Figure 134. Doughnut Radiation Pattern
of a Simple Dipole.
c. A simple dipole has very poor directivity because it radiates in
almost all directions.
A simple dipole antenna is used, then, to radiate
power in all directions.
It is a very efficient antenna for commercial
radio and TV stations.
d. Now suppose you want to transmit in only one direction, or you want
to receive from only one direction. For example, if you are transmitting to
ships out at sea, you want all the power from the transmitter to go out over
the ocean. Can you do this using a dipole antenna? Yes, you can, by adding
a reflector to the dipole.
8.
Adding a reflector to the dipole makes the antenna directional.
a. To make a dipole antenna directional, you merely add an element
called a reflector. A reflector may be nothing more than a conducting rod a
little longer than one-half wavelength.
Figure 135 shows a dipole with a
reflector; it also shows the antenna radiation pattern.
b. The elements of an antenna that radiate or receive energy are called
driven elements. Any other elements are called parasitic elements. Notice
in Figure 135 that the dipole is the driven element, and the reflector is
the parasitic element.
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