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Receive loop calculator

Why you would want one

A receive loop is not there for signal level — it gives less than a dipole. It is there for the null: the figure-of-eight pattern has two deep notches off the axis, and turning the loop can drop a local noise source by twenty or thirty decibels.

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The deep null off the axis of the loop is the whole reason a receiving loop is put up
Frequency (MHz)
Loop shape
Loop size (m)
Number of turns
Wire diameter (mm)
Field strength (µV/m)
Result
Loop inductance
Capacitance for resonance
Effective height
Induced EMF
EMF at resonance (× Q)
Tank Q
Tank bandwidth
Loop perimeter

📐 The formulas​

Inductance of a loop of N turns:

L = N² × μ₀ × r × [ ln(8r/a) − 2 ]

Tuning capacitance:

C = 1 / ( (2πf)² × L )

Effective height (aperture):

heff = 2π × N × A / λ

Induced EMF at a field strength E:

e = heff × E

At resonance the voltage across the tuned circuit is Q times greater.

Symbols:​

  • N — number of turns, A — loop area, a — wire radius
  • r — the equivalent loop radius derived from the perimeter
Where this calculator stops:
  • The aperture formula holds for a small loop: a perimeter up to 0.1 λ. Beyond that the calculator warns you
  • The self-capacitance of a multi-turn winding is not accounted for, and it caps the upper frequency
  • Q is worked out from the loss resistance of the wire; the real figure is lower because of the insulation and whatever is standing nearby
  • The induced EMF assumes the loop is aimed optimally; in the null it falls to zero

🎯 In practice​

  • 160 and 80 metres. A 1 × 1 m loop of 6–8 turns hears noticeably quieter than the transmit antenna, but without the town's noise.
  • Direction-finding on interference. Turning the loop points at the source — after that you can go and look for it on foot.
  • A magnetic antenna for a portable receiver. The same arithmetic, smaller dimensions.
Practical advice:
  1. Symmetry matters more than anything: feed the loop through a transformer or a choke, or the null smears out.
  2. A shielded loop, with the shield broken at the top, does better at rejecting the electric component of the noise.
  3. More turns give more signal but a lower top frequency, because of the winding's self-capacitance.
  4. A preamp right at the loop buys you more than a long cable to the receiver.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Receive loop calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/rx_loop (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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