The Yagi is a highly directional antenna invented in the 1920s by two Japanese scientists. A Yagi antenna uses three types of elements mounted on a boom: a reflector, a driven element and one or more director elements. A variety of easily obtained materials can be used to build a Yagi antenna. The size and spacing of the elements are calculated based on the radio frequency to be used.
- Skill level:
- Moderately Easy
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Things you need
- 3/8 or ½ inch aluminium tubing for antenna elements
- 2x2 lumber for antenna boom
- Two 2-hole pipe straps with screws
- 300 ohm TV antenna cable
- Drill with bits
- Pole for mast
- U clamp
Calculate the wavelength of the radio signal for which the antenna will be designed. Since radio waves travel at the speed of light, or 300 million meters per second, dividing 300 by the frequency in megahertz (millions of cycles per second) will give the wavelength in meters. For example, the high end of the FM radio band, or 108MHz, will have a wavelength of 300/108 = 2.79 meters, or 9.15 feet.
Determine the lengths of the three elements as follows: The reflector should be 0.495 x wavelength. The driven element should be 0.473 x wavelength. And the director should be 0.440 x wavelength. For our FM antenna, these work out to 4.5 feet, 4.33 feet and 4.03 feet. Cut pieces of the aluminium tubing to these lengths. Cut the driven element exactly in half to form a dipole antenna.
Attach the elements to the boom. Almost any material can be used for the boom. Wood is used in this example, as it is inexpensive and easy to work with. For the FM antenna, you will need a piece about 3 feet long. Drill a hole the same diameter as the aluminium tubing through the board at one end. Insert the reflector, the longest element, through this hole so that an equal length extends on each side. Secure the reflector in the hole with glue.
Calculate the spacing between the reflector and the driven element as 0.125 x wavelength. In this example, that would be 0.125 x 9.15 feet, or about 13 ¾ inches. Measure this distance from the reflector and mark it on the boom. Because the driven element is a dipole, attach each half to the top of the board with two-hole pipe straps so they are not touching. Before tightening the clamps, insert one lead of the 300 ohm TV antenna cable under the each clamp so it is held against the dipole element.
Measure the same spacing in front of the driven element (13 ¾ inches in this example) for the director element. Drill a hole through the boom and secure the director in the same manner as the reflector element. The antenna is now complete. Mount it to the mast with a U-clamp and set the mast upright in an appropriate location. Route the antenna cable to your radio and connect it.
Tips and warnings
- Additional director elements will increase the directionality of the antenna. Each additional director should be 0.005 x wavelength shorter than the preceding one, and should be spaced 0.250 x wavelength apart.
- Watch for overhead wires when setting up your antenna if you are using a tall mast.
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