Skip to main content

Full-wave loop calculator

How it works

The perimeter of a full-wave loop equals one wavelength, and that is the whole calculation. What is far more interesting is the feed point. The same wire, fed at a corner or a quarter of a side further down, gives horizontal or vertical polarisation — two different antennas out of one piece of wire.

feedpointperimeter = λ
The perimeter of the loop equals one wavelength; the feedpoint sets the polarisation
Frequency (MHz)
Loop shape
Feed point
Conductor diameter (mm)
Dimensions
Loop perimeter
Side length
Triangle height
Wavelength λ
Feed
Expected impedance
Polarisation
Quarter-wave transformer to 50 Ω
Gain referred to a dipole

📐 The formulas​

Loop perimeter:

P = k × 299.792458 / f    [m],   k ≈ 1.005…1.022

The classic 1005/f rule in feet is 306.3/f in metres.

Dimensions:

delta:  side = P/3,   height = side × √3/2
quad:   side = P/4
circle: diameter = P/π

Matching:

Z_t = √(50 × Z_loop)       ← impedance of the quarter-wave transformer

Impedance and polarisation by feed point:​

Feed pointImpedancePolarisation
At a corner≈ 110 Ωhorizontal
Middle of a side≈ 120 Ωhorizontal
A quarter along a side≈ 55 Ωvertical
Where this calculator stops:
  • The impedances are typical, not exact: the real figure depends on height and on the shape of the loop
  • For a delta, the orientation of the triangle — apex up or apex down — matters as much as the feed point
  • The shortening factor for a loop is smaller than for a dipole and barely depends on wire thickness
  • The +1…1.5 dB over a dipole assumes the loop hangs at 0.5 λ or higher

🎯 In practice​

  • A 20 m delta. Perimeter 21.7 m, side 7.2 m — it fits between two trees and one mast.
  • A quiet receive antenna. A closed loop picks up less of the electric component of local noise than a dipole does.
  • Vertical polarisation without a vertical. Feeding a quarter along a side gives a low radiation angle with no radials at all.
Practical advice:
  1. 110–120 Ω matches neatly through a λ/4 length of ordinary 75 Ω cable — the classic solution.
  2. You can also feed the loop directly with 50 Ω: an SWR around 2.2 is nothing a tuner cannot handle.
  3. Apex up is easier to hang; apex down gives a slightly lower radiation angle.
  4. Do not skip the choke on the feedline: on a loop fed asymmetrically, current on the braid is particularly noticeable.

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

How to cite this calculator
UR3PKI. «Full-wave loop calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/full_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.

Last updated on