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Math!!!! > EO-5 > Flashcards

EO-5 Flashcards

1
Q

Antenna

A

A structure associated with the region of transition between guided wave and free space wave.

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2
Q

What is the main function of an antenna?

A

To concentrate a radiated energy into the beam of required shape, (Aka antenna pattern) to transmit it in desired direction, and to receive energy from targets.

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3
Q

What is the most important characteristic of any type of antenna?

A

Antenna gain

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4
Q

Define antenna gain

A

The measure of the ability of an antenna to concentrate energy in the desired direction and to minimize energy waste.

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5
Q

Reciever Gain?

A

Designed to control the sensitivity of the receiver section of a radar system.

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6
Q

what are the two types of antenna gain?

A

directive and power

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7
Q

What is directive gain?

A

the measure of signal intensity radiated in a particular direction.

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8
Q

Directive gain is dependent on what?

A

the shape of the radiated pattern of a specific radar antenna.

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9
Q

Define power gain

A

aka gain. Is a ratio of the maximum radiation intensity to an isotopic antenna. in dBs

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10
Q

T/F Directive gain does take into account the dissapitive losses of the antenna.

A

False. Directive gain doesnt. But Power gain does.

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11
Q

What is an isotropic antenna?

A

Theoretical spherical antenna that radiates with equal power in all directions.

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12
Q

Monostatic antennas are used for what?

A

Transmitting and receiving. (pulsed radars, most common)

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13
Q

Bi-static antenna

A

2 separate antennas. one for transmitting, one for recieving. Continous wave.

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14
Q

what is polarization of a radio wave?

A

the orientation of the E-field of the wave with respect to the ground as it moves through space.

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15
Q

What is used to generate/determine orientation of transmitted waves?

A

Dipoles. (They can generate either linear or non-linear polarization.)

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16
Q

What are the linear types of polarization?

A

Vertical, horizontal, and diagonal

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17
Q

what are the non linear types of polarization?

A

Circular, and elliptical

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18
Q

what are the two types of circular polarization?

A

right, and left hand.

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19
Q

What determines the direction of right and left handed polarization?

A

Determined by which dipole is in front.

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20
Q

what happens when the polarization of a recieving antenna doesnt match that of the transmitting antenna?

A

loss of power

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21
Q

what happens when a circular polarization is transmitted and is received by a linearly polarized antenna?

A

The E-field will only match in one plane, there will be 50% power loss.

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22
Q

what happens if it is a linear polarization transmitted and is received by a horizontally polarized antenna?

A

loss will be 100%

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23
Q

what is a graphical representation of the radiation properties of the antenna as a function of space coordinates?

A

antenna pattern. (usually a plot of power gain vs angular coordinates)

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24
Q

what is the main lobe?

A

portion of radiation pattern with the highest power density and is directed toward the target.

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25
Q

What is the back lobe?

A

portion of radiation pattern that is 180 degrees from the main lobe. (Most antennas are designed to minimize these)

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26
Q

What are side lobes?

A

Small beams of greatly reduced intensity that are emitted at diverging angles from the primary beam, due to “spill over”.

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27
Q

what is beamwidth?

A

determines angular resolution of the radar, is measurement of the main lobe at the 1/2-power point.

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28
Q

what is a reflector antenna

A

an aperture-type antenna with a feed radiating toward a reflector that shapes the radiation to obtain the desired antenna pattern.

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29
Q

what are the 2 main types of reflectors?

A

parabolic and cassegrain

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30
Q

what are the 4 main types of antenna feeds?

A

dipole feed, slot feed, waveguide feed, and horn feed

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31
Q

What is a dipole?

A

an antenna consisting of 2 metallic rods, fed at its center by 2 wire transmission line.

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32
Q

Horn antenna

A

rugged, low gain antennas. simply a waveguide with a flared end. allowing the signal to be propagated into free space directly from the end of a waveguide. (AKA feed horns)

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33
Q

What antenna is often used to feed larger, higher grain antennas?

A

feed horns, horn antenna

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34
Q

Parabolic antenna consists of what 2 parts?

A

Made up of a reflector in the shape of a parabola and a feed element (typically a feed horn)

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35
Q

Cassegrain antenna

A

similar to parabolic antenna but has an extra reflector

36
Q

Antenna array, array antenna

A

set of multiple connected antennas which work together as a single antenna to transmit and receive

37
Q

Why are antenna arrays used?

A

To achieve higher gain, to give path to diversity which increases communications reliability, to cancel interference from specific directions, to steer the radio beam electronically to point in different directions.

38
Q

what are grating lobes?

A

when the spacing between radiating elements become bigger than the operating freq wavelength

39
Q

what is an array element

A

radiating element, set of which makes up the array apeture

40
Q

what are the 4 most widely used array elements?

A

dipoles, slots, small horns, waveguides

41
Q

linear array

A

An array in which a group of identical elements placed in one dimension (vertical or horizontal) along a given direction

42
Q

planar array

A

An array in which all elements located in a single plane occupying a definite area. different configurations. triangular, hexagonal…ect..

43
Q

what is an active array?

A

An array in which oscillator is connected to each radiator. (can act independently)

44
Q

what are 3 advantages of active arrays?

A

increase radiated power, decreases thermal loss, and increase reliability.

45
Q

passive array

A

An array in which all elements share the same oscillator

46
Q

phased array

A

An array antenna whose beam direction or radiation pattern is controlled primarily by the relative phases of the radiating elements.

47
Q

The 3 features used to classify a phased array?

A

scanning methods, radiator feed methods, position of radiators in the array.

48
Q

What are the most advanced type of antenna used in modern radars?

A

phased array

49
Q

Scan

A

Process of how a radar searches it’s environment by shifting it’s radiating pattern.

50
Q

scans are measured in what?

A

scan rate or scan period

51
Q

Scan Period

A

Represents time that it takes to complete one scan cycle. (normally used if time to complete one scan is greater than one second; ex. circular, bi-directional)

52
Q

Scan Rate

A

Indicates how many complete scan cycles occur within a given time interval, expressed in cycles per sec. (Normally used if scans complete in less than one second; ex. conical, uni-directional)

53
Q

what are the 2 methods of scanning?

A

mechanical and electronic

54
Q

Mechanical Scanning

A

Scan method that physically moves the antenna, or part of the antenna, to position the radiation pattern

55
Q

Electronic Scanning

A

Scan method that steers the radiation pattern through phase interactions or non-mechanical means. (search in one or more directions without ever physically moving the antenna)

56
Q

Non-Scanning Emitters

A

Emitters that do not have a scanning radiation patterrn, said to have “steady scans”

57
Q

what are the 3 primary ways to form an electronic scan?

A

phase shift scanning, time delay scanning, freq scanning

58
Q

Circular Scan

A

Often use a narrow, fan-shaped, vertical beam that yields acceptable azimuth resolution and sufficient altitude coverage. (Simplest and most common form of radar antenna scan)

59
Q

raster scan

A

single beam moves in horizontal and vertical planes. (used for air borne intercept and fire control.)

60
Q

what is the simplest and most common form of radar antenna scan used today?

A

circular

61
Q

Helical Scan

A

combines vertical motion with circular scan. (does the corkscrew up and down - Sinusoidal; or fly back down to the starting point - Sawtooth)

62
Q

Uni-directional scan

A

Radar beam moves in one direction only. (Normally used to attain a high data rate.)

63
Q

what are the 3 ways to get uni-directional scan?

A

Circular scan blanking, using a rotating feed, and electronically steering the beam.

64
Q

Bi-directional scans

A

2 way sweeping scan. (Used to search a specific region of space)

65
Q

Bi-directional scanning can occur where?

A

Either in the horizontal or vertical plane

66
Q

What are the sector limits for bi-directional scans?

A

Either fixed or variable

67
Q

Lobe switching scan

A

Generated when the transmitter radar beam is electronically switched between 2 or more overlapping points. (Among the earliest scans used for anti-aircraft fire control)

68
Q

Lobe on Receive Only (LORO)

A

Only recieved signals are scanned. (steady beam, receives the target echoes on either two or four separate feed horns that are sampled; results in low-no indication of target tracking ability)

69
Q

conical scans

A

Lobe concept that is expanded to continuously rotate the beam around a central axis.

70
Q

What is a nutating feed?

A

fixed feed, antenna moves

71
Q

What is rotating feed?

A

rotates the feed, changes polarization

72
Q

What is generally the preferred method of conical scans?

A

A nutating feed

73
Q

What is generally the less mechanically complex method of conical scans?

A

A rotating feed

74
Q

Conical Scan on Receive Only (CORSO)

A

Systems that do not modify the transmit signal sent from the antenna. (Signals multiplexed inside a waveguide device that has a rotating vane)

75
Q

Palmer scans

A

Any scanning scheme on which a conical scan is super imposed.

76
Q

What is the most common Palmer Scan?

A

Palmer Raster Scan

77
Q

Multi beam scans

A

Techniques that use several beams of different frequencies, radiated by the same antenna and often requiring more than one feed. (Stacked, switched, v-beam)

78
Q

Monopulse processing

A

not a scan type, but a method of tracking targets with out moving the radar beam.

79
Q

what “scan” subdivided the antenna into multiple sections by using multiple feed horms?

A

Monopulse Processing

80
Q

what is the main advantage of monopulse?

A

it is not affected by amp fluctuations of the target echo.

81
Q

what are the two main categories for electronic scans?

A

Passive Electronically Scanned Array (PESA) and Active Electronically Scanned Array (AESA)

82
Q

What is PESA?

A

uses a single power amplifier source, transmitting a single power signal then divided into multiple paths to phase shifter and elements.

83
Q

What is AESA?

A

radar elements each has their own transmit/recieve components

84
Q

What is frequency scanning (FRESCAN)?

A

Changing the transmit freq changes the relative phase at each of the elements and, therefore the direction of the beam

85
Q

Frequency scanning is accomplished with what kind of array?

A

linear array

86
Q

What is phase shift scanning?

A

Changes phase at different elements to change beam shape or direction. (does not rely on a linear array)

87
Q

What is time delay scanning?

A

Uses a network of delay line switches. modifying the time it takes for the input signal to get to the array elements. changing the initial phase of the signal at different elements. (can be expensive)

Math!!!! (7 decks)

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