THREE-ELEMENT YAGI BEAM
Figure 10.2 shows a Yagi antenna made up of a half wavelength driven
element, a reflector and a director. The gain of the array and the front-toback
ratio peaks at a particular boom length (boom not shown), which is
indicative of the spacing between the elements. Maximum gain occurs at a
boom length of 0.45λ. An example of a three-element Yagi antenna built on
a 0.3λ boom will provide 7 to 8 dBd forward gain, and a front-to-back ratio
of 15 to 28 dB depending on the element tuning.
The feedpoint impedance of the three-element beam is about 18 to
25 ohms, so some means must be provided for adjusting the impedance to
the 52 ohm coaxial cable.
Element lengths
Where:
Director is the length of the director in meters (m)
D.E. is the length of the driven element in meters (m)
Reflector is the length of the reflector element in meters (m)
Spacing is the spacing of the elements in meters (m).
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