A screwdriver antenna is an inverted-V, vertical antenna that can be adjusted to cover any frequency from 10MHz to 75MHz. It is powered by a remotely-adjustable DC electric motor and has a reversible configuration: the coil can either be placed at the top of the tower or at its base, depending on your location and needs.
The Screwdriver Antenna, which is described in a three-book series on “HF mobile rings,” was designed by Don Johnson, W6AAQ, in 1991.
An Operational Theory
The screwdriver antenna (102″-120″, as in 11-meter CB installations) is a versatile device utilizing a loading coil and whip antenna. Most HF antennas can only cover one frequency band, which is not the case with the loading coil.
The antenna bases are hollow tubes. Within the outside signal, there is a DC motor that can reverse direction. The tuning coil and whip are moved to encase a portion of the coil in the base, and the rest is used to tune the selected band with the motor, which is controlled remotely using a momentary DPDT switch.
When changing frequencies, antenna coils need to be extended or retracted in order to tune to a new band. An external antenna tuner isn’t necessary since inductive loading coils are adjusted to resonance this way.
Tuning antennas remotely was traditionally accomplished with variable capacitors or inductors synchronized with electric motors. An antenna with a screwdriver motor (or an antenna consisting of a vertical whip suspended in a tube) is usually a screwdriver antenna. Tuning coils are controlled by motors, typically with switch pads or remote switches. With these mechanisms, one switch toggles power to the antenna, which is then extended or retracted in an infinite array of lengths, with great variability. Different wavelengths can be transmitted and received using the antenna based on variations in height.
It differs from garden-variety antennas in a number of ways. With a powerful motor and the screwdriver’s tough torque ratings, it could provide continuous coverage of frequency ranges. Antenna stability may be improved by machining the motor shafts, or tuning noise might be reduced by adding filter capacitors.
Antenna length can be easily adjusted with screwdriver motors. In the event of a malfunction, these motors can be easily replaced. Screwdriver antennas can be constructed by enthusiasts themselves. Depending on the frequency and length that they require, they can design their own antennas. There is a large range in antenna designs and performance because the length of an antenna is directly proportional to, or a multiple of, the wavelengths it can pick up.
Remote controllers and switches are common accessories for screwdriver antennas. Some of these devices can offer digital displays and precision tuning, memory, antenna positioning and programable positioning, and sometimes dynamic braking; this enables more precise braking in the event a motor overshoots a custom setting. Various screwdriver antenna designs may be supported by controllers. Additionally, mounting brackets, weather covers, and various mounts are available for this model.
Antennas with screwdrivers offer the convenience of push-button control without sacrificing performance. The rugged design allows for high radiation efficiency in small packages. Some radio enthusiasts find these antennas difficult to resist because they provide powerful transceiving capabilities.