Also known as: loop, magnetic loop, resonant loop
A loop antenna is simply a closed loop of wire, and its behaviour splits sharply into two regimes depending on how big the loop is compared to the wavelength.1 A small loop — one whose circumference is much less than a wavelength — acts as a magnetic dipole: it responds to the magnetic part of the passing radio wave and shows a sharp figure-eight pattern with deep nulls, making it a classic direction-finding and receiving antenna. A resonant loop — one about a full wavelength around — behaves completely differently, radiating efficiently like a folded dipole bent into a ring. The same shape thus serves as both a tiny sensitive receiver and a full-size transmitting element.
How it works
Small loop. When the loop is electrically tiny, the current is essentially uniform all the way around, and the loop’s response is proportional to the rate of change of magnetic flux threading it. Two facts follow. First, it is a magnetic-field sensor, so it largely ignores the local electric-field noise (nearby power lines, dimmers, plasma TVs) that an electric antenna picks up — a small loop can be strikingly quiet. Second, its pattern is a figure-eight in the plane of the loop with sharp nulls broadside to the loop face; rotating the loop to null out an interferer or to find a bearing is the basis of radio direction finding. The penalty is efficiency: a small loop has a tiny radiation resistance, so as a transmitter it is poor unless carefully tuned (see the magnetic loop antenna).
Resonant loop. Grow the loop until its circumference is about one wavelength and it resonates. Now the current varies around the loop and the structure radiates well, with a gain slightly higher than a dipole and its main lobe broadside to the plane of the loop — the opposite of the small loop. A one-wavelength square loop is the “quad” element; two or more make a cubical-quad beam, a compact rival to the Yagi.
Variants
- Ferrite loopstick. Winding the loop on a ferrite rod concentrates magnetic flux through the coil, shrinking the antenna dramatically — the AM broadcast antenna inside nearly every portable radio.
- Tuned small loop. Adding a resonating capacitor turns the broadband small loop into a high-Q, narrowband receiving (or QRP transmitting) antenna, covered separately as the magnetic loop.
- Full-wave / quad loop. A resonant transmitting element, often used in beams.
Relevance to SDR
Loops are among the most useful receive antennas for an SDR at HF and below. Their magnetic pickup rejects nearby electrical noise, and their nulls let you steer away from a strong local interferer that would otherwise desensitise the receiver — valuable because SDRs have finite dynamic range. Small tuned and broadband loops are the standard antenna for low-frequency, MW, and shortwave listening on RTL-SDR and similar dongles, and loopsticks are what let a pocket radio hear AM at all.
GopherTrunk works at VHF/UHF on trunked land-mobile systems, where vertical whips dominate and full-size resonant loops are uncommon, so a loop is not a typical GopherTrunk antenna. It appears here as a foundational antenna type and the parent of the magnetic loop.
Sources
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Loop antenna — Wikipedia, for the small-versus-resonant-loop distinction, magnetic pickup, and pattern nulls. ↩