4-40
Figure 4-33.—A typical parasitic array used for transmitting and receiving.
Q42. What is the advantage of adding parasitic elements to a Yagi array?
Q43. The Yagi antenna is an example of what type of array?
SPECIAL ANTENNAS
In this section we will cover some special communications and radar antennas. Some of these
antennas we touch on briefly since they are covered thoroughly in other courses.
Previously discussed antennas operate with standing waves of current and voltage along the wires.
This section deals principally with antenna systems in which the current is practically uniform in all parts
of the antenna. In its basic form, such an antenna consists of a single wire grounded at the far end through
a resistor. The resistor has a value equal to the characteristic impedance of the antenna. This termination,
just as in the case of an ordinary transmission line, eliminates standing waves. The current, therefore,
decreases uniformly along the wire as the terminated end is approached. This decrease is caused by the
loss of energy through radiation. The energy remaining at the end of the antenna is dissipated in the
terminating resistor. For such an antenna to be a good radiator, its length must be fairly long. Also, the
wire must not be too close to the ground. The return path through the ground will cause cancellation of
the radiation. If the wire is sufficiently long, it will be practically nonresonant over a wide range of
operating frequencies.
