 10 meters, the antenna is close to a resonant
> 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 element. For
> this let's assume a constant ground from 80 to 10 meters.
>
> Is there any way to design a reasonable matching network for a
> screwdriver antenna??
>
One approach that has been used is to use a 4:1 (or other ratio, as
suitable) transformer, with capacitive coupling between primary and
secondary (either parasitic, or explicit). The idea is that as the frequency
goes up, the capacitance tends to dominate over the transformer. At low
frequencies, the impedance of the capacitor is high enough that it is
irrelevant.
Taking your example, a 4:1 would get the 80m impedance up to 24 ohms, not
too far from the 35 ohms at 10m. Some leakage inductance in the transformer
would effectively make the transformer "path" have a low pass, and the
capacitance would make the "straight through" path have a high pass, so it
might work out nicely (or, anyway, good enough).
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