Back-to-back Screwdrivers form Rotatable, Tunable Dipole

Terry Schieler writes "I recently responded to several posts on another ham site relating to antennas for balconys and attics. One post reminded me of an idea I had some years back but lacked the personal knowledge to follow through on. Now, with access to the creative minds of hundreds of talented amateurs via QRZ.COM, I thought I'd try to breathe new life into my idea by sharing it.


Since I live in an antenna restricted community, I have spent a great deal of time experimenting with attic and stealth antennas and find it fun and quite challenging. I have tried most everything I can think up or find in print. (I have an attic full of wire and bent-up aluminum and some of it actually works!)


One day while tinkering with "back-to-back" Hustler mobile antennas configured as a rotatable dipole, I was frustrated by my lack of ability to tune the dipole to another band without having to climb into the attic and change resonators.


Then I saw an article reviewing Don Johnson's "Screwdriver" antennas and the idea hit me. Why not configure two of Don's screwdriver antennas, horizonatlly, "back-to-back" (or "bottom-to-bottom" if you prefer) forming a tunable, rotatable dipole? They could be mounted to a piece of 2x3 treated wood and attached to any type of mast. I would need to find a way to tune them in and out in "harmony" so the left side of the "dipole" would always be exactly the same length as the right side?


At the time, I wrote to both Don Johnson and Lew McCoy (now SK) with my thoughts. Lew suggested wiring the two screwdrivers' tuning motors in parallel, but warned that the antenna may not retain its electrical integrity due to the two motors aging differently. Don responded with a nice note that included a rough drawing of a set of gears that could run the screwdrivers' coils up and down simultaneously, and in sync with each other, maintaining the rotatable dipole's electrical balance. Don said he had no plans at the time to produce such a product but provided me with his personal sketches showing how one gear might drive two others at the base of each screwdriver. Unfortunately, I don't know a worm gear from scuba gear! My personal mechanical skills left me some 40 years ago with the demise of the "Erector Set"...so I dropped the idea. Somewhere out there are hams with some degree of mechanical ability that might want to adopt, experiment with or collaborate on a project like this. Maybe someone has already tried it and succeeded. I'd enjoy hearing your thoughts and suggestions. I still have the original sketches Don sent me and would be happy to share them with anyone who might want to tackle the mechanics of the gear/drive system. Or, possibly, the answer is more electrical than mechanical: like maybe a logic controller that sends precise voltage commands to each of the two "screwdriver" motors to keep them perfectly in sync (ala "stepping motors") while tuning.


Imagine...a rotatable dipole that was remotely tunable from 80-10 meters! Or, (here's a stretch) how about six screwdrivers (I know..$$$, but stay with me here) configured as a three element yagi where the director and reflector change their respective lengths in sync with the tuning of the driven element? That could lead to a 80-10 Meter TUNABLE yagi! Longshot? Maybe not! What a great hobby.

73,
Terry, WØFM"

 

Where can I find Don Johnsons original article on how to build a screwdriver antenna.