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Loop antennas come in a remarkable range of forms — from tiny magnetic loops that sit on a desktop to full-wave horizontal loops strung around the perimeter of a garden. What they share is a continuous conductor formed into a closed loop. Beyond that, their behaviour, performance, and applications vary enormously depending on the loop's size relative to the operating wavelength.
This page covers the major categories of loop antennas used in amateur radio, with practical guidance on when each type makes sense.
Loop antennas divide naturally into two groups based on their size:
Small loops (magnetic loops): The loop circumference is a small fraction of the operating wavelength — typically one-tenth of a wavelength or less. These antennas are dominated by magnetic field coupling and behave very differently from wire antennas. They are compact, can be very effective despite their small size, and are popular for restricted-space and portable operation.
Large loops (full-wave and larger): The loop circumference is one full wavelength or more. These are full-size antennas that behave more like bent dipoles or arrays. They offer excellent performance and can rival or exceed a dipole in many situations. Full-wave horizontal loops, delta loops, and quad loops fall into this category.
The small transmitting magnetic loop (often called a "mag loop") has gained tremendous popularity among operators who face antenna restrictions, have limited space, or operate portable. A well-built magnetic loop can be surprisingly effective — many operators work DX regularly with these antennas from apartments, patios, and hotel rooms.
A small magnetic loop consists of a single-turn loop of large-diameter conductor (copper or aluminium tubing, typically 12–25 mm / 0.5–1 inch in diameter) formed into a circle or octagon, with a tuning capacitor connected across a gap in the loop. The loop diameter is typically 0.5 to 1.5 metres (1.5 to 5 feet).
The loop is tuned to resonance by the variable capacitor. At resonance, very high circulating current flows in the loop — far higher than the current delivered by the transmitter. This high current produces a strong magnetic field that radiates. The tuning is extremely sharp (narrow bandwidth), and the antenna must be retuned every time you move more than a few kilohertz.
The feedline is coupled to the loop via a smaller coupling loop (typically one-fifth the diameter of the main loop) positioned inside the main loop and connected to the coax.
Advantages:
Disadvantages:
The tuning capacitor is the most critical and often most expensive component. Requirements include high voltage rating (1 kV to 10 kV depending on power and band), low loss (air-spaced or vacuum), and smooth adjustment for precise tuning.
Vacuum variable capacitors offer the best performance — very high voltage rating, low loss, and wide capacitance range in a compact package. They are available as surplus components from commercial and military sources.
Butterfly capacitors use specially shaped vanes that eliminate the sliding contact of conventional variable capacitors. This is important because even small contact resistance in the capacitor can dominate losses in the system.
Split-stator capacitors (two sections in series) double the voltage rating for a given capacitor size.
Motor-driven tuning is popular because the loop must be retuned frequently and manual adjustment requires approaching the antenna (which changes the tuning due to body proximity). A small stepper motor or gear motor driving the capacitor, controlled from the operating position, makes operation much more practical.
Magnetic loops are popular homebrew projects. Key construction guidelines:
Safety warning: Magnetic loops generate very high voltages across the tuning capacitor and strong electromagnetic fields close to the antenna. Maintain safe distance from the antenna during transmission. Do not touch the antenna while transmitting. Follow your country's RF exposure guidelines, and be especially cautious when operating indoors or at higher power levels.
The full-wave horizontal loop — sometimes called a "sky loop" or "loop skywire" — is a loop of wire with a circumference of one full wavelength at the design frequency, supported horizontally above the ground. The loop can be any shape (square, rectangular, triangular, irregular) as long as the total wire length is approximately one wavelength.
At one wavelength circumference, the loop is resonant and the current distribution produces useful radiation. The feed point impedance depends on the loop shape and where the feedline is attached, but is typically around 100–130 ohms for a square loop fed at one corner. A 4:1 balun brings this to approximately 50 ohms, or the loop can be fed with ladder line and a tuner for multi-band use.
A horizontal full-wave loop at modest height (less than one-half wavelength above ground) radiates primarily straight up — a high-angle pattern that is excellent for NVIS communication and medium-distance contacts on 80 and 40 metres. As the antenna is raised higher, the pattern breaks into lower-angle lobes suitable for DX.
The horizontal loop is quieter on receive than a dipole at the same height, partly because its closed shape tends to reject some types of local noise, and partly because its high-angle pattern does not pick up as much distant noise from low angles.
A full-wave loop for 80 metres (approximately 86 metres / 282 feet of wire) also works on higher bands as a multi-wavelength antenna. Fed with ladder line and a tuner, it can operate on 80 through 10 metres. The radiation pattern changes significantly on each band — on higher bands the pattern develops multiple lobes — but overall it performs well as a general-purpose multi-band antenna.
The main challenge with a horizontal loop is the space required — you need supports at multiple points around the loop perimeter, typically trees, masts, or building attachment points. The loop does not need to be a perfect shape; irregular loops draped around available supports work well. Performance is remarkably tolerant of shape distortion.
A full-wave 80-metre loop requires roughly 86 metres (282 feet) of wire, which can fit in a garden of approximately 21 metres (70 feet) per side if arranged as a square. A 40-metre loop at about 43 metres (140 feet) is more manageable.
The delta loop is a full-wave loop arranged in a vertical triangle (delta shape). One side is typically horizontal at the top, with the apex pointing downward, though the opposite orientation (apex up) is also used.
A vertically oriented delta loop produces a different radiation pattern than a horizontal loop. Fed at the bottom apex, it radiates with predominantly vertical polarisation and a low radiation angle — excellent for DX on HF. Fed at one of the top corners or at the midpoint of the bottom side, the polarisation shifts toward horizontal.
The delta loop offers about 1–2 dB of gain over a dipole at the same height, with a useful omnidirectional pattern (slightly directional broadside to the plane of the loop). This gain advantage, combined with the low-angle radiation when vertically oriented and bottom-fed, makes the delta loop popular for DX-oriented stations that cannot install a Yagi.
A delta loop for 40 metres requires about 43 metres (140 feet) of wire and a vertical dimension of roughly 12 metres (40 feet). This usually means the top wire is supported between two high points (trees, masts, or buildings) and the bottom apex is anchored near or just above the ground.
A delta loop for 20 metres is half that size and more practical for many installations. Multi-band operation is possible with a tuner, though dedicated single-band delta loops are simpler to optimise.
The cubical quad antenna uses one or more full-wave square loops arranged on a boom, analogous to a Yagi. A single quad loop (one element) is a full-wave loop antenna that can be oriented horizontally or vertically. A multi-element quad array (with reflector and director loops) provides gain and directivity similar to a Yagi.
The quad has some theoretical advantages over a Yagi of equivalent boom length — slightly higher gain (about 1–2 dB), a broader frequency response, and better performance at lower heights. In practice, these advantages are debated and depend on the specific designs being compared.
The main disadvantage of the quad is mechanical complexity. Each element is a loop rather than a straight rod, requiring a support structure (spreaders) at each corner. This makes the quad heavier, more wind-resistant, and more difficult to maintain than a Yagi of similar gain. Spider-style spreaders made from fibreglass are common.
Quad antennas have a dedicated following, particularly for 2-element and 3-element designs on 20, 15, and 10 metres. Multi-band quads (with concentric loops for different bands on the same spreaders) are available commercially and as homebrew designs.
The right loop antenna depends on your situation: