There is a class of antenna that checks every box — low takeoff angle, direct 50-ohm match, no radials, low visual profile, and genuine gain over a dipole — yet most operators have never strung one up. The Half Square Antenna is that antenna. Russian operator UA9OS nicknamed it the “Secret” antenna for two reasons: despite being documented on the internet, it remains oddly obscure among hams; and when slung between a couple of trees, it is essentially invisible to neighbours who are already convinced your Yagi is intercepting their television signals.
This article covers the geometry, the feed method, why it works, how to trim it, and complete dimension tables for the most-used HF bands — including those most relevant to VU operators.
The Half Square is a wire bent into the shape of the letter Π (or a squared-off U turned upside down). The two vertical sides are each one quarter-wavelength long; the horizontal top section is one half-wavelength long. The feeder connects at one of the top corners — the junction between the horizontal run and one of the vertical drops.
The diagram below shows the three segments and their relationships:
- L₂ — horizontal phasing section = λ/2 × 0.98 (velocity factor correction)
- L₁ — near vertical (feed-side leg) = λ/4 × 0.98
- L₃ — far vertical (remote leg) = λ/4 × 0.98
The 50-ohm coaxial cable connects at the feed corner: centre conductor to the horizontal wire, braid/shield to the vertical leg.
The Half Square antenna is essentially two quarter-wave vertical monopoles spaced a half-wavelength apart and fed 180° out of phase with respect to each other. Because the current maximum sits at the top of each vertical element — well up in the air where it counts — the antenna radiates primarily at low elevation angles, which is exactly where DX signals live.
The key electrical properties that make it attractive:
- Radiation resistance ≈ 50 Ω — the antenna presents a close match to standard 50-ohm coax. No ATU is required under normal conditions.
- No radials or ground screen needed— unlike a conventional ground-mounted vertical, the Half Square is self-contained. The horizontal phasing section doubles as the return path.
- Bidirectional pattern — the radiation is broadside to the plane of the antenna (perpendicular to the vertical elements). Plan your orientation accordingly to aim the main lobes toward your target regions.
- Low takeoff angle — the vertical polarisation and the elevated current maxima combine to suppress high-angle radiation, flattening the vertical pattern toward the horizon.
UA9OS compared his Half Square against a sloper dipole on 24.9 MHz and found a consistent 4 dB advantage in the direction perpendicular to the antenna plane. Operators running the antenna on 20 metres for POTA activations have reported successful DX contacts into Europe and beyond using modest power with no matchbox. The antenna is also effective on 40 metres, where it outperforms a typical Inverted-V whose apex sits at similar height — and does so without the radial field that a ground-mounted vertical would demand.
Half Square Antenna – Feed Point Detail
The feed point of Half Square antenna is the upper corner — where the horizontal section meets one of the vertical legs. Coax centre goes to the horizontal wire; coax braid goes to the vertical wire that hangs below that corner. Some builders add a few turns of coax (a choke balun) near the feedpoint to prevent shield current from affecting the pattern. This is good practice, particularly when the coax run follows the antenna wire for any distance.
The antenna is normally fed at the left corner (near leg, L₁ side), though it can be fed at the right corner without changing the electrical behaviour — only the direction the coax runs away from the antenna changes.
Trim for Your Frequency
After cutting to calculated length, you will almost certainly need to prune for best SWR. Surrounding objects (trees, walls, roofline) and proximity to ground all shift the resonant frequency downward from the calculated value.
The trimming rule from UA9OS is:
- Near leg (L₁, the feed-side vertical): shorten or lengthen by ΔL = (ΔF / 300,000) / 4 metres, where ΔF is your desired frequency shift in Hz.
- Far leg (L₃, the remote vertical): adjust by 3 × ΔL — three times the correction applied to L₁.
In practice: start slightly long, measure SWR vs. frequency with an antenna analyser or NanoVNA, then trim in small increments (50–100 mm at a time) from the bottom of each leg. Cut a little, measure, repeat.
Half Square Antenna – Practical Construction Notes
Wire: Any 1–2 mm copper or copper-clad steel works well. For portable use, 0.5 mm enamelled wire is light and packs small. For a permanent installation, 1.5 mm PVC-insulated hookup wire or hard-drawn copper is more durable.
Top support: The horizontal run must be supported at both ends and at the centre (or at least at the ends). Two trees, two masts, or a tree and a mast — any combination works. For 40 m you need supports roughly 10–11 m high, which is within range of a typical mature tree.
Can’t reach full height? If your support falls short, the lower section of each vertical leg can be bent horizontal and run parallel to the ground at a couple of metres height. This does reduce radiation resistance somewhat, but the antenna will still perform well above a dipole at comparable height. Several operators have reported good results with the bottom 3–5 m of the legs run horizontally when the vertical portion alone could not be fully supported.
Feedline routing:Route the coax away from the antenna at a right angle from the feedpoint if possible. This minimises interaction between the feedline and the antenna’s near field. A simple 5-turn coil of coax (15–20 cm diameter) at the feedpoint acts as a useful RF choke.
Weatherproofing the feedpoint: Seal the SO-239 or solder joint with self-amalgamating tape. The feedpoint sits at the top of the antenna and will see rain directly.
Further Reading
- UA9OS — Original “Secret Antenna” article describing real-world comparisons against a sloper on 24.9 MHz: http://ua9os.qrz.ru/Secret.htm
- AA2SD — Half Square on 20 metres with radiation pattern diagrams and POTA activation experience: https://www.aa2sd.com/blog/half-square-20-mtr-antenna
- UT2FWF — Ukrainian operator’s build notes and trimming experience (Russian-language): https://ut2fwf.blogspot.com/2022/04/blog-post_57.html
Related Posts
- W5GI Multiband Collinear Antenna: A Proven HF All-Band Wire Antenna (December 24, 2025)
- 20–40–80 Meter Loaded Multiband HF Dipole Antenna (December 31, 2025)
- Multiband Dipole Antenna for 10–80 Meters (January 6, 2026)
- Half Loop Antenna for 80, 40, 20, and 15 Meter Ham Radio Bands (January 30, 2026)
- Extended Lazy-H Antenna: A Classic Multiband Wire Design from QST (January 31, 2026)
- Near Vertical Incidence Skywave (NVIS): The Definitive Guide to Reliable Regional HF Radio (January 31, 2026)
- DIY 80m Magnetic Loop Antenna: A Compact Solution for Low Bands (February 9, 2026)