VHF and UHF Mobile Antennas: NMO, Magnetic Mounts and Ground Planes

A mobile radio is only as good as the antenna on the car. A 50-watt VHF/UHF transceiver connected to a poor antenna in a poor position will lose to a handheld with a good one, and many disappointing mobile installations suffer from the same few mistakes: the wrong mount, a bad ground connection, a cable run through a door, or an antenna that was never checked for SWR. This guide explains how a VHF UHF mobile antenna works, why the metal body of the car is part of the antenna, how to choose between quarter-wave and 5/8-wave whips, what the difference is between an NMO mount and a magnetic base, and how to install and test the antenna so that it performs. It applies to the 2 meter and 70 centimeter bands that most mobile operators use for FM, repeaters and local contacts.

The Car Is Part of the Antenna

A mobile whip is a monopole antenna: a single vertical rod, fed against a conductive surface called the ground plane. The Wikipedia article on the monopole antenna explains that the current from the transmitter is applied between the rod and the ground plane, and that the most common form is the quarter-wave monopole, which is about a quarter of a wavelength long. The rod provides half of the radiator, and the ground plane acts as a mirror that provides the other half. On a car, the ground plane is the sheet metal of the body, usually the roof.

This explains the first rule of mobile installation: put the antenna where there is the most metal around it. The center of the roof is ideal, because the metal extends in all directions, and the pattern is symmetrical. A mount on a fender or at the edge of the trunk lid produces a lopsided pattern, with a weaker signal in the direction where there is less metal. A glass-fiber or plastic roof, or a large sunroof, reduces the ground plane and may require a different mounting point or a ground-plane kit. A good ground plane is also the reason why a handheld with a rubber duck antenna performs poorly inside a car: the body is a metal box, and the antenna is inside it. Our guide to the handheld antenna upgrade shows the alternatives for handheld operation.

Quarter-Wave, 5/8-Wave and Collinear Whips

The length of the whip decides much of the performance. At 146 MHz, one wavelength in free space is a little over 2 meters, so a quarter-wave whip is about half a meter long, and a 5/8-wave whip is about 1.2 meters. On 440 MHz the figures are about 17 centimeters and about 40 centimeters. Dual-band antennas combine the two bands in a single whip, usually with a loading coil or a phasing section between two radiating sections, and the whip antenna is the general name for this kind of flexible rod.

The Wikipedia article on monopoles notes that five-eighths-wavelength monopoles are common because, at this length, a monopole radiates a maximum amount of its power in the horizontal direction. That is what you want for a car: low-angle radiation that reaches the horizon, rather than power that goes up into the sky. In practice, a 5/8-wave whip gives a gain of a few decibels over a quarter-wave whip on a good ground plane, though the exact value depends on the antenna and the vehicle. A quarter-wave whip is more tolerant of a small or irregular ground plane, and many operators use it on the trunk lid or on a smaller vehicle.

Mounting Options

The mount is the most important choice you will make after the antenna. Each type has advantages and disadvantages.

• NMO mount. A permanent mount that requires a round hole, typically about 3/4 inch (19 mm), in the roof or trunk. The antenna screws onto the mount, and the mount has a coax connection underneath. It is the strongest and electrically best solution, and it is the choice of professional users. The drawback is the hole in the body, so plan the position carefully and protect the edges against rust.
• Magnetic mount. A magnet in the base holds the antenna to a steel roof. It is quick to fit and remove, and it needs no drilling. The cable must pass through a door or a window seal, which can wear the cable and let in water, and at higher speed or in strong wind the magnet can slide. The coupling to the roof is capacitive, so the ground connection is not as good as that of a direct mount.
• Trunk-lip or hood-lip mount. A clamp on the edge of the trunk or the hood. It is good for cars where drilling is not possible, but the ground contact depends on a clean bare metal point, and the pattern is less symmetrical.
• Glass-mount antenna. A mount that couples the signal through the glass. It needs no hole and no metal roof, but its efficiency is lower, and it is a compromise for rental cars.

A magnetic mount is perfectly acceptable for occasional use, and many amateurs use one to try a mobile setup before they commit to drilling. For a regular daily mobile station, an NMO mount on the roof is hard to beat. A mobile HF installation has different requirements, so read our article on mobile HF installation if you plan to operate on the shortwave bands as well.

Coax, Connectors and Routing

The cable matters more at UHF than at VHF, because the loss of the coax grows with frequency. A thin cable of RG-58 type loses noticeably more at 440 MHz than at 146 MHz, and a few meters of it can cost a significant fraction of your power on the higher band. For a normal car, a better low-loss cable such as RG-8X or a 240-size low-loss type is a good choice. Our guide to coaxial cable for ham radio compares the common types.

Route the cable with care. Keep it away from airbag systems, door sills with moving parts, pedals, the engine bay and hot surfaces, and secure it so it cannot move. Do not pinch the cable under a door seal, because this destroys it over time; run it along the edge of the headliner or through an existing grommet instead. Leave a loop of cable near the antenna to protect the connector from strain. Use a good crimped or soldered connector and weatherproof it where it is exposed to water.

Installing and Testing the Antenna

After you mount the antenna, test it before you rely on it. Measure the SWR with an SWR meter or an analyzer at the low, middle and high end of the band. A good installation gives a low SWR across the whole band. A high SWR on a new installation usually means a poor ground connection at the mount, a damaged connector or a cable fault, and not a badly cut whip. Dual-band antennas are tuned for both bands at the factory, so do not cut the whip unless the instructions tell you to. Quarter-wave whips that are supplied long, however, are usually cut to the frequency that you want, a little at a time.

Check that the mount makes a firm electrical contact with the body. On an NMO mount, the contact is made through the ring around the hole, and on a car with a painted roof the paint must be removed under the mount ring or the mount must have a ground strap. On a magnetic mount, clean the roof and the magnet from dust and grit, which scratch the paint and weaken the capacitive coupling. Our article on what an SWR meter tells you covers the measurement in more detail.

Common Problems and Quick Fixes

Most mobile antenna complaints fall into a short list. A signal that is good when the car is parked but poor when you drive is usually caused by ignition or alternator noise reaching the receiver, which shows as a whine that changes with the engine speed, and a power cable connected directly to the battery with a ferrite choke usually cures it. A signal that is weaker to one side of the car points to a poorly placed mount or a ground plane with a hole in it, such as a large sunroof. A signal that disappears in the rain or when you shut the door suggests water in the connector or a damaged cable at the door seal. A rising SWR as the car warms up in the sun can be caused by a magnetic mount that is moving a little on the roof, which changes the coupling.

If the antenna works on the bench but not on the car, test it on a metal sheet or a baking tray that serves as a temporary ground plane. This distinguishes an antenna problem from an installation problem in a few minutes. Keep a note of the SWR when the installation is new, and measure it again twice a year: a slow change usually means corrosion at the mount, a connector that is no longer tight, or a cable that has been damaged by flexing.

Radio Placement, Power and Safety

Place the transceiver where you can reach it without taking your eyes off the road, and connect the power cable directly to the battery with a fuse near the battery, not to the cigarette lighter socket, which cannot supply the current of a 50-watt radio. Keep the antenna as far as possible from the people in the car; a roof mount is better than a trunk mount for this reason, and the roof itself offers some shielding to the passengers. Avoid transmitting at high power while someone stands next to the antenna, especially for long periods. Local rules on the use of a radio and a microphone while driving differ from country to country, so follow the rules of your country, and use voice operation or a hands-free microphone.

Finally, remember the car wash and the garage. A tall whip can be damaged by low barriers, and a magnetic mount can be knocked off by a car wash brush. Take the antenna off or use a model that can be folded or unscrewed quickly. An NMO mount allows you to unscrew the antenna in seconds and to cap the mount with a plug if you want to keep the car clean.

Choosing for Your Use

For repeater use and local FM contacts, a 5/8-wave single-band or dual-band antenna on the roof is usually the best choice. For weak-signal SSB or for a small vehicle, a quarter-wave whip on a good ground plane may work better. For a mobile station that is used only on holidays or in a rental car, a magnetic mount with a quarter-wave whip is the simplest to carry. Whichever antenna you choose, the decisive factors are the position, the ground plane and the quality of the coax and connectors. A modest antenna, correctly installed on the center of the roof, will outperform an expensive one on a poor mount.

The Bottom Line

A VHF/UHF mobile antenna is a monopole that uses the car body as its ground plane. Put it on the center of a metal roof, choose a quarter-wave or 5/8-wave whip to suit your vehicle and your band, and use an NMO mount for permanent installations and a magnetic mount for occasional use. Use low-loss coax, route it safely, make sure that the mount grounds well, and test the SWR across the band. With these steps your mobile radio will work as well as the antenna and the car allow, and you will hear and be heard much better than with a handheld alone.

Frequently Asked Questions

Where is the best place for a mobile antenna?

The center of a metal roof. The metal extends in all directions, which gives a symmetrical pattern and a good ground plane.

What is an NMO mount?

A permanent antenna mount that needs a round hole of about 3/4 inch in the roof or trunk. The antenna screws onto it and the coax connects underneath.

Is a magnetic mount good enough?

For occasional use, yes. For permanent use, a drilled mount has a better ground connection and no cable passing through a door.

Quarter-wave or 5/8-wave?

A 5/8-wave whip radiates more power at low angles and is common for repeaters. A quarter-wave whip is shorter, simpler and works better with a small ground plane.

Why is my SWR high on a new mobile antenna?

Most often a poor ground at the mount, a damaged connector or a cable fault. Check the contact and the cable before cutting the whip.

Which cable should I use?

A low-loss coax such as RG-8X or a 240-size type, especially on 70 centimeters, where thin cable has more loss.