Moxon antenna
| Moxon antenna | |
|---|---|
| Also called | Moxon rectangle |
| Designer | Les Moxon, G6XN |
| Type | Two-element parasitic beam with the element tips bent toward each other |
| Elements | Driven element and reflector |
| Performance | |
| Front-to-back ratio | Unusually high for a two-element beam |
| Feed-point impedance | Close to 50 ohms |
| Size | A little over a third of a wavelength wide, about an eighth deep |
| Common bands | 20 through 10 m on HF; 6 m, 2 m and up on VHF and UHF |
A Moxon antenna, or Moxon rectangle, is a compact two-element beam whose elements are bent into a rectangle, with the ends of the driven element and the reflector folded back toward each other until they almost touch. It is named after the British amateur Les Moxon, G6XN. The design gives a forward gain close to that of a two-element Yagi, an unusually good front-to-back ratio, and a feed-point impedance near 50 ohms, all in an antenna about 70 percent as wide as a full-size Yagi. It is simple to build from wire, tubing or rod, which has made it a popular homebrew project.
History
The Moxon grew out of an earlier design, the VK2ABQ antenna of the Australian amateur Fred Caton, VK2ABQ, which bent the elements of a two-element beam into a square with their tips close together. Moxon refined the idea into a rectangle with carefully chosen proportions and described it in his writing on HF antennas. The design spread widely after L. B. Cebik, W4RNL, modeled it in detail and published dimension tables and formulas for building one on any band. The same tip-coupling idea is used in the hex beam.
How it works
Like a two-element Yagi, a Moxon has a driven element, connected to the feed line, and a slightly longer reflector behind it. The reflector is not connected to anything; it picks up energy from the driven element and reradiates it in a way that reinforces the signal in the forward direction and cancels it toward the rear.
What makes the Moxon different is the shape. Each element runs straight across the front or back of the rectangle and then bends 90 degrees, so its ends, or tails, point toward the other element. A small gap separates the tips of the two elements. The elements couple to each other along their parallel sections in the usual way, and also across that gap. With the dimensions right, this extra coupling produces a broad, deep null to the rear and a feed-point impedance close to 50 ohms, so the antenna can be fed with coax directly, with no matching network. A choke at the feed point is still a good idea.
The forward lobe is broad, so a Moxon doesn't need precise aiming. Its main strength is rejection from the back, useful for cutting interference and for hearing weak stations in one direction while unwanted ones are behind the antenna.
Size and dimensions
A Moxon is a little over a third of a wavelength wide and about an eighth of a wavelength deep. For the 20-meter band that works out to roughly 25 feet wide and 9 feet deep, compared with 33 feet or more for the elements of a full-size two-element Yagi.
The exact dimensions depend on the frequency, the diameter of the wire or tubing, and the size of the gap, and the front-to-back ratio is sensitive to them. Rather than scaling one set of figures from band to band, builders usually work from a Moxon calculator or from antenna modeling software, entering the actual wire size they will use. Insulated wire shifts the dimensions slightly as well.
Building and using one
On HF, a Moxon is usually built from wire stretched on a pair of crossed fiberglass spreaders, much like a hex beam, and mounted on a mast with a light-duty rotator. Its small size and low weight suit backyard towers, roof mounts and push-up masts. A wire Moxon can also be hung in a fixed direction from ropes between trees, or hung in a vertical plane for vertical polarization on the lower HF bands.
On 6 meters and the 2-meter band, Moxons built from aluminum rod or wire on a plastic pipe frame are light enough to hold or mount on a small mast. They are used for portable stations, fox hunting and working satellites, where a broad forward lobe and a clean rear null are both helpful.