Four-Square vs Half-Square Antennas: An Honest Deep Dive

The half-square, bobtail curtain, and four-square are all ways of getting vertically polarized, low-angle HF radiation without putting a full-size rotatable beam in the air. They overlap in purpose, but they are not the same kind of antenna.

A half-square is a two-vertical broadside wire array. A bobtail curtain is a three-vertical broadside wire array. A four-square is an actively phased four-vertical array with switchable direction and intentional rear cancellation.

The half-square uses two roughly quarter-wave vertical legs joined by a roughly half-wave horizontal wire. The horizontal section mainly acts as the phasing section between the two vertical radiators. The bobtail curtain extends the idea by adding a third vertical and another horizontal phasing section. Its total horizontal span is about one wavelength, and in the usual current-fed form the center vertical carries roughly twice the current of each outer vertical, giving a current distribution close to 1:2:1.

The four-square is a different animal. It uses four verticals arranged in a square, usually with spacing around a quarter wavelength per side, and a phasing/switching system to make the array fire in selected directions. When a four-square fires diagonally, the two middle elements act together like the center element of an effective three-element array. So the desired current distribution is related to the same 1:2:1 array idea, but now that distribution must be produced deliberately by the feed system.

Low-Angle Does Not Always Mean a Four-Degree Peak

A common oversimplification is to say that half-squares, bobtails, and four-squares all “peak at 4–6 degrees.” That can happen over exceptional terrain, salt water, or favorable ground reflection. It is not a safe general statement for flat average ground.

On average ground, modeled peak elevation angles are often closer to the high teens or low twenties. Published half-square and bobtail examples on 40 m often show peak angles around 18–23 degrees, depending on soil and height. A practical 21 MHz four-square described in QEX was also modeled with its main elevation lobe near 20 degrees over its practical ground system.

That does not make these poor DX antennas. Quite the opposite. They usually have useful energy at low elevation angles and less wasted high-angle radiation than many low horizontal wires. But the peak angle is site-dependent. Soil, slope, nearby objects, radial efficiency, feedline radiation, and terrain hundreds of meters away can all matter.

The better comparison is not “which antenna peaks at 5 degrees?” The better questions are:

• How much useful field does it produce from about 5 to 20 degrees?
• How clean is the azimuth pattern?
• How much does the feed system disturb the intended currents?
• How much does the environment change the match and pattern?

Half-Square: Two Verticals, Simple Phasing, Fixed Bidirectional Pattern

The half-square is best understood as two vertical radiators fed in the correct phase by the top wire. The two vertical sections do most of the useful low-angle vertically polarized radiation. The top horizontal wire is not perfectly non-radiating, but much of its radiation cancels because of the current directions on that wire.

A normal down-hanging half-square has two vertical legs hanging from the ends of a horizontal top wire. The bottom ends of the verticals are high-voltage, low-current points. The top corners are high-current, lower-voltage points.

For a current-fed half-square, the best practical feedpoint is one of the top corners. The coax center conductor goes to one side of the break, and the shield goes to the other side, with a good 1:1 current choke at or very near the feedpoint. The vertical section below that feedpoint is not merely a “counterpoise” in the casual sense. It is one of the radiating vertical elements and the return side of the feed.

Published examples show why this matters. A corner-fed half-square can present a feedpoint impedance in the practical coax range, often around 50–75 ohms depending on geometry and ground. The same antenna fed at a bottom high-voltage end can present several thousand ohms with large reactance. The radiation pattern may be similar, but the feedpoint behavior is completely different.

Bobtail Curtain: Three Verticals, More Aperture, Still Fixed Bidirectional

The bobtail curtain is the next step up from the half-square. It uses three vertical sections and a total horizontal span near one wavelength. In a current-fed bobtail, the clean feedpoint is at the top of the center vertical. The center vertical hanging downward is the main current return and the center radiator. The top horizontal wires phase-feed the two outer verticals.

The bobtail has more aperture than the half-square and a narrower broadside pattern. Practical models commonly place a well-built bobtail in the same broad gain class as small vertical arrays, without requiring four driven elements, a phasing box, or a radial field.

That is why the bobtail is such an interesting antenna on 20–10 m. It is not magic, but it is an efficient wire array. It gives more pattern control than a half-square while remaining mechanically and electrically much simpler than a four-square.

However, a single bobtail is still bidirectional. It has two broadside lobes. It does not have the rear cancellation of a four-square. Describing a half-square or bobtail as having “front-to-back” is usually the wrong metric. The better terms are front-to-side ratio, end-fire null depth, or side rejection. The “back” of a half-square or bobtail is normally the other useful broadside lobe.

Four-Square: Better Control, More Gain, More Failure Modes

A four-square normally uses four ground-mounted quarter-wave verticals. The array is switched in four directions by changing the current amplitude and phase relationships between the elements. The reward is unidirectional gain, switchable headings, and useful nulls.

A well-built four-square can outperform a half-square or bobtail in three important ways:

• It can put more field in one selected direction.
• It can reject signals and noise from the rear.
• It can switch directions without physically moving wires.

A practical 21 MHz four-square can land in the range of roughly 7–8 dBi modeled forward gain over practical ground, with measured front-to-back ratios above 20 dB when the system is correctly adjusted. That places the four-square roughly a few dB ahead of a good bobtail in many real installations, and often several dB ahead of a half-square.

But the more important advantage is not always raw gain. It is pattern control. A 20 dB rear null can be more valuable than 2 dB of forward gain when the problem is QRM, local noise, or a loud station behind you.

The cost is precision. A four-square is not just four verticals and a box. The element currents, phase relationships, feedline electrical lengths, mutual coupling, and ground losses all interact. Equal power division is not the same as correct four-square operation. The important quantity is the correct current and phase at the elements.

The Feedpoint Issue: High-Voltage Feed Versus High-Current Feed

This is one of the most important practical differences between a stable wire array and a touchy one.

On a down-hanging half-square, the high-voltage points are the two bottom ends of the vertical legs. On a bobtail curtain, the traditional high-voltage feedpoint is the lower end of the center vertical, usually with a tuned circuit or transformer.

Those bottom feedpoints are physically convenient because the matching box can be near ground. Electrically, however, they are among the most sensitive points on the antenna.

At a high-voltage, low-current point, the impedance may be several thousand ohms. That means tiny stray capacitances and leakage paths are no longer tiny in circuit terms.

At 14 MHz, 2 pF of stray capacitance is roughly 5,600 ohms of reactance. Five pF is roughly 2,300 ohms. At 28 MHz, the same 5 pF is roughly 1,100 ohms. That is a major shunt path if the antenna feedpoint is several thousand ohms. It is almost irrelevant if the feedpoint is 40–70 ohms.

This is why a voltage-fed half-square or bobtail can tune perfectly in one location and drift in another. Wet rope, damp fiberglass, tree branches, a nearby metal mast, the coax shield, the operator’s hand near the box, or changing soil moisture can all become part of the matching system.

The voltage levels also become serious. If a high-impedance feedpoint is around 4,000 ohms, then 100 W implies about 630 V RMS at the feedpoint before considering reactive voltage magnification in the tuner. At 1 kW, that rises to about 2 kV RMS. By contrast, 100 W into 50 ohms is about 71 V RMS, and 1 kW into 50 ohms is about 224 V RMS.

That voltage difference explains why high-Z feeds are more weather-sensitive. Leakage across wet surfaces is driven harder. Stray capacitance matters more. Insulators matter more. The matching box becomes part of the antenna.

Why Current Feeding Is More Predictable

A current feed is not “better” because it changes the basic radiation mechanism. At the fundamental frequency, a corner-fed and an end-fed half-square can produce very similar radiation patterns. The difference is feedpoint stability.

A low-voltage, high-current feed is more predictable because:

• The feedpoint impedance is low enough that small stray capacitances become far less important.
• The voltage stress on ropes, insulators, and nearby objects is much lower.
• The feedline shield can be isolated with a practical 1:1 current choke.
• The feed system is governed mostly by wire geometry instead of accidental capacitance to the environment.
• The coax is less likely to become an unintended radiator.

The last point matters. Any off-center or asymmetric coax feed can put common-mode current on the outside of the coax shield. A good current choke does not “balance” the antenna by magic. It simply makes the outside of the coax a high-impedance path so the intended antenna wires carry the RF current.

For a half-square, put the choke at the top corner feedpoint. For a bobtail, put the choke at the top of the center vertical feedpoint. Routing the coax away at about 45 degrees is good practice because it keeps the coax from running parallel to a high-current vertical wire and reduces coupling into the feedline. But routing is not a substitute for choking. Use both.

Material 43 ferrite can be useful in the upper HF range, especially from 20 m upward, but the actual choke impedance depends on core size, number of turns, winding layout, coax diameter, and power level. The choke should be designed and measured as a choke, not assumed from the material number alone.

Ground Dependence: No Radials Does Not Mean No Ground Effects

A half-square or bobtail does not need a ground radial system to complete the RF circuit. The antenna is self-contained. The return current is in the antenna structure itself.

But the ground still matters because radiation reflected from the earth combines with direct radiation. Soil quality changes the elevation pattern and the low-angle field. Nearby ground can also absorb energy from the near field.

A ground-mounted four-square is both ground-dependent and radial-dependent. Its verticals are monopoles using the earth and radial system as the return path. Poor or asymmetric radials create loss and disturb the feedpoint impedances. Disturbed impedances change the element currents, and changed element currents damage the pattern.

Elevated-radial four-squares can work well, but then each element needs its own tuned and symmetrical elevated radial system. That can reduce wire count, but it increases mechanical and tuning complexity.

Practical Performance Comparison

The following table is not a promise. It is a practical engineering comparison for well-built monoband antennas over ordinary land, assuming the half-square and bobtail are current-fed and the four-square has a competent ground and phasing system.

Approximate Dimensions for 10–30 m

These are approximate electrical starting dimensions using free-space wavelength. Real wire antennas normally need trimming because of wire diameter, insulation, height above ground, end effects, support materials, and nearby objects.

This table shows why half-squares and bobtails are attractive from 30 m through 10 m. The antennas are physically large enough to work efficiently, but still small enough to build with fiberglass poles, trees, light masts, or portable supports.

The bobtail becomes especially interesting on 20–10 m because a one-wavelength wire span is still manageable. On 30 m, a 30 m wide bobtail is already a real structure, but still possible. On 40 m, the same idea is roughly 40 m wide. On 80 m, it becomes roughly 80 m wide, which moves it from “wire antenna” to “property-scale installation.”

How the Three Compare by Band

30 m

On 30 m, the half-square is very practical: about 15 m wide and 7–8 m tall electrically. A bobtail is about 30 m wide, so it needs more real estate but gives a stronger, narrower broadside pattern. A four-square is also physically practical, with about 7.4 m side spacing, but it still needs four verticals, a ground or radial system, feedlines, switching, and phasing.

For a fixed DX direction, the bobtail is hard to beat for simplicity-versus-performance. For multiple headings or receive nulling, the four-square has the advantage.

20 m

On 20 m, all three are realistic. A half-square is compact and easy. A bobtail is still only about 21 m wide. A four-square has a small square footprint, but the radial field and phasing system dominate the work.

This is one of the most interesting bands for comparing them directly. A good bobtail can be close enough to a four-square in raw forward field that the difference may be only a few dB. But the four-square still has the big operational advantage: switchable directions and rear rejection.

If you only care about two opposite broadside directions, the bobtail is much simpler. If you need Europe, Africa, South America, and Asia from the same array, the four-square is the more complete system.

17 m and 15 m

On 17 m and 15 m, the wire arrays become easy to support. The half-square can be very light. The bobtail becomes a compact fixed broadside array. The four-square also becomes mechanically easy, but the feed network remains just as critical.

At these frequencies, a modest rotatable Yagi or Moxon also becomes a serious competitor. A four-square may still be attractive if you specifically want vertical polarization, low-angle response, no tower, or instant direction switching. But if a horizontal beam at useful height is available, it may deliver more gain and cleaner control for less ground-system work.

12 m and 10 m

On 12 m and 10 m, the half-square and bobtail are physically small. A 10 m half-square is only about 5 m wide, and a 10 m bobtail is about 10.5 m wide. They become easy monoband wire arrays.

A 10 m four-square is also compact, but at this point the question becomes whether the phasing system is worth it. A small rotatable beam is often easier and may outperform all three if mounted well. The four-square still has value where vertical polarization, instant switching, or ground-level construction is desired.

Why the Bobtail Is Closer to a Four-Square Than the Half-Square Is

In terms of raw aperture, the bobtail is the more serious wire-array competitor to the four-square.

A half-square has two vertical radiators separated by about half a wavelength. A bobtail has three vertical radiators across about one wavelength. A four-square has four verticals, but when firing diagonally, the two middle elements act together as an effective center element.

That makes the bobtail and four-square conceptually related to a three-element vertical-array current distribution, even though they implement it very differently. The bobtail is broadside and bidirectional. The four-square is phased for unidirectional, cardioid-like or end-fire behavior.

That is why a bobtail can have respectable gain and a clean broadside lobe, but cannot produce the four-square’s rear null without additional parasitic or phased elements.

Why the Four-Square Usually Wins on Receive

Transmit gain matters, but receive pattern often matters more.

A half-square or bobtail reduces response off the wire ends and often has less high-angle pickup than a low horizontal wire. That can improve signal-to-noise ratio for DX. But it still hears equally well in the opposite broadside direction.

A four-square can reject the rear direction by 15–25 dB when properly adjusted. That is a large receiving advantage when the noise or QRM is directional. It can also switch nulls instantly by changing direction.

However, this advantage only exists if the four-square pattern is actually correct. A four-square with poor radial symmetry, bad current balance, lossy feedlines, or incorrect phasing may still show a good SWR while having weak nulls and mediocre gain.

Bandwidth and Tuning

For 30, 17, and 12 m, bandwidth is usually not a major issue because the amateur allocations are narrow. For 20 and 15 m, all three can usually be made to cover the band well if built correctly.

For 10 m, the band is wide enough that a thin-wire resonant array may not cover everything with a perfect match. This is where feedpoint choice and wire diameter matter. A current-fed half-square or bobtail with a low-Z feedpoint is still manageable, but the antenna should be designed for the part of the band actually used.

Four-square bandwidth is not only about SWR. It is also about whether the current amplitudes and phases stay correct across frequency. A good feed system can be stable over a useful percentage bandwidth, but that comes from careful design and measurement, not from simply connecting four verticals to a generic box.

Why Lower Frequencies Often Change the Answer

Below 30 m, the antennas remain electrically valid, but the physical engineering trade-offs change.

On 40 m, a half-square is about 20 m wide and 10 m tall electrically. A bobtail is about 40 m wide. These are still possible, and many excellent 40 m wire arrays use these forms.

On 80 m, the half-square becomes about 40 m wide and 20 m tall. The bobtail becomes about 80 m wide. That is no longer a casual antenna. It is a large installation.

On 160 m, the dimensions become property-scale.

This is why other systems may become more practical on the lower bands, depending on the station. A single vertical or inverted-L with a serious radial system may be easier to support. A two-element phased vertical array may give switchable directivity with less span than a bobtail. A four-square becomes more attractive on 40, 80, and 160 m because high horizontal beams are impractical, but it demands space, radials, and phasing accuracy.

The practical lower-band answer is often split: use the best vertical or phased vertical system you can build properly for transmitting, and use a separate low-noise directional receive antenna if possible.

The Most Honest Comparison

A current-fed half-square is the simplest serious low-angle vertical wire array. It is compact, efficient, and predictable if choked correctly. It is not a four-square replacement, but it can deliver very good DX performance with a fraction of the complexity.

A current-fed bobtail curtain is the stronger wire-array option. It has more aperture, more gain, and a narrower broadside pattern than the half-square. On 20–10 m, it is mechanically very attractive. On 30 m, it is still realistic if the span is available. It is probably the best “no radial field, no phasing box” competitor to a four-square for fixed-direction operation.

A four-square is the superior system when switchable headings, real rear rejection, and maximum control of the azimuth pattern are required. It is also the most complex and the most dependent on correct currents, feedlines, phasing, and ground symmetry.

A bottom high-voltage feed can work, but it is inherently more exposed to weather, stray capacitance, leakage, and feedline participation. A top current feed is less convenient mechanically, but it is usually much more predictable electrically.

For 10–30 m, the clean technical summary is simple:

• Half-square: lowest complexity, fixed bidirectional DX pattern, very practical.
• Bobtail curtain: best fixed-direction wire-array performance without radials.
• Four-square: best controllable pattern and nulls, but requires a real phased-array installation.

The right choice depends less on advertised gain and more on what you can build accurately, keep stable in weather, and aim at the directions you actually need.

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