CHAPTER 5 MICROWAVE ANTENNA - PowerPoint Presentation

Slide1

CHAPTER 5MICROWAVE ANTENNASlide2
Slide3

MICROWAVE ANTENNA

Definition

A

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

). Slide4

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

Slide5

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.Slide6

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 Slide7

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

5.2Slide8

TYPES OF MICROWAVE ANTENNA

Horn / aperture antenna

Parabolic / dish antenna

Dipole antenna

Slotted (leaky-wave) antenna

Dielectric lens antenna

Printed (patch or

microstrip

) antenna

Phase Array

antennaSlide9

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

Slide10

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 radiatorSlide11

DIFFERENT TYPES OF HORN ANTENNASlide12

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.Slide13

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. Slide14

DIMENSION OF HORN ANTENNASlide15

DIMENSION OF HORN ANTENNA (cont)Slide16

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 receiverSlide17

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 AntennaSlide18

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 .Slide19

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 raysSlide20

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:Slide21

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 elevationSlide22

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

reflectorSlide23

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. Slide24

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. Slide25

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.Slide26

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.   Slide27

PARABOLIC RADIATION PATTERN

Figure 5.7 : Parabolic radiation patternSlide28

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

spaceSlide29

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.

Slide30

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). Slide31

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. Slide32

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

. Slide33

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. Slide34

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

. Slide35

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. Slide36

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. Slide37

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 radiationSlide38

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 waveguideSlide39

Slot antenna (cont)

The waveguide itself acts as a transmission line, and the slots in the waveguide can be viewed as parallel (shunt) admittances. Slide40

Slot antenna (cont)Slide41

Slot antenna (cont)Slide42

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.Slide43

D) DIPOLE ANTENNAS

TWO 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/V

c

)

λr

 Slide44

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

. Slide45

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

/ 2Slide46

D) DIPOLE ANTENNAS Cont.

Dipole geometry

Dipole configuration

 Slide47

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

.Slide48

D) DIPOLE ANTENNAS Cont.

The

3D plot of the radiation pattern of a dipole

antennaSlide49

D) DIPOLE ANTENNAS Cont.

The radiation pattern for the Electric field for a folded dipole antennaSlide50

D) DIPOLE ANTENNAS Cont.

The radiation pattern of the dipole all the field is electric as shown

Slide51

D) DIPOLE ANTENNAS Cont.

The radiation pattern of the dipole, the magnetic field equals zeroSlide52

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

.Slide53

D) DIPOLE ANTENNAS Cont. Slide54

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.Slide55

BASIC PRINCIPLE

E)

DIELECTRIC (LENS)

ANTENNAS cont.

Slide56

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

).

E)

DIELECTRIC (LENS)

ANTENNAS cont.

Slide57

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.

E)

DIELECTRIC (LENS)

ANTENNAS cont.

Slide58

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.

Slide59

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.

Slide60

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

Slide61

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.

Slide62

TYPES OF MICROSTRIP ANTENNAS:

F

)

PRINTED (MICROSTRIP OR PATCH)

ANTENNA Cont.

Slide63

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

.

F

)

PRINTED (MICROSTRIP OR PATCH)

ANTENNA Cont.

Slide64

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.

F

)

PRINTED (MICROSTRIP OR PATCH)

ANTENNA Cont.

Slide65

F)

PRINTED (MICROSTRIP OR PATCH) ANTENNA Cont.

Microstrip

Antennas

Reflector Antennas

Preferred for low directivity applications.

Lower efficiency.

Suffers low efficiency caused by

f

eed

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

i

s less.

Higher efficiency.

Suffers blockage caused by fixation Struts.

Uses mechanical scanning .

Less accuracy , sometimes parabolic

s

urfaces

are rough.

Uses other antenna (dipole , monopole, apertures ,

etc

) as

a

feed.

MICROSTRIP VS. REFLECTORS

Slide66

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 directionSlide67

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 interferenceSlide68

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.)Slide69

Phased Array Antenna (Cont)

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 calculatedSlide70

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 costsSlide71

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 performanceSlide72

Phased Array Antenna (Cont)

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.Slide73

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.Slide74

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.Slide75

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.Slide76

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 shiftersSlide77

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.Slide78

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.Slide79

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.Slide80

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.Slide81

MICROWAVE FEEDER SYSTEM (DRIVER ELEMENT)

TYPES OF FEEDER

81

Omnidirectional

Cassegrain

Gregorian

Horn feedSlide82

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.

82Slide83

DIPOLE FEEDER

83

SPHERICAL REFLECTOR TO DIRECT WAVE TO THE MAIN REFLECTOR

MAIN REFLECTOR

PRIMARY FEED DIPOLE AT FOCUS Slide84

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%.Slide85

85

AXIAL

OR

FRONT FEED

Slide86

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.

86Slide87

87

OFF-AXIS

OR

OFFSET FEED

Slide88

88

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 beamSlide89

89

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%.Slide90

90

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. Slide91

91

GREGORIAN FEED

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

Aperture efficiency over 70% can be achieved.Slide92

92

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 LINESlide93

93

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.Slide94

94

FACTORS AFFECTING THE ANTENNA

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. Slide95

95

FACTORS AFFECTING THE ANTENNA

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. Slide96

96

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.Slide97

97

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. Slide98

98

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.Slide99

99

GAIN

GAIN ; G =

4 π A

λ

2

Where;

G = gain;

A = area of parabolic dish (m

2

);

λ

= wavelength of operational frequency (m)

If the area of the dish, A

A =

π d

2

4

Where;

A = area of parabolic dish (m

2

);

d = diameter of dish opening (m)

Beamwidth α =

115 λ

°

d

α = antenna beamwidth or angle between half power points ( °)

λ = wavelength (m)

d

= diameter of dish opening (m)