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Side View

Side View

Fig 1—Hardwood element mounting block and Lexan plate.

Fig 2—Custom saddle dimensions.

Jan/Feb 1998 53

correctly, but I haven't really looked for a source of telescoping square tubing. Two of the element pins also hold the three boom sections together.

It's more traditional to use U bolts and saddles, but many people have problems with the elements rotating out of alignment. This results from using "universal" saddles, which aren't matched to the tubing being used.

You can get much better performance by machining saddles that maximize the contact area. I've made them out of 1 inch square tubing to fit standard IV4 inch mast sections. It may be a little easier to make them out of channel stock. This is a tedious process with a round file, but a lot cheaper than using a miniature milling machine with a rotary table. Fig 2 shows the basic design of the saddles. I've also made some wooden versions using a Forstner bit (designed for cutting clean holes), but haven't thoroughly tested them.

Here's another significant mechanical problem: What's the best way to make portable elements that fit inside a small car? Fortunately, aluminum tubing is conveniently sold in sizes that telescope together—0.058-inch-wall tubing works quite well for this purpose.

The challenge is joining them together. I did it the hard way, slotting the tubing and making custom clamps out of aluminum plate. I've made the slots with a bandsaw, but I get much better results with a metal reinforced wood fixture that guides a hacksaw

Photo 2—Machined clamp to match 17« inch mast.

Fig 3—V2 inch tubing clamp construction steps. 54 QEX

blade. A fixture using a miter box and backsaw might work even better.

Fig 3 shows an element clamp I've designed. I make them out of scraps of V4 inch 6061-T6 aluminum plate. Number 8 stainless-steel wing nuts work quite well; they allow quick assembly by hand. (I've also tried #6 hardware, but it is a little too small. I needed a flat washer to reduce the friction when using the smaller #6 hardware.) A version using #10 hardware might be advisable for someone with bigger fingers.

If the wing nuts get lost, it is still possible to use the clamps, by unscrewing the 1 inch screws and screwing them in backwards with an Allen wrench. Keep an Allen wrench handy by taping it to the boom.

You could simply drill holes in the assembled elements and put screws through the holes. The disadvantage is that you can't tweak the lengths, but

Photo 3—Machined element clamp.
Photo 4—Element clamp holding tubing together.

that isn't a problem if you are copying a design that doesn't need adjustments. Adjustment-free antennas make quite a bit of sense for portable work, since it isn't unusual for things to shift during transport.

You don't want to waste time figuring out the proper settings. To help identify the element pieces quickly, I color coded them by painting the element tips. I also painted the boom to assist in putting the elements in the proper location. Colored tape also works, as long as you don't have to slide a clamp or U-bolt over it. Tape can stop a clamp's movement or get shredded in the process. Paint will probably be scratched up the first time you use the antenna—so don't worry too much about how it looks.

You can also use stainless-steel hose clamps in place of my machined versions. If you go this route, it's a good idea to use clamps made entirely of stainless steel; some clamps have parts that rust.

In keeping with the idea of a simple design, I used a X/4 of somewhat exotic RG-83 coax to feed a split driven element, as shown in Fig 4 (before coiling to form the balun). I'm not sure if anyone besides Times Microwave makes this coax, but it is available from the Wireman.4 Because the antenna is balanced, I provide a balun by making a two-turn coil out of the matching section. To accommodate the large #10 center conductor of RG-83, I used a 9913 N male connector on one end and drilled out the center pin of a female UG-23/U connector for the other. Many standard N connectors are designed for #11 or #13 wire and won't fit without modification. Some people have filed down the center conductor of the coax to fit such connectors.

Alternatively, one could parallel connect two lengths of 75 i2 coax (connecting both the shields and center conductors at both ends) to make 37.5 ii coax. This does shift the optimum feedpoint impedance, however, from 24.5 to 28 £2. A perfectionist might want to tweak the electrical design of the Yagi a bit to fit this.

The driven-element connection is simpler—I swaged a pair of terminal posts to some brass strips and some unclad fiberglass circuit board. The brass strips are then attached to the driven element with #4-40 screws. The coax is taped to the fiberglass to provide strain relief for the soldered connections to the terminal strips. For the other end, I made an L bracket out of brass to hold a UG-58A/U panel jack.

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