No. 38: Notes on Home-Brewing 10-Meter Yagis

Over the last several columns, we have examined the electrical designs of a number of Yagis ranging from 3 to 7 elements. Along the way, we have noted some construction pointers and preferences. However, let's pause a bit to see if we can pull some of those ideas together in one place--here.

1. Element and Boom Stock: For casual and portable operations, where an antenna is not going to be in the wind for long periods of time, hardware depot aluminum tubing is usually satisfactory for elements. It comes--at least in my part of the country--in limited sizes, beginning at about 3/4" diameter. So designs using this span of tubing can rely on local purchase.

6063-T832 and 6061-T6 are the more commonly used aluminum tubing for Yagi elements. Commercial houses can use thin wall versions of this tubing for lighter elements, since they have the machinery to resize the ends for perfect lap joints. However, 0.55" to 0.58" wall thicknesses are the ones used by most hams--and these are the versions carried by vendors like Texas Towers and others. Usually, for shipping ease, the tubing comes in 6' lengths, so designs need to take this into account. If you need to put two lengths together at the center of an element to get, for example, 12' of 5/8" tubing, use a short piece (about 1') of 1/2 inch tubing inside the inner ends of both 6' pieces to form a good electrical joint and stiffen the element on its mounting. For split driven elements, use a piece of fiberglass rod.

For short boom, such as for a 2-element Yagi, 1 1/4" PVC works well, but longer than 5' or so, the material sags and is heavy. Aluminum tubing makes a good boom. For modest beams up to 12 to 14 feet long, you can nest 1.125 and 1.25 inch tubing to make a very durable boom. If you have only 6' lengths, you can stagger the sections, making sure that they meet tightly at their ends.

2. Boom-to-Element Mounting: Commercial antennas often use special assemblies to join elements to the boom. Plates are bent and curved as necessary by equipment to which the average home builder has no access. However, if you like to build antennas, consider accumulating old broken beams from hams in your local area, even offering a few dollars to remove the "junk" from their yards. You will acquire a lot of unusable bent tubing and a number of quite unharmed boom-to-element plates (and also some boom-to-mast plates). Most of the junction assemblies will be reusable, even if you have to replace the original hardware with stainless steel nut and bolts from the local hardware depot. For 10-meter beams, if you have to build your own boom-to- element assemblies, it is best to design for elements that are insulated from the boom. (Directly connected elements and insulated elements usually require slightly different lengths to do the same job, so do not convert one type of assembly to the other without some redesign work.) Fig. 1 shows a method that I prefer--meaning that it is certainly not the only method available.

Having used a lot of materials for the insulating plate, I have gone to polycarbonate, the generic name for Lexan. The material is UV resistant, has excellent RF properties, is exceptionally strong, and can be cut and drilled with woodworking tools. 6 by 12 inch plates of 1/4" material handle any 10-meter elements. Trimming the corners to save a little weight is feasible. Two U-bolts hold the boom and either 2 or 4 U-bolts hold the elements. The inner U-bolts are needed only if you have a split fed elements with no alignment rod running from end-to-end of the plate inside the aluminum.

You can replicate the plate using good outdoor plywood in 1/4 or 3/8 inch thicknesses. However, plywood requires special attention both in the beginning and every year. Initially, seal the edges with a good exterior-grade fill. Then apply several coats of good spar varnish. I normally have done this to the wood before drilling U-bolt holes and then revarnished the word, getting varnish into the holes. An annual sanding and recoating is necessary, just as it would be for a wooden boat.

Use stainless steel U-bolts, preferably with saddles. The saddles help prevent the U-bolts from collapsing the tubing when you tighten everything down. These will likely be a mail-order item. Drill the holes in the plate with precision to exactly fit the U-bolts with minimum amounts of free play (but no stress on the plate). This practice will keep your elements aligned as the wind tries to push them around.

Note also that the elements are mounted beneath the plates, with the boom on top. This arrangement let's gravity help keep the elements level.

There is a simpler method of mounting insulated elements. It involves a single U-bolt per elements that goes around the boom and through the element. It is best to use a curved plate that matches the element curve as a keeper. Between the U-bolt and the boom is a short section of split gray electrical PVC that acts as an insulator. (The gray PVC used for electrical conduit has the greatest UV resistance and hence will last longer in the sun before going brittle.) The tubing used in this system should be fairly large and should have an interior piece of tubing for reinforcement. Otherwise, the element will break off right at the U-bolt. This system requires even more careful machine work for good alignment and does weaken the element. So use the scheme, outlined in Fig. 2, with caution.

<>

3. Boom-to-Mast Plates: For very light antennas, you can use a non-metallic plate for the boom-to-mast junction. Again, plywood--with the proper treatment--can also do the job. However, I prefer metal plates either 1/4 or 3/8 inch thick, depending on the weight of the antenna and the diameters of both the boom and the mast. Predrilled plates are available from mail order sources, as is stock that lets you drill your own custom pattern of holes. The object of using a metal plate is to ensure a good electrical bond between the boom and the mast (and downward) so that the entire assembly (except for the elements) is a ground potential for bleeding charges off the antenna support system.

For the larger U-bolts in this assembly, you can use saddle- types or you might wish to use muffler-type clamps. These U-bolt assemblies have a saddle with edges that some folks believe grips the boom and the mast better. However, if you do use these auto store components, be certain that all of the pieces are stainless steel. It will take a pair of U-bolts for the mast and another pair for the boom.

You have a choice of shapes for the plate: a square (or rectangle) of a diamond. In principle, the diamond conserves the most weight and material. However, be certain that there is enough material beyond the U-bolt holes in a diamond plate to ensure that the material will not break under stress. Fig. 3 will reveal the areas of potential weakness.

<>

Alternatively, you might use for this assembly one of those commercially made fixtures with its complex bends and shapings to provide a maximum grip between the boom and the mast. Also, be sure the mount this fixture at the center of weight of the antenna boom.

4. A Note About Stainless Steel Hardware: For antenna work, use stainless steel hardware everywhere. Make no exceptions. Although once hard to find, stainless steel nuts, bolts, U-bolts, and other fixtures are easy to locate at hardware depots. Do not use aluminum hardware, as it is very weak. All other hardware will create a bi-metallic junction that will corrode one or both of the pieces joined. Stainless steel has proven to be the most successful hardware for antenna jobs ranging from joining element sections of tubing to boom-to-mast plate mountings.

The construction methods that we have noted will run the cost of a home brew Yagi only to about twice the cost of using junk-box components. However, the final cost will still be less than 1/3 the cost of a commercially made antenna--and it will last as long or longer than the commercial assembly. Even if the Yagi is home brew, if it is worth making, it is worth making well. Nothing is more frustrating than breaking a beam in the wind during a world-wide band opening.

Updated 10-10-2003. © 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.

Go to An-Ten-Ten-nas Page

Return to Amateur Radio Page