Loop Antenna

Part of Radio

A loop antenna is a closed conductor forming a complete circuit, offering directional reception, noise rejection, and compact size for a given frequency range.

Why This Matters

The loop antenna is underappreciated and underused in most radio installations, yet it offers capabilities that no wire antenna can match. Its figure-eight directional pattern has deep nulls broadside to the loop plane — rotating the loop can reject an interfering station or noise source by 20–40 dB while keeping a desired signal unaffected. This directional selectivity is available with a loop small enough to hold in your hands.

For a post-collapse community, loop antennas serve three critical functions: direction finding (determining where a signal originates, useful for locating transmitters, beacons, and people); noise rejection (pulling weak signals out of electrical noise, useful when generators and motors create interference); and compact receiving antennas for situations where a long wire is impractical.

The physics of loop antennas is elegantly different from wire antennas. A wire antenna responds to the electric field component of the electromagnetic wave; a loop responds primarily to the magnetic field component. Magnetic fields are less affected by nearby conductors and electrical interference sources, which partially explains the loop’s superior noise rejection in many environments.

Small Loop vs. Resonant Loop

A small transmitting loop (STL) is a single or few-turn loop with circumference much less than one wavelength (typically λ/10 or less). It has very low radiation resistance (milliohms) and very high Q factor, making it extremely narrowband but capable of very high field strengths in close proximity.

A receive-only small loop is the classic direction-finding and shortwave listening antenna. Wound with many turns on a small frame, tuned with a variable capacitor, it is highly sensitive and directional. The AM loopstick inside every portable AM radio is exactly this: a ferrite rod wound with fine wire, small enough to fit in a hand.

A resonant loop has circumference close to one wavelength. Its radiation resistance is much higher (around 100 ohms for a circular loop), making it a more efficient radiator and receiver. It is large — a full-wavelength loop for 7 MHz is about 42 meters in circumference — but it offers excellent performance on both transmit and receive.

For listening, the small multi-turn ferrite loop is most practical. For transmitting with modest power, the resonant single-turn loop is preferred. For direction finding, a small air-core frame loop with deep null characteristics is optimal.

Building a Ferrite Loopstick Antenna

The loopstick is the core receiving antenna for AM broadcast frequencies. Ferrite rod provides high permeability (μr = 100–5000 depending on grade), concentrating magnetic flux through the winding and multiplying the effective capture area of the loop.

Obtain ferrite rod from old AM radios, or from ferrite cores used in switching power supply transformers. The grade matters: for AM broadcast (0.5–2 MHz), use medium-permeability rod (μi = 400–1000). For HF (2–30 MHz), use lower-permeability types.

Wind 70–100 turns of 0.1–0.2 mm enameled wire on the rod for AM band reception. Space the winding over 50–60% of the rod length (wound section centered on the rod, not run to the ends). Add a secondary winding of 5–10 turns for impedance matching to the following circuit.

Connect a variable capacitor of 15–365 pF across the main winding. This forms a tank circuit resonant across the AM band. The resonant frequency can be moved through the band by adjusting the capacitor. The secondary winding couples to the detector circuit at low impedance.

Performance of a well-built loopstick can match or exceed a 5-meter wire antenna for AM reception, and the directional pattern adds selectivity against interference. Rotate the rod to find the null (broadside to rod axis) pointed at an interferer, and the signal-to-noise ratio improves dramatically.

Building a Frame Loop for Direction Finding

A direction-finding loop is an air-core frame wound with multiple turns, sized for a deep null rather than maximum sensitivity. Typical dimensions: 30–60 cm square or circular, 5–20 turns of 0.5–1 mm wire.

The null of a loop is sharp and deep — ideally 40 dB below the peak in the main directions. The null is broadside to the loop face: hold the loop vertically and rotate; when the signal drops to near-zero, the loop face is pointing toward (or directly away from) the transmitter. The ambiguity (null exists in both directions 180° apart) is resolved by a sense antenna — a short whip antenna combined with the loop output through a 90-degree phase shift. The combined pattern is a cardioid (heart-shaped) with a single null in one direction, allowing unambiguous bearing determination.

Frame construction: build the frame from dry hardwood or PVC pipe. Keep metal fasteners away from the winding — they distort the pattern. Wind the turns evenly, securing them with lacing cord or small cable ties. Bring both ends of the winding to a shielded center post (Faraday shield — an incomplete metal ring surrounding the winding, grounded to the circuit but not forming a complete turn that would short-circuit the loop). The Faraday shield prevents pickup of the electric field component, ensuring the antenna responds only to the magnetic field component and maintains a clean null.

Use with a preamplifier (even a simple single-transistor common-emitter stage) and tuning capacitor. Take bearings from multiple locations to triangulate the transmitter position — two bearings cross at the transmitter location on your map.

Resonant Receiving Loop for HF

For general HF receiving (3–30 MHz), a one or two-turn resonant loop of 1–2 meters diameter provides excellent performance with low noise pickup.

Wind one or two turns of coaxial cable (or heavy insulated wire) in a circle of 1–1.5 m diameter. Connect a variable capacitor at the gap in the loop conductor to resonate it. At resonance, the loop Q rises sharply and the signal output voltage peaks. A small coupling loop of 1–2 turns inside the main loop feeds the feedline without disturbing the resonance.

The high-Q resonant loop is narrowband — you must retune as you change frequency. This narrowband characteristic is actually an advantage in interference-rich environments: the loop is only sensitive to signals very close to its resonant frequency, rejecting everything else. In a noisy setting (generator running nearby, power lines), the resonant loop may outperform a long wire antenna even though its theoretical sensitivity is lower, because it hears less noise.

Mount the resonant loop on a rotating base for direction finding or interference nulling. Rotate until an interferer disappears; the desired signal remains. This technique is particularly valuable for pulling weak signals out from behind stronger ones on nearby frequencies.

Integration with Receivers

Antenna coupling is often overlooked. A loop antenna needs to be coupled to the receiver input correctly to transfer maximum power.

For direct coupling to a crystal radio or high-impedance input: tap the loop winding and connect via a few pF coupling capacitor. A small secondary winding of 3–5 turns inside the loop delivers the signal at low impedance, suitable for a 50-ohm coax-fed receiver.

A preamplifier between the loop and receiver compensates for the loop’s lower signal output compared to a long wire antenna. A simple BJT or JFET amplifier with 10–20 dB gain brings the loop output to a level that drives any receiver input well. The preamp should be as close to the antenna as possible to minimize feedline noise pickup.

For general shortwave receiving, many operators find a small indoor resonant loop combined with a good preamplifier outperforms a longer but electrically noisier wire antenna, particularly in urban and industrial environments where electrical interference is pervasive.