Building UHF Antennas

Published in QST magazine (August 2006).

This method of construction can be used on most UHF through "low" microwave Yagis, and is especially useful for the 33, 23 and 13 cm bands; These antennas were made with this process, and tested on this back-yard antenna range.

I like to use the basic DL6WU wideband Yagi designs whenever possible. A good design tool for this is the Yagi calculator written by VK5DJ. Here's how to make one using basic hand tools and readily available materials.

Parasitic Elements

Shown here are all of the parasitic elements prepared for the four 48" 23 cm Yagis I used on a 23 cm beacon array. It does help to organize the elements like this prior to attachment to the boom.

For material, I usually use 1/8" aluminum welding filler rod. it is inexpensive, easily cut to size with wire or bolt cutters, and holds up well in weather. Once cut to size, trim the ends flat with a file.

These antennas were designed for full metal-to-metal contact between the boom and the parasitic elements, so be certain to keep that in mind when designing an antenna with whichever calculator you use.

A simple method for attaching the elements to the boom is described later in the process.

Mark the Boom Ends

Prepare the boom by cutting it to the proper length. Then,

I used another piece of tubing as a guide, but one can use almost anything that is straight, even a piece of wood.

In the next step, you will be drawing a straight line down the length of the boom.

Mark the Boom

Place the tip of your marker in the joint between the boom and the guide, and mark a line down the entire length of the boom. You will probably need to tape the boom to the guide in several places to prevent the marker from forcing the boom away from the guide.

Un-tape the boom from the guide, and rotate the boom to the opposite side using the other locating mark you made at the end of the boom as a reference. Re-tape the boom to the guide and mark a line down this side also.

Now that you have both lines drawn, place tick marks on the lines (both sides of the boom) where the holes for the parasitic elements must be drilled, as shown in the picture below. Measure from the end of the boom to each mark to eliminate cumulative errors.

Finally, use a center punch to prep the locations (both sides) that will be drilled for the parasitic elements. If you will use 1/8" diameter elements, use a 1/8" drill bit.

Drill the holes one side at a time...a common mistake is to drill all the way through the boom in one pass. This will always result in crooked elements unless you are using a drill press, and even then it's difficult to do properly that way.


Attach the Elements

Secure the parasitic elements to the boom:


Align the Elements

If you did a good job with your humble hand tools, your antenna should look something like this.

You may find an element or two out of alignment with the others as you sight down the boom... it never goes perfectly, especially for me.

If you need to adjust an element, grasp the end and bend it slightly to put it in alignment with the others.

If you look closely, you can see a slight 'zig' in the alignment of the reflector on this antenna. I bent this into position after discovering it was just out of alignment with the first director.

These small adjustments will not affect the performance of the antenna.

Driven Element Parts

For the driven element, you will need:

The smooth part of a screwdriver handle makes a good form for bending both the balun and the dipole element.

Driven Element Closeup

I usually wrap the barrel and threads of the N connector with several layers of paper, and hold it in a small vise. The paper protects the threads and slows heat loss to the vise while soldering.

Solder the shield of the .141 coax balun to the connector as shown.

Next, trim the solder cup of the N connector a bit, and solder one end of the dipole into it. Making a small bend into the cup of the pin is helpful.

Last, bend the center conductors of the balun under and around the ends of the dipole and solder as shown. Be careful to leave a small gap between the outer conductor of the balun and the ends of the dipole (about 2 mm).

Almost Done - DE

OK, to the right is what it should look like now.

Apply a generous coat of epoxy to the connections for strength and weatherproofing (see the left picture at the bottom).

Not all epoxies are created equal, be certain to test a cured lump in a microwave oven before using it on the antenna. Place your epoxy sample next to a cup of water in the microwave, and cook for 1 minute. If the epoxy is hot, try another brand. Just warm is OK.

The second picture below shows a final coat of white latex paint covering the epoxy seal and the copper dipole.
 


Finishing Up

All that is left to do now is mount the driven element to the boom. For this I used 1/2" aluminum spacers and 6-32 machine screws.

Make certain you have the dipole placed at the correct distances from the reflector and first director before drilling the mounting holes in the boom.

The picture below shows the completed 4' antenna, held by my trusty XYL (she's still shaking her head).

You may not need to tweak at all, but if you do, SWR is affected by the shape of the dipole (height). You can also bend the ends toward or away from the reflector as an additional adjustment. You should be able to get the antenna down below 1.2/1 at mid-band, and below 1.5/1 at the band edges.

If you plan on stacking Yagis, and need to flip one of the antennas over, you can put that antenna back in phase without having to make a 'mirror' dipole. Notice that I have the DE mounted so that the balun loop is toward the reflector? Just mount the DE so the balun loop points the other way. This will put it back in phase with the antenna on the top. You can see this done here.