CH 6: Antenna Characteristics and Scans

Question 1

3 primary antenna types

Answer 1

Parabolic, Cassegrain, and phased array antennas

Question 2

Parabolic antenna definition

Answer 2

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.

Question 3

Parabolic cylinder antenna definition

Answer 3

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.

Question 4

Height finder parabolic antenna definition

Answer 4

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.

Question 5

Multiple feed parabolic antenna

Answer 5

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.

Question 6

Cassegrain Antenna definition

Answer 6

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.

Question 7

Flat Plate Cassegrain Antenna definition

Answer 7

Built to reduce the aperture blockage by the subreflector and to provide a method to rapidly scan the radar beam.

(See textbook)

Question 8

Phased array antenna definition

Answer 8

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.

Question 9

Principle of radar phase definition

Answer 9

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.

Question 10

Broadside array definition

Answer 10

An array where most of the radiation is in the direction that is broadside to the line of the antenna array. (y axis)

Question 11

End fire array definition

Answer 11

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).

Question 12

Planar array antenna definition

Answer 12

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.

Question 13

Antenna gain definition

Receiver gain definition

2 types of antenna gain

Answer 13

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.

Question 14

Directive gain definition.

Answer 14

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

Question 15

Power gain def

Answer 15

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

Question 16

Isotropic antenna def and power density equation

Answer 16

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)

Question 17

Power density of a practical antenna equation

Answer 17

Power density (practical antenna)=PtG/4pir2

Pt= power transmitted 
G= antenna gain

Question 18

Primary main beam definition

Answer 18

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.

Question 19

Backlobe definition

Answer 19

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.

Question 20

Sidelobe definition

Answer 20

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.

Question 21

Circular scan radar definition

Characterized by?

Designed for?

Scope display used?

Answer 21

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.

Question 22

Scan rate def

Scan duration def

Answer 22

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.

Question 23

Modified circular scan radars that determine range, azimuth and elevation:

Answer 23

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.

Question 24

Linear scan definition

Answer 24

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.

Question 25

Unidirectional scan definition

Answer 25

The helical scan is a unidirectional scan pattern that allows a pencil beam to search a 360° pattern. After each complete revolution, the antenna elevation is increased. The scan pattern is repeated for a specific number revolutions, and at the end of the scan pattern, the antenna elevation is reset to the initial elevation and the scan is repeated. A helical scan pattern is commonly used as a target acquisition mood for radar systems with narrow vertical and horizontal beamwidths.

Question 26

Bidirectional scan definition

Answer 26

A bidirectional linear scan, such as a raster scan, sweeps both horizontally and vertically. A raster scan uses a thin beam to cover a rectangular area by horizontally sweeping the area. The angle of elevation is incrementally stepped up or down with each horizontal sweep of the desired sector. After the sector has been covered, the angle of elevation is reset to the original value and the process is repeated. The number of raster bars is set by the number of horizontal sweeps in the basic raster pattern.

Question 27

Conical scan definition and characteristics

Answer 27

A conical scan, or conscan, is generally used for precision target tracking. A concise scan radar employs a pencil beam of radar energy that is continuously rotated around the target. This circular rotation of a pencil beam generates a cone shaped pattern with the apex of the cone located at the antenna.

Designed for precision target tracking and usually operate at high frequency, high PRF, narrow pulse width and narrow beamwidths.

Question 28

Palmer-raster scan definition

Answer 28

The combination of conical scan and raster scan. A Palmer-raster scan uses a thin beam, while employing a conical scan searching pattern, for a specific sector of airspace. With each sweep of the sector, the angle of elevation is incrementally stepped up or down. After the vertical sector has been covered, the angle of elevation is set at the original elevation and the process is repeated.

Question 29

Palmer scan definition

Answer 29

The combination of a conical scan and a circular scan. Palmer scans incorporate a circular scanning antenna to search the entire horizon while simultaneous performing a conical scan.

If the radar antenna is also performing a unidirectional altitude search in conjunction with this scan, it is also employing a Palmer-helical scan.

Question 30

Track while scan def

Answer 30

A track while scan (TWS) system uses a technique that allows a radar to track one or more targets while scanning for others. Radar systems with a TWS capability must be able to generate two or more distinct radar beams.

A conventional TWS radar employs two antennas that work with each other to perform the scan function. Each antenna produces a separate unidirectional beam. Each beam is transmitted at a different frequency.

The track function is accomplished in the area where the two beams pass through each other. A target that is within this center area is tracked, and positional info on range, elevation, and azimuth is updated each time that beams sweep through the area.

Question 31

Radar horizon def

Answer 31

Radar horizon is the max range a radar system can detect a target due to the curvature of the earth. The distance (d) to the horizon for a radar antenna at a height (h) can be computed by the basic radar horizon equation (see textbook).

Distance to horizon (d) =1.06(square root of h)

Question 32

Direct terrain masking def

Answer 32

When terrain, such as a mountain, acts as a radar horizon and limits target LOS.

Placing prominent terrain features between the aircraft and threat radar systems effectively negate these systems and is an integral part of threat avoidance during combat mission planning.

Question 33

Indirect terrain masking def

Answer 33

When both the aircraft and a prominent terrain feature are illuminated by a radar beam, a pulse radar system may not be able to differentiate the target return from the return generated by the terrain. Indirect terrain masking is most effective when the aircraft is flying a beam the radar site.