A plot view of the dimensions
Click the photo’s thumbnail for a larger view,
This is a 4-element Yagi that works well for direction finding as well as a high gain directional antenna for general use. The elements are made from 1/8″ brass welding rod available from any welding supply such as Piner’s.
For the boom I used a piece of scrap aluminum tubing 3/8″ diameter but virtually any material, metal, wood, plastic will work as well. If you use metal, there is no need to insulate the elements from the boom. The spacing should be kept as close as possible to the figures shown above. The length of the elements is sized to resonate at 446mHz so they should be kept within 1/16th of an inch.
The elements are held in place using JB Weld although any epoxy would work as well. The coax is matched to the driven element using a 2 pF variable capacitor although a fixed capacitor can be used as well. It’s not critical. The shield of the coax is attached to the boom just behind the driven element. If you use a non metallic boom, attach it to the driven element as close as possible to the boom, but opposite the capacitor. The capacitor is soldered to the driven element about 1-1/4″ out from the boom.
The nearly finished antenna. It either needs a handle or a mast mount clamp. I used 75 ohm coax because that’s what I had. I can live with a 1.2:1 SWR for such a short cable.
If the holes aren’t drilled “just right” like in a machine shop, the elements will stick out in all directions like a hair brush. Drill the holes slightly oversize, then tape it down to a board with the needed adjustments. The epoxy will hold the elements straight.
In this photo the feedline and the matching capacitor are ready to attach to the driven element (DE). Using this type of capacitor requires soldering.
This antenna, like most will be affected by nearby objects. Keep the coax dressed along the boom until it clears the reflector. After that you can pretty much route it anywhere. Keep your radio behind the reflector as well. Waving your hand within 6 inches or so of the driven element will make the SWR jump from 1.2:1 to nearly 3:1
If you can’t find a small variable capacitor you can make your own from copper flashing or brass shim stock. The latter is available at “The loose caboose” at 3rd and Soscol. Cut a small strip about 3/4″ x 3/16″ and form one end into a sleeve using a scrap of the element rod. Insulate the driven element using cellophane tape between the boom and about 2″ out. Slip the copper or brass “sleeve” over the tape and solder the coax center conductor to the sleeve. It’s final resting place should be about 1-1/4″ out from the boom. The cellophane tape serves as the capacitors dielectric, be sure the sleeve is not shorted to anything. It should fit tightly enough so it doesn’t move around.