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Small Transmitting (Magnetic) Loop

Loop Antennas HF Portable 🔧 Buildable at home

A compact tuned loop, far smaller than a wavelength, with sharp tuning and high efficiency.

Band
HF (typically 80-10 m)
Gain
~ -3 to +1 dBi (size dependent)
Polarization
Linear (plane of the loop)

Photos

Real-world photo of a Small Transmitting (Magnetic) Loop in use
Real-world example. Source: Wikimedia Commons (CC BY-SA 3.0; Kuha455405).

Radiation / wave patterns

New to these diagrams? How to read a radiation pattern →

Radiation pattern of the Small Transmitting (Magnetic) Loop
Pattern lies in the plane of the loop: a figure-eight with sharp, deep nulls broadside to the loop — handy for nulling out interference by rotating it.

How & why it works

A small transmitting loop is a single turn of large-diameter tubing, much less than a wavelength around, resonated by a high-voltage vacuum or butterfly capacitor. Because it is electrically small, almost all the stored energy is in the magnetic near field, giving it a very high Q: tuning is razor-sharp and bandwidth is narrow, but a well-built loop can be surprisingly efficient and is far less sensitive to nearby electrical noise than a comparable short whip. Its compact size and deep nulls make it ideal where space is limited.

Real-world uses

Apartment and stealth HF stations, portable operation, and direction finding.

Make one at home

A practical starting point — always trim and check SWR for your exact frequency.

See the finished antenna pictured above ↑.

A small transmitting (magnetic) loop is a compact HF antenna for tight spaces — more involved because of the high-voltage tuning capacitor.

You’ll need

  • A loop of large-diameter copper tubing or thick coax braid (circumference ~1/8 wavelength, e.g. ~3 m loop for 20–10 m)
  • A wide-spaced or vacuum variable capacitor (the loop develops thousands of volts — ordinary capacitors will arc over)
  • A small coupling loop (~1/5 the main loop diameter) for the feed

Build

  1. Form the main loop and connect its two ends across the variable capacitor.
  2. Mount the small coupling loop at the bottom, opposite the capacitor, and feed it with coax.

Tune & caution

  • Tuning is very sharp — retune the capacitor for even small frequency changes.
  • Do not touch the loop or capacitor while transmitting — the RF voltage is dangerous even at modest power. Tune at low power first.

Safety

  • Keep antennas well clear of power lines — never put one where it could fall onto or contact mains wiring.
  • Don’t transmit while anyone is close to the antenna; mind RF-exposure limits and your local licensing rules.
  • Ground masts and use coax surge protection where appropriate.
  • Cutting/soldering: eye protection and a ventilated space.