Half Square Antenna Calculator & Design Guide

The dimensions below are nominal free-space starting values. They use one quarter wavelength for each vertical element and one half wavelength for the horizontal element. Actual construction dimensions normally require final adjustment after installation.

Note: The 11 meter half square antenna should be calculated for the actual operating frequency you intend to use rather than assuming one fixed channel or frequency.

40 Meter Half Square Antenna

A 40m half square is physically large: the horizontal wire is approximately half a wavelength long and each vertical section is approximately one quarter wavelength. At 7.150 MHz, the nominal dimensions are about 20.96 m for the horizontal element and 10.48 m for each vertical element before applying any shortening or tuning correction.

The antenna needs enough horizontal span for the top wire and enough height to accommodate the vertical sections without placing the lower ends where they can be touched.

For single-band use, a corner-fed current-fed arrangement is the simpler configuration. An end-fed 40 meter half square can also be explored when a convenient low feedpoint or multiband operation is a priority, but feedpoint impedance and common-mode behavior become more important.

20 Meter Half Square Antenna

A 20m half square provides the same basic geometry in a much smaller footprint. At a reference frequency of 14.175 MHz, each nominal vertical section is about 5.29 m and the horizontal wire is about 10.57 m.

Its vertical polarization and broadside radiation make this geometry particularly interesting where the goal is long-distance HF operation without installing a conventional ground-mounted vertical and radial field.

Final resonance should always be checked at the actual installation height.

15 Meter Half Square Antenna

At 21.225 MHz, a nominal 15 meter half square uses vertical sections of about 3.53 m and a horizontal section of about 7.06 m.

Because the complete structure is smaller, installation becomes easier, but nearby metal, support ropes and other conductors can represent a larger fraction of a wavelength and may noticeably affect tuning and the radiation pattern.

10 Meter Half Square Antenna

A 10 meter half square is compact enough for installations where a 20m or 40m version would be impractical. At 28.400 MHz, the nominal vertical elements are about 2.64 m each and the horizontal element is about 5.28 m.

At these shorter wavelengths, small physical changes can represent a meaningful change in electrical length. Make final adjustments with the antenna in its intended operating position rather than tuning it close to the ground.

11 Meter Half Square Antenna

An 11 meter half square antenna should be designed around the exact operating frequency. Using 27.185 MHz only as a reference, each vertical section is approximately 2.76 m and the horizontal section is approximately 5.51 m before physical correction.

If your operating frequency differs, enter it directly into the half square antenna calculator. Even relatively small frequency changes alter the required antenna length.

A half-square antenna is a wire antenna formed by two vertical sections connected across their upper ends by a horizontal wire. In the classic configuration, each vertical element is approximately one quarter wavelength long and the connecting horizontal element is approximately one half wavelength.

The geometry resembles three sides of a rectangle:

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The two vertical sections are the primary contributors to the vertically polarized broadside field. The horizontal wire also carries RF current and helps establish the required spacing and phase relationship between the vertical sections.

Unlike a conventional ground-mounted quarter-wave vertical, a half-square does not normally require a large ground-radial system. This makes it useful where vertical polarization and low-angle HF radiation are desired but installing radials is inconvenient.

The two vertical elements are separated by approximately half a wavelength. RF current travels through the vertical and horizontal sections as one continuous radiator.

The electrical length and current distribution cause the fields from the two vertical sections to reinforce in broadside directions. The result is a bidirectional radiation pattern with the strongest radiation generally perpendicular to the plane of the half-square.

The basic geometry is:

• Left vertical element: approximately ¼ wavelength
• Horizontal element: approximately ½ wavelength
• Right vertical element: approximately ¼ wavelength
• Total wire length: approximately one wavelength

These are electrical starting dimensions. A real wire antenna rarely resonates at exactly the free-space dimensions because insulation, conductor diameter, end effect, support materials and surrounding objects change its electrical length.

A half-square can be fed in more than one way. The two most useful configurations are a corner-fed current-fed version and a high-impedance end-fed version.

Actual impedance is installation-dependent. Do not select a matching transformer solely from a theoretical resistance value; measure the installed antenna whenever possible.

Corner-Fed Half Square

In the traditional current-fed half-square, the feedpoint is located at one upper corner where a vertical section joins the horizontal wire.

Around the intended resonance, this point can provide a feedpoint impedance that is much easier to match to 50 Ω coaxial cable than feeding the antenna at a voltage maximum.

This configuration is attractive for a dedicated single-band antenna because it can minimize the matching hardware required at the feedpoint.

Route the feedline so that it disturbs the antenna as little as practical. If RF current is detected on the outside of the coax shield, add an appropriate common-mode choke.

End-Fed Half Square Antenna

Feeding the half-square at the end of a vertical section moves the feedpoint to a location where voltage is high and current is low. The resulting feedpoint impedance can be thousands of ohms rather than approximately 50 Ω.

A matching transformer is therefore normally needed between the radiator and the coaxial feedline.

A 49:1 unun is commonly associated with high-impedance end-fed wire antennas. It should be treated as a practical starting choice rather than a promise of a perfect 50 Ω match. Feedpoint resistance and reactive impedance vary with frequency, height, geometry and surroundings.

An end-fed installation also needs an RF return path. Depending on the design, this may involve a short counterpoise wire, the outside of the coax shield, or a combination of both.

If the feedline becomes part of the antenna unintentionally, common-mode current can increase RF in the station, change the radiation pattern and make SWR sensitive to coax length or routing. A properly selected ferrite choke can help isolate the feedline.

A typical half square antenna radiation pattern is bidirectional in azimuth. Maximum radiation is generally broadside to the plane containing the two vertical elements, while reduced radiation or nulls occur toward the ends of the horizontal span.

The exact azimuth pattern, elevation pattern, gain and take-off angle depend on installation height, ground conductivity, nearby structures and the feed configuration.

Diagram caption: Typical patterns shown on this page are representative rather than predictions for every installation.

Azimuth Pattern

Viewed from above, a half-square generally produces two principal lobes perpendicular to the line between its vertical sections.

Simplified top view:

reduced radiation ← verticals / horizontal span → reduced radiation

maximum broadside radiation ↑

maximum broadside radiation ↓

For installation planning, orient the plane of the half-square so that its broadside directions face the areas where stronger coverage is desired.

Real null depth is affected by asymmetry, surrounding conductors, ground and feedline current.

Diagram label: Typical Half Square Azimuth Pattern

Elevation Pattern

One reason operators consider the half-square for HF DX is its potential for useful low-angle radiation from vertically polarized elements.

The elevation pattern is not fixed. Raising or lowering the structure changes its interaction with the ground and can alter the take-off angle, lobe structure and gain.

A pattern generated for one antenna height should therefore not be treated as an exact prediction for an installation at a different height or over different ground.

Diagram label: Typical Half Square Elevation Pattern

Polarization and Gain

A normally installed half-square is predominantly vertically polarized because the two vertical sections make the major contribution to the desired broadside field.

The horizontal section still carries current, so the complete electromagnetic behavior is more complex than treating the antenna as two isolated verticals.

Do not assign one universal gain figure to every half-square. Gain depends on frequency, element dimensions, installation height, ground properties, losses and pattern direction.

Which Direction Does a Half Square Radiate?

The strongest radiation is generally broadside to the antenna.

If the horizontal wire runs north–south, the principal broadside directions will normally be approximately east and west. If the antenna runs east–west, the primary broadside directions will be approximately north and south.

This makes orientation an important design choice before supports are installed.

Step 1: Choose the operating frequency

Decide which part of the band matters most. Enter that design frequency into the calculator rather than relying only on a band name.

For example, a 20m antenna optimized around one part of the band may require slightly different final dimensions than one optimized elsewhere in the band.

Step 2: Calculate the starting dimensions

Calculate:

• two quarter-wave vertical sections;
• one half-wave horizontal section;
• approximate total wire length;
• any additional wire required for connections and final trimming.

Do not cut the radiator to an irreversible final length before testing. Leaving a small amount of adjustment wire is usually more practical than trying to add wire after cutting too short.

Step 3: Select the feed configuration

Choose between:

Corner-fed / current-fed Best suited to a straightforward single-band half-square where the upper feedpoint is accessible.

End-fed Useful where feeding from a lower end is mechanically convenient or where a multiband high-impedance arrangement is being explored.

Step 4: Prepare the supports

Install supports that can hold the horizontal section and allow both vertical sections to hang in the intended geometry.

Keep the wire clear of power lines and other hazardous conductors. Allow safe clearance from places where people can touch the antenna during transmission.

Step 5: Install the radiator

Keep the geometry reasonably symmetrical unless the design intentionally uses different dimensions.

Small deviations will not automatically prevent operation, but significant bends, sloping verticals or nearby conductive objects can change current distribution, resonance and the radiation pattern.

Step 6: Install the feedline and matching components

For a corner-fed antenna, connect the feedline at the selected corner and route the coax away from the radiator as consistently as practical.

For an end-fed design, install the impedance transformer at the feedpoint and provide the intended counterpoise or return path.

Add a common-mode choke where appropriate.

Step 7: Raise the antenna to operating height

Resonance can change significantly when a wire antenna is moved from tuning height to its final position.

Measurements made with the antenna lying on the ground or hanging only a short distance above it are not reliable predictions of its final operating condition.

Step 8: Measure impedance and SWR

Use an antenna analyzer or suitable measurement equipment to find:

• resonant frequency;
• standing wave ratio;
• resistive impedance;
• reactive impedance.

Do not evaluate the antenna from SWR alone. Knowing resistance and reactance makes tuning decisions much clearer.

Step 9: Adjust the wire length

If resonance is too low in frequency, the antenna is generally electrically too long and should be shortened gradually.

If resonance is too high, the antenna is electrically too short.

Make small, symmetrical adjustments where practical, then repeat the measurement at full installation height.

The calculator provides theoretical starting dimensions. Tuning converts those numbers into a resonant antenna for the actual installation.

Start by measuring the feedpoint impedance over a frequency range that includes your target operating frequency.

At resonance, the reactive component approaches zero. The remaining impedance is primarily resistive, although the exact resistance may not equal 50 Ω.

A low SWR reading and resonance are related but not identical concepts. An antenna may be resonant with a resistive impedance different from 50 Ω, or it may show an acceptable SWR while still having some reactance.

If resonance is below the target frequency

The radiator is generally electrically too long.

Shorten it gradually and remeasure.

If resonance is above the target frequency

The radiator is generally electrically too short.

Lengthening the antenna is preferable where practical.

If the reactive impedance is capacitive

A radiator that is electrically short often presents capacitive reactance around the intended resonance.

Increasing electrical length can move the system toward resonance.

If the reactive impedance is inductive

A radiator that is electrically long often presents inductive reactance around the intended resonance.

Reducing electrical length can move the resonant point upward in frequency.

These relationships are useful diagnostic guides, but the complete feed system and surrounding environment can make real measurements more complex.

Feedpoint impedance has both resistive and reactive components:

Z = R + jX

Where:

R = resistive impedance

X = reactive impedance

At or near resonance, reactance approaches zero and the impedance becomes primarily resistive.

The radiation resistance of the antenna is only part of the system. Wire loss, ground interaction, matching transformer loss and feedline loss can all contribute to the measured resistance.

Corner-fed matching

A correctly dimensioned corner-fed half-square can operate in a relatively low-impedance region suitable for connection to 50 Ω coax after practical tuning.

If the measured feedpoint impedance is significantly different from 50 Ω, investigate antenna dimensions, height, feedline routing and common-mode current before assuming an antenna tuner is the only solution.

End-fed matching

An end-fed half-square operates at a high-impedance point and requires impedance transformation.

The transformer must handle both the transformation ratio and the voltage present at the feedpoint. Transformer design, ferrite material, winding method, operating frequency and power level can all influence loss and SWR.

Antenna tuner

An antenna tuner can transform the impedance seen by the transmitter, but it does not physically change the resonant frequency or eliminate losses in the antenna, transformer or coaxial cable.

Whenever possible, tune the radiator and matching system first and use a tuner for the remaining transformation rather than relying on the tuner to conceal a severe mismatch.

A half-square is simplest to predict when designed as a single-band antenna. Multiband operation is possible in some end-fed configurations because a long wire can present additional resonances on harmonically related frequencies.

A 40m end-fed configuration, for example, may offer usable resonant behavior on higher HF bands such as 20m, 15m or 10m depending on its exact electrical length and installation.

However, resonance on another band does not mean the antenna behaves like a scaled half-square on that band.

As operating frequency changes:

• the radiator becomes multiple wavelengths long;
• current distribution changes;
• feedpoint impedance changes;
• additional radiation lobes can appear;
• null directions can shift;
• transformer loss may change;
• coax loss can become more significant.

For multiband use, measure each intended band individually. Check both SWR and impedance, and do not assume that a low SWR guarantees the same radiation pattern as on the fundamental design band.

The geometry of a half-square makes installation height important for both electrical and mechanical reasons.

The top horizontal wire must be high enough to allow the two vertical sections to extend downward safely. The lower ends should remain clear of people, animals and conductive objects.

There is no single universal installation height that produces the same radiation pattern at every location. Ground properties and wavelength both matter.

Orientation

Decide which directions matter most before placing the supports.

The strongest azimuth radiation is generally broadside to the plane of the antenna. The directions along the horizontal span tend to receive less radiation.

Nearby objects

Try to keep the radiator away from:

• metal roofs;
• towers and masts not intended as part of the antenna;
• gutters;
• fences;
• large conductive structures;
• parallel power or communication wiring.

Trees and nonconductive supports may still affect the antenna when wet or when hardware is close to high-voltage points.

The feedline should transfer RF energy between the transmitter and antenna without unintentionally becoming a major part of the radiator.

Common-mode current occurs when RF flows on the outside of the coaxial cable shield. This can change the antenna pattern, alter feedpoint impedance and make SWR depend on feedline length or routing.

Possible symptoms include:

• SWR changing when the coax is moved;
• RF interference in station equipment;
• different analyzer readings when feedline length changes;
• unpredictable radiation pattern;
• RF voltage on equipment.

A ferrite common-mode choke placed at an appropriate point can increase the impedance presented to unwanted shield current.

End-fed systems deserve particular attention because the antenna requires an RF return path. If no intentional counterpoise is provided, part of the coax may perform that function.

Coax loss

Feedline loss increases with frequency, cable length and SWR.

A lossy coax cable can make the SWR measured at the transmitter appear better than the actual mismatch at the antenna because some reflected energy is dissipated before it returns to the meter.

For meaningful troubleshooting, measure as close to the antenna feedpoint as practical.

Choose a half-square when

A half-square is worth considering when you want vertical polarization, broadside directivity and a wire antenna that does not depend on a conventional ground-radial system.

Choose a dipole when

A dipole is often simpler when horizontal polarization, easy center feeding and broadside coverage fit the installation.

Choose a conventional vertical when

A vertical may be preferable when approximately omnidirectional azimuth coverage is more important than the directional broadside pattern of a half-square and a suitable radial or ground system can be installed.

Choose an EFHW when

An EFHW may be more convenient when a single end feedpoint and multiband harmonic operation are priorities and the installation can accommodate the required matching transformer and common-mode management.

Advantages

Vertical polarization without a conventional ground radial field The antenna can provide vertically polarized radiation without requiring the radial system normally associated with a ground-mounted quarter-wave vertical.

Useful broadside directivity Its bidirectional pattern can favor two opposite directions rather than distributing energy equally around the horizon.

Simple wire radiator The main antenna can be made from one continuous length of wire arranged as two vertical sections and one horizontal section.

Multiple feed options A corner-fed version can simplify single-band matching, while an end-fed arrangement can provide a mechanically convenient lower feedpoint.

Potential for low-angle HF radiation The geometry is attractive for installations where low-angle radiation toward selected broadside directions is desired.

Limitations

Requires horizontal span and vertical clearance A 40 meter half square is a physically large antenna.

Not omnidirectional Nulls or reduced radiation in the directions along the antenna may be undesirable for some operating goals.

Installation affects tuning Height, nearby conductors and feedline routing can noticeably change resonance and impedance.

End-fed versions require careful RF management High feedpoint impedance, transformer losses, counterpoise behavior and common-mode current must be considered.

Multiband patterns can become complex An antenna that shows a usable SWR on a harmonic band does not necessarily preserve the fundamental-band radiation pattern.

The calculator begins with free-space wavelength:

Wavelength λ (meters) = 299.792458 / frequency in MHz

For a basic half-square:

Each vertical section = λ / 4

Horizontal section = λ / 2

Approximate total radiator length = λ

Example at 14.175 MHz

Wavelength:

299.792458 / 14.175 ≈ 21.15 m

Each vertical:

21.15 / 4 ≈ 5.29 m

Horizontal section:

21.15 / 2 ≈ 10.57 m

Approximate total wire:

5.29 + 10.57 + 5.29 ≈ 21.15 m

These values represent nominal electrical geometry before practical shortening or trimming.

Applying a correction factor

If a physical-length correction factor is selected:

Corrected vertical length = λ / 4 × correction factor

Corrected horizontal length = λ / 2 × correction factor

A factor below 1.00 shortens the physical dimensions relative to free-space values.

Do not assume one universal correction factor for every antenna. Insulated wire, bare wire and different installation environments can produce different results.

Electrical length

Electrical length describes a conductor in terms of phase rather than physical distance alone. Two wires with identical physical dimensions can behave slightly differently when conductor construction and surrounding materials differ.

This is why the calculated antenna length should be treated as the starting point for measurement.

Velocity factor

Velocity factor is especially important when calculating the electrical length of a coaxial cable, transmission-line section or matching stub because waves travel more slowly inside the cable than in free space.

Do not automatically apply the coax cable's published velocity factor to the radiating antenna wire. For the radiator, use an appropriate antenna shortening or correction factor based on its construction and measured behavior.

End effect

The ends of a real antenna do not behave like perfectly abrupt theoretical boundaries. End effect, conductor diameter and nearby dielectric materials can make resonance occur at a physical length different from an exact free-space fraction of a wavelength.

Inductive and capacitive loading

If there is not enough physical space for a full-size antenna, inductive loading can increase electrical length without using the full calculated wire dimension. Capacitive techniques can also modify the electrical behavior of a shortened radiator.

Loading changes current distribution, impedance, efficiency and potentially the radiation pattern. A full-size half-square is therefore the simpler reference design whenever sufficient space is available.

Reverse frequency calculation

If you know the measured effective wavelength:

Frequency in MHz = 299.792458 / wavelength in meters

For practical tuning, however, it is usually more useful to measure the resonant frequency directly and then adjust the radiator dimensions proportionally.