A screwdriver antenna is a remote-operated variable-tuning antenna favored by radio and ham operators and enthusiasts. Characterized by an oversized cylindrical housing at its lower end and a whip antenna at its upper end, it is usually fixed on a heavy-duty mount to a vehicle or base. This antenna derives from a principle that a motor from an electric screwdriver could be used to drive a mobile antenna for remote tuning. Developed by Donald K. Johnson in 1991, the antenna features a large cylindrical base designed to increase surface area and greatly improve radiation capability in a compact size. Able to automatically shorten or lengthen for a wider range of tuning, this antenna can be adjusted by remote switch or digital controller.
Traditionally, antennas tuned by remote relied upon variable capacitors or inductors tuned by electric motors. The screwdriver antenna, in contrast, is usually a vertical whip suspended in a tube housing a screwdriver motor. Its tuning coil is adjusted via the motor, usually by remote switches or pads of varying complexity. Essentially, these switches toggle power to the antenna and drive the mechanism up or down, extending and retracting its length with great variability and bandwidth range. This height variation adjusts the wavelengths the antenna is capable of transmitting and receiving.
The screwdriver antenna has certain unique characteristics that distinguish it from garden-variety antennas. It may provide continuous coverage of frequency ranges while running on a powerful motor with the tough torque ratings screwdrivers are designed to handle. Converting the motor might involve additions such as machining output shafts for antenna stability or including filter capacitors to reduce motor noise when tuning.
Screwdriver motors permit quick adjustments across the length of the antenna. These types of motors are also easily replaced if problems occur. Many enthusiasts prefer to construct their own screwdriver antennas. They may build the antennas to their preferred frequencies and lengths. The length of an antenna is directly proportional to, or a multiple of the size wavelengths it can pick up, which affords a great variety in designs and performances.
Common accessories to the screwdriver antenna include remote controllers or switches. The more complex of these devices offer digital displays and precision tuning, memory, antenna positioning readings, programmable positioning, and sometimes dynamic braking; this can provide more precise stopping points to counter any delayed reactions by a motor that might overshoot tuning. Controllers may be designed to work with any number of screwdriver antenna designs. Other accessories can include mounting brackets, weather covers, and various mounts.
In short, screwdriver antennas provide push-button convenience for versatile performance. Their design permits high radiation efficiency in a tight package. These antennas provide users with powerful transceiving capability difficult for some radio enthusiasts to resist.
