1-21
radar. A two-dimensional search radar, however, does use a single-lobe that is scanned in a 360-degree
pattern because automatic tracking circuits are not normally used in 2D radars.
Single-lobe scanning is unsuitable for use as a tracking radar for several reasons. For example, let’s
assume that a target is somewhere on the lobe axis and the receiver is detecting signals reflected from the
target. If these reflected signals begin to decrease in strength, the target likely has flown off the lobe axis.
In this case, the beam must be moved to continue tracking. The beam might be moved by an operator
tracking the target with an optical sight, but such tracking is slow, inaccurate, and limited by conditions of
visibility. An automatic tracking system would require that the beam SCAN, or search, the target area in
such a case.
Again, assume that a missile is riding (following) the axis of a single beam. The strength of the
signals it receives (by means of a radar receiver in the missile) will gradually decrease as its distance from
the transmitter increases. If the signal strength decreases suddenly, the missile will know, from built-in
detection circuitry, that it is no longer on the axis of the lobe. But it will not know which way to turn to
get back on the axis. A simple beam does not contain enough information for missile guidance.
Methods of Beam Scanning
The two basic methods of beam scanning are MECHANICAL and ELECTRONIC. In mechanical
scanning, the beam can be moved in various ways: (1) The entire antenna can be moved in the desired
pattern; (2) the energy feed source can be moved relative to a fixed reflector; or (3) the reflector can be
moved relative to a fixed source. In electronic scanning, the beam is effectively moved by such means as
(1) switching between a set of feeder sources, (2) varying the phasing between elements in a multielement
array, or (3) comparing the amplitude and phase differences between signals received by a multielement
array. A combination of mechanical and electronic scanning is also used in some antenna systems.
MECHANICAL SCANNING.—The most common type of mechanical scanning is the rotation of
the antenna through 360 degrees to obtain azimuth coverage. Most search radar sets use this method. A
common form of scanning for target tracking or missile beam-rider systems is CONICAL (cone-like)
SCANNING. This is generally accomplished mechanically by NUTATING the rf feed point.
Nutation is difficult to describe in words but easy to demonstrate. Hold a pencil in two hands. While
holding the eraser end as still as possible, swing the point in a circular motion. This motion of the pencil
is referred to as nutation; the pencil point corresponds to the open, or transmitting, end of the waveguide
antenna. The important fact to remember is that polarization of the beam is not changed during the
scanning cycle. This means that the axis of the moving feeder does not change either horizontal or vertical
orientation while the feeder is moving. You might compare the feeder movement to that of a ferris wheel;
that is, the vertical orientation of each seat remains the same regardless of the position of the wheel.
Recall that a waveguide is a metal pipe, usually rectangular in cross section, used to conduct the rf
energy from the transmitter to the antenna. The open end of the waveguide faces the concave side of the
reflector and the rf energy it emits is bounced from the reflector surface.
A conical scan can be generated by nutation of the waveguide. In this process the axis of the
waveguide itself is moved through a small conical pattern. In an actual installation of a nutating
waveguide, the three-dimensional movement is fast and of small amplitude. To an observer, the
waveguide appears merely to be vibrating slightly.
By movement of either the waveguide or the antenna, you can generate a conical scan pattern, as
shown in figure 1-17. The axis of the radar lobe is made to sweep out a cone in space; the apex of this
cone is, of course, at the radar transmitter antenna or reflector. At any given distance from the antenna,
