EO-5 Flashcards
Antenna
A structure associated with the region of transition between guided wave and free space wave.
What is the main function of an antenna?
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.
What is the most important characteristic of any type of antenna?
Antenna gain
Define antenna gain
The measure of the ability of an antenna to concentrate energy in the desired direction and to minimize energy waste.
Reciever Gain?
Designed to control the sensitivity of the receiver section of a radar system.
what are the two types of antenna gain?
directive and power
What is directive gain?
the measure of signal intensity radiated in a particular direction.
Directive gain is dependent on what?
the shape of the radiated pattern of a specific radar antenna.
Define power gain
aka gain. Is a ratio of the maximum radiation intensity to an isotopic antenna. in dBs
T/F Directive gain does take into account the dissapitive losses of the antenna.
False. Directive gain doesnt. But Power gain does.
What is an isotropic antenna?
Theoretical spherical antenna that radiates with equal power in all directions.
Monostatic antennas are used for what?
Transmitting and receiving. (pulsed radars, most common)
Bi-static antenna
2 separate antennas. one for transmitting, one for recieving. Continous wave.
what is polarization of a radio wave?
the orientation of the E-field of the wave with respect to the ground as it moves through space.
What is used to generate/determine orientation of transmitted waves?
Dipoles. (They can generate either linear or non-linear polarization.)
What are the linear types of polarization?
Vertical, horizontal, and diagonal
what are the non linear types of polarization?
Circular, and elliptical
what are the two types of circular polarization?
right, and left hand.
What determines the direction of right and left handed polarization?
Determined by which dipole is in front.
what happens when the polarization of a recieving antenna doesnt match that of the transmitting antenna?
loss of power
what happens when a circular polarization is transmitted and is received by a linearly polarized antenna?
The E-field will only match in one plane, there will be 50% power loss.
what happens if it is a linear polarization transmitted and is received by a horizontally polarized antenna?
loss will be 100%
what is a graphical representation of the radiation properties of the antenna as a function of space coordinates?
antenna pattern. (usually a plot of power gain vs angular coordinates)
what is the main lobe?
portion of radiation pattern with the highest power density and is directed toward the target.
What is the back lobe?
portion of radiation pattern that is 180 degrees from the main lobe. (Most antennas are designed to minimize these)
What are side lobes?
Small beams of greatly reduced intensity that are emitted at diverging angles from the primary beam, due to “spill over”.
what is beamwidth?
determines angular resolution of the radar, is measurement of the main lobe at the 1/2-power point.
what is a reflector antenna
an aperture-type antenna with a feed radiating toward a reflector that shapes the radiation to obtain the desired antenna pattern.
what are the 2 main types of reflectors?
parabolic and cassegrain
what are the 4 main types of antenna feeds?
dipole feed, slot feed, waveguide feed, and horn feed
What is a dipole?
an antenna consisting of 2 metallic rods, fed at its center by 2 wire transmission line.
Horn antenna
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)
What antenna is often used to feed larger, higher grain antennas?
feed horns, horn antenna
Parabolic antenna consists of what 2 parts?
Made up of a reflector in the shape of a parabola and a feed element (typically a feed horn)
Cassegrain antenna
similar to parabolic antenna but has an extra reflector
Antenna array, array antenna
set of multiple connected antennas which work together as a single antenna to transmit and receive
Why are antenna arrays used?
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.
what are grating lobes?
when the spacing between radiating elements become bigger than the operating freq wavelength
what is an array element
radiating element, set of which makes up the array apeture
what are the 4 most widely used array elements?
dipoles, slots, small horns, waveguides
linear array
An array in which a group of identical elements placed in one dimension (vertical or horizontal) along a given direction
planar array
An array in which all elements located in a single plane occupying a definite area. different configurations. triangular, hexagonal…ect..
what is an active array?
An array in which oscillator is connected to each radiator. (can act independently)
what are 3 advantages of active arrays?
increase radiated power, decreases thermal loss, and increase reliability.
passive array
An array in which all elements share the same oscillator
phased array
An array antenna whose beam direction or radiation pattern is controlled primarily by the relative phases of the radiating elements.
The 3 features used to classify a phased array?
scanning methods, radiator feed methods, position of radiators in the array.
What are the most advanced type of antenna used in modern radars?
phased array
Scan
Process of how a radar searches it’s environment by shifting it’s radiating pattern.
scans are measured in what?
scan rate or scan period
Scan Period
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)
Scan Rate
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)
what are the 2 methods of scanning?
mechanical and electronic
Mechanical Scanning
Scan method that physically moves the antenna, or part of the antenna, to position the radiation pattern
Electronic Scanning
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)
Non-Scanning Emitters
Emitters that do not have a scanning radiation patterrn, said to have “steady scans”
what are the 3 primary ways to form an electronic scan?
phase shift scanning, time delay scanning, freq scanning
Circular Scan
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)
raster scan
single beam moves in horizontal and vertical planes. (used for air borne intercept and fire control.)
what is the simplest and most common form of radar antenna scan used today?
circular
Helical Scan
combines vertical motion with circular scan. (does the corkscrew up and down - Sinusoidal; or fly back down to the starting point - Sawtooth)
Uni-directional scan
Radar beam moves in one direction only. (Normally used to attain a high data rate.)
what are the 3 ways to get uni-directional scan?
Circular scan blanking, using a rotating feed, and electronically steering the beam.
Bi-directional scans
2 way sweeping scan. (Used to search a specific region of space)
Bi-directional scanning can occur where?
Either in the horizontal or vertical plane
What are the sector limits for bi-directional scans?
Either fixed or variable
Lobe switching scan
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)
Lobe on Receive Only (LORO)
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)
conical scans
Lobe concept that is expanded to continuously rotate the beam around a central axis.
What is a nutating feed?
fixed feed, antenna moves
What is rotating feed?
rotates the feed, changes polarization
What is generally the preferred method of conical scans?
A nutating feed
What is generally the less mechanically complex method of conical scans?
A rotating feed
Conical Scan on Receive Only (CORSO)
Systems that do not modify the transmit signal sent from the antenna. (Signals multiplexed inside a waveguide device that has a rotating vane)
Palmer scans
Any scanning scheme on which a conical scan is super imposed.
What is the most common Palmer Scan?
Palmer Raster Scan
Multi beam scans
Techniques that use several beams of different frequencies, radiated by the same antenna and often requiring more than one feed. (Stacked, switched, v-beam)
Monopulse processing
not a scan type, but a method of tracking targets with out moving the radar beam.
what “scan” subdivided the antenna into multiple sections by using multiple feed horms?
Monopulse Processing
what is the main advantage of monopulse?
it is not affected by amp fluctuations of the target echo.
what are the two main categories for electronic scans?
Passive Electronically Scanned Array (PESA) and Active Electronically Scanned Array (AESA)
What is PESA?
uses a single power amplifier source, transmitting a single power signal then divided into multiple paths to phase shifter and elements.
What is AESA?
radar elements each has their own transmit/recieve components
What is frequency scanning (FRESCAN)?
Changing the transmit freq changes the relative phase at each of the elements and, therefore the direction of the beam
Frequency scanning is accomplished with what kind of array?
linear array
What is phase shift scanning?
Changes phase at different elements to change beam shape or direction. (does not rely on a linear array)
What is time delay scanning?
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)
