| The Yagi Beam
The yagi is very simple. The basic yagi consists of
three elements, as shown in figure 1. The middle element is an antenna
you are already familiar with, the simple 1/2 wave dipole antenna. This
element is generically called the "driven element". This is
because this is the only element that is connected directly to the
radio, it actually drives the whole antenna. The other two outer
elements are generically called parasitic elements. One is called the
Reflector and the other one is called the director element. These
elements get their name from the job they do. The reflector reflects RF
energy, the director directs RF energy. There is no magic circuit
located inside the elements, they are simply straight rods! The
reflector element is typically 5 % longer than the driven element and
the director is typically 5 % shorted than the driven element. How it
works. See figure 1. As signal A comes in it strikes all three elements
hence generates a current on each element., the signals are re-radiated
by the director and reflector and arrive at the driven element in-phase
with one another (the two re-radiated signals and the original signal).
This basically means, the signals reinforce each other...and make the
incoming signal much stronger coming from direction A.
When the signal comes from direction B and C, the same
thing happens, except the signals arrive at the driven element out-of-phase
with one another which simply means they cancel each other out,
significantly reducing signals from direction B and C.
This very useful effect (signals arriving
in-phase/out-of-phase) is caused by the special spacing and length of
the director and reflector element in relation to the driven element.
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