APPLICATIONS OF THE ARRAY IN RADAR

                

              The phased-array antenna has been of considerable interest to radar systems engineer because its properties are different from those of other microwave antennas.The array antenna takes several forms 

Mechanically scanned array.  :  The array antenna in this configuration is used to from a fixed beam that is scanned by mechanical motion of the entire antenna.No electronic beam steering is employed.This is an economical approach to air-surveillance radars at the lower radar frequencies, such as VHF. It is also employed at higher frequencies when a precise aperture illumination is required, as to obtain extermely low sidelobes. At the lower frequencies, the array might consist of slotted waveguides.

Linear array with frequency scan:. The frequency - scanned, linear array feeding a parabolic cylinder or a planer array of slotted  waveguides has seen wide application as a 3D air-surveillance radar. In this application, a pencil beam is scanned in elevation by use of frequency and scanned in azimuth by mechanical rotation of the entire antenna.

Linear array with phase scan. :      Electronic phase steering instead of frequency scanning, in the 3D air-surveillance radar is generally more expensive, but allows the use of the frequency domain for purposes other than beam steering.The linear array configuration is also used to generate multiple, contiguous fixed beam (stacked beam ) for 3D radar.Another application is to use either phase- or frequency- steering in a stationary linear array to steer the beam  in one angular coordinate, as the CGA radar.

Phase-frequency planar array :    A two-dimensional (planer) phased array can utilize frequency scanning to steer the beam in one angular coordinate and phase shifters to steer in the orthogonal coordinates, but as with any frequency-scanned array the use of the frequency domain for other purposes is limited when frequency employed for beam steering

Phase-phase planer array :             The planar array which utilizes phase shifting to steer the beam in two orthogonal coordinates is the type of array that is of major interest for radar application because of its inherent  versatility.Its application, however, has beam limited by its relatively high cost. The phase-phase array is what is generally implied when the term electronically steered phased array is used 

The phased array antenna has seen application in radar for such purpose as aircraft surveillance  from on board ship (AN/SPS-33), ballistic missile defense (PAR, MSR), air defense (AN/SPY-1 and patriot), aircraft landing systems (AN/TPN-19 and AN/TPS-32) mortar (AN/TPQ-36)and artillery (AN/TPQ-37) location tracking of ballistic missiles (Cobra Dane), and airborne bomber radar (EAR)

There have been many  developmental array radars, built in the United States, including ESAR, ZMAR, MAR, Typhon, Hapdar ADAR, MERA,  RASSR, and others. Although much effort and funds have been expended, except for limited-scan arrays there has been no large serial production of such radars comparable to serial production  of radars with mechanically rotating reflector antennas.

    

THE ELECTRONICALLY STEERED PHASED ARRAY ANTENNA IN RADAR

         

      The phased array is a directive antenna mode up of individual radiating antenna, or elements, which generate a radiation pattern whose shape and direction is determined by the relative phases and aptitudes 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 wave guide, or any other type of antenna.The inherent flexibility offered by the phased-array antenna in steering the beam by means of electronic control is what has made it if interest for radar.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 a bout many targets at a flexible,rapid date 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,                  

   

The concept of directive radiation from fixed (non steered) phased-array antennas was known during world war 1. The first use of the phased-array antenna in commercial broad casting transmission was in the early thirties and the first large steered directive array for the reception of transatlantic short-wave communication was developed and installed by the  Bell Telephone Laboratories in the late thirties, In world war 2, the United States, Great Britain,and Germany all used radar with fixed phased-array antenna in which the beam wad scanned by mechanically actuated phase shifters. In the United States,this was an azimuth scanning  S-band fire control radar, the Mark 8, that was widely used on cruisers and battleship's, and the AN/APQ-7 ( Eagle) high-resolution navigation and bombing radar at X band that scanned a 0.5 (deg) fan beam over a 60(deg) sector in 1 1/3 seconds. The British used the phased array in two height-finder radars, one at VHF and the other at S band.The Germans employed VHF radars with fixed planer phased arrays in significant numbers. One of these called the Mammut, was 100 ft  wide and 36 ft high and scanned a 10(deg) beam over a 120(deg) sector                                            

                                                                                                               

A major advance in phased-array technology was made in the early 1950"s with the replacement of mechanically actuated phase shifters by electronic phase shifters. Frequency applied.in one angular coordinate was the first successful electronic scanning technirqe to be applied.In terms of numbers of operational radars, frequency scanning  has probably seen more application than any other electronic scanning method.The first major electronically scanned phased arrays that performed beam steering without frequency scan employed the Huggins phase shiftier which, in a sense, used the principle of frequency scan without the necessity of changing the radiated frequency.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

LENS ANTENNAS

•     The most common type of radar antenna is the parabolic reflection in one of its various forma the microwave paraboloid reflection is analogous to an automobile headlight or to searchlight mirror. The analogy of an optical lens is also found in radar.Three types of microwave lenses applicable to radar are  
• dielectric lens
•     metal-plate lenses

• lenses with nonnuifrom index of refractin                                                                                       

     Dielectric lenses. The homogeneous, solid, dielectric-lens antenna of  is similar to the conventional optical lens. A point at the focus of the lens produces a plane wave on the opposite side of the lens. Focusing action is a result of the difference in the velocity of propagation inside the dielectric as compared with the velocity of propagation in air. The index of refraction n of a dielectric is defined as the speed of light in free space to the speed of light in the dielectric medium. It is equal to the square root of the dielectric constant. Materials such as polyethylene, polystyrene, plexiglas, and Teflon are suitable for small microwave lenses. They have low loss and may be easily shaped to the desired contour. Since the velocity of propagation is greater in air than in the dielectric medium, a converging lens is thicker in the middle than at the outer edges, just as in the optical case.

One of the limitations of the solid homogeneous dielectric lens is its thick size and large weight.Both the thickness and the weight may be reduced considerably by stepping or zoning the lens. Zoning is based on the fact that a 360(deg ) change of phase at the aperture has on effect on the aperture phase distribution. Starting with zero thickness at the edge of the lens the thickness of the dielectric is progressively increased toward the lens axis as in the design of a normal lens. However, when the path length introduced by the dielectric is equal to a wavelength, the path in the dielectric can be reduced to zero without altering the phase across the aperture.The thickness of the lens is again increased in the direction of the axis according to the lens design until the path length in the dielectric is once more 360(deg), at which time another step may be made. The optical path length through each of the zones is one wavelength less than the next outer zone.

 

      Artificial dielectrics    Instand of using ordinary dielectric materials dielectric materials for lens,it is possible to construct them of artificial. The ordinary dielectric consists of molecular particles of microscopic size, but the artificial dielectric consists of discrete metallic or dielectric particles of macroscopic size. The particles may be spheres, disks,or rods imbedded in a material of low dielectric constant such as polystyrene foam. The particles are arranged in some particular configuration in a three-dimensional lattice. The dimension of the particles in the direction parallel to to the electric field as well as the spacing between particles should be   small compared with a wave length.If those  conditions are met, the lens will be insensitive to frequency.When the particles are metallic spheres of radius a  and spacing s between centers, the dielectric of the artificial dielectric is approximately assuming no interaction between the spheres. An artificial dielectric may also be constructed by using a solid dielectric material with a controlled pattern of voids. This is a from of Babinet inverse of the more usual artificial dielectric  composed of particles imbedded in a low-dielectric-constant material. The voids may be either spheres or cylinders,but the latter are easier to machine.

    Metal-plate lens     An artificial dielectric may be constructed with parallel-plate wave guides.The phase velocity in parallel-plate wave guide is grater than that in free space:  hence the index of refraction is less than unity.This is opposite to the usual optical refraction medium. A converging metal-plate less is therefore thinner at the center than at the edges, as opposed to a converging dielectric lens which is thinner at the edges .The metal-plate lens is an E-plane lens since the electric- field vector is parallel to the plates.Snell"s law is obeyed in an  E-plane lens,and the direction of the rays through the lens is  governed by the usual optical laws involving the idex of refraction.The surface contour of a metal-plate lens is in general,not parabolic as in the case of the reflector.For example, the surface closest to the feed is an ellipsoid of revolution if the surface  at the opposite face of the lens is plane                                                                                                                                        

Even with an index of refraction in the vicinity of 0.5 to 0.6 the thickness  of the metal-plate lens becomes large unless inconveniently long focal lengths are used.The thickness may be reduced by zoning just as with a dielectric lens.The bandwidth of a zoned metal-plate lens is larger than that of an un zoned the gain, and increase the sidelobe level.An example of an X-band metal-plate zoned lens                                                                                                                                                             

Another class of metal-plate lens is the constrained lens or path-length lens, in which the rays are guided or constrained by the metal plate. In the H-plane metal -plate constrained lens, the electric field is propagates through the plates is relatively unaffected provided the plate spacing is greater than     The direction of the rays if the not affected by the refractive index. and Snell"s law dose not apply .Focusing action is obtained by constraining the waves ti pass between the plates in such manner that the path length can be increased above that in free space. In one type of cylindrical constrained lens with the E field parallel to the plate ,A 1 beam could be scanned over a 100(deg)sector by positioning the line feed.The lens was 72 wavelength in size had. and operated at a wavelength of 1.25 cm

Evaluation of lenses as antennas.   One of the advantage of a lens over a reflector antenna is the absence of aperture blocking. Considerable equipment can be placed at the focus of the lens with out interfering with the resultant antenna pattern.The first monopulse radars used lenses for this purpose, but with time the monopulse RF circuitry  was reduced in size and the reflector antenna came to be preferred over the lens or the homogeneous sphere can beam  over a wide angle. The lens is usually less efficient than comparable reflector antennas because of loss when propagating through the lens medium and the reflecting from the two lens surface.In zoned lens there will be additional, undesired scattering from the steps The lack of suitable  solid or artificial dielectric materials has limited the development of lenses.The problem of dissipating heat from large dielectric lenses can sometimes restrict their use to moderate-power or to receiver applications.Conventional lenses are usually large and heavy, unless zoned.To reduce the loss caused by scattering from the steps in a zoned lens, the ratio of focal length Fto the antenna diameter D must be made large f/d ratio. The mechanical support of a lens is usually more of a problem than with a reflector                                                                          

The wide- angle scanning capability of a lens would be of interest in radar as a competitor for a phased array if there were available a practical means for electronically switching the transmitter abd receiver among fixed feeds so as to achieve a rapidly scanning beam 

ANTENNA PARAMETERS

                         

            The purpose of the radar antenna is to act as a transducer between free-space propagation and guided-wave (transmission-line) propagation.The function of the antenna during transmission is to concentrate the radiated energy into a shaped beam which points in the desired direction in space. On reception the antenna collects the energy contained in the echo signal and delivers in to the receiver.Thus the radar antenna is called upon to fulfill reciprocal but related roles.In the radar equation derived these two roles were expressed by the transmitting gain and the effective receiving aperture.The two parameters are proportional to one mitting gain and the effective receiving aperture implies a large transmitting gain another.An antenna with a large effective receiving aperture gain.            

        The large apertures required for long-range detection results in narrow beam widths, one of the prime characteristics of radar.Narrow beam widths are important if accurate angular measurements are to be made or if targets close to one another are to be resolved.The advantage of microwave frequencies for radar application is that with apertures of relatively small physical size, but large in terms of wavelengths, narrow beam widths can obtained conveniently                                                             

        Radar antennas are characterized by directive beams which are scanned, usually rapidly.The parabolic reflector, well know in optics has been extensively employed in radar. The vast majority of radar antenna use the parabolic reflector, in one from or another. microwave lenses have also found some radar application,as have mechanically rotated array antennas.The electronically scanned phased array, is an antenna with unique properties that has been of particular interest for radar application                                                                                                                                                   

        The radar antenna will be considered either as a  transmitting or a receiving  device,depending on which is more convenient for the particular discussion.Results obtained for one may be readily  applied to the because of the reciprocity theorem of antenna theory.

         Directive gain     

•   Power gain 
• Effective aperture
• polarization 
• Sidelobe radiation 
• Aperture efficiency          
  

MODULATORS

                                                                                                                                                                    The function of the modulator is to turn the transmitting tube on and off to generate the desired wave from. When the transmitted wave from is a pulse, the modulator is some times called a pulser. Each RF power tube has its own peculiar characteristics which determine the particular type of modulator to be used.The magnetron modulator, for instance, must be designed to handle the full pulse power. On the other  hand, the full power of the klystron and the traveling-wave tube can be switched by a modulator handling only a small fraction of the crossed-field amplifier (C F A) is often cathode-pulsed, requiring a full-power modulator. Some CA F s are d-c operated, which means they can be turned on by the start of the RF pulse and turned off by a short, low-energy pulse applied to a cutoff electrode. Some C A F s can be turned  on and off by the start and stop of the RF pulse, thus requiring no modulator at all. Triode and trtrode grid-controlled tube may be modulator by modulated by applying a low-power pulse to the grid. plate modulation is also used when the radar application cannot tolerate the inter pulse noise that results from those few electrons that escape the cutoff of the grid                                                                                                                                                               

        Energy from an external source is accumulated  in the energy- storage element at slow ate during the inter-pulse period. The charging impedance limits the rate at which energy can be delivered to the storage element. At the proper time, the  switch is closed and stored energy is quickly discharged through the load, or RF tube, to  from the pulse.During the discharge part of the cycle, the charging impedance prevents energy from the storage element from being dissipated in the source.                        

        Line-type modulator  . A delay line, or pulse-forming network (P F N),is sometimes used as the storage element since it can produce  a rectangular pulse and can be operated by a gas- tube switch.This combination of  delay -line storage element and gas-tube switch is called a line-type modulator It has seen wide application in radar because of its simplicity, compact size, and its ability to tolerate abnormal load conditions such of magnetron sparking.                                                             

      The charging impedance is shown as an inductance. The pulse-forming network is usually a lumped-constant delay line.It  might consist of an air-core inductance with taps along its length to which are attached capacitance to ground.A perfect match is not always possible because of nonlinear impedance characteristic of microwave tubes.A gas tube such as a thyratron or ignitrn is capable of handling high power and presents alow impedance when conducting.However a gas tube cannot be turned off it has been turned on unless the plate current is reduced to a small value.The switch initiates the start of  the  modulator pulse by discharging the pulse-forming network,and the shape and duration  of the pulse are determined by the passive circuit elements  of the pulse-forming network. Since the trailing edge of the pulse depends on how the pulse- forming network discharges into the nonlinear load, the trailing edge usually not sharp and it may be difficult to achieve the desired pulse shape.                                                                                                                                         

 

   

 The charging inductance L ch and the capacitance C of the pulse-forming network forming  a resonant circuit, whose frequency of oscillation approaches .(The inductance of the pulse-forming network and the load are assumed small.) With a d-c energy source the pulse repetition frequency Fp will be twice the resonant frequency if the thyratron is switched at the peak of maximum voltage. This method of operation, ignoring the effect of the charging diode, is called d-c resonant charging. A disadvantage of d-c resonant charging is that the pulse repetition frequency is fixed once the value of the charging. inductance and the pulse-forming-network delay-line capacitance permits the modulator to operated at any pulse repetition frequency less than that determined by the resonant frequency F o.    

Hard-tube modulator :The hard-tube modulator is essentially a high-power video pulse amplifier.It derives its na,e from the fact that the switching is accomplished with "hard vacuum" tubes rather than gas tubes.Semiconductor devices such as the S C R (silicon controlled rectifiers) can also be used in this application.Therefore, the name active-switch modulator is sometimes used to reflect the that the function of a hard-tube modulator can be obtained without  vacuum tubes. Active-switch pulse modulators can be cathode pulsars that control the full power of the RF tube but with little current, or grid pulsars   that operate at a far smaller voltage than of the RF beam                                                        

Tube protection ; power tubes can develop internal flash  arcs with little warning even though they are of good design. When a flash arc occurs in an u n protection  tube, the capacitor bank discharges large currents through the arc and tube can damaged.One method for protecting the tube is to direct the arc-discharge currents with a device called an electronic crowbar. It places a virtual short circuit across the momentary short-circuit conditions.The name is derived from the analogous action of placing a heavy conductor, like a crowbar directly across the  capacitor bank . Hydrogen  thyrateons, ignitions and spark-gaps have been used as switch.The sudden surge of current due to a fault in a protected power tube is sensed and the crowbar switches.the line-type modulator does not usually require a crowbar  since it stores less energy than the hard- tube modulator and  it is designed to discharge safely all stored each time it is triggered.

TRAVELING-WAVE-TUBE AMPLIFIER

        The traveling wave tube (T W T) is another example of a linear-beam,or O-type, tube.It differs from the Klystron  amplifier by the continuous interaction of the electron beam and the RF field over the entire length of the propagating structure of the traveling -wave tube rather than the interaction occurring at the gaps of a relatively few resonant cavities. The chief character-is tic of the TWT of interest to the radar system engineer is its relatively wide bandwidth. A wide bandwidth is necessary in applications where good range-resolution is required or where it is desired to avoid deliberate jamming or mutual interference with near by radars. Although low power T W Ts are capable of octave bandwidths of the order of 10 to 20 precent are more typical at the power levels required for long-range radar applications.The gain, efficiency,and power levels of T W Ts are like those of the klystron :but,in general,their values are usually slightly less than can be obtained with a klystron of comparable design.

      A diagrammatic representation of a traveling-wave tube.The electron optics is similar to the klystron. Both employ the principle of velocity modulation to density-modulate the electron beam current.Electrons emitted by the cathode of the traveling wave tube are focused into a beam and pass through the RF interaction circuit known ass the slow-wave structure, or periodice delay line.An axial magnetic field is provided to maintain the electron-beam focus as in the klystron. A shadow grid to pulse-modulate the beam can also be included.After delivering their d-c energy to the RF field, the electrons are removed by the collector electrode. 

      The simple helix was used as the slow-wave structure in the early T W Ts and is still preferred  in traveling-wave tubes at power levels up to a few kilowatts. It is capable of wider bandwidth than other slow-wave structures.but its power limitations do not make it suitable fore most high-power radar applications.A modification of the helix known as the ring -bar circuit as has bean used in T W Ts to achieve higher power and efficiencies between 35 and 50 percent.The Raytheon  Q K W -1671 A,which utilizes a ring -bar circuit,has a peak power of 160 kW,a duty cycle of 0.036,pulse width at L band.This tube is suitable for air -search radar similar T W Ts have bean used in phased -array radar.The Air Force Cobra Dane phased-array radar.

        The ring-loop slow-wave circuit which consists of equally spaced rings and connecting bars,is also related to the helix and the ring-bar.It is chained to be preferred for tubes in the power range from 1 to 20 kW, as for lightweight airborne radar or as drivers for high-power tubes. The ring-loop circuit is not bothered by the backward-wave oscillations of the ordinary helix or the "rabbit ear" oscillations which can appear in coupled-cavity circuits.

         The helix  has bean operated at high average power by passing cooling fluid through a helix constructed of copper tubing The bandwidth of this type of fluid-cooled helix. TWT con be almost an octave. and it is capable of several tens of kilowatts average power at L band with a duty cycle suitable for radar applications.

        A popular from of slow-wave structure for high-power TWTs is the coupled-cavity circuit.It is not derived from the helix as are ring-bar or ring-loop circuits.The individual unit cells or the coupled -cavity  circuit resemble the ordinary klystron resonant cavities.There is no direct coupling between the cavities of a klystron :but in the traveling wave tube coupling is provided by along slot in wall of each cavity.The coupled-cavity circuit is quite compatible with the use of lightweight PPM focusing, a desired feature in some air borne applications.

       Although the TWT and the klystron are similar in many respects, one of the major differences between the two is that feedback along the sloe-wave structure is possible in the TWT, but the back coupling of in the klystron negligible. The attenuation may be distributed , or it may be lumped:but it the usually the middle third of the tube. loss introduced  to attenuate the backward wave also reduces the  power of the forward wave which results in a loss of efficiency. This loss in the forward  wave can be avoided by the use of  discontinuities called severs, which are short internal terminations designed to dissipate the reverse-directed power without seriously afficiency.                                            
This loss in the forward wave can be avoided by the use of discontinuities called severs, which are short internal terminations designed to dissipate the reverse-directed power without seriously affecting the forward power.The number of severs depends on the gain of the tube: one sever is used for each 15 to 20 dB of gain.in addition to reflection-type oscillations, backward-wave oscillations can occur. These frequently occur outside the passband so that they can be reduced by loss that is frequency selective.

 in principle, the traveling-wave tube should be capable of as large a power output as the klystron. The cathode,RF interaction region, and the collector  are all separate and each can be designed  to perform their required functions independently of the others.In practice, however, it is found that there are limitations to high power. The necessity for attenuation or severs in the structure, as mentioned above, tends to make the traveling-wave tube less efficient than the klystron. The slow-wave structure can also provide a limit to TWT capability. It seems that hose snow- wave structures can also provide a limit to TWT capability. It  seems that those slow-wave structures best suited for broad band width (like the helix) have poor power capability and poor heat dissipation. A sacrifice in bandwidth must be made if high-power is required of a TWT. If the bandwidth is too small, however. there is little advantage to be gained with a traveling-wave tuba as compared with multi cavity keystrokes.

NON COHERENT MTI

   

   The composite echo signal from a  moving target and clutter fluctuates in both phase and amplitude.The coherent MTI and the pulse -doppler radar make use of the phase fluctuations in the echo signal to recognize the doppler component produced by a moving target. In these systems,amplitude fluctuations are removed by the phase detector.The operation of this type of radar, which may be called coherent MTI, depends depends upon a reference signal at the radar receiver that is coherent with the transmitter signal                                                                                                           

It is also possible to use the amplitude fluctuations to recognize  the doppler component produced by a moving target. MTI radar  which uses amplitude instead of phase fluctuations is called noncoherent .It has also been  called externally coherent, which is a more descriptive name.The non coherent MTI radar dose not require an internal coherent reference signal or a phase detector as dose the coherent from of MTI. Amplitude limiting cannot be employed in the noncoherent MTI receiver, else the desired fluctuations would be lost.Therefore the IF amplifier must be linear, or if a large dynamic range is required , it can be logarithmic. A logarithmic gain characteristic not only provides protection from saturation, but it also tends to make the clutter fluctuations at its output more uni from  with variations in the clutter input amplitude detector. The detector following the IF amplifier is a conventional amplitude detector. The phase detector is not used since phase information is of no interest to the non coherent radar.The local oscillator of the oscillator of the noncoherent radar dose not have to be as frequency-stable as in the coherent MTI. The transmitter must be sufficiently stable over the pulse duration to prevent beast between overlapping ground clutter, but this is not as severe  a requirement as in the case of coherent radar.                                                                                          

The output of the amplitude contained in the amplitude fluctuations may also be detected by applying component of the amplitude   detector to an A-scope,Amplitude fluctuations due to doppler produce a butterfly modulation similar to that, but in this case, they ride on top of the clutter echoes.Except for the inclusion of means to extract the doppler amplitude component, the noncoherent MTI block diagram is similar to that of a conventional pulse radar.                                                                                    

The advantage of the noncoherent MTI is its simplicity: hence it is attractive for those applications where space and weight are limited. Its chief limitation is that the target must be in the presence of relatively large clutter signal if moving -target detection is desired . The clutter echoes of serves the same function as dose the reference signal in the coherent MTI. If clutter were not present, the desired target would not be detected. It is possible, however, to provide a switch to disconnect the noncoherent. MTI operation  and revert to normal radar whenever sufficient clutter echoes are not present.If the radar is stationary, a map of the clutter might be stored in a digital memory and used to determine when to switch in or the noncoherent                                                                                           

MTI    :   The improvement factor of a noncoherent MTI will not, in general, be as good as can be obtained with a coherent MTI that employs a reference oscillator (coho) The reference signal in the noncoherent case is the clutter itself, which will not be as stable as a reference oscillator because of the finite width of the clutter spectrum caused by its own internal motion. If a nonlinear IF amplifier is used. it will also limit the improvement that can be achieved