What a horizontal loop is
A horizontal loop, or sky loop, is a single loop of wire one wavelength long at its lowest frequency, strung horizontally between several supports. It's popular because it's quiet on receive, works on many bands with a tuner, and doesn't need to be very high. At a typical height of 8 to 15 m on 80m, most of its signal goes upwards, which makes it excellent for UK and European contacts on 80m and 40m. On the higher bands, where it's several wavelengths around, it develops low-angle lobes that work DX well.
Total length (metres) = 306.3 ÷ f (MHz)
A full-wave loop is about 2% longer than a free-space wavelength, the opposite of a dipole. The length factor of 1.02 gives the traditional formula.
Shape and height
The shape matters less than enclosing the biggest area you can, which gives slightly more gain. A square or circle is ideal, a triangle works well with three supports, and a rectangle is fine as long as it isn't too long and thin. Use whatever trees and masts you have, and let the shape follow them. Get it as high as practical, but even 6 m works on the low bands for short-range contacts.
Feeding the loop
At its fundamental, a horizontal full-wave loop has a feed impedance of roughly 100 to 200 Ω, depending on height and shape. On the harmonics it varies widely. The most flexible way to feed it is with 450 Ω ladder line to a balanced tuner, or to a 4:1 current balun at the tuner. Ladder line keeps losses low even with a high SWR on the line, so the loop works on almost every band from its fundamental up. A 4:1 balun at the feedpoint with coax is simpler but lossier on bands where the match is poor.
The loop is close to resonant on its harmonics, which is why an 80m loop also works well on 40m, 20m, 15m and 10m. On the in-between bands, the tuner does more of the work.
Questions
How long is an 80m sky loop?
About 84 m (275 ft) of wire at 3.65 MHz, so a square of about 21 m on each side.
How high should a horizontal loop be?
As high as practical. At 8 to 15 m on 80m, it radiates mainly upwards, which is ideal for UK and European contacts. Higher improves DX.
How do I feed a sky loop?
450 Ω ladder line to a balanced tuner, or to a 4:1 balun at the tuner, works on almost every band. A 4:1 balun and coax at the feedpoint is simpler but lossier.
Does the loop have to be square?
No. Any shape works. Enclosing a larger area gives slightly more gain, so aim for something close to a square or circle.