A Short Tale About the Family, the Fox, and the Moxon
The number of amateur radio activities in which one can involve all family members, whether or not licensed, is quite large. Field Day comes to mind, with a strong communal sense pervading the operating positions, the picnic tables, and the play area for children. However, my thoughts are drawn to the idea of some special activities in which there is mutual activity between parent and child, to the near exclusion of all else. The activities should let the child participate and give plenty of time for simple togetherness between the family members. Of course, "simple togetherness" is never simple, but involves all of the details of living that make family relationships so natural in their complexity and so complex in their naturalness.
One such activity is fox-hunting: seeking out the hidden transmitter on VHF--usually 2 meters--by direction finding techniques that range from the simple to the sophisticated. There are periods of detection, in which one listens for and finds the direction of the hidden transmitter. Then there are periods of driving toward it--or at least thinking that one is driving toward the transmitter. These periods are perfect for that combination of ham and non-ham conversations that weave together a hobby and whatever is most important to the family members at the moment. The alternating detection and driving periods build toward a climax: the approach and actual discovery of the transmitter location. Finally, there is family triumph, as all shore the successful climax of the effort. Even if the work does not fall into the prize-winning category, the success is sufficient for all to share.
If this little account sounds a bit glorified, let me share a bit of correspondence: Just a quick note to say thanks for your contribution to the Amateur Radio Community. I built a variation of your 2-meter Moxon antenna design and used it at the local Amateur Radio Club fox hunt - on Saturday, May 5, 2001.
My son (who has Downs Syndrome) and I came in second place in only 35 minutes! This was our first Fox Hunt. We were beat only by a team of engineering students who do this sort of thing as a regular pass time. The third place team came in at three times the time.
Much of our success was due to the Moxon antenna, which I scaled to the exact fox frequency using EZNEC. I was actually startled at the depth of the null. Your antenna design and my son's compass brought us to within 20 feet of the "fox" with only 4 readings!
Thanks again. My son is very proud of the second place award he was given for our effort!
I have removed the names of the individuals, but left in all of the other details. Similar parent-child mutual activities are certainly possible, and the benefits--for fully able and for challenged children--are likely impossible to catalog. Even sharing the frustration of not winning a prize and demonstrating the will to try again next time can benefit a youngster of any age.
The antenna and support details appear in Fig. 2. Looking at details B-B and C-C, we can see the plan of assembly and feeding. The antenna will consist of 4 rods, each threaded to 10-24 on one end. The driver will have pairs of 10-24 stainless or aluminum nuts to hold terminal rings from the feedline. The reflector half-elements will join in a stainless steel coupling nut (a short piece threaded all the way through). You can purchase these or make one from a small piece of 1/2" by 1/2" aluminum stock. Simply drill though the piece from end to end and tap the required 10-24 threading.
My favorite antenna support material is Schedule 40 PVC. As frequency increases, I like to minimize contact with the material. It would appear that Schedule 40 PVC varies in exact composition from one part of the US to another. In some places, UV retardants are effective, while in others, they are either ineffective or non-existent. Likewise, RF characteristics may vary from one manufacturer to another. By minimizing contact with the elements, any RF deficiencies in the PVC have little or no effect on the antenna.
In
, you can see the scheme I used. 1/2" nominal PVC is glued into a T, with elbows pointing upward in the sketch. The elements pass through end caps. These caps and the pipe stubs necessary to cement them to the elbows permit the use of set screws that pass through threaded holes in the double thickness of PVC. With threading that deep, set screws work well, although other methods can be used to ensure that the element do not move. One system that also works is to use short pieces of plastic tubing over the rods between the PVC supports and the nuts.
Fig. 2 also shows a center Tee fitting for connection to a mast. This fitting is optional and would be suited more to the horizontal use of the antenna. Lets look at the alternative that I actually used in Fig. 3. When vertically orienting the Moxon, a rear support system is very useful. We can make one up out of more PVC fittings and pipe. For my test antenna, I used a single PVC pipe section between elements, with the reflector center point holding a 4-way cross fitting. Another pipe stub proceeds rearward and friction fits into the Tee in the support. I drilled two sets of holes through the stub so that I could change the antenna's orientation from vertical to horizontal and back by removing only a single bolt.
The support consists of a straight section and an angular section, forming a strong triangle. The terminating sections are 1.25" to 1/2" adapters sections. The antenna support pipes fit into the 1/2" side ports, while the 1.25" through sections receive piping of the same size to fit over a standard TV mast. Clamps mounts are shown, although one might also use set screws. The space between the upper and lower adapters can consist of the short clamps pipe sections shown, or the space can be filled with a single section of 1.25" nominal PVC. The resulting structure has a bit of flexibility, but stands up to abuse very well.
One of the keys to building a good PVC structure is to be prepared in advance for aligning portions of the structure that must be at right angles or parallel to each other. PVC cement may give you as little as 15 seconds before glued parts become immovable. I have cataloged a number of right-angle junctions in my shop that are suitable for aligning PVC pieces. Among them are legs and other supports for the work bench. If a Tee or an elbow requires fitting, I usually put a scrap of pipe dry into the open end. The longer section makes alignment much easier than a junction stub. I simply glue the junction in question and then press it against my sturdy pre-formed angle.
For the element mountings in their caps, I first cemented the junction pipe stubs in the caps. (These stubs will be glued into the elbows later.) Then I drilled the caps and stubs together. A drill press makes easy work of centering the holes on both sides of each cap. Next, I pushed each pair of caps over a scrap of 3/16" diameter rod to keep them aligned while cementing each one into its corresponding elbow joint. These and similar techniques simplify making almost any conceivable structure from PVC parts.
Bending rod elements is most simply done with a copper tubing bender, a small device that can be found at most hardware depots. The bender will make a 1" radius bend. 3/16" rod and smaller will easily handle bends of this radius with no noticeable weakening or visible cracks. I first cut each element section to length and mark the point (away from the threaded end) that corresponds to one half the side-to-side width. I placed this point in the middle of the 90-degree bend. This technique has yielded results identical to those predicted by sharp-cornered computer models of the antenna.
There are many ways to construct the Moxon for VHF use. The sample shown here is only one of them. For an alternative construction method, with many useful additional notes on the design, see "A Compact Two-Element, 2-Meter Beam" by Lee Lumpkin, KB8WEV, and Bob Cerrito, WA1FXT. The article appeared in the January, 2000, QST, pp. 60-63. Their version used #10 AWG wire, which is just over 0.1" in diameter.
The Moxon can be scaled for 220 or 440 MHz use--so long as one remembers also to rescale the element diameter or to make appropriate adjustments in dimensions if a scaled element cannot be used. The thinner the wire relative to the original, the smaller the gap; the fatter the element, the wide the gap. And other dimensions will also alter slightly to obtain the maximum null at the new design frequency, with the resonant point slightly lower to obtain full operational coverage of a band or the subsection of interest.
For general use--horizontally or vertically, the front-to-rear ratio--accounting for the full rear quadrant--is very good anywhere in the band. Moreover, as Fig. 5 shows, the antenna has a very broad operating bandwidth. The SWR curve never reaches 1.4:1 anywhere in the band.
The first task after placing all of the Moxon element pieces in their PVC holders and attaching the feedline is to adjust and temporarily lock the gaps between element ends. The first rod Moxon showed a 50-Ohm resonance at about 145.75 MHz, just about where the design placed it.
It is usually dangerous to assume that, because an antenna is tuned to provide a good match for the feedline, it will also show the modeled gain and front-to-back ratio. However, in this case, the antenna geometry that determines the source impedance also determines the pattern shape. Since the elements were uniform in diameter, there was no reason to expect any performance surprises. Therefore, I locked the assembly tight and moved on to performance tests.
The Knoxville, TN, area has repeaters from close to 145 MHz up to nearly 148 MHz. All of them are easily accessed and heard at full quieting from my location with only a low elevation ground-plane vertical. In fact, a telescoping whip on my hand-helds will access all of our main repeaters. On a 15' mast, the Moxon made "telephone" copy of all of the repeaters when the antenna pointed anywhere near the forward direction to them. A distant repeater that I hear poorly on a vertical was now full quieting. The Moxon's gain can make a difference for signals at the FM threshold.
When the antenna faced away from the repeaters, only 1 of 6 repeaters that I checked could still be heard, and then at far less than full quieting. I could not access any of the repeaters with 5 watts of power--the limit of my gear at the time. I ran additional tests using two hand-helds on simplex across the FM portion of the band. Differences in the patterns at the band edges and at the design frequency were not especially detectable at distances of a quarter mile. Face forward, communications was easy and full quieting. With the antenna pointed in the other direction, communications was virtually impossible.
The proto-type vertical rod Moxon has proven a very effective antenna at blocking unwanted signals from the rear. In fox-hunting, it is precisely the deep null to the rear that let's us take a bearing on the hidden fox transmitter. Whether distant from the fox or close in, the deep null--combined with a compass--will serve well in locating the fox's direction without any ambiguity.
I shall leave the details of setting up the Moxon for mobile fox-hunting to your ingenuity. Some folks use a system involving a auto-window mount support, and it may also contain a large compass rose. Set the compass rose with a compass at each stop or reading period and then take the null of the fox. Using an antenna backwards takes a little getting used to, but quickly results in accurate bearings on the fox. Now if only the roads went to the fox as the crow flies, we could win every fox hunt in a flash (well, a slow flash within the speed limits).
As in the communications that I received, you can let the child operate the antenna and compass, building their confidence in their abilities to contribute to family success. I remain convinced that the self-confidence of children as they grow older stems not solely from succeeding in games and activities with their peers. It also grows with every successful contribution they make to the success of any activity that the family deems to be important. As well, successful contributions to the fox hunt may also intensify their interest in amateur radio as a life-long means of making contributions to the larger family that we call community and society.
Fox-hunting is not the only way to achieve such goals, but it is a good case in point. Amateur radio is filled with opportunities for service and for full family participation that includes both the licensed and non-licensed members. I am indebted to a father and son for reminding me of this important aspect of our avocation.
From Proceedings of the 2002 National ARRL Education Workshop, (Newington: ARRL, 2002), pp. 95-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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