3.
What is an antenna?
a. An antenna is a conductor or system of conductors that radiates and
receives electromagnetic energy. In communications systems, an antenna may
be very simple.
For example, an ordinary broadcast receiver uses just a
length of wire. If you want good long distance communications, especially
at UHF and microwave frequencies, you have to use a better antenna such as a
dipole or even a whole system of dipoles.
b. Radar sets need different types of antennas because radar serves a
different function than a communications system.
We use radar for
searching, tracking, navigation, and height finding.
Each one of these
applications needs a specific type of antenna. But even though they may be
constructed somewhat differently, most radar antennas consist of a feed and
reflector. The feed may be a dipole or a horn that couples energy to the
reflector.
c. Now let's review what an antenna does.
4.
An antenna radiates or picks up energy.
a. You have learned how radar generates an RF carrier and combines
information with the carrier.
That's the purpose of the transmitter and
modulator. You have also learned how the set detects and amplifies a small
amount of RF current to provide target information. That's the purpose of
the receiver.
b. Antennas serve a dual purpose, they transmit energy and receive
energy.
A transmitting antenna takes the energy from the transmitter,
changes it into electromagnetic waves, and radiates the waves out into
space. A receiving antenna picks up the electromagnetic waves and changes
them into voltage and current.
c. Transmitting and receiving energy does not require special types of
antennas; any antenna can be use to transmit or receive.
In fact, most
radar sets use the same antenna to radiate energy from the transmitter, pick
up the echoes of this energy, and direct the echoes to the receiver.
d. Now, let's review the operation of a simple dipole antenna.
5.
Dipole antennas.
a. You are already familiar with the simple dipole antenna shown in
Figure 132.
You have seen this type of antenna many times on roof tops;
it's one kind of TV antenna.
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