Also known as: small tuned loop, STL, mag loop
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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.
Where to buy
For low-band and HF reception in a small space, an active loop like the MLA-30+ (around $38) is the popular budget pick: a broadband receive loop with a built-in low-noise amplifier covering roughly 0.5–30 MHz, quiet against local electric noise and small enough for a balcony. It is receive-only and amplified, trading a tuned loop’s razor selectivity for no-retune convenience.
This is an HF/MW listening antenna, not a scanning antenna — GopherTrunk’s VHF/UHF trunking wants a vertical, discone, or Yagi instead. See the best SDR antenna guide for the scanning line-up.
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Sources
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Loop antenna — Wikipedia, for the small tuned loop, capacitor resonance, high Q, and narrow bandwidth. ↩
Frequently asked questions
Which magnetic loop antenna should I buy for SDR listening?
For low-band and HF reception in a small space, the MLA-30+ active loop (around $38) is the popular budget pick: a broadband receive loop with a built-in low-noise amplifier that covers roughly 0.5–30 MHz, mounts on a balcony or rooftop, and stays quiet against local electric noise. It is a receive-only, active (amplified) loop, so it trades a tuned loop’s razor selectivity for no-retune convenience. Note it is an HF/MW antenna, not a VHF/UHF scanning antenna.
Is a magnetic loop useful for GopherTrunk trunking?
Not directly. GopherTrunk decodes VHF/UHF land-mobile trunking (P25, DMR, NXDN, TETRA), where wavelengths are short and simple verticals work well, and a high-Q loop would need constant retuning. Mag loops shine at HF/MW shortwave listening on an SDR, where they act as a low-noise, compact antenna and a tracking preselector. For scanning, use a vertical, discone, or Yagi instead.
Why use a magnetic loop instead of a long wire?
Two reasons: size and noise. A tuned or active loop a fraction of a wavelength across fits on a balcony where a full-size wire cannot go, and because it responds to the magnetic field it stays quieter against the local electric noise that swamps HF in a city — often a bigger win than raw gain. A tuned loop’s high Q also preselects, protecting the SDR’s front-end dynamic range.
Passive tuned loop or active (amplified) loop?
A passive tuned loop like the classic small transmitting/receiving loop has the highest selectivity and the best preselection, but you must retune it every time you move in frequency. An active broadband loop such as the MLA-30+ drops the tuning and adds a low-noise amplifier, so it is plug-and-play across the whole HF band at the cost of some out-of-band rejection. For casual SDR listening the active loop is far more convenient.