microwave_antenna

CHAPTER 5MICROWAVE ANTENNA

MICROWAVE ANTENNA

DefinitionA conductor or group of conductors used either for radiating electromagnetic energy into space or for collecting it from space.

or Is a structure which may be described as a metallic object, often a wire or a collection of wires through specific design capable of converting high frequency current into EM wave and transmit it into free space at light velocity with high power (kW) besides receiving EM wave from free space and convert it into high frequency current at much lower power (mW).

Basic operation of transmit and receive antennas

On the receiving end, electromagnetic energy is converted into electrical energy by the antenna and fed into the receiver

• Electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into space.

Figure 5.1 : Basic operation of transmit and receive antennas

Basic operation of transmit and receive antennas (cont)

• Transmission - radiates electromagnetic energy into space

• Reception - collects electromagnetic energy from space• In two-way communication, the same antenna can be

used for transmission and reception.• Short wavelength produced by high frequency

microwave, allows the usage of highly directive antenna. For long distant signal transmission, the usage of antenna at microwave frequency is more economical. Usage of waveguide is suitable for short distant signal transmission.

FUNCTION OF ANTENNA• Transmit energy with high efficiency .• Receive energy as low as mW.• Provide matching between transmitter and free space and between

free space and receiver, thus maximum power transfer is achieve besides preventing the occurrence of reflection.

• Directs radiation toward and suppresses radiation• Two common features exist at the antenna Tx and Rx antenna is the

radiation pattern and impedance, but it is different in terms of transmission power and reception power.

• Figure 5.2 below, shows the energy transmitted into free space via an open ended λ/4 transmission line. The proportion of wave escaping the system is very small due

FUNCTION OF ANTENNA (cont)• Mismatch exist that is surrounding space as load.• Since the two wires are closed together and in opposite direction (180°),

therefore it is apparent that the radiation from one tip will cancelled that from the other.

• Figure 5.2 below, shows the energy transmitted into free space via an open ended λ/4 transmission line. The proportion of wave escaping the system is very small due

Figure 5.2

TYPES OF MICROWAVE ANTENNA

A. Horn / aperture antenna B. Parabolic / dish antennaC. Dipole antennaD. Slotted (leaky-wave) antennaE. Dielectric lens antennaF. Printed (patch or microstrip) antennaG. Phase Array antenna

A - HORN / APERTURE ANTENNA• Like parabolic reflectors,

HORN RADIATORS can use to obtain directive radiation at microwave frequencies

• Horn radiators are used with waveguides because they serve both as an impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial and other types of lines

Figure 5.3 : Horn antenna

Horn Antenna• Horn radiators are constructed in a variety of shapes, as illustrated

in figure 5.4• The shape of the horn determines the shape of the field pattern.

The ratio of the horn length to the size of its mouth determines the beam angle and directivity. In general, the larger the mouth of the horn, the more directive is the field pattern.

Figure 5.4 : Horn radiator

DIFFERENT TYPES OF HORN ANTENNA

THREE TYPES OF HORN ANTENNA

• Horn antenna tapered / flared in one dimension only i.e in E-plane or H-plane (known as sectoral horn).

• Horn antenna tapered / flared in two dimension i.e in E-plane and H-plane (known as pyramidal horn).

• Conical taper / flares uniformly in all direction i.e in circular form.

THE DIFFERENCES BETWEEN THE E-, H-PLANE & PYRAMIDAL HORN SECTORAL

ANTENNA

E- PLANE HORN SECTORAL

ANTENNA

Radiation pattern exhibits side lobe

H- PLANE HORN SECTORAL

ANTENNA

Radiation pattern exhibits no side

lobe, thus more popular.

PYRAMIDAL HORN ANTENNA

Radiation pattern flares in 2 direction

i.e in E-plane and H-plane. Therefore

improves directivity.

DIMENSION OF HORN ANTENNA

DIMENSION OF HORN ANTENNA (cont)

B- PARABOLIC (REFLECTOR / DISH) ANTENNA

• Is a big dish like structure made from metal or wire mesh / grid.

• Mesh hole ≤ λ / 12. • Widely used in microwave propagation via free

space. • Also known as secondary antenna since it

depends on primary antenna which acts as a feeder at the focal point (horn antenna or dipole antenna) to enhance the performance quality of the transmitter and the receiver

Introduction of parabolic antenna• A parabolic antenna is a high-gain

reflector antenna used for radio, television and data communications, and also for radiolocation (radar), on the UHF and SHF parts of the electromagnetic spectrum

• With the advent of TVRO and DBS satellite television, the parabolic antenna became a ubiquitous feature of urban, suburban, and even rural landscapes.

Figure 5.5 : Parabolic Antenna

Why is it used?• At higher microwave frequencies

the physical size of the antenna becomes much smaller which in turn reduces the gain and directivity of the antenna

• The desired directivity can be achieved using suitably shaped parabolic reflector behind the main antenna which is known as primary antenna or feed .

Working rules• A parabolic reflector follows the

principle of geometrical optics.

• When parallel rays of light incident on the reflector they will converge at focus or when a point source of light is kept at focus after reflection by the reflector they form a parallel beam of rays

Basic Parabolic

• The basic paraboloid reflector used to produce different beam shapes required by special applications. The basic characteristics of the most commonly used paraboloids are presented as below:

TRUNCATED PARABOLOID• Since the reflector is parabolic in the horizontal plane, the energy is

focused into a narrow beam. With the reflector TRUNCATED (cut) so that it is shortened vertically, the beam spreads out vertically instead of being focused. This fan-shaped beam is used in radar detection applications for the accurate determination of bearing. Since the beam is spread vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. The truncated paraboloid also works well for surface search radar applications to compensate for the pitch and roll of the ship

• Truncated paraboloid may be used in target height-finding systems if the reflector is rotated 90 degrees, as shown in figure 3-5B. Since the reflector is now parabolic in the vertical plane, the energy is focused vertically into a narrow beam. If the reflector is truncated, or cut, so that it is shortened horizontally, the beam will spread out horizontally instead of being focused. Such a fan-shaped beam is used to accurately determine elevation

ORANGE-PEEL PARABOLOID • A section of a complete circular paraboloid, often called an

ORANGE-PEEL REFLECTOR because of its orange-peel shape. Since the reflector is narrow in the horizontal plane and wide in the vertical plane, it produces a beam that is wide in the horizontal plane and narrow in the vertical plane. In shape, the beam resembles a huge beaver tail. The microwave energy is sent into the parabolic reflector by a horn radiator (not shown) which is fed by a waveguide. The horn radiation pattern covers nearly the entire shape of the reflector, so almost all of the microwave energy strikes the reflector and very little escapes at the sides. Antenna systems which use orange-peel paraboloids are often used in height-finding equipment.

Orange-peel paraboloid Cylindrical paraboloid Corner reflector

CYLINDRICAL PARABOLOID • When a beam of radiated energy that is noticeably wider in one

cross-sectional dimension than in another is desired, a cylindrical paraboloidal section which approximates a rectangle can be used. A PARABOLIC CYLINDER has a parabolic cross section in just one dimension which causes the reflector to be directive in one plane only. The cylindrical paraboloid reflector is fed either by a linear array of dipoles, a slit in the side of a waveguide, or by a thin waveguide radiator. It also has a series of focal points forming a straight line rather than a single focal point. Placing the radiator, or radiators, along this focal line produces a directed beam of energy. As the width of the parabolic section is changed, different beam shapes are obtained. You may see this type of antenna system used in search radar systems and in ground control approach (gca) radar systems.

CORNER REFLECTOR

• The CORNER-REFLECTOR ANTENNA consists of two flat conducting sheets that meet at an angle to form a corner, as shown in figure 5.6. The corner reflector is normally driven by a HALF-WAVE RADIATOR located on a line which bisects the angle formed by the sheet reflectors.

Figure 5.6 : Parabolic reflector radiation.

CORNER REFLECTOR (cont)• A microwave source is placed at focal point F. The field leaves this

antenna as a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is phase-shifted 180 degrees. Each part is then sent outward at an angle that results in all parts of the field traveling in parallel paths. Because of the special shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, when the parts of the field are reflected from the parabolic surface, they travel to line XY in the same amount of time.

CORNER REFLECTOR (cont)• A point-radiation source is placed at the focal point F. The field

leaves this antenna with a spherical wavefront. As each part of the wavefront moving toward the reflector reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths. Because of the shape of a parabolic surface, all paths from F to the reflector and back to line XY are the same length. Therefore, all parts of the field arrive at line XY at the same time after reflection.

• A parasitic array to direct the radiated field back to the reflector, or a feed horn pointed at the paraboloid is used to make the beam sharper and to concentrates the majority of the power in the beam.

• The radiation pattern of the paraboloid contains a major lobe, which is directed along the axis of the paraboloid and several minor lobes. Very narrow beams are possible with this type of reflector.

PARABOLIC RADIATION PATTERN

Figure 5.7 : Parabolic radiation pattern

PARABOLIC (REFLECTOR / DISH) ANTENNA as TRANSMITTER

• The wave at the focus point will be directed to the main reflector and will be reflected parallel to the parabola axis. Thus the wave will travel at the same the and phase at A`E` (XY) line and the plane wave produce will be transmitted to the free space.

• Waves are emitted from the focal point of the wall and bounced back in line with the axis of the parabola and will arrive on time and with the same phase of the line and will form the next plane waves emitted into free space

PARABOLIC (REFLECTOR / DISH) ANTENNA as RECEIVER

• The plane wave received which is parallel to the parabola axis will be reflected by the main reflector to the focus point.

• All received waves parallel to the axis of the parabola will be reflected by the wall to the point of convergence.

• This characteristic makes the parabola antenna to possess high gain and a confined beam width.

• These features causes a parabola has a high gain and width of the focused beam.

C- SLOTTED (LEAKY-WAVE) ANTENNA

• Can be fabricated from a length of a waveguide. They are simple to fabricate, have low-loss (high efficiency) and radiate linear polarization with low cross-polarization.

• Slotted antenna arrays used with waveguides are a popular antenna in navigation, radar and other high-frequency systems. These antennas are often used in aircraft applications because they can be made to conform to the surface on which they are mounted. The slots are typically thin (< 0.1 ʎ) and 0.5 ʎ (at the center frequency of operation).

SLOT ANTENNA

What is SLOT Antenna:-A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot, as well as the driving frequency, determine the radiation distribution pattern.

Figure 5.8 : Slot antenna.

SLOTTED (LEAKY-WAVE) ANTENNA (CONT)

• The slots on the waveguide will assumed to have a narrow width. Increasing the width increases the bandwidth (recall that a fatter antenna often has an increased bandwidth); the expense of a larger width is a higher degree of cross-polarization. The Fractional Bandwidth for thin slots can be as low as 3-5%; wide slots can have a FBW on the order of 75%.

• An example of a slotted waveguide array is shown in Figure 5.9 (dimensions given by length a and width b)

Figure 5.9 : slot waveguide with dimensions given by length a and width b.

SLOTTED (LEAKY-WAVE) ANTENNA (CONT)

• As in the cavity-backed slot antenna, each slot could be independently fed with a voltage source across the slot. This would be very difficult to construct especially for large arrays. The waveguide is used as the transmission line to feed the elements.

• The position, shape and orientation of the slots will determine how (or if) they radiate. In addition, the shape of the waveguide and frequency of operation will play a major role.

Slot antenna (cont)• EXAMPLE; • The dominant TE10 mode will be assumed to exist

within the waveguide. Radiation occurs when the currents must "go around" the slots in order to continue on their desired direction. As an example, consider a narrow slot in the center of the waveguide, as shown in Figure 5.10

Figure 5.10 : example slot waveguide with dimensions given by length a and width b.

Slot antenna (cont)• In this case, the z-component of the current will not be

disturbed, because the slot is thin and the z-current would not need to travel around the slot.

• Hence, the x-component of the current will be responsible for the radiation. However, at this location (x=a/2), the x-component of the current density is zero - i.e. no current and therefore no radiation. As a result, slots cannot be placed in the center of the waveguide as shown in Figure 5.10.

• If the slots are displaced from the centerline as shown in Figure 5.9, the x-directed current will not be zero and will need to travel around the slot. Hence, radiation will occur.

Slot antenna (cont)

• If the slot is oriented as shown in Figure 3, the slot will disturb the z-component of the current density. This slot will then radiate. If this slot is displaced away from the center line, the amount of power that it radiates can be adjusted.

Slot antenna (cont)• If the slot is rotated at an angle about the centerline as

shown in Figure 4, it will radiate. The power it radiates will be a function of the angle (phi) that it is rotated - specifically given by . Note that the z-component of the current is still responsible for radiation in this case. The x-component is disturbed; however the currents will have opposite magnitudes on either side of the centerline and will thus tend to cancel out the radiation

Slot antenna (cont)

• The most common slotted waveguide resembles that shown in Figure 5:

• The front end (the open face at the y=0 in the x-z plane) is where the antenna is fed. The far end is usually shorted (enclosed in metal). The waveguide may be excited by a short dipole (as seen on the cavity-backed slot antenna) page, or by another waveguide

Slot antenna (cont)

• The waveguide itself acts as a transmission line, and the slots in the waveguide can be viewed as parallel (shunt) admittances.

Slot antenna (cont)

Slot antenna (cont)

Slot antenna (cont)

• The end of the waveguide is terminated in a pyramid terminator to avoid line reflections.

• The radiating field pattern depends on the spacing of the slots (phase relationship) and their orientation with reference to the waveguide.

• A slot cut in the wall of the waveguide, transverse to the direction of the interior boundary currents ( due to the interior em wave) will couple the em energy from inside the wave guide to a radiant free-space wave.

• The length of slot is cut to be a resonant one-half (ʎ/2) wavelength.

D) DIPOLE ANTENNASTWO TYPES OF DIPOLE ANTENNAS:•Half-wave (ʎ/2) dipole antenna (or Hertz antenna)•Quarter-wave (ʎ /4) vertical antenna (or Marconi antenna)

•Maxwell equations, the strength of the radiated field is ;

Є = 60 π dl I cosӨ cos w ( t – r/Vc) λr

D) DIPOLE ANTENNAS Cont.• A for a free space short-dipole and the radiation pattern (polar

diagram) in the vertical plane and a circular in a horizontal plane.

• The electric field, Є is directional in the vertical plane but is

omnidirectional on the horizontal plane.

D) DIPOLE ANTENNAS Cont.• Dipole antenna consists of 2 wires (ʎ/4 for its

length) , the two wires are separated by a gap and their terminals are connected to the transmitter or the receiver.

This type of dipoles is called half wave length dipole as the total length is ʎ / 2

D) DIPOLE ANTENNAS Cont.

Dipole geometry Dipole configuration

D) DIPOLE ANTENNAS Cont.RADIATION PATTERN•The dipole is an electric field antenna, means that the magnetic field is zero at the near field. •The radiation pattern is like a donut cake with the maximum perpendicular to the dipole, and a null along it. •The polarization is along the dipole.


The 3D plot of the radiation pattern of a dipole antenna

D) DIPOLE ANTENNAS Cont.The radiation pattern for the Electric field for a folded

dipole antenna

D) DIPOLE ANTENNAS Cont.The radiation pattern of the dipole all the field is electric

as shown

D) DIPOLE ANTENNAS Cont.The radiation pattern of the dipole, the magnetic field

equals zero

D) DIPOLE ANTENNAS Cont.• When the length of the dipole exceeds lambda

the radiation pattern takes a new shape due to the appearance of the grating lobes where the major lobes divides into multiple lobes .


E) DIELECTRIC (LENS) ANTENNAS• Lenses play a similar role to that of reflectors in reflector antennas: they

collimate divergent energy.• Used at the higher microwave frequencies (often preferred to reflectors at

frequencies > 100 GHz) and are useful in mm microwave region.

BASIC PRINCIPLE

E) DIELECTRIC (LENS) ANTENNAS cont.

• The velocity of em wave through a dielectric materal is less than that in free space.

• The section of spherical em wave that travels through the center (the greatest thickness) of the dielectric material will travel most slowly compared to both end.

• The velocities of the spherical wave entering the lens will be controlled and the curved wavefront will become a plane wavefront with constant phase in front of the dielectric antenna (refraction based on Snell’s law).


• Are contructed from polistyrene, teflon or any denser dielectric material to produce large diffraction (belauan) although its size and weight is small. The material use will cause the wave to attenuate greatly (losses and absortion of signal - greatest attenuation at center – thickest lens).

• To avoid this situation, zoned and stepped dielectric antennas are used so that the optical path can be divided into paths differing by integral multiples of a wavelength from one zone to another.


• Basic dielectric lens :-• Requires a specific wavelength due to its thicness. • Its usage is not practical as compared to the stepped or

zoned dielectric lens antenna which has different path for different wavelength.

E) DIELECTRIC (LENS) ANTENNAS Cont.

• Stepped or zoned dielectric lens antenna :-• Used to reduced the lens thickness and to decrese

the curveture of the spherical wave.

E) DIELECTRIC (LENS) ANTENNAS Cont.

• A patch antenna is a narrowband, wide-beam antenna. • Fabricated by etching the antenna element pattern in metal

trace bonded to an insulating dielectric substrate, such as a printed circuit board, with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane.

• Microstrip antenna shapes :- ex : square, rectangular, circular and elliptical

• Some patch antennas do not use a dielectric substrate and instead made of a metal patch mounted above a ground plane using dielectric spacers; the resulting structure is less rugged but has a wider bandwidth.

F) PRINTED (MICROSTRIP OR PATCH) ANTENNA

http://en.wikipedia.org/wiki/Light_beam

http://en.wikipedia.org/wiki/Dielectric

http://en.wikipedia.org/wiki/Printed_circuit_board

http://en.wikipedia.org/wiki/Ground_plane

http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)

• Microstrip antenna a very low profile, are mechanically rugged and can be shaped to conform to the curving skin of a vehicle, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile radio communications devices.

F) PRINTED (MICROSTRIP OR PATCH) ANTENNA Cont.

http://en.wikipedia.org/wiki/Mobile_radio

TYPES OF MICROSTRIP ANTENNAS:


ADVANTAGES•High accuracy in manufacturing , the design is executed by Photo etching.•Easy to integrate with other devices.•An array of microstrip antennas can be used to form a pattern that is difficult to synthesize using a single element.•We can obtain high directivity using microstrip arrays. •Have a main radiating edge , this makes it useful for mobile Phones to avoid radiation inside the device . •Small sized applicable for handheld portable communication.•Smart antennas when combined with phase shifters .


DISADVANTAGES•Narrow band width ( 1% ) , while mobiles need ( 8% ).•Low efficiency , especially for short circuited microstrip antenna.•Some feeding techniques like aperture and proximity coupling are difficult to fabricate. •An array suffers presence of feed network decreasing efficiency , also microstrip antennas are relatively expensive.



Microstrip AntennasReflector Antennas

Preferred for low directivity

applications.

Lower efficiency.

Suffers low efficiency caused by feed

network for arrays.

Smart antennas, uses electronic

scanning when combined with phase

shifters.

More accurate manufacturing by

photo etching.

Feeding is by coupling or coax feed

lines.

Performed for high directivity

applications as the effect of blockage is

less.

Higher efficiency.

Suffers blockage caused by fixation

Struts.

Uses mechanical scanning .

Less accuracy , sometimes parabolic

surfaces are rough.

Uses other antenna (dipole , monopole,

apertures , etc) as a feed.

MICROSTRIP VS. REFLECTORS

G- PHASED ARRAY ANTENNA• Is an array of antennas in which the relative phases of the

respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.

• Phased array transmission is use to enhance transmission of radio waves in one direction.

• A phased array antenna is composed of lots of radiating elements each with a phase shifter. Beams are formed by shifting the phase of the signal emitted from each radiating element, to provide constructive/destructive interference so as to steer the beams in the desired direction

Phased Array Antenna (Cont)• Areas of the antenna matrix can act as separate

antennas. This allows many antenna beam patterns to be individually controlled at the same time. A large,phase-steered antenna system could be used to control the positions of many aircraft as at larger airport.

• In the figure 1 (left) both radiating elements are fed with the same phase. The signal is amplified by constructive interference in the main direction. The beam sharpness is improved by the destructive interference

Phased Array Antenna (Cont)

• In the figure 1 (right), the signal is emitted by the lower radiating element with a phase shift of 22 degrees earlier than of the upper radiating element. Because of this the main direction of the emitted sum-signal is moved upwards.

• (Note: Radiating elements have been used without reflector in the figure. Therefore the back lobe of the shown antenna diagrams is just as large as the main lobe.)


• The main beam always points in the direction of the increasing phase shift.

• If the signal to be radiated is delivered through an electronic phase shifter giving a continuous phase shift, the beam direction will be electronically adjustable. However, this cannot be extended unlimitedly.

• The highest value, which can be achieved for the Field of View (FOV) of a phased array antenna is 120° (60° left and 60° right). With the sine theorem the necessary phase moving can be calculated

Phased Array Antenna (Cont)Advantages Disadvantages

high gain width los side lobes

Ability to permit the beam to jump from one

target to the next in a few microseconds

Ability to provide an agile beam under computer

control

arbitrarily modes of surveillance and tracking

free eligible Dwell Time

multifunction operation by emitting several

beams simultaneously

Fault of single components reduces the

capability and beam sharpness, but the system

remains operational

the coverage is limited to a

120 degree sector in azimuth

and elevation

deformation of the beam while

the deflection

low frequency agility

very complex structure

(processor, phase shifters)

still high costs

Phased Array Antenna (Cont)• CONCLUSION:• Beamforming antenna systems improve wireless network

performance• increase system capacity• improve signal quality• suppress interference and noise • save power

• Beamforming antennas improve infrastructure networks performance. They may improve ad hoc networks performance. New MAC protocol standards are needed.

• Vector antennas may replace spatial arrays to further improve beamforming performance


• The relative amplitudes of — and constructive and destructive interference effects among — the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan rapidly in azimuth or elevation.

DIFFERENT TYPES OF PHASED ARRAYS

• There are two main types of beamformers:• time domain beamformers • frequency domain beamformers• A graduated attenuation window is

sometimes applied across the face of the array to improve side-lobe suppression performance, in addition to the phase shift.

TIME DOMAIN BEAMFORMER • works by introducing time delays. • The basic operation is called "delay and sum". It

delays the incoming signal from each array element by a certain amount of time, and then adds them together.

• The most common kind of time domain beam former is serpentine waveguide.

• Active phase array uses individual delay lines that are switched on and off. Yttrium iron garnet phase shifters vary the phase delay using the strength of a magnetic field.

FREQUENCY DOMAIN BEAMFORMERS

• TWO DIFFERENT TYPES OF FREQUENCY DOMAIN BEAMFORMERS:• separates the different frequency components that are present in the

received signal into multiple frequency bins (using either an DFT or a filterbank). When different delay and sum beamformers are applied to each frequency bin, the result is that the main lobe simultaneously points in multiple different directions at each of the different frequencies. This can be an advantage for communication links, and is used with the SPS-48 radar.

• makes use of Spatial Frequency. Discrete samples are taken from each of the individual array elements. The samples are processes using a Discrete Fourier Transform (DFT). The DFT introduces multiple different discrete phase shifts during processing. The outputs of the DFT are individual channels that correspond with evenly spaced beams formed simultaneously. A 1 dimensional DFT produces a fan of different beams. A 2 dimensional DFT produces beams with a pineapple configuration.

FREQUENCY DOMAIN BEAMFORMERS (CONT)

• These techniques are used to create two kinds of phase array.

• Dynamic - an array of variable phase shifters are used to move the beam

• Fixed - the beam position is stationary with respect to the array face and the whole antenna is moved

• There are two further sub-categories that modify the kind of dynamic array or fixed array.

• Active - amplifiers or processors in each phase shifter element

• Passive - large central amplifier with attenuating phase shifters

Dynamic Phased Array• Each array element incorporates an adjustable phase shifter that are

collectively used to move the beam with respect to the array face.• Dynamic phase array require no physical movement to aim the beam. The

beam is moved electronically. This can produce antenna motion fast enough to use a small pencil-beam to simultaneously track multiple targets while searching for new targets using just one radar set (track while search).

• As an example, an antenna with a 2 degree beam with a pulse rate of 1 kHz will require approximately 16 seconds to cover an entire a hemisphere consisting of 16,000 pointing positions. This configuration provides 6 opportunities to detect a Mach 3 vehicle over a range of 100 km (62 mi), which is suitable for military applications.

• The position of mechanically steered antennas can be predicted, which can be used to create electronic countermeasures that interfere with radar operation. The flexibility resulting from phase array operation allows beams to be aimed at random locations, which eliminates this vulnerability. This is also desirable for military applications.

Fixed Phase Array• Fixed phase array antennas are typically used to create an antenna with a

more desirable form factor than the conventional parabolic reflector or cassegrain reflector. Fixed phased array radar incorporate fixed phase shifters. This kind of phase array is physically moved during the track and scan process. There are two configurations.

• Multiple frequencies with a delay-line• Multiple adjacent beams• The SPS-48 radar uses multiple transmit frequencies with a serpentine

delay line along the left side of the array to produce vertical fan of stacked beams. Each frequency experiences a different phase shift as it propagates down the serpentine delay line, which forms different beams. A filter bank is used to split apart the individual receive beams. The antenna is mechanically rotated.

• Semi-active radar homing uses monopulse radar that relies on a fixed phase array to produce multiple adjacent beams that measure angle errors. This form factor is suitable for gimbal mounting in missile seekers.

Active Phase Array

• Active phase arrays elements incorporate transmit amplification with phase shift in each antenna element (or group of elements). Each element also includes receive pre-amplification. The phase shifter setting is the same for transmit and receive.

• Active phase array do not require phase reset after the end of the transmit pulse, which is compatible with Doppler radar and Pulse-Doppler radar.

Passive Phase Array• Passive phase arrays typically use large amplifiers that produce all

of the microwave transmit signal for the antenna. Phase shifters typically consist of waveguide elements that contain phase shifters controlled by magnetic field, voltage gradient, or equivalent technology.

• The phase shift process used with passive phase array typically puts the receive beam and transmit beam into caddy-corner quadrants. The sign of the phase shift must be inverted after the transmit pulse is finished and before the receive period begins to place the receive beam into the same location as the transmit beam. That requires a phase impulse that degrades sub-clutter visibility performance on Doppler radar and Pulse-Doppler radar. As an example, Yttrium iron garnet phase shifters must be changed after transmit pulse quench and before receiver processing starts to align transmit and receive beams. That impulse introduces FM noise that degrades clutter performance.

MICROWAVE FEEDER SYSTEM (DRIVER ELEMENT)

TYPES OF FEEDER

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•Omnidirectional •Cassegrain •Gregorian •Horn feed

Parabolic antennas are also classified by the type of feed, i.e. how the radio waves are supplied to the antenna.

• The primary antenna is placed at the parabolic focus point.

• Reason: produce better transmission and reception. (enhance directivity and gain)

• The primary antenna has to be used together with the reflector to avoid the flaring of the radiation pattern and thus reduced the directivity.

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DIPOLE FEEDER

83

SPHERICAL REFLECTOR TO DIRECT WAVE TO THE MAIN REFLECTOR

MAIN REFLECTOR

PRIMARY FEED DIPOLE AT FOCUS

AXIAL OR FRONT FEED

84

• The most common type of feed, with the feed antenna located in front of the dish at the focus, on the beam axis.

• A disadvantage of this type is that the feed and its supports block some of the beam, which limits the aperture efficiency to only 55 - 60%.

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AXIAL OR FRONT FEED

OFF-AXIS OR OFFSET FEED

• The reflector is an asymmetrical segment of a paraboloid, so the focus, and the feed antenna, is located to one side of the dish.

• The purpose of this design is to move the feed structure out of the beam path, so it doesn't block the beam.

• It is widely used in home satellite television dishes, which are small enough that the feed structure would otherwise block a significant percentage of the signal.

• Offset feed is also used in multiple reflector designs such as the Cassegrain and Gregorian.

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87

OFF-AXIS OR OFFSET FEED

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CASSEGRAIN FEED

• The feed is located on or behind the dish, and radiates forward, illuminating a convex hyperboloidal secondary reflector at the focus of the dish.

• The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the outgoing beam

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CASSEGRAIN FEED

•The advantage of this configuration is that the feed, with its waveguides and "front end" electronics does not have to be suspended in front of the dish, so it is used for antennas with complicated or bulky feeds, such as large satellite communication antennas and radio telescopes.

• Aperture efficiency is on the order of 65 - 70%.

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CASSEGRAIN FEED

• Focus points for the secondary and primary reflectors will meet at the same point.

• Radiation from the horn antenna will be reflected by the secondary reflector and transmitted to the primary reflector to collimate the radiation.

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GREGORIAN FEED

• Similar to the Cassegrain design except that the secondary reflector is concave, (ellipsoidal) in shape.

• Aperture efficiency over 70% can be achieved.

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HORN FEED

• It is widely used as a primary feeder, because of the flaring directivity pattern , thus preventing refraction.

MAIN REFLECTOR

PRIMARY FEED HORN

WAVEGUIDE/TRANSMISSION LINE

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FACTORS AFFECTING THE ANTENNA

RADIATION PATTERN

Radiation pattern refers to the performance ot the antenna for example when it is mounted far away from objects such as buildings or mountain ( earth) by which reflecting signal might affect the shape of the pattern.

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Figures below show the 3-dimensional models (polar graf/diagram) of field strength or power density measurements made at a fixed distance from an antenna in a given plane.

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Figures below show the 3-dimensional models (polar graf/diagram) of field strength or power density measurements made at a fixed distance from an antenna in a given plane.

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BEAM WIDTH (BEAM / FLARED ANGLE)

•It is the angle subtended by the points at which the radiation power falls to the half of its maximum power. •In other words, the field strength has fallen to 1/√2 (70.7 % ) of its maximum voltage or the angle measured between the -3dB (half power) points on the major lobe of an antenna’s radiation pattern.

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ANTENNA GAIN

• It is defined as the ratio of power per unit area received from the antenna at a point in space to the power received from an isotropic antenna at the same point in space.

• The capability of a directive antenna to concentrate power in a given direction is the capability to direct radio frequency energy into a given region and not in all direction.

• For transmitting antenna, it refers to how far is the concentration of transmission power in a given direction.

• For receiving antenna, it refers to how far its receive the best signal in a given direction rather than in all direction.

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CHARACTERISTIC OF PARABOLOID ANTENNA

• To convert the spherical waveform produced at a focus point to the plane wave.

• All the energy received from the free space which is the same as the parabolic axis (Rx) will be reflected to the focus point.

ADVANTAGES• The gain can be increased whenever needed.• Can be operated at any frequency in the microwave zone.• Simple Installation.DISADVANTAGES• Difficult to install with high accuracy.• Operational frequency limited to the types of dish used.

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GAINGAIN ; G = 4 π A λ2

Where;G = gain; A = area of parabolic dish (m2); λ = wavelength of operational frequency (m)If the area of the dish, A A = π d 2 4Where; A = area of parabolic dish (m2); d = diameter of dish opening (m) Beamwidth α = 115 λ ° dα = antenna beamwidth or angle between half power points ( °)λ = wavelength (m) d = diameter of dish opening (m)

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