Half-Square Antenna: A Low-Angle Vertical Wire for 40 and 80 m

Plenty of amateurs want a vertically polarised antenna that works for DX but do not want to bury a hundred radials or erect a tall tower. The half-square antenna is a quiet answer to that problem. It is a single wire, a wavelength long, bent into an upside-down U shape and hung between two supports. It radiates with vertical polarisation at a low angle, it does not depend on a ground radial system the way a quarter-wave vertical does, and it is cheap to build from ordinary antenna wire. It also has quirks that catch first-time builders: a very high feedpoint impedance, high RF voltage at the wire ends and a pattern that favours two directions instead of all of them. This guide explains how the antenna works, how to size it for the 40 and 80 metre bands, how to feed it, and when it is a better choice than a dipole, an inverted L or a conventional vertical.

What a Half-Square Antenna Is

Start with one wire that is one full wavelength long on your band. Bend it so that a quarter wavelength points straight up from the ground, a half wavelength runs horizontally across the top, and the last quarter wavelength drops back down to the ground. The result looks like a square with the bottom edge missing and is half a square in the sense that the vertical sides are a quarter wave tall and the top is a half wave wide. The two lower ends of the wire are left open, not grounded.

Along a wire a full wavelength long, the current peaks a quarter wavelength in from each end, which here is exactly where the top corners are. Each vertical section therefore carries current that rises from nothing at the bottom to its maximum at the top. The two vertical sections carry current in the same direction, so their radiation adds up. The horizontal top wire carries current in opposite directions on its two halves, so its radiation largely cancels in the far field. What remains is two in-phase vertical radiators half a wavelength apart, with their best current in the upper half of each vertical, well away from the lossy ground.

That is the whole secret. The antenna behaves like a pair of vertical dipoles fed in phase, and the horizontal wire is just the phasing line that connects them. It is also why the half-square antenna belongs with the voltage-fed wires: the ends are the high-voltage points, and the feed system has to cope with that.

Pattern, Polarisation and Take-Off Angle

Two vertical radiators spaced half a wavelength apart and fed in phase give a bidirectional pattern. The strongest signals leave perpendicular to the plane that the wire occupies, in both directions, so if the wire runs north to south the best radiation is toward the east and the west. The directions along the line of the wire are weak. The lobes are broad, and a half-square covers a useful sector on each side rather than a narrow pencil beam. The general way to read this kind of figure is covered in the radiation pattern article, and for a vertically polarised wire it is the azimuth cut that shows the two broadside lobes.

The elevation pattern is the reason many people build one. Because the current is concentrated high up on the verticals and there are two radiators in phase, the vertical lobe is pressed down toward the horizon. On the DX bands a low take-off angle is what carries a signal over long distances with few hops. The same reasoning is behind the advice in our guide to antenna height and take-off angle, where a low angle matters more than raw gain once the path is long. A half-square also compares well with a single quarter-wave vertical in peak gain, simply because it has two radiators. How large the advantage is depends on the ground and on what you compare against, so treat any single figure with caution and judge it by contacts and reports.

Polarisation is vertical. On the lower bands that is a real asset for DX, as ground reflection losses at low angles are smaller for vertical polarisation over typical soil, and it is a liability for local contacts, because many stations nearby use horizontal wires and you will lose a little through cross-polarisation. For the full story on why this happens, see our comparison of vertical and horizontal polarisation.

Dimensions for 40 and 80 Metres

The working figures are simple. Each vertical section is about a quarter wavelength, the top wire is about a half wavelength, and the total wire is a little over one wavelength once you allow for the velocity factor of insulated wire and for the effect of the bends. Start from the standard rules of thumb and plan to trim with an antenna analyser, because every site is different.

• 40 metres, around 7.1 MHz: top wire about 20 m, each vertical about 10 m. Total wire about 40 m.
• 80 metres, around 3.6 MHz: top wire about 39 m, each vertical about 20 m. Total wire about 80 m.

The 80 metre version needs tall supports or a modified shape, and that is where the half-square often gets compromised. If you cannot reach a full quarter wavelength in height, you can let the verticals sag or slope, bring the lower ends in toward each other, or accept a shorter vertical section with some loading. Each compromise costs some of the low-angle performance. Practical builders often find that a 40 metre half-square is the sweet spot: the supports are reachable, the wire is a manageable length, and the band rewards low-angle vertical polarisation.

Use insulated wire if you can, or at least be consistent, because insulation lowers the resonant frequency compared with bare wire. A lightweight stranded wire is easier to hang and to repair. Our HF antenna wire guide covers the materials in detail and explains why a strong wire that does not stretch matters when the top is held at two points.

Supports and Layout

You need two supports about half a wavelength apart and tall enough to lift the top wire a quarter wavelength or a little less. Trees, masts, a mast on a garage and a pair of fibreglass poles are all common. Run the top wire as straight and level as you can; a sag in the middle changes the pattern and the resonant frequency. Hang the verticals with a light weight or tie their lower ends to a non-conducting anchor with rope or a stretch of dacron cord so they stay taut in wind. The lower ends carry high RF voltage and should be well above the reach of children and animals, so keep the bottom of each wire at least a couple of metres above ground, or terminate them on a robust insulator out of reach.

Feeding a Half-Square Antenna

This is where most projects either succeed or end up on a shelf. Because the wire ends are voltage maxima, a feed at one lower end, which is the usual choice, sees a very high impedance, typically thousands of ohms, and that is far from the 50 ohm output of a transceiver. A direct connection to a coaxial cable would produce an extreme SWR and heavy loss. You need a way to transform that impedance down.

The common options are these:

• A parallel-tuned matching network at the base. An inductor and a variable capacitor, connected so that the high-impedance antenna end sees a tuned circuit and the coaxial line taps in at a lower impedance. It is adjustable, covers a little of the band and works well on one band at a time.
• A remote antenna tuner at the feedpoint. A weatherproof automatic tuner that sits at the bottom of the wire and tunes for the lowest SWR on the coax. It is the simplest solution for many stations, though the tuner must be able to match very high impedances; check the specification before buying.
• A high ratio transformer. Some builders use a broadband transformer to bring the impedance down. This is more difficult with a very high feedpoint impedance than with an end-fed half-wave, so treat it as an experiment and measure the losses. Our guide to baluns and ununs explains how the ratios work and where they run out of range.
• Feeding at another point. Moving the feed away from the end changes the impedance, and some builders feed near the top corner with a matching section. This adds complexity and is worth trying only after the basic version works.

Whatever you choose, put the matching device at the antenna end, with a short connection to the wire, and run coaxial cable back to the shack. Use a common-mode choke on the coax near the feedpoint, because a voltage-fed antenna with no ground system can push RF onto the shield. The goal is to keep the antenna current on the antenna and the coax quiet.

Why There Are No Radials

A quarter-wave vertical, known formally as a monopole antenna, is half an antenna and uses the ground, or radials that stand in for the ground, as the other half. If the ground return is poor, much of the transmitted power is lost as heat in the soil. A half-square is a complete resonant wire radiator and does not depend on a ground return in that way. The current maxima are high above the ground and the ends, where the voltage is high but the current is small, are the only parts near it. For that reason the antenna can be erected over almost any ground without a radial field.

That does not mean the ground is irrelevant. The earth still shapes the elevation pattern, because the signal that leaves downward is reflected by the ground and combines with the direct signal. Soil conductivity changes how strongly this happens, and it explains why a half-square on a wet or coastal site often performs better than one over dry sand. For a comparison with the radial-based option, read our guide to vertical antenna radials, which shows how much effort a conventional vertical needs for similar efficiency.

Half-Square Antenna Compared With Other Wires

The table below puts the half-square next to the most common alternatives. It is a qualitative guide, because real results depend on height, ground and clutter.

The half-square gives you vertical polarisation without a radial field and with more directivity than a plain vertical, at the cost of a trickier feed and the need for two supports. A dipole is easier. A vertical covers all directions at once. If you want the low-angle behaviour of a vertical on 80 metres and have the room for long radials, the inverted L is a good alternative that needs only one support, although it brings back the radial question.

Directions, Variants and Ideas

Because the antenna has two lobes, the direction you hang it matters. Pick the line that places the two broadside lobes toward your most important targets. For a station in Europe, a north-south wire favours directions toward the east and west, which may suit Asia and the Americas. If you work all directions equally, a vertical is still a safer choice, and you can later add a second half-square at right angles for a switchable pair.

There are a few well-known relatives. A bobtail curtain adds a third vertical to make two half-square sections with a single feed at the middle vertical, and it gains directivity and a narrower beam. A sloping half-square tilts the verticals when supports are shorter than a quarter wave. A half-square with a horizontal top bent to fit a small plot is possible at some cost to the pattern. These variants give you ways to adapt the idea, but start with the simple shape and measure it before changing anything.

Building and Tuning Steps

1. Cut the wire about five percent longer than the calculated length, so there is room to trim.
2. Erect the two supports at about half a wavelength apart and hang the top wire at the highest practical height.
3. Hang the verticals straight down and keep their lower ends clear of people and of other conductors.
4. Connect the matching device at one lower end and run the coax back through a common-mode choke.
5. Use an antenna analyser to find the resonant point with the matching network in place, and adjust the network for the lowest SWR on your target frequency.
6. Trim the wire in small steps if the resonance falls far from the band segment you want, and record the final dimensions.

An analyser such as a NanoVNA makes this much easier, and our NanoVNA guide explains how to calibrate it and read the sweeps so that you can judge the matching network as well as the wire.

Safety, Noise and Practical Cautions

The ends of the wire carry high RF voltage. At 100 W the voltage at a high-impedance end can be very high, enough to give a painful burn and to arc at a sharp point. Keep the ends high and insulated, and never leave them where people or pets can touch them while you transmit. Protect the feed point from rain with a plastic box or a good enclosure, because water and high voltage are a poor combination.

A vertically polarised antenna picks up more man-made noise than a horizontal wire in many urban settings. That includes switching power supplies, LED lighting and other household electronics. If your noise floor is already high, expect the half-square to listen to it as well. Putting the antenna away from the house helps, and a common-mode choke at the feedpoint keeps the feedline from acting as a noise antenna.

Lightning is another reason for care. A wire strung between two trees is exposed to it, as is the matching box. Disconnect the coax during storms and follow the station grounding practices in our guide to station grounding and lightning protection.

Finally, remember that the antenna is built for one band. A half-square cut for 40 metres does not work well on 20 metres without a tuner, and the pattern changes on other bands because the electrical lengths are different. If you want several bands from one wire, a fan dipole or a multiband doublet is a better tool, and a half-square is the right choice when one band matters most.

The Bottom Line

A half-square antenna is a simple wire that behaves like two in-phase vertical radiators. It offers vertical polarisation, a low take-off angle and a useful bidirectional pattern, and it gets its performance without a radial field. The price is a high-impedance feedpoint that needs a proper matching network, high RF voltage at the wire ends and a need for two supports half a wavelength apart. On 40 metres it is a very reasonable project, and on 80 metres it is rewarding when you can get the wire high enough. Plan the direction of its broadside lobes, build it carefully, measure it with an analyser and keep it safe, and it can be one of the better DX antennas you can make from a roll of wire.

Frequently Asked Questions

Does a half-square antenna need radials?

No. It is a complete resonant wire radiator, so it does not rely on a ground return the way a quarter-wave vertical does. A station safety ground is still sensible, but it is not part of the radiating system.

What is the feedpoint impedance of a half-square?

When fed at the lower end of a vertical, it is very high, typically in the thousands of ohms. That is why a matching network, such as a parallel-tuned circuit or a remote tuner, is needed to reach 50 ohms.

Which way does a half-square radiate?

The strongest signals leave perpendicular to the plane of the wire, in both directions, and there is little radiation along the line of the wire. Orient the antenna so that the two broadside lobes point toward the regions you want to work.

Is a half-square better than a dipole for DX?

For low-angle vertical polarisation it can be, particularly on 40 and 80 metres where a low horizontal dipole radiates mostly at high angles. A dipole is easier to feed and works better for nearby contacts, so the answer depends on the paths you care about.

Can I use a half-square on more than one band?

Not easily. It is resonant on one band, and on others the pattern and impedance change. A tuner can load it elsewhere, but the performance is less predictable than on its design band.

How high should the half-square be?

The vertical sections should be about a quarter wavelength tall, which is roughly 10 metres on 40 metres. Lower versions still work, but they lose some of the low-angle advantage, and the lower wire ends must be kept out of reach because of the high voltage.