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Yagi Antenna

3 Element Yagi



5 Element Yagi

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.
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