First, a word about Moxon Rectangle antennas. The Moxon Rectangle was originally designed by Mr. L.A. Moxon, who modified an existing square design by VK2ABQ called the “VK2ABQ Square.” Both designs essentially “shrank” a 2-element Yagi by bending the ends of the driven element and reflector so the ends were practically touching, as shown in Figure 1.
As it turned out, this considerably reduced the size and turning radius of the antenna, making it easier to erect and to rotate. It could also better withstand extreme weather. A Moxon Rectangle's forward gain is somewhat less than that of a full-size two-element Yagi, but its front-to-back ratio is better, providing a worthwhile tradeoff.
The PAR Electronics' Stressed Moxon (SM) is an ingenious variation on the Moxon Rectangle. The SM consists of three main components: A square aluminum driven element, a reflector element, and tubular end arms (see Figure 2).
The driven element is divided into two sections and is separated by a sturdy solid fiberglass insulator. The mast clamp and antenna feed block also bolt onto this insulator. The ends of the driven element are drilled to provide a snug friction-fit for the end rods. Each end rod actually forms part of the driven element and part of the reflector. A plastic insulator in the center separates the two halves as shown in the figure.
The ends of the rods opposite the driven element are threaded to accept a small screw, which holds the reflector element in place. The reflector is made of insulated flex-weave copper wire with factory-installed lugs at each end.
The stress of this antenna comes from the reflector wire, which is slightly shorter than the driven element. When the SM is assembled, the wire pulls on the ends of the end rods, locking them into place on the main element and putting stress on the entire antenna, as if it were an archer's bow.
The beauty of the PAR Electronics design is that as you move away from the feedpoint, each subsequent section of the antenna is made of thinner, lighter (but strong) material. This gives the SM a great capacity to flex and absorb punishment from the elements – whether they be high winds, snow or ice – while keeping weight to a minimum.
The SM accepts a standard PL-259 UHF connector. Adjustment for minimum SWR is made by varying how far the end rods pass through the square main element. A Moxon Rectangle's bandwidth is quite wide, more so than a Yagi, and the SM we tested worked fine with the end rods inserted just far enough to slide the provided plastic end caps into place.
The SM-50 can be assembled from scratch in 20 minutes with only a screwdriver. The assembly method makes it a perfect antenna for “hill-topping,” rover work, or emergency operations. It can be kept partly disassembled in the trunk of a car and can be put together without tools in less than 5 minutes. Its small size and light weight make for easy “Armstrong method” rotation, or you can use an inexpensive TV rotor to do the job.
To test a quickly erected portable setup, we clamped the SM-50 to an MFJ fiberglass telescoping mast by extending only the lower six sections. This provided a height above ground of about 18 feet (roughly one wavelength on the 6 meter band). On the air, local 6-meter propagation beacons that were S-9 off the front of the SM-50 were typically still audible but didn’t move the S-Meter off the back of the antenna. This was an impressive testimony to the SM-50’s front-to-back ratio, mentioned earlier.
During on-the-air contacts, we received excellent signal reports with the SM-50, and found that it compared favorably with a four-element Yagi installed at the same height. As you might expect, the Yagi had an edge in signal strength most of the time, but the difference was not extreme, and considering the easy assembly and portability of the SM-50, we believe this is an antenna well worth exploring for the VHF operator. We can all do with a little less stress in our lives, but just this once you'll be better off by adding a little!
The approximate cost of the SM-50 is $79.95 (U.S.). You can learn more about the antenna and other Par Electronics offerings by visiting the firm’s website at http://www.rf-filters.com.
Table 1. PAR SM-50 Technical Specifications
Parameter Specification
Polarity: Horizontal
Gain: 5.8 dBi
F/B Ratio: 17 dB
Design Impedance: 50 Ohms
VSWR Bandwidth: 1.4 MHz between 1.5:1 points
Power Handling: 1000 Watts
Weight: 3 Pounds
Size: Rectangular, 84" X 31"
Hardware: Stainless Steel
Mounting: Mast mounted. Supplied bracket accommodates 0.75" to 1.5" masts.