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
Isotropic antenna def and power density equation
The term Isotropic Antenna describes a theoretical spherical antenna that radiates with equal intensity in all directions. This results in a spherical radiation pattern. The power density for any point on an isotropic antenna is the radiation intensity and can be calculated by dividing the total power transmitted (Pt) by the total service area of the sphere.
Power density (isotropic antenna)= Pt/(4phr2)
(Unusable for radar applications since an isotropic antenna would provide neither azimuth or elevation resolution)
Power density of a practical antenna equation
Power density (practical antenna)=PtG/4pir2
Pt= power transmitted G= antenna gain
Primary main beam definition
The beam where the radar has the most power and where target detection usually occurs. The dimensions of this main beam are highly dependent on the design of the antenna.
Backlobe definition
A lobe that is directly opposite to the location of the Mai. Beam. The sensitivity and signal strength associated with the backlobe is significantly less than that in the main beam.
The backlobe is caused by diffraction effects of the reflector and direct signal leakage. It is an undesirable radiation that severely affects the max radar range and increases the vulnerability of the radar to certain jamming techniques.
Sidelobe definition
Sidelobes do not have the signal strength or sensitivity associated with the main beam. The radar signal weakness in the sidelobes of the main beam make these areas of the radar signal vulnerable to jamming.
Circular scan radar definition
Characterized by?
Designed for?
Scope display used?
A circular scanning radar uses an antenna system that continuously scans 360° in azimuth. Usually has large vertical beamwidth and small horizontal beamwidth, long pulse width and low PRF, resulting in a large resolution cell at long ranges.
Circular scan radars provide accurate target range and azimuth info. Ideal for EW and initial target acquisition roles.
Circular scan radars designed for EW usually have a low PRF so that the radar has a long, unambiguous range capability.
Plan position indicator (PPI) scope display usually used with a circular scan radar.
Scan rate def
Scan duration def
Scan rate: the time required for the antenna to sweep one complete 360 degree cycle.
Scan duration: the number of ‘hits per scan’ or the number of pulses, reflected by a target as the radar beam crosses it during one full scan.
Modified circular scan radars that determine range, azimuth and elevation:
V beam radar: transmits two fan-shapes beams that are swept together. A vertical beam provides range and azimuth info. A second beam, rotated at an angle, provides a measure of altitude of the target.
Stacked beam radar: employs a vertical stack of fixed elevation ‘pencil’ beams which rotate 360 degrees. Elevation info is obtained by noting which beam contains the target return. Range and azimuth info is determined in the same manner as in an EW radar.
Linear scan definition
Linear scan is a method used by some radar systems to sweep a narrow radar beam in a set pattern to cover a large volume of air space. Linear scans can be oriented in a vertical direction for heightfinder radars or in a horizontal direction, or raster, for acquisition and target tracking radars.
