Loop antennas

What Are Loop Antennas?

Loop antennas are radiating elements formed from a closed conducting path, circular, square, rectangular, or triangular in outline, that is driven at a break in the conductor. They are one of the two canonical antenna families alongside the linear dipole, and the electrically small loop is the exact electromagnetic dual of the short dipole: where the dipole behaves as an electric current element, the small loop behaves as a magnetic dipole oriented normal to the plane of the loop. Both produce the same sine-shaped far-field pattern and the same directivity of 1.5, with the electric and magnetic field roles interchanged.

Loop behavior divides sharply according to electrical size. A loop whose circumference is small compared with the wavelength, conventionally under one tenth of a wavelength, carries nearly uniform current and radiates weakly. A loop whose circumference approaches or exceeds one wavelength supports a standing-wave current distribution, radiates efficiently, and has a pattern with its maximum along the axis normal to the loop plane. The two regimes are designed and analyzed by different methods, and they serve entirely different purposes in practice.

Electrically Small Loops

The radiation resistance of a small loop scales with the fourth power of its circumference divided by the wavelength, which drives it to a fraction of an ohm at typical receiving frequencies. Ohmic loss in the conductor then dominates, and radiation efficiency is poor. Designers compensate by adding turns, since radiation resistance rises with the square of the turn count, by winding the loop on a high-permeability ferrite rod, and by resonating the loop with a series or parallel capacitor to overcome its large inductive reactance. Work on electrically small loop antennas for RFID applications shows how distributing capacitors around the loop keeps the current uniform along the perimeter, which produces the strong and even magnetic field that near-field tag coupling requires. Small loops also radiate most strongly in the plane of the loop and have a sharp null along the axis normal to that plane, which is what makes them useful for direction finding.

Electrically Large Loops and Arrays

Once the circumference reaches roughly one wavelength, the loop becomes a practical transmitting antenna with an input impedance near 100 ohms and a gain slightly above that of a half-wave dipole. The full-wave loop is the element used in cubical quad arrays, where driven and parasitic loops are stacked along a boom in the manner of a Yagi-Uda array. Loops are also combined into planar arrays and folded into compact meandered forms for handset and wearable use. Because the loop is a closed structure, it can be fed in balanced fashion without the ground plane dependence that constrains monopoles.

Field Probes and Near-Field Coupling

Loops are the standard sensor for magnetic field measurement because their induced voltage is proportional to the time derivative of flux through the loop area. Electrostatically shielded loops suppress the electric field response, and a two-turn shielded-loop near-field probe design shows the tradeoff between sensitivity and spatial resolution that governs printed circuit board emissions scanning. Metrology laboratories build on the same principle: NIST has described a high-frequency electromagnetic field probe using a semiconductor laser in a small loop antenna. Coupled loops also underpin inductive power transfer, and analysis of mid-range wireless power transfer with compensated radiation loss treats the transmitter and receiver coils explicitly as small loop antennas.

Applications

Loop antennas are used across a range of systems, including: