The moxons will fed out phase with phasing line coaxial cable

By L. B. Cebik, W4RNL
A Simple Fixed Antenna for VHF/UHF Satellite Work Explore the low-Earth orbiting amateur satellites with
this effective antenna system.

W vesting in a complex azimuth-elevation ested in amateur satellite operation, the thought of in-hen we are just getting inter-
			will be exactly half the impedance of one element alone. So we will obtain an im-pedance of about 35 Ω. For the dipole-based turnstile antenna, we’ll either have to accept an SWR of about 1.4:1 or we’ll have to use a matching section to bring the antenna to 50 Ω. A parallel set of RG-63 1/4-λ lines will yield about 43 Ω im-pedance, about right to bring the 35-Ω antenna impedance to 50 Ω for the main coax feed line. For all such systems, we must remember to account for the veloc-ity factor of the transmission line, which will yield a line length that is shorter than a true quarter wavelength.
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The elevation patterns show the con-siderably smoother pattern dome of the Moxon pair over the traditional turnstile. The middle of the turnstile dome has nearly 2 dB less gain than its peaks, while the top valleys are nearly 3 dB lower than	the peaks. The peaks and valleys can make the difference between successful commu-nications and broken-up transmissions. So, for the purpose of obtaining a good dome, the Moxon pair may be superior.

Building the Moxon Pairs

The Moxon rectangle is a modification of the reflector-driver Yagi parasitic beam. However, instead of using linear elements, the driver and reflector are bent back to-ward each other. The coupling between the ends of the elements combined with the coupling between parallel sections of the elements combine to produce a pattern with a broad beamwidth. By carefully selecting the dimensions, we can obtain both good performance (meaning adequate gain and an excellent front-to-back ratio) and a 50-Ω feed point impedance.1
In fact, a single Moxon rectangle might be used on each band for reason-ably adequate satellite service. When pointed straight up, the Moxon rectangle pattern is a very broad oval, although not a circle. The oval pattern also gives the Moxon another advantage over dipoles in a turnstile configuration. If the phasing-line between dipoles is not accurately cut, the normal turnstile near-circle pattern degrades into an oval fairly quickly be-

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Figure 4—A comparison of elevation patterns for 2-element turnstiles (crossed 2-element Yagis, shown in blue) and a Moxon pair (shown in red), both at 2λ height.

Figure 5—The basic dimensions of a Moxon rectangle. Two identical rectangles are required for each “turnstiled” pair.

cause the initial single dipole pattern is a figure 8. The single Moxon oval pattern allows both dimensional inaccuracies and phasing-line inaccuracies of considerable amounts before degrading from a nearly perfect circle.				The driver consists of two pieces, since we’ll split the element at its center for the feeding and phasing system. I usu-ally make the pieces a bit longer before bending and trim them to size afterwards. The total length of the driver, including the open area for connections, should be A + (2 ⋅ B). Perhaps the most critical dimension is
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It is one thing to make a single Moxon and another to make a working crossed pair. Figure 6 shows the general scheme that I used for the prototypes, using CPVC. (Standard schedule 40 or thinner PVC or fiberglass tubing can also be used.) The support stock is 3/4 inch nominal. The reflectors go into slots at the bottom of the tube and are locked in two ways.

Whether or not the two reflectors make contact at their center points makes no difference to performance, so I ran a very small sheet screw through both 2-meter reflectors to keep their relative positions firm. I soldered the centers of the 435-MHz reflectors. Then I added a coupling to the bottom of the CPVC to support the double reflector assembly and to connect the boom to a support mast. Cementing or pressure fitting the cap is a user option.

The 435-MHz Moxons.

You can contact the author at 1434 High Mesa Dr, Knoxville, TN 37938;cebik@ cebik.com.

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