A Direction Finding Yagi for UHF that you can build
by N6XN
A plot view of the dimensions
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 elements are 1/8" in diameter and of the following length: R1=12-7/8" DE=12-1/2" D1=12-1/4" D2=12-1/8"
Click thumbnail for a larger view
Note A
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 A close-up of the matching capacitor and the shield termination. I used a solder lug on this one; probably should have used star washers instead,
See note belowThe "kit" ready to assemble. Notice each element is slightly different in length. The longest is R1 and the shortest is D2 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. A close-up of the JB Weld. These will hold forever! Note B
How's it work? I was able to bring up the Benicia repeater using low power on my handheld and the antenna held in my hand. Ed said I was mostly white noise so I increased power to 5 Watts. By adjusting the azimuth I was petty fair copy. Compared to a rubber duck, I couldn't even hear the repeater! Experimenting with various other repeaters I was able to get some pretty noticeable peaks in the received signal by rotating the antenna.
On the last fox hunt, I used this antenna. From the River Terrace off Soscol avenue it gave me a very positive reading on the transmitter hidden at Fairview park. Unfortunately, once I got to the park it was useless as were all the antennas there. The point is, it works and works well as a DF antenna.
Notes:
Note A: 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
Note B: (See photo above) 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.
