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RADAR > Antennas > Flashcards

Antennas Flashcards

1
Q

What is an Antenna?

A

It concentrates a radiated energy into a beam of required shape into a desired direction.
It also receives the energy returned from targets.

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2
Q

What is Antenna Gain?

A

The measure of the ability of an Antenna to concentrate energy in the desired direction.
Aims to minimize energy waste.

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3
Q

What are the two types of Antenna Gain?

A

Directive and Power.

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4
Q

What is Directive Gain?

A

The measure of signal intensity radiated in a certain direction.
It is dependent on the shape of the radiation pattern of a specific RADAR antenna.
It does not take into account antenna dissipative losses.

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5
Q

What is Power Gain?

A

Often expressed as Gain, it is a ratio of the maximum radiation intensity to an isotopic antenna.
Typically expressed in decibels (dB).
It takes into account antenna dissipative losses.

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6
Q

What is an Isotropic Antenna?

A

It is a theoretical antenna that radiates equal power in all direction, shaped like a sphere.

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7
Q

What are the two types of Antennas that RADARs can use?

A

Monostatic and Bistatic.

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8
Q

What is a Monostatic Antenna?

A

Sends out pulsed signals instead of continuous signals.
During the RADARs “On-Time’, the RADAR sends out high power pulses.
During the RADARs “Off-Time”, the RADAR receives the low-power returned pulses.
Monostatic RADAR systems are the most common type.

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9
Q

What is a Bistatic Antenna?

A

These use two separate antennas for transmission and receiving signals.
They normally use Continuous Wave applications.

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10
Q

What is Polarization?

A

It is the orientation of the E-field of the wave with respect to the ground as it moves through space.
Based off placement of the Dipole, polarization can be horizontal, vertical, diagonal, or circular.

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11
Q

How is circular polarization achieved?

A

It’s achieved by applying alternating current to two crossed dipoles, perpendicular to each other.

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12
Q

What are the two types of circular polarization and how is the direction determined?

A

Right-handed circular and Left-handed circular. It is determined based off which dipole is in front.

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13
Q

What is a polarization mismatch?

A

This happens when the polarization of the receiving antenna is different from the transmitting antenna.

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14
Q

List all types of polarization mismatches and their power loss.

A

Horizontal Vertical RH Circular LH Circular

Horizontal 0% 100% 50% 50%

Vertical 100% 0% 50% 50%

RH Circular 50% 50% 0% 100%

LH Circular 50% 50% 100% 0%

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15
Q

What is a Radiation Pattern?

A

A graphical representation of the radiation properties of the antenna.
It has 3 parts; the Main lobe, the Side lobes, and the Back lobe.

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16
Q

What is the Main Lobe?

A

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

17
Q

What are the Side Lobes?

A

These are adjacent to the main lobe and are due to “spill over”, from the antenna feed.
They are normally suppressed as much as possible to reduce susceptibility to jamming and ensure accurate target detections.

18
Q

What is the Back Lobe?

A

This is 180 degrees from the main lobe.
It is much lower in amplitude and is only a small portion of the radiation pattern.

19
Q

What is beamwidth?

A

This determines the angular resolution of the RADAR.
It is the measurement of the Main Lobe at 1/2-power point.

20
Q

What is a Reflector Antenna?

A

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

21
Q

What are the main types of reflectors and antenna feeds?

A

Reflectors: Parabolic and Cassegrain
Antenna Feeds: Dipole Feed, Slot Feed, Waveguide feed, and Horn Feed.

22
Q

What is a Dipole?

A

A dipole is an antenna consisting of two metallic rods, fed at its center by a two-wire transmission line.
The length of the Dipole is approximately half the wavelength of the frequency transmitted.

23
Q

How does the Dipole work?

A

It creates radio waves when alternating current is applied.
This current causes electrons to move between the two legs of the Dipole.
One leg becomes negatively charged and the other becomes positively charged.
The attraction between the two forces results in an oscillating electric field (E-field), producing a corresponding magnetic field (B-field) perpendicular to the “E-field”.
Together, the two fields constitute an EM Wave (radio wave) that propagates out from the antenna.

24
Q

What is a Horn Antenna?

A

These are rugged, low-gain antennas.
Simply a waveguide with a flared end, allowing a signal to be propagated into free space.
It’s often used to feed larger, higher-gain antennas, but can also be used as a stand-alone antenna.

25
Q

What is a Parabolic Reflector?

A

Most recognizable type of RADAR antenna.
Components are a large reflector and typically a feed horn.
The feed is placed at the focus of the parabolic dish to ensure the same phase is reradiated from all points on the antenna.

26
Q

What is a Cassegrain Reflector?

A

Very closely resembles a Parabolic Reflector.
Instead, the feed is located at the center of the Parabolic Dish, rather than suspended in front of it, on an arm.
An additional reflector is suspended at the focus, reflecting the signs, power transmitted from the feed, towards the Parabolic dish.

27
Q

What is an Array Antenna?

A

A set of multiple connected antennas which work together with a single antenna, to transmit or receive radio waves.
These radio waves combine and superpose, adding together to enhance the power radiated in a desired direction.
Generally, the more antenna elements used, the higher the gain and more narrow the beam.

28
Q

What is something to consider with Array Antennas?

A

It could lead to a phenomenon known as “grating lobes”.
This happens when the spacing between radiating elements become bigger than the operating frequency’s wavelength.

29
Q

What is an array element?

A

It is a small independent, microwave antenna, the most widely used are;
Dipoles
Slots
Small horns
Waveguides

30
Q

What is a Linear Array?

A

A group of identical array elements, placed in one dimension along a given direction.
They can have equidistant or non equidistant spacing.
They are the building blocks for forming array of arrays.

31
Q

What is a Planar Array?

A

This has all array elements located in a single plane, occupying a definite area.
They have different configurations of elements; rectangular, triangular, or hexagonal.

32
Q

What’s the difference between and Active and Passive Array?

A

Active Array: Each array element is connected through each oscillator element.

Passive Array: All array elements are powered through one common oscillator.

33
Q

What are some advantages of an Active Array?

A

It has the capability to increase radiated power.
It can decrease thermal losses.
It can increase reliability. (If one oscillator goes out, only one array element goes out, not all of them).

34
Q

What is an advantage of a Passive Array?

A

They are simpler to fabricate.

35
Q

Which RADAR normally uses Passive Arrays?

A

Navigation RADARs.

36
Q

What is a Phased Array?

A

This is an array antenna, whose beam direction or radiation pattern is controlled primarily by the relative phase of the radiating elements (array elements).
They are arranged in either a linear or planar configuration.

37
Q

What basic features are used to classify Phased Arrays?

A

Scanning Methods.
Radiator feed methods.
Positions of radiators in the array.

38
Q

Why are Phased Arrays the most advanced type of antenna?

A

They provide the RADAR with the flexibility and ability to simultaneously operate multiple functions.

39
Q

What is a con about the Phased Array Antenna?

A

They are the most complicated and expensive types of modern antennas.

RADAR (3 decks)

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