Horizontal loop antenna
| Horizontal loop antenna | |
|---|---|
| Also called | Loop skywire, full-wave horizontal loop, "sky loop" |
| Type | Full-wave wire loop mounted parallel to the ground |
| Perimeter | About 1005 ÷ frequency in MHz, in feet |
| Shape | Any shape enclosing as much area as practical; square or near-square is common |
| Typical sizes | |
| 80 m (3.6 MHz) | About 280 ft (85 m) of wire |
| 160 m (1.85 MHz) | About 540 ft (165 m) of wire |
| Feed line | Ladder line to a tuner for multiband use, or coax through a balun |
| Supports | Three or more, at the corners |
A horizontal loop antenna is a single loop of wire, one wavelength around on its lowest band, strung parallel to the ground from three or more supports. Often called the loop skywire, after a 1985 QST article by Dave Fischer, W0MHS, that popularized it, the full-wave horizontal loop is a favorite with hams who have a large yard and want one wire antenna for every HF band. Built for the 80-meter or 160-meter band and fed with ladder line and a tuner, it covers its fundamental and all the higher bands, and many owners find it quieter on receive than other wire antennas.
How it works
The perimeter of a full-wave loop is about 1005 divided by the frequency in megahertz, in feet. For an 80-meter loop cut for 3.6 MHz, that is about 280 feet of wire, a square roughly 70 feet on a side. A 160-meter loop needs about 540 feet, a square of about 135 feet per side. The shape need not be regular: any outline that fits the available trees will work, as long as it encloses a reasonable area. Squares and circles enclose the most area for a given length of wire. A very long, narrow loop behaves more like a folded dipole than a loop.
On its fundamental, at the heights most hams can reach, a horizontal loop is low in terms of wavelength: 40 feet is only about a seventh of a wavelength on 80 meters. At that height the loop radiates mostly straight up, which makes it an excellent antenna for NVIS, the short-range skywave that gives strong, steady signals out to a few hundred miles on 80 and 160 meters.
On the higher bands, the same loop is several wavelengths around and much higher in terms of wavelength. Its pattern breaks up into a number of lobes, some of them at the lower angles useful for DX. The loop is not a beam, and its lobes and nulls shift from band to band, but on 20 meters and above it can work long distances well.
Feeding for multiband use
The feed-point impedance of a horizontal loop is roughly 100 ohms or more on its fundamental, depending on height, and it varies widely on other bands. The usual multiband arrangement is 450-ohm window line or other ladder line running all the way to an antenna tuner in the shack, either a balanced tuner or one with a 4:1 balun at its output. Ladder line has very low loss even at the high SWR it sees on some bands, which is what makes the arrangement efficient.
Ladder line needs a little care. It should be kept a few inches away from metal, gutters and other wiring, held off walls with standoffs, and given a gentle twist along its length to keep it from fluttering in the wind. Where it must change to coax, the balun should be outside or just inside the wall.
A loop fed with coax through a balun is also common. It gives a good match on the fundamental and on some harmonics, but less flexibility than ladder line.
Quieter receive
Many owners report that a horizontal loop is noticeably quieter than a vertical or a dipole, with a lower noise floor and signals that are easier to copy. A common explanation is that much of the noise from household electronics near the ground is vertically polarized and is picked up less well by a horizontal antenna, especially one strung high and away from the house. Results vary with the location and noise sources, but the reputation is one of the main reasons the antenna is so popular for listening on the low bands.
Putting one up
A horizontal loop needs three or more supports, usually trees, at heights of 30 feet or more where possible. Each corner has an insulator or a smooth ring the wire can pass through. Because trees sway in the wind, many builders hang each corner from a rope over a pulley with a weight at the other end, so the wire stays taut without snapping. Stranded copper wire, insulated or bare, is the usual choice; insulation lowers the resonant frequency slightly.
The feed point can be at a corner or anywhere along a side, wherever the feed line can run most directly to the house. As with any large wire, the loop should be kept well away from power lines, and the feed line should enter the building through proper lightning protection.