Delta loop antenna
| Delta loop antenna | |
|---|---|
| Type | Full-wave wire loop in a triangular shape |
| Perimeter | About 1005 ÷ frequency in MHz, in feet, then trim |
| Feed-point impedance | Roughly 100 ohms or a little more, depending on shape and height |
| Polarization | Horizontal or vertical, set by the feed point position |
| Practical details | |
| Supports | One tall support at the apex, two low ones at the bottom corners |
| 40-meter loop | About 141 ft (43 m) of wire at 7.15 MHz |
| Matching | Quarter-wave section of 75-ohm coax, or a suitable balun |
| Common bands | 40 m and higher on HF; larger loops for 80 m; also VHF beams |
A delta loop antenna is a loop of wire about one wavelength around, formed into a triangle and usually hung in a vertical plane. The name comes from the Greek letter delta (Δ). Hung with one corner up, it needs only a single tall support plus two low tie-off points, and it gives a little more gain than a dipole. Simply by choosing where to connect the feed line, the builder can make it horizontally or vertically polarized, which makes the delta loop popular for DX on the 40-meter and lower bands.
How it works
A full-wave loop is closed, with no free ends. Current peaks at the feed point and again at the point halfway around the loop, and the loop behaves roughly like two short dipoles fed in phase: stacked one above the other when it is horizontally polarized, side by side when vertically polarized. That arrangement gives a modest amount of gain over a single dipole. A vertically hung loop radiates best broadside to the plane of the loop, in both directions, and least edge-on.
The loop's perimeter sets its resonant frequency. A widely used starting figure, the same one used for quad loops, is about 1005 divided by the frequency in megahertz, giving the perimeter in feet. For 7.15 MHz that comes to about 141 feet, or three sides of about 47 feet. The figure is only a starting point: insulated wire, nearby objects, the height of the bottom wire and the shape of the triangle all shift the resonance, so the loop is cut a little long and trimmed.
An equilateral triangle is the classic shape, but the loop tolerates distortion. A triangle that is wider than it is tall, or one that is squashed to fit between trees, still works, though the impedance and pattern change somewhat. A loop that encloses very little area, such as a long, thin triangle, behaves more like a folded dipole than a loop.
Feed point and polarization
In the common apex-up arrangement, with the flat side at the bottom:
- Fed at the middle of the bottom side, the loop is horizontally polarized. The current maximums fall at the bottom center and at the apex.
- Fed on one of the sloping sides, about a quarter wavelength down from the apex, the loop is vertically polarized.
- Fed at a bottom corner, the loop produces a mixture of both.
Vertical polarization is the usual choice when the apex can't be raised very high. A horizontally polarized antenna needs to be roughly a half wavelength above the ground to produce the low-angle radiation that favors DX, which on 40 meters means about 70 feet. A vertically polarized delta loop sends a useful share of its signal toward the horizon even with its bottom wire only a few feet up, much like a pair of short phased verticals. It doesn't need radials, but like any vertically polarized antenna its low-angle performance depends on the ground in front of it.
The loop can also be hung apex down, with the flat side at the top between two supports. Fed at the bottom point it is horizontally polarized; fed partway up one side, vertically polarized.
Feeding a delta loop
At resonance the feed-point impedance is typically around 100 ohms or a little more, too high for a direct connection to 50-ohm coax without some SWR. A classic match is a quarter-wave section of 75-ohm coax between the loop and the 50-ohm feed line, which transforms the loop's impedance down to near 50 ohms. The section must be an electrical quarter wavelength, so its physical length is a free-space quarter wavelength multiplied by the coax's velocity factor. A balun with a suitable ratio, or a choke at the feed point, keeps RF off the outside of the coax.
Fed with ladder line and an antenna tuner, the same loop can be used on several bands, though the pattern and polarization change from band to band.
Variations
Two delta loops on a boom, one driven and one acting as a reflector, make a directional beam similar in principle to a two-element quad. On VHF, small delta loops of wire or rod are easy to build. On 80 meters, a full-size delta loop needs about 280 feet of wire and a support of 80 feet or more, so many low-band versions are compressed or bent to fit the space available.