Moxon

The Moxon antenna is a compact two element beam antenna with amazing specifications. Free space performance claims about 6db forward gain with around 35db front to back rejection. The antenna can be built from aluminum tubing like conventional yagis but it also easily made from wire and bamboo spreaders similar to quad construction. In fact, when compared to the quad, the Moxon can deliver the same results as a quad yet the Moxon only requires half the parts. The Moxon puts the driven element AND the reflector on one set of four spreaders. Half the parts equal half the weight allowing a smaller, less expensive rotator. Don’t need no prop pitch motors here!

The ‘normal’ Moxon orientation is horizontal. That is the elements are parallel to the ground. Even so, a vertical orientation is also possible, more easily implemented, and just as effective as the horizontal. A rope or wire is strung between the tops of two tall trees. The two shorter sides of the Moxon are implemented using fiberglass insulators and aluminum tubing forming two rigid sturctures. The sides are made from wire which is held in tension by the weight of the lower structure. The upper structure is tied to the high horizontal rope from its midpoint. The lower structure is then rotatable toward any direction and kept in place with two thin ropes attached to the ends on the lower section. Now instead of bothering to feed this antenna at the midpoint of the driven element, it can be fed at the lower corner, in an off-center fed manner similar to a windom. The higher impedance at this corner can be matched to 50 ohms using a 6:1 balun. Or just connect two 4:1 baluns in series. Take off angle on such a vertical is around 18 degrees and IT NEEDS NO RADIALS AT ALL BECAUSE IT IS A COMPLETE TWO ELEMENT BEAM. ITS VERTICAL PROPERTIES ARE DUE TO ORIENTATION ALONE.. In the example given, the antenna can be turned by hand to favor any compass direction. Since the Moxon’s radiation pattern is very broad, accurate positioning to a specific direction is unnecessary.

The Moxon is perhaps the first answer I have ever seen to the question, ‘is there a cheap and easy solution to providing a rotatable beam antenna with decent gain and front to back’?

Moxons for multiple bands can be mounted to the same four spreaders required for this antenna making it a multiband device. There is also no reason a Moxon cell could not be used with conventional directors although Moxon directors would make more sense since they are only 70percent the size of the conventional directors. Smaller size, less material, less weight.

It is also possible to use open wire line, OR 300ohm twin lead, OR 450 ohm ladder line to feed the driven element and provide multiple band operation by using an antenna tuner. In this case the antenna would work as a true Moxon at only the lower design frequency but it would provide useful coverage of frequencies above that lower band as long as the tuner could provide a match.

At this location we have been using a KLM KT-34 beam for nealy 20 years. Recently the HD-78 rotator broke allowing the beam to freewheel in the wind. The freewheeling beam broke its feedline. While attempting repairs it was noted that the capacitor caps used on the linear loading sections were deteriorated beyond repair and needed replacing. After investigating the Moxon, we are now considering modifying the KT-34 to a single band (20meter) two element Moxon allowing east and west coverage either by providing seperate feedlines and flipping reflector and driven element functions or adding a variable capacitor to a center stub on the reflector and tuning it to be a reflector or director.

A two element 20 meter Moxon requires a 9 foot boom. The KT-34 has a 16 foot boom. The seven foot left over is not enough distance to install a director for 20 meters but it is enough to install a second Moxon for 15 meters (maybe). Interaction between the 20 and 15 meter beams may be a problem. In which case we can always add a 15 meter and even a 10 meter Moxon inside the 20 meter beam. Will have to look at that and see if perhaps a third element can be added to the 15 meter cell. Adding a 10 meter cell would undoubtedly allow two 10 meter directors.

The problem with the multiple cell Moxon is that we could not easily make it directionally reversible unless we rotate the antenna. Currently we have no plans to replace the HD-78 rotator. Can’t afford it. Even if we could afford it, we are adding a small rotator to support a couple of vhf antennas above the HF beam. We figure that removing the HD-78 and reducing some weight on the KT-34 will allow us to make do with the present mast without having to add more pipe to reinforce it.

In addition to the KT-34 modifications we are adding a bi-directional 40 meter Moxon, a 20 meter Moxon vertical, and a 30 meter bobtail in place of the 40 meter reduced bobtail installed now.

The 20 meter vertical Moxon will be hung from the center of a wire support strung from between the main mast and the smaller tilt over at the east side of the house. The 40 meter Moxon can be supported on the same supports holding the 80 meter loop without interferring with the 80 meter loop.

Now, on top of all this we can set up the 20 meter Moxons to also cover 17 meters.

To sum up we will have Moxons for 40, 20, 17, 15, and 10 meters. Roatable beams for VHF and a bobtail for 30 meters. Last but not least we will also still have the 80 meter loop.

Looks like a full summer of antenna work.

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