Screwdriver Antenna: A Motorized Tunable Vertical for HF

An HF antenna that covers 80 to 6 metres from a pole of two to three metres sounds too good to be true, and in a sense it is. Still, the screwdriver antenna is one of the most popular antennas for HF mobile operation and for small gardens, because it lets you change band by pressing a button instead of climbing out to change a coil. A small motor moves a contact along a coil, or moves the coil itself, and thus changes the inductance of the antenna. This guide explains how it works, what the physics allows, how to install it and tune it, and how to avoid the noise problems that its motor and control cable may cause.

How It Works

A vertical antenna is a quarter of a wavelength long when it is resonant on its own. On 40 metres, that is about 10 metres of wire, far too long for a vehicle. A shorter antenna can be made resonant by adding inductance in series, which cancels the capacitive reactance of the short radiator. This is the classic loading coil, described in our guide to the loading coil and capacity hat. A fixed coil works on one band, so mobile operators used to carry a set of whips, one for each band.

The screwdriver antenna makes the coil variable. A small DC motor, named after the screwdriver motors used in the first versions, drives a mechanism that either slides a contact along the winding of the coil or moves a metal slug in and out of it. The longer the part of the coil in use, the larger the inductance and the lower the resonant frequency. A controller in the vehicle sends the voltage to the motor in one direction or the other, and you watch the SWR meter until it reaches the minimum. Within a minute or less you can move from 20 metres to 40 metres, and later to 80, without leaving the seat.

What the Physics Allows

An honest article must mention the limits. A short vertical has a very low radiation resistance, which falls with the square of its height measured in wavelengths. On 40 metres, a radiator of 2.5 metres is only about six percent of a wavelength, and its radiation resistance is of the order of an ohm or less. The resistance of the coil and of the ground connection add to this, often a few ohms, so a large part of the transmitter power is converted into heat instead of radio waves. On the higher bands, such as 20 and 15 metres, the same antenna is a much larger fraction of a wavelength, and its efficiency is much better. The article on antenna efficiency, ground loss and radiation resistance explains the arithmetic.

The result is a pattern known to every mobile operator. A screwdriver antenna works very well on 20 to 10 metres, acceptably on 40 metres and noticeably less well on 80 and 160 metres, where the efficiency can be a few percent or less. It is still possible to make contacts on 80 metres with such an antenna, particularly with CW and digital modes, but you must expect to be weaker than a full-size station by many decibels. Understanding this stops disappointment (the general behaviour of a monopole antenna is well documented), and it explains why the antenna is a compromise and not a miracle.

Narrow Bandwidth and Retuning

The high Q of a loaded short antenna means a narrow bandwidth. On 80 metres the usable range with an acceptable SWR may be only 20 to 50 kilohertz wide, so if you move across the band you must retune. This is where the motor pays for itself: you press the button, watch the SWR and adjust. Many controllers offer memories or an automatic mode, in which a processor measures the SWR and moves the motor until it is at the minimum. The same narrow response has a benefit on receive, because the antenna acts as a preselector and rejects strong out-of-band signals.

The narrow bandwidth means that the tuning accuracy matters. A high-Q circuit can reach very high voltages across the coil, particularly at power levels of 100 watts, so the coil, the insulation and the mechanical parts must be rated for it. A small mismatch can cause arcing or heating, so use the antenna within its stated power rating, and for digital modes with continuous carriers reduce the power further, since they heat the coil more than speech does. The effect of heating in digital modes is the subject of our article on the FT8 duty cycle.

The Ground Is Half of the Antenna

A vertical monopole needs a counterpoise or ground plane, which is half of the antenna. On a vehicle the body acts as the ground plane, and on a base installation you must supply radials. The poorer the ground, the more power is lost in the earth. On a car, bond the antenna mount to the body with a short, wide strap, and connect the hood, the trunk lid and the other panels together if they are separated by paint or rubber seals. Fibreglass or plastic bodies do not work as a ground plane at all and require wires or a metal sheet.

In a garden, a screwdriver antenna on the ground with a set of radials is a decent solution, and an elevated base with a few tuned radials is better. How many radials and how long they must be is explained in our guide to vertical antenna radials. More radials reduce ground loss, which for a short antenna is the main component of loss, so the improvement is larger than for a full-size vertical.

Installation on a Vehicle

The mounting is the most important mechanical question. The antenna weighs a few kilograms, is tall and moves under vibration and wind, so a strong, rigid mount is required. A ball mount on the bumper, a bracket on the trunk lip or a plate on a trailer hitch are typical solutions. The mount must be electrically solid, with a continuous path to the body. Keep the antenna away from other metal objects and as high and as free as possible, and consider the height of the vehicle in garages and under trees. Our overview of mobile HF installation covers the antenna position, the power wiring and the noise sources in the vehicle.

Use a heavy coax cable from the antenna to the radio, such as RG-213 or LMR-400, because the short antenna has a low impedance and losses in the cable matter. Place a common mode choke at the radio end to keep RF off the outside of the cable and out of the car electronics, as described in the article on common mode chokes.

The Control Cable and Noise

The control line to the motor is a source of problems. It carries the DC to the motor, and the motor brushes produce noise. In addition, the control cable can pick up RF from the antenna and carry it into the controller and the vehicle wiring. Use a shielded cable with the shield grounded at the controller, add ferrite chokes at both ends and bypass capacitors at the motor. Keep the control cable away from the coax and from the radio’s power cable, and test the radio for noise with the motor running and not running. If the noise is only present while the motor moves, it is the brushes, and filtering at the motor solves it. If it is present all the time, check the grounds and the chokes.

Limit switches are another detail. The motor must stop at the ends of the coil, or the mechanism will be damaged. Most antennas contain such switches, and the controller shows when the end has been reached. Do not hold the button when the antenna is at its end, and do not push the motor against the stop. Also protect the mechanism from water, because moisture in the motor housing is the commonest cause of failure. Check the seals and lubricate the moving parts as the manual describes.

Tuning Procedure

Begin with a quiet frequency, low power and an SWR meter. Select the band, send a carrier of a few watts, and move the motor in the direction that reduces the SWR. Because the response is narrow, move in short pulses and watch the meter, since overshooting is easy. When the SWR is at the minimum, move the frequency across the band and note how the SWR changes; you will see how often you need to retune. Many users keep a table of the motor positions for each band and frequency, or use the memories of the controller. When the SWR is no lower than about 1.5 to 1, the antenna is tuned, and the transmitter can work at full power.

Do not expect an SWR of 1.0 on all frequencies. A small residual mismatch is normal and not important. If the minimum SWR is high, say above 2, the ground connection is poor or the matching is incorrect, and the antenna loses power in the ground. Check the ground strap first. A tuner at the radio can correct small residual mismatches, but it does not recover the power lost in the antenna system.

Practical Tips From Long Use

A few habits make life with a screwdriver antenna easier. Tune on the lowest practical power, and increase it only when the SWR is low. Never change bands while transmitting, since the coil may arc during the movement. Park the antenna at a convenient setting before you drive, and secure it if the vehicle goes through low areas, or lower it where the design allows. Wash the mechanism area gently after driving on salted roads in winter, because salt is the enemy of the motor, the contacts and the mount.

Keep a log of your tuned positions. In a few weeks you will know the numbers for the bands you use most, and you can drive the motor straight to the right place, which saves time and avoids needless transmitter carriers on the band. Listen before you tune: the antenna acts as a preselector, and the receiver noise rises sharply when the coil is at the correct setting, so you can even find the resonance by ear. This habit is useful on a cold morning when you want to save your power for the contact itself.

Remember also that the vertical is a vertical-polarised antenna with a low radiation angle, which suits DX on the higher bands but is less suitable for short-range contacts on 80 or 40 metres, where a high-angle antenna is better. If your interest is in regional rag-chews at night, think about a different antenna for the home station. For travelling and DX, the screwdriver is a convenient tool, as long as you know what you are asking of it.

Alternatives

A full-size or multiband wire antenna is better than a screwdriver antenna wherever there is space. For a small lot or a restrictive housing association, the screwdriver is a useful candidate, as discussed in our guide to antenna restrictions and stealth options. Another approach is a set of fixed-band whips, which are cheaper and simpler but need a manual change for each band. A magnetic loop is quieter and can have good efficiency on a limited bandwidth, again with motorised or manual tuning. The choice depends on space, bands, budget and the kind of operation.

The Bottom Line

A screwdriver antenna is a short vertical with a motor-driven variable loading coil, which lets you change band and retune from the operating position. It works well on 20 to 10 metres, acceptably on 40 and with a heavy penalty on 80 and 160, because the radiation resistance of a short radiator is very low. Give it a good ground plane or radials, a rigid mount, heavy coax with a choke, a shielded and filtered control cable and a power level within its rating. Retune whenever you move more than a small part of the band, and treat it as a convenient compromise rather than a full-size antenna.

Frequently Asked Questions

What is a screwdriver antenna?

It is a short vertical antenna whose loading coil is adjusted by a small motor, so that it can be tuned to different HF bands from the operating position.

How efficient is a screwdriver antenna?

Good on the higher bands from 20 to 10 metres, fair on 40 metres and low on 80 and 160 metres, because the short radiator has very low radiation resistance and the coil adds loss.

Why is the bandwidth so narrow?

A heavily loaded short antenna has a high Q. On 80 metres only a few tens of kilohertz are usable before you must retune.

Does it need radials?

Yes, or a good ground plane. On a car the metal body serves, with the mount bonded to it. At home, a set of radials reduces ground loss.

Why do I hear noise when the motor runs?

The brushes of the motor and the control cable can radiate noise. Use a shielded control cable, ferrite chokes and bypass capacitors at the motor.

Can I run full power on a screwdriver antenna?

Only within the rating of the coil and mount. The high voltage across the coil makes arcing possible, and continuous digital modes need a lower power than speech.