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[TowerTalk] Using Screwdriver Antenna as a Base Antenna

To: <>
Subject: [TowerTalk] Using Screwdriver Antenna as a Base Antenna
From: (Jim Lux)
Date: Sat Jun 21 17:09:45 2003
It will work better than it does on your car, but not as well as, say, a
dipole.  It is, after all, physically short, with a loading coil. The
loading coil does have some loss.  You're essentially looking at a vertical
dipole with the center 9 feet off the ground.

I've fooled around a bit with mounting a screwdriver on a tripod made from 3
painters extension poles...

The impedance is a bit funky in that situation, however, I suspect that
fooling with the length of the whip on top would find something that is a
happy medium.  The usual screwdriver matching box is a 4:1 transformer that
matches the low impedance of the screwdriver to 50 ohms.  The problem comes
in when you mount it on a real good ground plane (i.e. that metal roof).
It's physically short, so the radiation resistance is quite low.  The
inductor tunes out the capacitive component, but doesn't act as a
transformer (unlike, say, a "shorty 40"), so you're left with an antenna
with a real low resistive impedance.  The loss resistance of the whip, coil,
and your car actually helps the match, by bringing up the feedpoint
resistance (kind of like how the MAXCOM miracle match box worked...)

Enough analysis already... in the time it takes you to read all the emails
you'll get, you could just climb up on the roof and try it. If it works,
great, if not, ....

----- Original Message -----
From: "K" <>
To: <>
Sent: Saturday, June 21, 2003 1:50 PM
Subject: [TowerTalk] Using Screwdriver Antenna as a Base Antenna

> I have a hf mobile station using the Icom 706 MKII G
> and an Screwdriver antenna.  Seems to work great.  I
> have been thinking about mounting another Screwdriver
> antenna on top of my 10x20 foot and 9 ft high metal
> top garage in the middle.  I was wondering how well it
> might work 80-10m without any other radials?
> All comments or suggestions are greatly appreciated.
> Thank You
> 73
> Kenny
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