Also known as: small tuned loop, STL, mag loop
A magnetic loop antenna (small tuned loop, or “mag loop”) is a loop antenna far smaller than a wavelength — typically an eighth of a wavelength or less around — brought to resonance by a capacitor across a small gap in the loop.1 Tuning cancels the loop’s large inductive reactance, so a physically tiny antenna presents a usable match and its weak radiation resistance is momentarily amplified by a very high Q. The payoff is a compact, sharply tuned, low-noise receiving antenna that works indoors and on a balcony where a full-size wire is impossible — which is why the mag loop is a favourite of apartment-bound and portable HF listeners.
How it works
A small loop is mostly inductance with a very small radiation resistance. Left alone it is a hopeless match. Placing a capacitor across a break in the loop forms a series (or parallel) resonant circuit: at one frequency the capacitor’s reactance exactly cancels the loop’s, leaving only resistance, and a large circulating current builds up. That current is what actually radiates or receives, so the tuned loop performs far better than an untuned one of the same size.
The circulating current is large because the circuit Q is very high — often several hundred. High Q brings the mag loop’s characteristic virtues and vices in one package:
- Narrow bandwidth. The antenna is only well matched over a few kilohertz to tens of kilohertz, so it must be retuned whenever you move more than a little in frequency. A remotely driven variable capacitor is standard.
- Built-in preselection. That same sharpness rejects out-of-band signals before they reach the receiver, easing intermodulation and protecting the SDR’s front-end dynamic range.
- High voltages. The resonant current develops kilovolts across the capacitor when transmitting, so mag loops need wide-spaced or vacuum capacitors — a receive-only loop is far more forgiving.
Like any small loop it keeps the figure-eight pattern and magnetic-field pickup, so it stays quiet against local electric noise and can be rotated to null an interferer. A small coupling loop (about a fifth the diameter) or a gamma/Faraday feed transfers energy between the main loop and the coax without directly loading it.
In practice
Receive-only mag loops are cheap to build and forgiving of construction, and are widely sold as compact “wideband” active loops (a broadband loop plus a low-noise amplifier, trading the tuned loop’s selectivity for no-retune convenience). Transmitting mag loops demand careful high-voltage construction and lose efficiency as they are made smaller, but reward the effort with a genuinely portable HF antenna. Orientation matters: because pickup is magnetic and the nulls are sharp, a few degrees of rotation can markedly change the signal.
Relevance to SDR
The magnetic loop is one of the best partners for a wideband SDR at HF. SDRs are exposed to the entire band at once, so a strong shortwave broadcaster or local transmitter can overload the receiver; the mag loop’s high-Q resonance acts as a tracking preselector, knocking down everything but the wanted signal before it hits the ADC. Combined with its low noise and small footprint, that makes it a standard indoor and field antenna for RTL-SDR and higher-end receivers doing MW and shortwave work.
GopherTrunk decodes VHF/UHF land-mobile trunking, where wavelengths are short, verticals are easy, and high-Q retuning would be a nuisance, so mag loops are not part of a GopherTrunk station. The antenna is documented here as the practical, tuned member of the loop family and a textbook example of trading bandwidth for selectivity via Q.
Sources
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Loop antenna — Wikipedia, for the small tuned loop, capacitor resonance, high Q, and narrow bandwidth. ↩