5/8 Wave Vertical Antenna: Gain, Matching and Radials Explained

Walk around any rooftop in a town with active VHF repeaters and you will see simple vertical whips on masts. Some are a quarter of a wavelength long, and many are almost two and a half times as long, five eighths of a wavelength. The 5/8 wave vertical is one of the most popular antennas for 2 meters, 70 centimeters, 6 meters and 10 meters, and it is sold for CB radio as well. It promises more gain and a lower angle of radiation than the quarter-wave whip, but it needs a matching coil and it behaves differently in the presence of ground. This guide explains why that length was chosen, how the antenna works, what you must do to install it properly and when a quarter wave is the better choice.

Why Five Eighths of a Wavelength?

A vertical monopole radiates in all horizontal directions, but how much of its power goes toward the horizon and how much into the sky depends on its length. The Wikipedia article on the monopole antenna notes that monopoles five eighths of a wavelength long, that is 0.625 wavelength, are common because at this length a monopole radiates the maximum amount of its power in horizontal directions. It adds that monopoles longer than 0.625 wavelengths are almost never used, because additional lobes appear that are directed into the sky.

That is the whole idea of the antenna. For a station that wants to talk to other stations on the ground, whether through a repeater or on simplex, the energy that goes up is wasted. A quarter-wave vertical already has a low takeoff angle, but the 5/8 wave concentrates the pattern a little more toward the horizon. The gain figure that is often quoted is between 1 and 3 decibels over a quarter-wave in ideal conditions, with real installations getting less. The gain is not free: it comes from squeezing the pattern in the vertical plane, so the antenna sees fewer signals from high angles, which can be an advantage or a disadvantage.

How Long Is Five Eighths of a Wavelength?

The length depends on the band. The table gives the radiating element length for a quarter wave and for a 5/8 wave, based on the free-space wavelength at typical frequencies and without a velocity factor correction. Practical antennas are cut a little shorter, because of end effect and the shape of the whip, and the maker’s instructions should always be followed.

The numbers show immediately why the antenna is so common on VHF and UHF and rare on the lower HF bands. A 5/8 wave on 20 meters would need more than 13 meters of vertical radiator, which is not a practical thing for most gardens, while on 2 meters the element is only 1.3 meters long and fits easily on a car or a balcony. On 10 meters, 6.6 meters of aluminum tube is realistic for a base station mast.

The Matching Problem and the Base Coil

A quarter-wave monopole is resonant: at its design frequency the reactance is zero and the resistance is roughly 36 ohms over a perfect ground, which is easy to match to 50 ohm coaxial cable. A 5/8 wave is not resonant. At 0.625 wavelength the feed point shows a capacitive reactance in addition to a resistance, and the resistance itself is higher than that of a quarter wave. To use the antenna with ordinary coaxial cable you therefore need a matching network at the base.

The usual solution is a small coil, often a few turns of wire, in series with the element or in a combination with a tap or a shunt. The inductance cancels the capacitive reactance and brings the resistance down toward 50 ohms. In the commercial antennas that you buy, the coil is built into the base under the radome or a plastic cover, and the factory has already adjusted it. If you build your own, you will need an antenna analyzer or a NanoVNA to set the coil, in the way we describe in our guide to NanoVNA calibration and sweeps. The related technique of matching with an inductor and a capacitor is explained in our article on L-network matching.

Some designs add a phasing coil in the middle of a longer element to make a collinear antenna that stacks two or more half-wave sections. The 5/8 wave with a base coil is simpler and the coil is easy to protect from the weather, but a damaged or detuned coil is the most common failure of the antenna in practice, so inspect it if the SWR rises.

Radials and the Ground Plane

A monopole needs a counterpoise. The signal on the antenna is a half of a dipole, and the other half is the image in the ground or the radials. The 5/8 wave vertical depends more strongly on a good ground plane than a quarter wave does, because the current at the base is lower and the matching is sensitive to the return path. On VHF and UHF the usual practice is to use three or four radials, each about a quarter wavelength long, sloping downward at about 45 degrees. The slope raises the feed point resistance a little and makes the match to 50 ohms better, so the angle is a design variable, not a mechanical convenience. Our guide to vertical antenna radials covers the principles for HF, which also apply here with shorter lengths.

For mobile use the roof or the trunk lid of the vehicle acts as the ground plane. A magnetic mount in the middle of the roof is therefore a good compromise, and the antenna works best when the roof is large compared with the wavelength. On 2 meters and 70 centimeters the metal roof of a car is large enough to function as a decent plane. A 5/8 wave on the edge of the roof or on a glass mount without a plane will not perform as designed, and the pattern may tilt in the direction of the larger metal mass.

5/8 Wave Versus Quarter Wave

The choice between the two depends on how and where you use the antenna. The quarter-wave is shorter, simpler, cheaper and less dependent on the coil, and it has a very low angle of radiation. The 5/8 wave is taller and has a little more gain at low angles, but it also suffers if the ground plane is poor. In very hilly terrain, where signals arrive from above or below the horizon, the extra gain may not help, and the quarter-wave can be better. In flat country with distant repeaters, the 5/8 wave often has an advantage.

• Gain: the 5/8 wave offers about 1 to 3 dB over a quarter wave at best, while the quarter wave is the reference.
• Matching: the quarter wave connects almost directly to 50 ohm cable with radials, whereas the 5/8 wave needs a base coil or matching network.
• Height: the 5/8 wave is two and a half times taller than the quarter wave.
• Ground plane: both need one, but the 5/8 wave depends on it more strongly.
• Best use: the quarter wave suits portable work, small vehicles and tight spaces; the 5/8 wave suits base and mobile stations in flat terrain.

If you want to understand how these two patterns compare, our article on antenna radiation patterns explains lobes, beamwidth and the way a plot should be read. A different option, favoured for fixed 2 meter stations, is the collinear or the J-pole, both discussed in our guide to 2 meter antenna options.

The Pattern in Practice

The extra gain of the longer element shows up as a flatter, more compressed pattern in the vertical plane. A good ground plane, as the Wikipedia article on the ground plane explains, acts as a reflecting surface that completes the antenna and decides how much of the energy goes toward the horizon. If the plane is small, the lobe tilts upward and the advantage of the 5/8 length is partly lost. This is the reason why you may see two different reports about the same antenna: one operator with a large metal roof praises its gain, while another with a small plane finds it no better than a quarter wave.

Terrain matters for the same reason. On the top of a hill, the horizon is below the antenna, and the best signal may arrive from a slightly negative elevation angle; a pattern that is squeezed toward the horizontal plane may then miss the stations in the valley. Before you buy the longer antenna for a hilltop site, look at the profile of the surrounding land, or try both types if you can borrow them, and compare the reports over several days.

Installing a 5/8 Wave Vertical

Whether you buy a ready-made antenna or build one, the installation rules are the same. Place the antenna as high as practicable and away from metal masts, gutters and other antennas by at least a wavelength where possible. Mount the radials or the ground plane horizontally or sloping down, never upward. Use a good low-loss feed line, since on VHF and UHF the cable loss is often larger than the gain of the antenna; a 3 dB loss in a long run of thin cable cancels any advantage from the 5/8 length. Waterproof all connectors, as our guide on weatherproof coax connectors describes, because moisture in the base coil is a common source of failure.

Check the SWR at the antenna band edges. A 5/8 wave typically has a bandwidth of a few percent, which on 2 meters is enough to cover the whole band, but on 10 meters the antenna may be adjusted for the part of the band where you operate. If the SWR is high across the whole band, suspect a poor ground plane, a faulty connector or a damaged coil before you cut the element.

Common Problems

• High SWR everywhere. Usually a missing or poor ground plane, a bad connector, or water in the coil.
• Poor performance on one side. On a car, the antenna is off center, or the roof has a metal rack that distorts the pattern.
• Noise on receive. Common-mode current on the feed line; a ferrite choke near the antenna often helps.
• Short range despite good SWR. The SWR only shows the match, not the radiation. Check the height, the feed line loss and the surroundings.
• Broken radials. The thin rods used as radials fatigue in the wind. Inspect them in the spring and in the autumn.

The Bottom Line

A 5/8 wave vertical is a monopole 0.625 wavelength long, the length at which a vertical radiates the largest share of its power toward the horizon. It offers roughly 1 to 3 dB over a quarter wave in good conditions, needs a base coil or matching network because it is not resonant, and depends on a decent ground plane or radials. It is excellent for VHF and UHF base and mobile stations in open terrain and for 10 meter and 6 meter verticals on a mast, but it is too tall for the lower HF bands and not always better than a simple quarter wave in hilly country or with a poor ground plane. Use good coax, protect the base, and judge the antenna by the signal reports rather than only by the SWR.

Frequently Asked Questions

Why is a 5/8 wave antenna better than a quarter wave?

It has a lower angle of radiation and concentrates more power toward the horizon, which gives a gain of a few decibels at best in good conditions.

Do I need radials for a 5/8 wave vertical?

Yes. It needs a ground plane, such as radials or a metal roof, because the antenna works against it, and a poor plane reduces performance and spoils the match.

What is the coil at the base for?

A 5/8 wave is not resonant, so the feed point has a capacitive reactance. The coil cancels it and brings the impedance close to 50 ohms.

How long is a 5/8 wave on 2 meters?

About 1.29 meters at 145 MHz, in free space. Commercial antennas are a little shorter, so follow the maker’s dimensions.

Can I use a 5/8 wave on HF?

On 10 meters and 6 meters, yes. On the lower bands the element is too tall for most stations.

Is a longer antenna always better?

No. Beyond about 0.625 wavelength the pattern develops lobes into the sky, so longer elements lose gain at the horizon.