3-3
antenna increases as the degree of directivity increases because the power is concentrated into a narrow
beam and less power is required to cover the same distance.
Since microwave antennas are predominantly unidirectional, the examples you will study in this
chapter are all of the unidirectional type.
Reciprocity
You read in this chapter that an antenna is able to both transmit and receive electromagnetic energy.
This is known as RECIPROCITY. Antenna reciprocity is possible because antenna characteristics are
essentially the same regardless of whether an antenna is transmitting or receiving electromagnetic energy.
Reciprocity allows most radar and communications systems to operate with only one antenna. An
automatic switch, called a DUPLEXER, connects either the transmitter or the receiver to the antenna at
the proper time. Duplexer operation will be explained in later NEETS modules dealing with radar and
communications systems. Because of the reciprocity of antennas, this chapter will discuss antennas from
the viewpoint of the transmitting cycle. However, you should understand that the same principles apply
on the receiving cycle.
Radar Fundamentals
Radio, television, radar, and the human eye have much in common because they all process the same
type of electromagnetic energy. The major difference between the light processed by the human eye and
the radio-frequency energy processed by radio and radar is frequency. For example, radio transmitters
send out signals in all directions. These signals can be detected by receivers tuned to the same frequency.
Radar works somewhat differently because it uses reflected energy (echo) instead of directly transmitted
energy. The echo, as it relates to sound, is a familiar concept to most of us. An experienced person can
estimate the distance and general direction of an object causing a sound echo. Radar uses microwave
electromagnetic energy in much the same way.
Radar transmits microwave energy that reflects off an object and returns to the radar. The returned
portion of the energy is called an ECHO, as it is in sound terminology. It is used to determine the
direction and distance of the object causing the reflection. Determination of direction and distance to an
object is the primary function of most radar systems.
Telescopes and radars, in terms of locating objects in space, have many common problems. Both
have a limited field of view and both require a geographic reference system to describe the position of an
object (target). The position of an object viewed with a telescope is usually described by relating it to a
familiar object with a known position. Radar uses a standard system of reference coordinates to describe
the position of an object in relation to the position of the radar. Normally ANGULAR measurements are
made from true north in an imaginary flat plane called the HORIZONTAL PLANE. All angles in the UP
direction are measured in a second imaginary plane perpendicular to the horizontal plane called the
VERTICAL PLANE. The center of the coordinate system is the radar location. As shown in figure 3-1,
the target position with respect to the radar is defined as 60 degrees true, 10 degrees up, and 10 miles
distant. The line directly from the radar to the target is called the LINE OF SIGHT. The distance from
point 1 to point 2, measured along the line of sight, is called TARGET RANGE. The angle between the
horizontal plane and the line of sight is known as the ELEVATION ANGLE. The angle measured in a
clockwise direction in the horizontal plane between true north and the line of sight is known as
BEARING (sometimes referred to as AZIMUTH). These three coordinates of range, bearing, and
elevation determine the location of the target with respect to the radar.
