CH 6: Antenna Characteristics and Scans Flashcards
3 primary antenna types
Parabolic, Cassegrain, and phased array antennas
Parabolic antenna definition
One of the most widely used radar antennas where the parabola-shaped antenna is illuminated by a source of radar energy, from the transmitter, called the feed.
The feed is placed at the focus of the parabola, and the radar energy is directed at the reflector surface. Because a point source of energy, located at the focus, is converted into a wavefront of uniform phase, the parabola is well suited for radar antenna applications.
Parabolic antenna generate a nearly symmetrical pencil beam that can be used for target tracking.
Parabolic cylinder antenna definition
Elongating the horizontal dimensions of the parabolic antenna creates a radar antenna called the parabolic cylinder antenna. The pattern of this antenna is a vertical fan-shaped beam.
Combining this antenna pattern with a circular scan technique creates a radar system well suited for long-range search and target acquisition.
Height finder parabolic antenna definition
Elongating the vertical dimensions of the parabola creates a radar antenna that generates a horizontal fan-shapes beam with a small vertical dimension. This type of antenna is generally used in height-finishing radar systems.
Multiple feed parabolic antenna
A variation of the basic parabolic antenna that uses an array of multiple feeds instead of a single feed. This type of parabolic antenna can produce multiple radar beams, either symmetrical or asymmetrical, depending on the angle and spacing of the individual feeds.
Cassegrain Antenna definition
A Cassegrain antenna uses a two-reflector system to generate and focus a radar beam. The primary reflector uses a parabolic contour and the secondary reflector, or su reflector, has a hyperbolic contour.
The antenna feed is located at one of the two foci of the parabola.
Flat Plate Cassegrain Antenna definition
Built to reduce the aperture blockage by the subreflector and to provide a method to rapidly scan the radar beam.
(See textbook)
Phased array antenna definition
A phased array antenna is a complex arrangement of many individual transmitting and receiving elements in a particular pattern. A phase the rate antenna can radiate more than one beam from the antenna by using a computer to rapidly and independently control groups of these individual elements. Multiple beams and computer processing of Radar returns give the phased array radar the ability to track while scanning and engage multiple targets simultaneously.
Most common employment of TWS capability is in air-to-air arena.
Principle of radar phase definition
A phased array radar uses the principle of radar phase to control the individual transmitting and receiving elements. Went two transmitted frequencies are in phase, their amplitudes add together, and the radiated energy is double. When two transmitted frequencies are out of phase, they cancel each other out.
Broadside array definition
An array where most of the radiation is in the direction that is broadside to the line of the antenna array. (y axis)
End fire array definition
A type of array where the same antenna elements are fed out of phase and the principle direction of radiation is along the axis of the antenna elements (x axis).
Planar array antenna definition
A planar array antenna uses transmit and receive elements in a linear array, but, unlike the phased array radar, the elements are smaller and are placed on a movable flat plate.
The ability to simultaneously track several targets is one advantage of this type of radar.
Antenna gain definition
Receiver gain definition
2 types of antenna gain
The measure of the ability of an antenna to concentrate energy in the desired direction.
Antenna gain should not be confused with receiver gain, which is designed to control the sensitivity of the receiver section of a radar system.
Two types of antenna gain: directive and power.
Directive gain definition.
The directive gain of a transmitting antenna is the measure of signal intensity radiated in a particular direction. It is dependent on the shape of the radiation pattern of a specific radar antenna. It does not take into account the dissipative losses of the antenna.
Directive gain (GD)=maximum radiation intensity (desired direction)/ average radiation intensity
Power gain def
Power gain of a transmitting antenna is the measure of signal intruding radiated in a particular direction and does include the antenna dissipate losses.
G (power gain)=max radiation intensity (practical antenna)/radiation intensity of an isotropic antenna