Halo and Loop Transmit Antennas on HF (80–10m)
What Is a Halo Antenna?
- A halo is a ring-shaped dipole (about half-wavelength circumference) — essentially a dipole bent into a circle, usually with a small gap and matching/feeding network.
- Polarization is horizontal. Azimuth pattern is broad and near-omnidirectional.
- On HF, a true half-wavelength halo is physically large: on 20 m it is about 3.4 m diameter; on 10 m about 1.7 m; on 80 m about 13.6 m — which is why halos are more common on VHF/UHF.
- A full-size halo (half-wavelength circumference) can be quite efficient if built with low-loss materials. When people shrink or load halos to fit tiny spaces, efficiency and bandwidth drop sharply.
What Is a Small HF Transmitting Loop?
- A small transmitting loop (STL) is typically 1–3 m diameter on HF — much smaller than a wavelength — and is tuned with a high-Q capacitor.
- Strong circulating current in the loop builds a magnetic field that couples into free space.
- The far-field pattern is a “doughnut” with deep nulls off the loop’s sides.
- Most STLs are mounted in a vertical plane, yielding horizontal polarization at low elevation angles.
- On 20 m and up, carefully built loops can reach around 10–40% efficiency (build dependent). On 80 m at indoor sizes, efficiency can sink to well below a few percent.
Efficiency Comparison
The limitation for both halos and loops is efficiency vs. size:
- Halo on HF: Practical and efficient only when full-size (half-wavelength circumference). On lower bands the diameter becomes unwieldy; if you resort to loading to make it smaller, losses rise and bandwidth narrows.
- Small loop on HF: Physically feasible from 80–10 m, but efficiency collapses as wavelength grows relative to loop size; very narrow bandwidth demands frequent retuning.
Note: A roughly 1.5 m loop on 80 m driven with 100 W might radiate only a watt or less. The same physical loop on 20 m can radiate an order of magnitude more, depending on conductor size, capacitor Q, and build quality.
Efficiency is ultimately tied to wavelength: the longer the band’s wavelength compared to your antenna size, the lower the efficiency. For a deeper dive, see It All Starts with Lambda.
Comparison at a Glance
| Antenna Type | Size on 20 m | Bandwidth | Efficiency | Polarization | Best Use Case |
|---|---|---|---|---|---|
| Full dipole | About 10 m span | Wide | High (90%+) | Horizontal | Standard base station |
| Quarter-wave vertical | About 5 m tall | Moderate | High (60–90%) | Vertical | DX, limited horizontal space |
| HF halo (half-wavelength circumference) | About 3.4 m diameter | Moderate | High when full-size; low if loaded | Horizontal | Balcony/mobile where a full-size ring fits |
| Small HF loop | About 1–2 m diameter | Very narrow | Around 10–40% (build-dependent) | Horizontal (vertical-plane mount) | Indoor/portable, restricted lots |
Takeaway: Both halos and small loops work and make QSOs, but they trade efficiency and bandwidth for compact size. A full-size dipole or quarter-wave vertical still wins on raw performance.
A true half-wavelength halo can be efficient; “mini-halos” that rely on heavy loading are much lossier and behave like other shortened antennas.
Mini-FAQ
- Do halo antennas work on HF? — Yes. When built at about half-wavelength circumference they can be efficient, but the required diameter is large on lower bands.
- Are small transmitting loops efficient? — On 20–10 m, careful builds can reach around 10–40%. On 80 m at small diameters, efficiency is usually very low.
- Why are STLs so narrow-band? — Very low radiation resistance leads to very high Q, so even small frequency changes require retuning.
- Which suits small spaces? — STLs fit the smallest spaces; halos are great if a full-size ring actually fits. If not, loading costs efficiency.
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