Build a Half-Square Antenna
The half-square is one of the most overlooked and underappreciated wire antennas in amateur radio — a simple two-element structure made entirely from wire that delivers genuine low-angle DX gain of 3–4 dBd without a ground plane, without radials, and without a tower. Shaped like an inverted U with the two vertical legs hanging down from a single horizontal section, the half-square combines the low-angle radiation of a vertical with the convenience of a wire antenna needing only two supports. On 40m it fits between two trees 33 feet apart; on 20m it spans just 16 feet. Compared to a dipole at the same height, the half-square delivers noticeably stronger signals at low elevation angles — the angles that matter for DX. This guide covers the theory, dimensions, feedpoint options, matching, installation, and a two-element phased half-square array for serious DX operators.
Half-Square Geometry and Operation
The half-square is two quarter-wave verticals connected at their tops by a half-wave horizontal wire — forming an inverted U shape. Each vertical leg hangs downward from the ends of the horizontal section, with the bottom of one leg being the feedpoint and the bottom of the other left open. The name comes from the shape: it is half of a full-square (quad) loop:
Radiation Pattern — Why It Beats a Dipole for DX
The half-square's gain advantage over a dipole is entirely at low elevation angles — the angles that matter for long-distance HF propagation. At high angles (NVIS, regional) a dipole is competitive or superior. At the low angles needed for DX, the half-square wins clearly:
Feedpoint Impedance and Matching Options
The half-square feedpoint is at the bottom of one vertical leg — a high-current, low-impedance point. The feedpoint impedance is approximately 50–70 Ω in free space, but can vary significantly with installation height above ground and ground conductivity:
Orientation and Radiation Direction
The half-square radiates broadside to its plane — perpendicular to the horizontal wire. Understanding this is critical for aiming the antenna toward the desired DX target:
- Broadside pattern: maximum radiation occurs broadside to the plane of the antenna — perpendicular to the horizontal wire and the two vertical legs. If the horizontal wire runs north-south, the antenna radiates primarily east and west.
- Bidirectional: the half-square is bidirectional — it radiates equally in both broadside directions. A north-south wire radiates east AND west simultaneously. To favour one direction, a second half-square can be phased (see phased array section).
- Orientation for DX: for a US station wanting European contacts on 40m, orient the horizontal wire north-south so it radiates east toward Europe (and west toward the Pacific). For Japan contacts, orient east-west for north-south radiation — but there is no ideal single orientation that favours all DX directions simultaneously.
- Height of horizontal wire: the higher the horizontal section, the better the low-angle radiation. The vertical legs below must hang freely — they should not be tied to masts or walls that would disturb the current distribution. Keep at least 3 feet of clearance between each vertical leg and any metal object.
- Ground below the verticals: unlike a vertical with radials, the half-square does not require a ground plane. Ground conductivity below the vertical legs does affect the feedpoint impedance and slightly affects pattern, but the antenna works well over typical garden ground without any ground system.
| Band | Frequency | Horizontal wire (λ/2) | Each vertical leg (λ/4) | Total wire | Min. support height | Support span |
|---|---|---|---|---|---|---|
| 40m | 7.150 MHz | 65.6 ft (20.0 m) | 32.8 ft (10.0 m) | 131 ft (40.0 m) | 35–40 ft | 67 ft |
| 30m | 10.125 MHz | 46.3 ft (14.1 m) | 23.2 ft (7.1 m) | 92.6 ft (28.2 m) | 25–30 ft | 47 ft |
| 20m | 14.150 MHz | 33.1 ft (10.1 m) | 16.6 ft (5.0 m) | 66.2 ft (20.2 m) | 18–25 ft | 34 ft |
| 17m | 18.100 MHz | 25.9 ft (7.9 m) | 12.9 ft (3.9 m) | 51.8 ft (15.8 m) | 15–20 ft | 27 ft |
| 15m | 21.200 MHz | 22.1 ft (6.7 m) | 11.1 ft (3.4 m) | 44.2 ft (13.5 m) | 13–18 ft | 23 ft |
| 12m | 24.940 MHz | 18.8 ft (5.7 m) | 9.4 ft (2.9 m) | 37.6 ft (11.5 m) | 11–15 ft | 19 ft |
| 10m | 28.500 MHz | 16.4 ft (5.0 m) | 8.2 ft (2.5 m) | 32.8 ft (10.0 m) | 10–13 ft | 17 ft |
Materials for a single-band 40m half-square — scales directly to any HF band
Building the 40m Half-Square
This guide builds a single-band 40m half-square. The same procedure applies to any band — substitute dimensions from the table above. Work from the top down: cut all wire, assemble the complete antenna on the ground, then raise. Orienting the horizontal wire perpendicular to your primary DX direction before raising saves repositioning later.
Cut and Lay Out the Wire
Cut the wire as a single continuous run of 131 feet — do not cut it into three separate pieces. Working with a single continuous wire eliminates two solder joints in the antenna and produces a smoother current distribution through the corners. Mark the wire at two points: 32.8 feet from one end (the feedpoint end), and 32.8 feet from the other end (the open end). The central 65.6-foot section between these marks is the horizontal wire; the 32.8-foot sections at each end are the vertical legs.
Lay the wire on the ground in its installed shape — horizontal section stretched out, vertical legs folded downward at 90° at each end. This allows you to check all dimensions and identify where corner insulators and support attachments will go before the antenna goes in the air.
Prepare the Feedpoint
The feedpoint is at the bottom tip of one vertical leg — the end of the wire that is 32.8 feet below one corner. At this point, strip 2 inches of insulation and connect the 1:1 current choke balun. The balun's balanced terminal connects to the wire end, and the coax connects to the balun's unbalanced (SO-239) terminal. The coax shield connects to the balun's ground terminal — which in this case goes to a short wire connected back to the feedpoint wire itself to close the circuit, or simply to earth via a short ground stake at the feedpoint location.
The open end (other vertical leg bottom) needs only a small end insulator to terminate the wire cleanly. It carries no feedline connection and hangs free — do not ground it, do not connect a counterpoise, and do not let it touch the mast or ground. It must be electrically floating.
Select Supports and Plan the Raise
The half-square requires two supports separated by 67 feet (the horizontal wire length plus a small catenary allowance) at a height of 35–40 feet. Trees are ideal — the antenna's weight is light and the support point needs only to handle the wire catenary load. The two corner insulators hang from the horizontal wire at the transition to the vertical legs, and the horizontal wire itself attaches to the support halyards at two points above the corners.
Orient the horizontal wire perpendicular to your primary DX direction before raising. For a US east-coast station wanting European contacts on 40m, the horizontal wire runs north-south so the antenna radiates eastward (and westward). Mark the support points and confirm the orientation with a compass before committing to the installation.
Raise the Antenna
Raise both ends of the horizontal wire simultaneously using halyards over the two support points. As the horizontal wire rises, the vertical legs hang freely below the corners — allow them to swing freely during the raise, they will settle into position as the horizontal wire reaches full height. Do not pull the vertical legs taut or restrain them — they must hang freely and vertically.
Once the horizontal wire is at full height, check that both vertical legs hang straight down and are not twisted, wrapped around each other, or in contact with any support structure. The feedpoint end (bottom of one leg) should be accessible for the coax connection — route the coax away from the vertical leg at the feedpoint so the coax does not run parallel to the leg for any significant distance, which would capacitively couple into the antenna.
Connect Coax and Measure Feedpoint Impedance
Connect the coax to the feedpoint balun and run it to the NanoVNA at the shack end. Sweep 6–8 MHz and locate the resonance point — the SWR minimum. For a correctly built 40m half-square at 35–40 ft height, the resonance should fall near 7.15 MHz and the SWR at resonance should be 1.2:1 to 2.5:1 directly into 50 Ω coax.
Verify Low-Angle Performance and Weatherproof
Once SWR is confirmed, verify the antenna's low-angle performance by monitoring DX signals on 40m and comparing to another antenna. The half-square's advantage shows most clearly on signals arriving from the broadside direction at distances of 5,000–10,000 miles — these signals arrive at elevation angles of 5°–15° where the half-square has its peak gain advantage over a dipole.
Run WSPR on 40m for 24–48 hours and compare the spot map to nearby stations. The half-square should produce more spots at intercontinental distances in the broadside direction compared to a dipole at the same height. This provides an objective, quantitative comparison that confirms the antenna is performing as expected.
Phasing Two Half-Squares for Directional Gain
A single half-square is bidirectional — it radiates equally in both broadside directions. Two half-squares spaced λ/4 apart and fed 90° out of phase form a cardioid pattern: maximum gain in one broadside direction and a deep null in the other. This two-element array adds approximately 3–4 dB of gain over a single half-square and provides front-to-back ratio of 20+ dB:
Half-Square vs Other DX Wire Antennas
Understanding where the half-square fits among other wire DX antennas helps decide when it is the right choice:
- vs dipole at the same height: the half-square wins by 5–8 dB at low elevation angles (DX) while losing at high angles (regional). Clear choice for DX-focused operation.
- vs delta loop: a full-wave delta loop fed at the bottom corner also produces low-angle radiation. The delta loop requires three supports and more wire but is self-supporting from a single top support. The half-square needs only two supports and simpler construction. Performance is comparable on DX angles.
- vs vertical with radials: a quarter-wave vertical with a good radial system produces omnidirectional low-angle radiation — useful in all directions simultaneously. The half-square is bidirectional (or unidirectional when phased) — better gain in the favoured directions but no coverage off the ends. If you need coverage in all directions, the vertical wins. If you have a primary DX direction, the half-square (or phased pair) wins.
- vs EFHW at low heights: an end-fed half-wave at low heights (under 30 ft) radiates primarily at high angles (NVIS). The half-square at the same height radiates at lower angles. For DX from modest heights, the half-square is clearly the better choice.
- vs 2-element Yagi: a 2-element wire Yagi has similar gain to a single half-square but is unidirectional and requires a boom structure. The half-square's simplicity — a single wire needing only two standard supports — is a significant construction advantage.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| No resonance visible in sweep — flat high SWR | Open circuit — wire break or feedpoint connection failure | Check DC continuity from feedpoint through full wire length; check balun terminal connections | Repair wire break; re-solder feedpoint connection; verify coax centre connects to wire end and shield to balun ground |
| Resonance present but no DX advantage over dipole | Open end grounded or touching metal; vertical legs not hanging freely; horizontal wire too low | Verify open end floats freely; check both legs hang vertically; confirm height of horizontal wire | Remove any ground connection from open end; add clearance from metal objects; raise horizontal wire higher |
| RF in shack on 40m | Missing or inadequate balun — coax shield carrying RF | RF tingle on equipment chassis confirms common-mode current | Install or replace 1:1 current choke balun at feedpoint; add ferrite choke on coax at shack entry |
| SWR at resonance 3:1 or higher | Feedpoint impedance significantly above or below 50 Ω due to height or ground effects | Measure Z with NanoVNA at resonance — note R and X components | If R < 40 Ω: add series capacitor (100–200 pF). If R > 60 Ω: add series inductor or L-network. Adjust while monitoring SWR. |
| Resonance shifts 50–100 kHz day to day | Wire elongation with temperature; or vertical legs blowing in wind and changing effective length | Compare resonance on hot vs cold days; observe leg movement in wind | Accept temperature shift as normal (±20 kHz is typical); add light tension to vertical legs with non-conductive cord tied to stakes to reduce wind movement |
| Pattern appears symmetrical — no benefit from phasing | Phasing cable wrong length or velocity factor error; or coax connections swapped at one element | Calculate electrical length of 75 Ω delay cable using actual measured VF; verify cable connections | Re-measure actual coax VF with NanoVNA; recalculate physical cable length; verify which element has the delay cable |
| One direction works well, other direction has poor performance | In a phased array: delay cable connected to wrong element for desired direction; or asymmetric vertical leg hang | Swap delay cable between elements; compare signal from both sides | Swap delay coax to reverse preferred direction; verify both vertical legs hang vertically with equal clearance from supports |
Does the half-square need a ground plane or radials?
No — this is one of the half-square's most significant advantages over a conventional vertical. The half-square is a self-contained resonant structure that does not depend on ground conductivity for its operation. The open end of the antenna acts as a capacitive termination, and the antenna's radiation is a result of the in-phase currents in the two vertical legs rather than a ground-return current. A simple earth stake at the feedpoint provides a local reference for the coax shield — this is not a radial system and does not need to be extensive. Three 6-foot radials at the feedpoint base are more than adequate for reference purposes.
Can I build a half-square for multiple bands?
A single half-square is resonant on one band only — unlike a dipole, there is no useful harmonic relationship between the half-square's operating band and other bands. For multi-band use, the most practical approach is to build separate half-squares for each desired band and feed them from a coax switch. On 20m and 15m the antenna is small enough that two half-squares sharing the same two supports (one for 20m, one for 15m) with wires woven at slightly different angles is quite practical. Trap half-squares have been described in amateur literature but the traps add loss and complexity that reduce the antenna's main advantage — its simplicity and efficiency.
How does the half-square perform compared to a full-wave loop?
A full-wave horizontal loop at 40 feet radiates primarily at high angles on 40m — good for regional NVIS but not for DX. A full-wave vertical loop (delta or square) fed at the bottom has similar low-angle radiation to the half-square but requires three or four support points and more wire. The half-square achieves similar or slightly better low-angle gain than a full-wave vertical loop with simpler construction — two supports and a single wire. For a DX-focused station wanting the best wire antenna performance from simple supports, the half-square competes favourably with loops of comparable size and complexity.
What is the minimum height for the horizontal wire?
The horizontal wire should be at least λ/4 above ground for the half-square to produce meaningful low-angle radiation. On 40m this means at least 33 feet. Below this height the ground reflection increasingly degrades the low-angle radiation pattern. At 25 feet on 40m the half-square still produces better low-angle radiation than a dipole at the same height, but the advantage is reduced. Every additional foot of height for the horizontal wire improves the DX performance. The practical minimum for a useful half-square on 40m is 25 feet; 35–40 feet produces significantly better results and should be the target where tree or mast support allows.
Can I use the half-square for portable or SOTA/POTA operation?
Yes — on 20m and 15m the half-square is compact enough for portable use. A 20m half-square uses 66 feet of wire total and can be supported on two 18-foot fibreglass poles with the horizontal wire at 18 feet — a realistic portable setup. On 15m the total wire is only 44 feet. The half-square's low-angle radiation is particularly valuable for SOTA activations where you want to reach distant chasers efficiently with limited power. Its no-radial requirement means faster deployment compared to a portable vertical. A 20m or 15m half-square on two 18-foot poles, oriented broadside to your target region, is one of the most effective portable HF DX antennas per pound of gear carried.
Is the gain figure of 3–4 dBd real or theoretical?
The 3–4 dBd gain figure is relative to a dipole at the same height at low elevation angles — and it is real, confirmed by antenna modelling software (EZNEC, 4nec2) and by on-air operator reports. The key qualifier is "at low elevation angles" — at the 10°–20° elevation angles used for intercontinental DX on 40m. At high angles, the half-square is 5–8 dB worse than the dipole. In practice this means DX signals from 5,000–12,000 miles are noticeably stronger on the half-square than on a dipole, while nearby stations at 200–1,000 miles sound weaker. This trade-off is exactly what a DX-focused operator wants — and the gain is real enough to make a practical difference on marginal DX contacts.