Four-Square Vertical Array: Phasing, Radials and Gain Explained

Few antennas have such a good reputation among low-band DXers as the four square vertical array. It uses four ordinary vertical radiators, placed at the corners of a square and fed so that their combined signal is strong in one direction and weak in the opposite one. Add a relay box and the same four wires can point towards any of four directions at the push of a button. The result is a low-angle, directional antenna with no rotating parts, and the receive benefit is often just as valuable as the transmit gain on noisy 160, 80 and 40 metres. It is also a project that rewards careful work and punishes carelessness, because the whole idea depends on currents in four elements having exactly the right magnitude and phase. This guide explains how the array works, how it is laid out and phased, what the ground system needs, how to tune it and what to expect in return.

The Idea Behind the Array

A single vertical monopole radiates equally in all horizontal directions. That is useful, but it also means it hears noise and strong signals from every side, and it does not concentrate your power towards the part of the world you want. Combining several elements changes this. When radiators are spaced a fraction of a wavelength apart and driven with controlled phase differences, their fields add in some directions and cancel in others. The general principle is described in the article on phased arrays, and a four-square is one of the neatest amateur applications of it.

The reason verticals suit this approach on the low bands is practical. A horizontal antenna must be high, measured in wavelengths, to radiate at a low angle, and on 160 and 80 metres that is rarely possible. A vertical radiates at a low angle naturally, and a ground-mounted array is as tall as its elements and no more. The explanation of how height affects the takeoff angle in our article on antenna height and takeoff angle shows why this is such an attraction for long-distance work.

Layout: Four Elements, One Square

The classic layout places four identical vertical monopoles at the corners of a square. The side of the square is usually one quarter of a wavelength. Each element is typically a quarter-wave vertical, which is the standard monopole working against a ground system. Because the square is defined in wavelengths, the physical dimensions depend on the band. As an illustration, at 3.6 MHz a wavelength is about 83 metres, so the side of the square comes out near 21 metres; at 7.1 MHz the wavelength is about 42 metres and the side is about 10.5 metres; at 1.83 MHz the side is around 41 metres. These are simple free-space calculations to size your plot, not construction dimensions, and the actual element lengths will come out a little shorter once the velocity effect of the conductor is taken into account.

The main direction of the array points along a diagonal of the square, towards one corner. That is why switching is easy to implement: the array has four corners, so four diagonal directions are available, covering all the compass with a broad lobe in each. With careful design the pattern is fairly broad in the forward direction and shows a pronounced reduction to the rear, which is the property that makes a four-square so good at hearing a weak signal in noise.

Phasing: How the Pattern Is Created

In the classical arrangement all four elements carry currents of equal magnitude, but with different phases. Relative to the element at the back of the array, the two side elements are fed 90 degrees behind and the front element 180 degrees behind. The main lobe points towards the element with the largest delay. If you want to turn the array around, you swap which element has which role, and that is exactly what the switching relay box does.

There are two common ways of producing the required currents:

• Delay lines and power dividers. A feed system splits the transmitter power and sends it to each element through coaxial lengths chosen to give the needed phase shifts. This is the traditional approach and can be very effective, but the lengths must be exact and the system is band-specific.
• Hybrid couplers. A network of 90-degree hybrid couplers splits power and creates the phase relationships electrically. It is more elegant, makes direction switching straightforward and tends to be more tolerant of variations in element impedance.

Both approaches have to deal with a basic complication. Elements close to each other couple strongly, so the impedance of each element is not the same as it would be alone, and it differs from one element to another. The feed system must therefore control the currents, not simply the voltages. This is the reason that a casual attempt to wire four verticals with equal lengths of coax often disappoints: without current forcing, the phase and amplitude at each element will drift away from the design values, and the pattern degrades.

Ground System: Where Most Performance Is Won or Lost

A vertical monopole is only half an antenna; the ground supplies the other half, and any resistance in that return path turns power into heat instead of radiation. The more efficient the ground system, the more of your power ends up in the air. Each element of a four-square needs its own ground system, and because the elements are only a quarter of a wavelength apart on a side, the radials of neighbouring elements overlap and interact.

Two practical approaches are used. On a buried or surface radial field, you lay as many radials as you can afford and bond the fields of adjacent elements together where they cross. The more radials the better, and doubling a small number of them helps far more than adding to a large number. The alternative is elevated radials, which work as a counterpoise raised above the ground; a small number of tuned radials per element, with their ends carefully separated from one another, can be effective and are easier to install on difficult terrain. Our guide to vertical antenna radials explains the trade-offs, and the same rules apply here, multiplied by four.

Switching and Control

The beauty of the four-square is that direction control is electrical. A control box contains relays that connect each element to the appropriate branch of the phasing network. In a hybrid system, rotating the pattern means changing which ports of the coupler feed which elements. The relays are controlled by a small cable and a switch at the operating position, or by a computer. Use good-quality relays rated for the power and the voltage you run, because relay contacts in a low-impedance network carry significant current. Keep control wiring decoupled from the RF, with chokes and bypass capacitors, so that RF does not travel back along the control cable into the shack.

Tuning and Checking the Array

A step-by-step procedure keeps the work manageable. First, build and tune each element on its own, with the other three disconnected or open-circuited at the base, so that every element has the same resonant frequency. An antenna analyzer, such as the one in our NanoVNA calibration guide, makes this easy. Second, cut the phasing lines or build the coupler network to specification and verify them on the bench before connecting them. Third, connect everything and measure the impedance seen at the main feed point in each of the four directions. If the numbers differ from direction to direction, one of the elements or one of the lines is not identical to the others.

Finally, check the pattern. Without a test range, the practical method is comparison. On the air, compare the signal strength of a stable distant station in the forward and reverse directions, and compare with a simple reference antenna. Spot reports from other stations are another useful reference. A front-to-back difference that is far smaller than you expected suggests a phase or amplitude error somewhere, and the fault is usually a damaged cable, a poor connector or a poor ground at one element.

Planning the Site

Before buying any wire, draw the plot. The square needs a clear area, and every conductor near it, including fences, gutters, other antennas and overhead lines, will interact with the pattern. Keep the array away from the house if possible, and remember that a single four-square needs a path for the control and feed cables that does not run parallel to the radials for long distances. Think about access too: you will want to walk between the elements to measure each base impedance and to inspect the radial junctions after a storm. Mowing is another real consideration. Elements are thin and the ground wires are easy to damage, so mark the radials and keep the grass short or bury them shallowly.

Finally, check the local planning and safety rules. Four tall vertical radiators can be a visible structure, and the power you feed them produces radio-frequency fields close to the base that you must keep people and animals away from. The principles in our article on station grounding and lightning protection apply as well: a good ground system and sensible surge protection protect both the array and the equipment in the shack.

What Gain Should You Expect?

Figures quoted for four-square arrays vary with the feed system, the ground and the spacing, so treat any single number carefully. Compared with a single, well-grounded vertical, the array typically offers several decibels of forward gain, often quoted at around 5 to 6 dB, together with a much improved rear rejection. The exact numbers depend on losses in the ground system and the feed network. More important than the transmit gain is the receive improvement: the ability to null a strong noise source or interfering station in the rear direction often decides whether a weak DX signal is readable at all. Our article on low-band receive antennas discusses why that matters.

Four-Square Compared With Other Vertical Options

Common Mistakes

• Unequal elements. Different lengths, different heights of the base or different loading change the currents and spoil the pattern.
• A weak ground system. Skimping on radials gives up gain that no phasing network can recover.
• Poor coax quality or mismatched lengths. Phase depends on length, and velocity factor differs between cable types; use the same cable for all lines. See the explanation of velocity factor for how lengths are calculated.
• Common-mode current on the feedline. Without a good choke at the feed point, the feedline itself becomes part of the antenna.
• Ignoring nearby conductors. Metal fences, buried pipes and other antennas close to the array distort the pattern.

The Bottom Line

A four square vertical array is a serious project, but it solves a real problem: how to have a directional, low-angle antenna on the bands where horizontal antennas cannot be put high enough. The results depend on equal elements, accurate phasing and a thorough ground system, in that order of importance. Get those right and the array gives you several decibels of gain, a useful reduction in noise from the rear and the ability to switch direction in a second. If you do not have room for a quarter-wave square or the time for careful tuning, start with a good single vertical and a receive antenna, and treat the four-square as the step you take when the rest of the station is ready for it.

Frequently Asked Questions

How far apart should the elements of a four-square be?

The classic spacing is one quarter of a wavelength along each side of the square. At 3.6 MHz that is about 21 metres, at 7.1 MHz about 10.5 metres.

How is the direction changed?

By switching which element plays which role in the phasing network. A relay box connects the elements to different ports, so the main lobe can point towards any of the four corners.

How many radials does a four-square need?

As many as you can install. Each element needs its own ground system, and adjacent systems overlap. Elevated radial systems with a few tuned radials per element are a common alternative where burying radials is impractical.

How much gain does a four-square give?

Compared with one good vertical, a commonly quoted figure is about 5 to 6 dB in the forward direction, with strong rejection to the rear. Real results depend on ground losses and the feed system.

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

The dimensions and phasing lines are band-specific, so a separate array or a more complicated switchable network is needed for each band. Most builders optimise for one band at a time.

Is a four-square better than a Beverage for receiving?

They solve different problems. A Beverage is a very quiet receive-only antenna that needs a long run of wire; a four-square works on transmit and receive in a smaller footprint. Many low-band stations use both.