Loop antenna

Lore & Background

Large loop antennas, also called self-resonant or full-wave loops, have a perimeter close to one or more whole wavelengths, making them self-resonant. At their first resonance, they exhibit a two-lobe dipole-like radiation pattern peaking perpendicular to the plane of the loop. The most popular shape in amateur radio is the quad antenna, a square loop constructed of wire on an ×-shaped frame, often used with parasitic elements for unidirectional gain. Triangular and rectangular shapes are also used, with a rectangle twice as high as wide providing slightly increased gain and a 50 Ω match.

Halo antennas are often described as half-wave dipoles bent into a circle, with ends not quite touching. Some writers exclude them from loop antennas, considering them bent dipoles, while others treat them as an intermediate category between large and small loops. The halo has an omnidirectional radiation pattern similar to a small loop but is more efficient due to its larger radiation resistance, and it presents a good impedance match to 50-ohm coaxial cable.

Small loop antennas, also called magnetic or tuned loops, have a perimeter smaller than half the operating wavelength. They are used mainly as receiving antennas due to low efficiency, though they are sometimes used for transmission. Loops with a circumference smaller than about 1/10 wavelength become so inefficient they are rarely used for transmission. A common example is the ferrite (loopstick) antenna in AM broadcast radios. Their radiation pattern is maximum within the plane of the loop, perpendicular to the maxima of large loops.

Reader's Guide

Loop antennas are notable for their versatility across frequency ranges and applications. Large self-resonant loops are physically large at lower shortwave frequencies, often installed horizontally for NVIS communication, but become more practical for vertical mounting above 10 MHz, where they can be rotated for directional use. Their radiation pattern changes with frequency: at first resonance it peaks perpendicular to the loop, while at higher harmonics it develops multiple lobes at lower angles, beneficial for long-distance communication. Arrays of loops, such as the quad configuration with parasitic elements, provide additional gain and unidirectional patterns.

Halo antennas are particularly useful on VHF bands and above, where their small diameter allows mobile use. Their nearly omnidirectional horizontal pattern can be evened out by adjusting loop size and adding capacitance between element tips, which also reduces wasted upward radiation. Halos pick up less electrical spark interference than monopoles and dipoles, making them advantageous in noisy environments.

Small loops, despite their low efficiency for transmission, are widely used as receiving antennas due to their compact size and directional properties. The ferrite loopstick antenna is a ubiquitous example in AM broadcast radios. The categorization of loop antennas into large, halo, and small types reflects their distinct design principles and performance characteristics, with the halo occupying a debated intermediate position.

Did You Know?

• Large loop antennas are also called self-resonant or full-wave loops.
• A halo antenna is often described as a half-wave dipole bent into a circle.
• Small loop antennas are also known as magnetic loops or tuned loops.
• The radiation pattern of a large loop peaks perpendicular to the plane of the loop, while a small loop peaks within the plane.

The Three Families of Loop Antennas

Loop antennas are radio devices built from a continuous loop or coil of conductor—wire, tubing, or similar material—fed by a balanced source for transmission or a balanced load for reception. They fall into three distinct categories based on their perimeter relative to the operating wavelength. Large loops, sometimes called self-resonant or full-wave loops, have a circumference close to one or more whole wavelengths, making them naturally resonant at that frequency. Small loops, also known as magnetic or tuned loops, keep their perimeter well below half a wavelength, typically no more than a quarter wave, and are primarily receiving devices due to their low efficiency; the ferrite loopstick in an AM radio is the classic example. Between these two extremes sit halo antennas, which some describe as dipoles bent into a circle with the tips nearly touching. Writers disagree on whether halos truly belong in the loop family or represent an intermediate boundary, but they share the self-resonant character of large loops while resembling small loops in shape and performance, distinguished mainly by their significantly higher radiation resistance.

Radiation Patterns and the Polarization Puzzle

One of the most counterintuitive aspects of loop antennas is how their radiation direction and polarization depend on factors beyond simple orientation. A large loop at its first resonance radiates strongest at right angles to the plane of the loop, producing a two-lobe pattern that resembles a dipole. Small loops behave almost oppositely: their radiation peaks within the plane of the loop, perpendicular to where large loops peak. Polarization adds another layer of complexity. Unlike a dipole, where the wire's orientation straightforwardly indicates polarization, a resonant loop's polarization is governed by where the feedpoint is placed. A vertically oriented loop fed at its bottom radiates horizontally polarized waves, while feeding the same loop from its side flips the polarization to vertical. As the operating frequency climbs to the second and third resonances of a large loop, the familiar perpendicular lobes fade and new strong lobes emerge near the loop's plane, fundamentally altering the antenna's directional behavior.

Geometry, Shape, and the Quad Tradition

A loop antenna need not be circular. Any closed geometric shape—circle, square, triangle, rectangle, or arbitrary polygon—works as long as the total perimeter slightly exceeds one wavelength for resonance. In amateur radio, the square form is especially popular, giving rise to the quad antenna, where a driven square loop is strung across an X-shaped support frame. Additional parallel loops can be added as parasitic directors or reflectors, creating a unidirectional array whose gain grows with each extra element. Rotating the square by forty-five degrees yields a diamond shape on a plus-shaped frame. Triangular loops find a niche as vertical installations because a single mast can support them, while a rectangle twice as tall as it is wide offers slightly higher gain and the convenience of matching fifty ohms directly. The key engineering constraint is that the perimeter must remain just above one wavelength; the shape itself is largely a matter of structural convenience and mounting preference.

From Gigantic to Compact: Size, Frequency, and Deployment

The physical scale of a large loop is dictated by the operating frequency. At the lowest shortwave band around 1.8 MHz, a circular loop's diameter stretches to roughly 175 feet, making it a substantial structure. As frequency rises, the required size shrinks dramatically, reaching about 11 feet in diameter at 30 MHz. This size gradient shapes how the antenna is deployed. At lower frequencies, the loop is so large that it must lie flat, with its plane horizontal and wire supported by masts at a low height. This orientation produces horizontally polarized radiation peaking nearly straight up, useful for regional NVIS communication but poorly suited for long-distance continental contacts. Above roughly 10 MHz, the loop becomes small enough to stand vertically, directing its main beam toward the horizon. At still higher frequencies, the antenna can be mounted on a rotator for directional control. Compared to a dipole, a vertical large loop wastes less energy into the sky or ground, yielding about 1.5 dB more gain in its two favored horizontal directions.