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Displays-Types
• The purpose of the display is to visually present the information contained in the
radar echo signal in a form suitable for operator interpretation and action.
• When the display is connected directly to the video output of the receiver, the
information displayed is called raw video. This is the "traditional” type of radar
presentation.
• When the receiver video output is first processed by an automatic detector or
automatic detection and tracking processor (ADT), the output displayed is
sometimes called synthetic video.
• The cathode-ray tube (CRT) has been almost universally used as the radar display.
There are two basic cathode-ray tube displays. One is the dejection-modulated CRT,
such as the A-scope, in which a target is indicated by the deflection of the electron
beam.
• The other is the intensity modulated CRT, such as the PPI, in which a target is
indicated by intensifying the electron beam and presenting a luminous spot on the
face of the CRT.
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• In general, deflection-modulated displays have the advantage of simpler circuits
than those of intensity- modulated displays, and targets may be more readily
discerned in the presence of noise or interference.
• On the other hand, intensity-modulated displays have the advantage of presenting
data in a convenient and easily interpreted form.
• The deflection of the beam or the appearance of an intensity-modulated spot on a
radar display caused by the presence of a target is commonly referred to as a blip.
Types of display presentations: The various types of CRT displays which might be
used for surveillance and tracking radars are defined as follows:
• A-scope: A deflection-modulated display in which the vertical deflection is
proportional to target echo strength and the horizontal coordinate is proportional to
range.
• B-scope: An intensity-modulated rectangular display with azimuth angle indicated
by the horizontal coordinate and range by the vertical coordinate.
• C-scope: An intensity-modulated rectangular display with azimuth angle indicated
by the horizontal coordinate and elevation angle by the vertical coordinate.

• D-scope: A C-scope in which the blips extend vertically to give a rough estimate of
distance.

• E-scope: An intensity-modulated rectangular display with distance indicated by the

horizontal coordinate and elevation angle by the vertical coordinate. Similar to the
RHI in which target height or altitude is the vertical coordinate.

• F-Scope: A rectangular display in which a target appears as a centralized blip when

the radar antenna is aimed at it. Horizontal and vertical aiming errors are
respectively indicated by the horizontal and vertical displacement of the blip.

• G-Scope: A rectangular display in which a target appears as a laterally centralized

blip when the radar antenna is aimed at it in azimuth, and wings appear to grow on
the pip as the distance to the target is diminished; horizontal and vertical aiming
errors are respectively indicated by horizontal and vertical displacement of the blip.

• H-scope: A B-scope modified to include indication of angle of elevation. The target

appears as two closely spaced blips which approximate a short bright line, the slope
of which is in proportion to the sine of the angle of target elevation.
 141

• M-scope: A type of A-scope in which the target distance is determined by moving

an adjustable pedestal signal along the baseline until it coincides with the horizontal
position of the target signal deflections; the control which moves the pedestal is
calibrated in distance.
• N-scope: A K-scope having an adjustable pedestal signal, as in the M-scope, for the
measurement of distance.
• O-scope: An A-scope modified by the inclusion of an adjustable notch for
measuring distance.
• PPI or Plan Position Indicator (also called P-scope): An intensity-modulated
circular display on which echo signals produced from reflecting objects are shown
in plan position with range and azimuth angle displayed in polar (rho-theta)
coordinates, forming a map-like display. An offset, or off center, PPI has the zero
position of the time base at a position other than at the center of the display to
provide the equivalent of a larger display for a selected portion of the service area.
A delayed PPI is one in which the initiation of the time base is delayed.
• R-scope: An A-scope with a segment of the time base expanded near the blip for
greater accuracy in distance measurement.
• RHI or Range-Height Indicator: An intensity modulated display with height
(altitude) as the vertical axis and range as the horizontal axis.
• PPI, A-scope, B-scope, and RHI are among the more usual displays employed in
radar.
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Duplexers
The duplexer is the device that 1) switches the radar antenna to either
transmitter or the receiver 2) serves to protect the receiver from burnout or damage
during the transmission.

Branch type Duplexer

Figure : Principle of branch-type duplexer

• The branch-type duplexer, shown in above Fig is one of the earliest duplexer configurations.
It consists of a TR (transmit-receive) switch and an ATR (anti-transmit receive) switch, both
of which are gas-discharge tubes.
• When the transmitter is turned on, the TR and the ATR tubes ionize; that is, they break down,
or fire. The TR in the fired condition acts as a short circuit to prevent transmitter power from
entering the receiver.
• Since the TR is located a quarter wavelength from the main transmission line, it appears as a
short circuit at the receiver but as an open circuit at the transmission line so that it does not
impede the flow of transmitter power.
• Since the ATR is displaced a quarter wavelength from the main transmission line, the short
circuit it produces during the fired condition appears as an open circuit on the transmission
line and thus has no effect on transmission.
• During reception, the transmitter is off and neither the TR nor the ATR is fired.
• The open circuit of the ATR, being a quarter wave from the transmission line, appears as a
short circuit across the line.
• Since this short circuit is located a quarter wave from the receiver branch line, the transmitter
is effectively disconnected from the line and the echo signal power is directed to the receiver.
• The branch-type duplexer is of limited bandwidth and power handling capability, and has
generally been replaced by the balanced duplexer and other protection devices. It is used,
inspite of these limitations, in some low-cost radars.
 143

• The balanced duplexer is based on the short-slot hybrid junction which consists of two
sections of waveguides joined along one of their narrow walls with a slot cut in the common
narrow wall to provide coupling between the two.
• The short-slot hybrid may be considered as a broadband directional coupler with a coupling
ratio of 3 dB.
• In the transmit condition (Figure a) power is divided equally into each waveguide by the first
short slot hybrid junction. Both TR tubes break down and reflect the incident power out the
antenna arm as shown.
• The short-slot hybrid has the property that each time the energy passes through the slot in
either direction, its phase is advanced 900. Therefore, the energy travels as indicated by the
solid lines.
• Any energy which leaks through the TR tubes (shown by the dashed lines) is directed to the
arm with the matched dummy load and not to the receiver. In addition to the attenuation
provided by the TR tubes, the hybrid junctions provide an additional 20 to 30 dB of isolation.
• On reception the TR tubes are unfired and the echo signals pass through the duplexer and into
the receiver as shown in Fig.6b. The power splits equally at the first junction and because of
the 900 phase advance on passing through the slot, the energy recombines in the receiving arm
and not in the dummy-load arm.
• The power-handling capability of the balanced duplexer is inherently greater than that of the
branch-type duplexer and it has wide bandwidth, over ten percent with proper design. A
receiver protector, is usually inserted between the duplexer and the receiver for added
protection.
 144

CIRCULATORS AS DUPLEXERS
CIRCULATOR AND RECEIVER PROTECTOR
• The ferrite circulator is a three or four-port device that can in principle, offer
separation of the transmitter and receiver without the need for the conventional
duplexer configurations explained earlier.

• The circulator does not provide sufficient protection by itself and requires a
receiver protector like duplexers.

• The isolation between the transmitter and receiver ports of a circulator is seldom
sufficient to protect the receiver from damage.

• However, it is not the isolation between transmitter and receiver ports that usually
determines the amount of transmitter power at the receiver, but the impedance
mismatch at the antenna which reflects transmitter power back into the receiver.

• The VSWR is a measure of the amount of power reflected by the antenna. For
example, a VSWR of 1.5 means that about 4 percent of the transmitter power will
be reflected by the antenna mismatch in the direction of the receiver, which
corresponds to an isolation of only 14 dB. About 11 percent of the power is
reflected when the VSWR is 2.0, corresponding to less than 10 dB of isolation.
Thus, a receiver protector is almost always required.
 145
• It also reduces to safe level radiations from nearby transmitters.
• The receiver protector might use solid-state diodes for an all solid-state
configuration, or it might be a passive TR-limiter consisting of a radioactive primed
TR-tube followed by a diode limiter.
• The ferrite circulator with receiver protector is attractive for radar applications
because of its long life, wide bandwidth, and compact design.

Figure : circulator and receiver protector. A four-port circulator is shown with the fourth port
terminated in a matched load to provide greater isolation between the transmitter and the receiver than
provided by a three-port circulator

Introduction to phased array antennas

• The phased array is a directive antenna made up of individual radiating antennas, or
elements, which generate a radiation pattern whose shape and direction is
determined by the relative phases and amplitudes of the currents at the individual
elements.
• By properly varying the relative phases it is possible to steer the direction of the
radiation.
• The radiating elements might be dipoles open-ended waveguides, slots cut in
waveguide, or any other type of antenna.
• Became of interest to Radar due to the inherent flexibility it has offered in steering
the beam by means of electronic control rather than by physical movement of the
antenna.
• It has been considered in those radar applications where it is necessary to shift the
beam rapidly from one position in space to another, or where it is required to obtain
information about many targets at a flexible, rapid data rate.
• The full potential of a phased-array antenna requires the use of a computer that can
determine in real time, on the basis of the actual operational situation, how best to
use the capabilities offered by the array.
 146
• Initially In World War 2, the United States, Great Britain, and Germany used radar
with fixed phased-array antennas in which the beam was scanned by mechanically
actuated phase shifters.

• A major advance in phased array technology was made in the early 1950s with the
replacement of mechanically actuated phase shifters by electronic phase shifters.

• Frequency scanning in one angular coordinate was the first successful electronic
scanning technique to be applied.

• The introduction of digitally switched phase shifters employing either ferrites or

diodes in the early 1960s made a significant improvement in the practicality of
phased arrays that could be electronically steered in two orthogonal angular
coordinates.

Basic concept
• An array antenna consists of a number of individual radiating elements suitably
spaced with respect to one another.

• Two common geometrical forms of array antennas used in radar are the linear array
and the planar array.

Linear array

• A linear array consists of elements arranged in a straight line in one dimension.

• The linear array generates a fan beam when the phase relationships are such that the
radiation is perpendicular to the array.

• When the radiation is at some angle other than broadside, the radiation pattern is a
conical-shaped beam.

• The linear array can also act as a feed for a parabolic-cylinder antenna.
 147
Planar array

• A planar array is a two dimensional configuration of elements arranged to lie in a

plane. The planar array may be thought of as a linear array of linear arrays.

• The two-dimensional planar array is the most commonly used in radar applications
since it is fundamentally the most versatile of all radar antennas.

• A rectangular aperture can produce a fan shaped beam. A square or a circular

aperture produces a pencil beam.

• The array can be made to simultaneously generate many search and/or tracking
beams with the same aperture.

Broadside array

• A broadside array is one in which the direction of maximum radiation is

perpendicular, or almost perpendicular to the line (or plane) of the array.

• The broadside linear-array antenna may be used where broad coverage in one plane
and narrow beam width in the orthogonal plane are desired.

End fire
• An end fire array has its maximum radiation parallel to the array.
• The end fire array is a special case of the linear or the planar array when the beam
is directed along the array.
• End fire linear arrays have not been widely used in radar applications.
• They are usually limited to low or medium gains since an end fire linear antenna of
high gain requires an excessively long array.
• Small end fire arrays are sometimes used as the radiating elements of a broadside
array if directive elements are required.
Other types of array antennas
• An array whose elements are distributed on a non planar surface is called a
conformal array.
• An array in which the relative phase shift between elements is controlled by
electronic devices is called an electronically scanned array.
• In an electronically scanned array the antenna elements, the transmitters, the
receivers, and the data-processing portions of the radar are often designed as a unit.
 148

Radiation pattern
• Consider a linear array made up of N elements equaIIy spaced a distance d apart as
shown in Fig.

Figure : N-element linear array.

• The elements are assumed to be isotropic point sources radiating uniformly in all
directions with equal amplitude and phase.

• The outputs of all the elements are summed via lines of equal length to give a sum
output voltage Ea.
• Element 1 will be taken as the reference signal with zero phase. The difference in
the phase of the signals in adjacent elements is Ψ = 2π (d/λ) sin θ, where θ is the
direction of the incoming radiation.
• It is further assumed that the amplitudes and phases of the signals at each element
are weighted uniformly. Therefore the amplitudes of the voltages in each element
are the same and, for convenience, will be taken to be unity.
• The sum of all the voltages from the individual elements, when the phase difference
between adjacent elements is Ψ, can be written as

where ω is the angular frequency of the signal. The sum can be written as
 149
• The first factor is a sine wave of frequency ω with a phase shift (N - 1) ψ/2. The
second term represents the amplitude factor of the form sin (Nψ/2)/sin (ψ/2). The
field intensity pattern is the magnitude of the equation 2 , or

• The pattern has nulls whenever the numerator is zero.

• NΠ(d/λ)sinθ = 0, ± Π, ± 2Π..., ± nΠ, where n = integer. The denominator, however,

is zero when Π (d/λ) sinθ, = 0, ± Π, ± 2Π..., ± nΠ. Note that when the denominator
is zero, the numerator is also zero.

• The value of the field intensity pattern is indeterminate when both the denominator
and numerator are zero. However, by applying L'Hopital's rule (differentiating
numerator and denominator separately) it is found that |Ea |is a maximum
whenever sinθ = ± nλ/d.

• These maxima all have the same value and are equal to N. The maximum at
sinθ = 0 defines the main beam. The other maxima are called grating lobes. They
are generally undesirable and are to be avoided.

• The radiation pattern is equal to the normalized square of the amplitude, or

• When directive elements are used, the resultant array antenna radiation pattern is

• where Ge (θ) is the radiation pattern of an individual element. The resultant

radiation pattern is the product of the element factor Ge(θ) and the array factor
Ga(θ).
• Grating lobes caused by a widely spaced array may therefore be eliminated with
directive elements which radiate little or no energy in the directions of the
undesired lobes.
• For example, when the element spacing d = 2λ, grating lobes occur at θ = ±30° and
±90° in addition to the main beam at θ = 0°. If the individual elements have a
beamwidth somewhat less than 60°, the grating lobes of the array factor will be
suppressed.
 150
• In a two-dimensional, rectangular planar array, the radiation pattern may sometimes be
written as the product of the radiation patterns in the two planes which contain the
principal axes of the antenna.
• If the radiation patterns in the two principal planes are G1(θe) and G2(θa) the two-
dimensional antenna pattern is

• Thus, the normalized radiation pattern of a uniformly illuminated rectangular array is

• Where N = number of radiating elements in θa dimension with spacing d and M the

number in θe dimension.

Beam steering phased array antennas

• The beam of an array antenna may be steered rapidly in space without moving large
mechanical masses by properly varying the phase of the signals applied to each
element.

• Consider an array of equally spaced elements. The spacing between adjacent

elements is d, and the signals at each element are assumed to be of equal amplitude.

• If the same phase is applied to all elements, the relative phase difference between
adjacent elements is zero and the position of the main beam will be broadside to the
array at an angle θ = 0.

• The main beam will point in a direction other than broadside if the relative phase
difference between elements is other than zero.

• The direction of the main beam is at an angle θ0, when the phase difference is Ø =
2π (d/λ) sin θ0. The phase at each element is therefore (Øc + m Ø) where m = 0,
1,2. . . (N - I) and Øc is any constant phase applied to all elements.
 151
• The normalized radiation pattern of the array when the phase difference between
adjacent elements is Ø is given by:

• The maximum of the radiation pattern occurs when sin θ= sin θ0

• The above Equation states that the main beam of the antenna pattern may be
positioned to an angle θ0 by the insertion of the proper phase shift Ø at each
element of the array. If variable, rather than fixed, phase shifters are used, the beam
may be steered as the relative phase between elements is changed

FIG: Steering of an antenna beam with variable phase shifters (parallel-

fed array).

• Using an argument similar to the non scanning array described previously, grating
lobes appear at an angle θg whenever the denominator is zero, or when

or

• If a grating lobe is permitted to appear at -900 when the main beam is steered to
+90°, it is found from the above that d = λ/2.

• Thus the element spacing must not be larger than a half wavelength if the beam is
to be steered over a wide angle without having undesirable grating lobes appear.

• Practical array antennas do not scan +/- 90°. If the scan is limited to +/- 600 the
element spacing should be less than 0.54λ.
 152

Change of beam width with steering angle

• The half-power beam width in the plane of scan increases as the beam is scanned
off the broadside direction. The beam width is approximately inversely proportional
to cos θ0, where θ0 is the angle measured from the normal to the antenna.
• This may be proved by assuming that the sine in the denominator of G(θ) discussed
earlier can be replaced by its argument, so that the radiation pattern is of the
form(sin2u)/u2, where u = NΠ (d/λ)(sinθ - sinθo).
• The (sin2u)/u2 antenna pattern is reduced to half its maximum value when u = ±
0.443Π. Denote by θ+ the angle corresponding to the half-power point when θ > θo
, and θ- , the angle corresponding to the half-power point when θ <θo; that is, θ+
corresponds to u = +0.443Π and θ- to u = -0.443Π.
• The sinθ - sinθo, term in the expression for u can be written

• The second term on the right-hand side of Eq. above can be neglected when θo is
small (beam is near broadside), so that

• Using the above approximation, the two angles corresponding to the 3-dB points of
the antenna pattern are

• The half-power beamwidth is

• Therefore, when the beam is positioned an angle θo off broadside, the beamwidth in
the plane of scan increases as (cos θo) -1.
 153

• The variation of the beam shape with scan angle is graphically shown in Fig below.

• The antenna radiation pattern is plotted in spherical coordinates as a function of the two
direction cosines, cos αx and cos αy of the radius vector specifying the point of
observation.

• The angle Ø is measured from the cos αx axis, and θ is measured from the axis
perpendicular to the cos αx and cos αy axes.
• In Fig. Ø is taken to be a constant value of 900 and the beam is scanned in the θ
coordinate.

• At θ= 0 (beam broad side to the array) a symmetrical pencil beam of half-power width
B0 is assumed.

• The shape of the beam at the other angular positions is the projection of the circular
beam shape on the surface of the unit sphere.

• It can be seen that as the beam is scanned in the θ direction, it broadens in that direction,
but is constant in the Ø direction.

• For θ≠0, the beam shape is not symmetrical about the center of the beam, but is
eccentric.

Beam width and eccentricity of the scanned beam

 154

Applications of phased array antennas

The phased array antenna has seen application in radar for a wide variety of purposes:
• Aircraft surveillance from on board ship
• Satellite surveillance
• Ballistic missile defense
• Air defense
• Aircraft landing systems
• Mortar and artillery location
• Tracking of ballistic missiles and Airborne bomber radar (EAR).
• Many developmental array radars have been developed and built in USA. Although
much effort and funds have been spent on this activity, except for limited-scan
arrays there has not been any large serial production of such radars compared to the
serial production of radars with mechanically rotating reflector antennas.

Advantages phased array antennas.

• Inertialess rapid beam-steering: The beam from an array can be scanned, or
switched from one position to another, in a time limited only by the switching speed
of the phase shifters. Typically, the beam can be switched in several microseconds,
but it can be considerably shorter if desired.

• Multiple, independent beams: A single aperture can generate many simultaneous

independent beams. Alternatively, the same effect can be obtained by rapidly
switching a single beam through a sequence of positions.

• Potential for large peak and / or average power: If necessary, each element of
the array can be fed by a separate high-power transmitter with the combining of the
outputs made in space to obtain a total power greater than can be obtained from a
single transmitter.
 155

• Control of the radiation pattern. A particular radiation pattern may be more

readily obtained with the array than with other microwave antennas since the
amplitude and phase of each array element may be individually controlled. Thus,
radiation patterns with extremely low side lobes or with a shaped main beam may
be achieved conveniently. Separate monopulse sum and difference patterns, each
with it’s own optimum shape, can also be generated.

• Graceful degradation. The distributed nature of the array means that it can fail
only gradually and not l at once (catastrophically).

• Convenient aperture shape. The shape of the array permits flush mounting and it
can be strengthened to resist blast.

• Electronic beam stabilization. The ability to steer the beam electronically can be
used to stabilize the beam direction when the radar is on an unstable platform, such
as a ship or aircraft that is subject to roll, pitch, and yaw disturbances.

Limitations of phased array antennas

1. The major limitation that has limited the widespread use of the conventional
phased array in radar is its high cost, which is due in large part to its complexity.

2. When graceful degradation has gone too far a seperate maintenance is needed.

3. When a planar array is electronically scanned, the change of mutual coupling that
accompanies a change in beam position makes the maintenance of low sidelobes
more difficult.

4. Although the array has the potential for radiating large power, it is seldom that an
array is required to radiate more power than can be radiated by other antenna types
or to utilize a total power which cannot possibly be generated by current high-
power microwave tube technology that feeds a single transmission line.
 156

Series Vs Parallel feeds

• The phase relationship between the adjacent elements of the array can be obtained
with either series fed or parallel fed arrangement.
• In series fed arrangement, the energy may be transmitted from one end to the line or
it may be fed from the center out to each end.
• The adjacent elements are connected by a phase shifter with phase ϕ.
• All the phase shifters are identical and introduce the same amount of phase shift
which is less than 2Π radians.
• In parallel fed the energy to be radiated is divided between the elements by a power
splitter.
• When a series of power splitters are used to create a tree like structure is called a
corporate feed.
• Equal lengths of line transmit the energy to each element so that no unwanted phase
difference are introduced by the lines themselves.
• The maximum phase change required of each phase shifter in the parallel-fed array
is many times 2Π radians.

• In a series-fed array containing N phase shifters, the signal suffers the insertion loss
of a single phase shifter N times. In a parallel-fed array the insertion loss of the p h
s e shifter is introduced effectively but once.

• Since each phase shifter in the series-fed linear array of below Fig has the same
value of phase shift, only a single control signal is needed to steer the beam. The N-
element parallel-fed linear array similar to that of below Fig requires a separate
control signal for each phase shifter or N - 1 total.

Parallel Feed
 157

Series arrangements for applying phase relationships in an array.

(a) fed from one end; (b) center-fed.