Often one needs to improve reception of a particular radio or television station. One effective way to do this is to build a Yagi-Uda, or Yagi, antenna. This is a traveling-wave structure which, as the number of elements increases, has improved directivity, gain, and front-to-back ratio (and additional sidelobes). The basic antenna is composed of one reflector (in the rear), one driven element, and one or more directors (in the direction of transmission/reception). The "zero-order" version of the Yagi has all elements one-half wavelength long and spaced one-quarter wavelength apart. The two designs presented here - a 3-element and a 6-element antenna - have been optimized for improved all-around performance, so their lengths and spacings are non-uniform.
Each design's parameters (element lengths and spacings) are given in terms of wavelength, so an antenna for any given frequency is easy to design. Moreover, these antennas' gains rise slowly up to the design frequency and fall off sharply thereafter. It is therefore easier (and smarter) to make the design frequency a little higher (dimensions a little smaller) than desired, so that the antenna will work despite "manufacturing tolerances". One final note: all the following performance figures are theoretical calculations! That means, for instance, that the actual gain will be slightly less than that given.
The first design is a 3-element Yagi [1]. For a 50-ohm feed, the gain is 7.6 dBi, the F/B is 18.6 dB, and the input impedance is 33-j7.5 ohms. Bandwidth is 15 percent.
reflector ----------------- 0.500 lambda
0.2 lambda
driven ------------- 0.460 lambda
0.2 lambda
director --------- 0.419 lambda
The second antenna is a 6-element Yagi [2]. The gain is 14.73 dBi and
the F/B is 10.04 dB. These figures assume the element diameter to be 0.003369
wavelength. HPBW is 37 degrees, with first sidelobes down 10.9 dB. Total
length is about 1.69 wavelengths; when cutting the boom, be sure to leave a
little extra at the ends.
reflector ------------------- 0.476 lambda
0.250 lambda
driven --------------- 0.452 lambda
0.289 lambda
director ----------- 0.436 lambda
0.406 lambda
director ----------- 0.430 lambda
0.323 lambda
director ----------- 0.434 lambda
0.422 lambda
director ----------- 0.430 lambda
Now for construction. A relatively inexpensive but robust method will be
described. Of course, for even less money, one could use scrap wire or coat
hangers, and an old broom handle or some other piece of insulating scrap. My
favored materials are 300-ohm twin-lead (for television applications),
3/4" x 1/2" wood stock (from McGuckins, for example), and 1/8" copper-plated
steel brazing rod (also from a hardware store). These materials are cheap; a
piece of wood 8 feet long is less than $2, and 3-foot-long metal rods are
about 20 cents apiece. The wood is cut with a saw, and the rods are cut with
heavy wire cutters and the ends are filed smooth. Holes are drilled in the
wood (preferably with a drill press, so that they are perpendicular); they
should be a little snug fit for the rod. After the rods are cut, they are
pushed into the wood and centered. That is the easy part.
The driven element is a litte more difficult. Since I use these a lot for television, I feed them with twin-lead; therefore I use a folded-dipole driven element. By itself, a folded dipole has a 277-ohm impedance, which is a good match to 300-ohm twin-lead. Of course the presence of the other elements will reduce the input impedance some, but there is still a good match. Note that no balun is required, either, since twin-lead is a balanced transmission line. Now if the frequency is high enough, the driven element can be made of one piece of rod. Push a long enough piece of rod through the driven element hole and bend it in at each end. Bend up the rods at the center (don't let them touch) and cut them off, leaving 1/4" or so sticking up. Solder the twin-lead to the cut ends and tape it down to the wooden boom. If the driven element is too long, then cut a rod to the right length, but extend it about 3/8". Put it into the hole. Then take a piece of cardboard or plastic and put two holes into it about 1/4" apart (make two). Cut two pieces of rod slightly longer than half of the dipole; bend up the ends about 1/2" or so. Those bends will go in the middle. Slide the plastic or cardboard spacers onto the rods to keep them from shorting. Wrap some wire around the two rods at the ends and solder. Wrap tape around the rods near the boom. Solder the twin-lead as for the other type of driver, and the antenna is finished.
2. Cheng, David K. Field and Wave Electromagnetics, 1983, Addison-Wesley Publishing Company, pp. 535-537.