When I first heard the term screwdriver antenna, I thought it was a joke. I immediately thought of stories about loading wet noodles, bedsprings, and random trees in the forest as antennas. Or perhaps the screwdriver antenna was a myth, something like the Wouff-Hong.
Many of us have used loading coils to match a vertical antenna, and that’s the principle behind the screwdriver antenna. A screwdriver is a vertical antenna with a remotely adjustable center-loading coil. Intended for mobile operation, it may be adjusted to cover any band from 6m to 75m. The antenna takes its name from the use of a reversible DC electric motor like those found in electric screwdrivers. It takes the hassle out of moving clips manually along the coil.
History
The original DK-3 Screwdriver Antenna design was created by Don Johnson (not the Miami Vice star). Don, W6AAQ (sk), was also author of a book on HF mobile operation, 40 years of HF Mobileering. When the FCC first authorized HF mobile, a group in the San Francisco Bay area decided to go mobile on 75 meters. They chose 3995 KHz as an operating frequency to reduce the possibility of high-power fixed stations interfering.
Johnson and the group conducted field tests with home-brewed antennas using a field strength meter to see who had the strongest signal. He came up with the idea of using a metal tube with a whip on top of a coil that fit inside the tube. During his search for a method of tuning the antenna remotely, Denney Moore, W6MHP (sk), suggested using the motor from a cordless screwdriver—and the screwdriver mobile antenna was born. Johnson received a patent in 1991 for the device and thousands were sold.
Theory of Operation
Screwdriver antennas combine a loading coil with a whip antenna, typically 32 to 102 inches long. The loading coil is adjustable to cover multiple HF bands, and the unused portion slides into a hollow tube. Concealed inside is a reversible DC motor. The motor, operated remotely by a momentary switch or an electronic controller, moves the whip and loading coil so that part of the coil is inside the base (and electrically bypassed, or inactive) and the rest of the coil is in use as a loading coil tuned for the selected band.
When changing frequencies, the antenna loading coil is extended or retracted to tune the antenna to the new frequency. A gear-motor turns a threaded rod (typically 1/4×20 all-thread) in and out of a nut or threaded boss attached to the bottom of the coil. This in turn moves the coil in and out of the mast. Contacts at the top of the mast slide along the outside of the coil, adjusting the resonant point.
Since this adjusts the loading coil to resonance, no external antenna tuner is required. Very low SWR can be achieved on any frequency in the HF ham bands 80-10 meters, depending on the length of the whip that is used. Shorter whips work better on the higher bands such as 6-15 meters; longer whips will work on 80 and 40 meters.
Stealth Antenna for Home
Screwdriver antennas aren’t just for mobile use. Ground mounting the screwdriver antenna in an HOA neighborhood is rather simple since the base of the antenna can be mounted to a readily available 4-foot water pipe which is driven into the ground with 6 to 12 inches extending above the ground level. A good ground plane is essential to making the antenna perform well. Ground planes can consist of multiple wires spread out in multiple directions from the base of the antenna; the more the better. Alternatively, one can use 2-foot-wide galvanized wire mesh screen which is laid out in four directions from the base of the antenna running approximately 12 feet in length.
You can mount a screwdriver antenna above ground, such as on a patio roof, but you should cut your ground plane wires to match the bands that you intend to operate on. This is true for all vertical antennas—once you mount a vertical antenna above ground level, then the ground plane radial wires need to be cut to specific lengths, whereas ground mounting does not require resonant radial wires.
Tuning
With the toggle switch, you can watch an SWR meter dip as the antenna is driven to resonate at a desired frequency. However, this method does not provide for any memory presets for favorite frequencies.
Some screwdriver antennas incorporate a turn counter to allow an electronic controller to retune the antenna to memory presets. The antenna uses a small magnet that spins past a reed switch which allows the controller to measure the switch pulses (i.e., turns) as the magnet rotates with the motor. Several controllers which allow the antenna to move to selected present frequencies include:
- Ameritron SDC-102 controller with 10 memory presets and manual operation
- West Mountain TARGETuner: TARGETuners sense the RF signal going to the antenna and will work with any transmitter or transceiver. They do not depend on a radio data link for band selection and do not need to control the radio for activation.
- Ameritron SDC-104 Series screwdriver antenna controllers automatically adjust the screwdriver antenna to the transceiver’s frequency. The controller uses your radio’s control port and reads the frequency from your transceiver, then moves the antenna to the correct location. There are models for Icom, Yaesu, and Kenwood radios. Your screwdriver antenna must have a pulse sensor (turns counter) installed for the antenna controller to operate properly.
- Yaesu ATAS system: Designed for use with Yaesu transceivers that support ATAS (Yaesu FT-857D, FT-897D, FT891, FT-991, FT-991A, FTDX10, and the FT-450D), microprocessor control signals are sent on the coaxial cable to adjust the antenna radiator length for best SWR.
Any of the Yaesu rigs listed above have the ATAS option, and the ATAS-120A antenna covers the 7, 14, 21, 28, 50, 144 and 430 MHz amateur bands.
Antennas
Screwdriver antennas come in a variety of sizes, ranging from 4 feet to more than 12 feet when fully extended. Unless you have a pickup or large SUV, you’ll probably want to stick with compact versions. The larger ones require heavy-duty mounts to support antenna weight and provide stability when driving. Here are some examples of the compact versions available at DXEngineering.com:
- Yaesu ATAS-120A: This is a mobile antenna that provides HF, VHF, and UHF coverage with automatic motorized tuning. Its popularity is in part due to the relatively small size (75 inches extended), light weight, and easy tuning with several Yaesu radios—no controller required. They can be adapted to portable operation with an appropriate mount and counterpoise.
- Diamond SD330: Covering all HF bands from 80 through 10 meters, this compact mobile antenna extends to a maximum length of 73 inches. Manual adjustment switch and control cables are included.
- MFJ-1668: This portable/mobile manual screwdriver antenna lets you operate on all bands, 80-6 meters (including 60 meters), on a budget. It comes with an extra-long 10-foot telescopic whip for portable operation and a shorter 4.5-foot stainless steel telescopic whip for mobile operation. You manually adjust and secure in place with a thumbscrew prior to operation (and driving!).
No matter what brand, all short screwdriver antennas have common mode issues—RF present on the control leads and coax cables. Choking common mode current is especially important if you want an automatic controller to work properly. Split bead or toroid chokes need to be mounted outside the vehicle as close to the base of the antenna as possible. Controller wire is wound around and through them to help suppress stray RF.
Mountings
Even many of the “compact” antennas weigh two or more pounds, so choose your mounting wisely. Typical choices include heavy-duty magnetic mounts like the MFJ-336 Series; lip mounts for hatchback doors, hoods, or trunks such as the Diamond K400; and the Diamond Antenna K540KM luggage rack mount. If you have a trailer hitch, the pin used for the towing bar can be used to hold the MFJ-2822 mount in place. All of these can be installed without putting holes in your vehicle.
Some require sheet metal surgery. L-brackets or gap-mounts, typically used inside the hood or trunk seams, require holes for screws but are usually hidden from sight. Comet Antennas “no hole” fender brackets use existing fender bolts—no drilling. The Hustler Antenna SSM-2 ball mount is a sturdy option for mounting on body panels but requires some substantial drilling—not an easy DIY repair if you later sell your vehicle.
Location, Bonding, and Other Factors
A vehicle’s body makes a poor ground plane at HF, so you also need to consider antenna placement, grounding, and bonding in the installation process. Maximizing the limited ground plane a vehicle offers is worth the effort. It’s the metal mass directly under the antenna, not what’s alongside, that counts.
The best possible scenario would be to center the antenna on the roof. Unfortunately, it may not be practical considering the height and the possibility of disturbing the side curtain airbags. Alternatives would include mounting around the hood or in the trunk area.
Minivans, SUVs, RVs, Jeeps, station wagons, and crossovers present a challenge. Except for front mounting, the body of these vehicles can shadow a large portion of the antenna. This causes tuning problems and reduces efficiency. If shadowing can’t be avoided, make sure the coil is as far away from metal as possible to minimize coil losses. Also, trailer hitch mounting is the least desirable location due to shadowing.
Of course, properly grounding the antenna itself is especially critical with luggage rack mounting. Many luggage racks are not bonded to the vehicle and will require some extra effort to do so. Clamp mounts like the K400 might save you from drilling a visible hole, but they can cause body damage if the antenna is stressed by highway speeds or hits a low-hanging object. Since the return path for the coax depends on the integrity of the mount’s hinges and set screws, they can also increase ground losses.
Continue by bonding the hood and trunk lid to the car body—horizontal surfaces are more important than vertical ones. Depending on the vehicle, there can be several dozen other places where ground straps will help. These include, but are not limited to, bumpers, suspension parts, rear axles, exhaust system, tailgates, and just about any bolted-on piece of hardware.
But Do They Work?
Yes they do, but there is a tradeoff between convenience and performance. Realize that the screwdriver is a compromise antenna design with a radiator that’s electrically short. You’ll find that the lower bands 80/40m have limitations, but as you go to the higher frequencies, 20-10m, they’ll perform better. You’ll even work some DX when the bands are open.
