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Before seeing about the antenna lets refresh our memory on the EM wave or called as the Electro Magnetic wave , now consider that a electrical wave is transmitted in the X-Axis and a Magnetic wave is transmitted in the Y-Axis , then a wave perpendicular to both the X and Y axis is generated containing the property of both the Electric and Magnetic component called as our EM wave , since further discussion may drag into equations let me stop here. Examples of EM waves include : Light,IR,Alpha,Beta,Gamma,X-Ray,Micro Wave, LF,HF,MF,UHF??..etc. Radar Frequency What the hell is frequency and why it is being used so often ? The answer to that my friends is a simple equation Frequency=Velocity of Light/Time taken for one cycle. What is a cycle ? This is definitely not our bicycle , since one cannot say to what extent time can go , all the signals are restricted in a graph between 0 to 2 Pi which called as a cycle. Why use the Term frequency ? The answer here is ?Just for easy computation? , ie to represent a time signal in a graph is tedious , thus we use a graph called as ?Semi Log? Graph where the entire spectrum can be easily plotted if frequency is used as a parameter. Is Defense Technology always so tough to understand ? Unfortunately yes , but let me try to make it simpler for you. Conventional Radars generally operate in what is termed as the ? Micro Wave region? , which is a term not rigidly defined. Operational Radars in the past have been from 100 MHz to 36 GHz , which covers more than 8 octaves but these are not necessarily the limits since HF OTH Radars operate at frequencies as low as a few MHz and on the other hand millimeter Radars have been at frequencies as high as 240 GHz. During World War-II , the letters X,S and L were used to designate the distinct frequency bands being used to maintain secrecy but latter became an convenient term but the International Telecommunication Union [ ITU ] has specified frequency ranges within with the Radars must be designed and operated , these are The Ranges are Band Designation--------Nominal Frequency Range-------Specific Range by ITU HF------------------------3-30 Mhz VHF--------------------- 30-300 MHz-------------------------138-144 MHz UHF---------------------- 300-1000 MHz--------------------- 216-225 MHz L --------------------------1-2 GHz ----------------------------1215-1400 MHz S --------------------------2-4 GHz---------------------------- 2300-2500 MHz C-------------------------- 4-8 GHz ----------------------------2700-3700 MHz X-------------------------- 8-12 GHz---------------------------- 8500-10,600 MHz Ku -------------------------12-18 GHz --------------------------13.4-14.0 GHz 15.7-17.7 GHz K ---------------------------18-27 GHz-------------------------- 24.05-24.25 GHz Ka --------------------------27-40 GHz --------------------------33.4-36 GHz V ---------------------------40-75 GHz ---------------------------59-64 GHz W---------------------------- 75-110 GHz------------------------- 76-81 GHz 92-100 GHz mm ---------------------------110-300 GHz---------------------------- 126-142 GHz 144-149 GHz 231-235 GHz 238-248 GHz The Different Antenna Parameters which are involved are 1} Directive Gain: Gain is the measure of the ability of the antenna to concentrate the transmitted energy in a particular direction , Thus the Directive Gain of the Antenna is given by , Gd = Maximum Radiation Intensity / Average radiation Intensity Needless to say this parameter must be high for any antenna . 2} Power Gain : Its is same as Directive gain , but the difference here is that the Power gain takes into account the Dissipative losses in the antenna Thus Power gain can be given as G= Maximum Radiation Intensity from the subject antenna/ radiation intensity from a lossless isotropic radiator with same power output. 3} Antenna Radiation Pattern : This forms a very, very important part of any antenna since the Antenna is has one Main Beam and Vestigial beams of the main beam , these vestigial beams form the side lobes which affect the illumination pattern of the antenna thus drastically reducing its performance. 4} Effective Aperture : The Effective aperture of an antenna is the measure of the effective area presented to the incident wave by the antenna. 5} Side Lobe Radiation : Side Lobe radiation is the radiation from the antenna that is not radiated by the main beam . It is possible in theory to have a single main beam but it is impractical and impossible to achieve [ since the main lobe will be exceptionally large] , in general lower the side lobes , lower will be the antenna gain and the aperture efficiency and greater will be the width of the main beam. 6} Aperture efficiency : It is the measure of the energy dissipated as the signal travels through the antenna . 7} Polarization : Polarization of an EM wave is defined as the orientation of the Electric Field and most Antennas are Linearly Polarized , Air-Surveillance Radar are generally Horizontally Polarized and Tracking Radars are vertically polarized , there is also circular and elliptic polarization but these are rare cases. Types Of Antennas: a) Dipole antenna : This antenna is assumed to be of a electrical dipole (ie two separate charges ) and as you can notice it has zero directivity and thus is never used (well Marconi used this anyway) Example : Your Metal Table Spoon can act as a Dipole Antenna Yagi-UDA : The most famous antenna in INDIA (remember TV antenna anyone ??the 3 metal tubes ,well you gussed right) .The middle tube is the driven element which converts electrical energy into EM waves (not in MW spectrum) THE long tube is what is called as the reflector which as the name suggests in phase opposing to the incoming EM waves from center thus pushing the waves in one direction .The director (small tube) pulls the EM waves and transmits it (it is in phase with the EM waves from the driven element) Thus this YAGI ?UDA provides high directivity and forms the basis of future antennas (don?t under estimate this one even the PH uses millions of microscopic yagis) Example : First used in LUFTWAFFE to shoot down enemy bombers flying in night and now used to receive DD in INDIA as the famous TV antenna. c)Reflector Antennas: Now why a Parabola? The answer is any waves from the focal point of the parabola travels in a line parallel to the bola ,thus providing high directivity ,high polarization also can be used at MW freq(note that the important part in Parabolic reflector (example : dish antennas anyone!!!) is not the Dish but the minute small point protruding is the actual antenna and the big dish is just a reflector!!!) .because of it?s high power coupling ,directivity is still used in many defense radars especially the low cost and the mobile ones(remember the antenna on TRISHUL vehicle and tungusca ,that is this type) These are also called as the Parabolic Reflectors , the parabolic surface is illuminated by a source of Radiated Energy called as the FEED and based upon the feeds there are different types of antennas like --------Parabolid : In this Rotating Curve type the parabola rotates around its axis producing a circular parabola or specifically a parabolid , when properly illuminated by a by the source at the Focus , the parabolid generates a nearly symmetrical pencil-shaped beam and is the most popular antenna for tracking Radars. Ex: Signal LW08 2D [ Jupitor ] L- band Fan- Beam air and surface surveillance Radar has an elliptical shaped fan beam antenna fed by a horn . The reflector surface is a mesh , the horizontal antenna mounted on the Top is for IFF. At the middle Right can be seen a fin which is added to the back of the Antenna to counter-balance the wind forces on the Reflector in the position of worst yawing moment. [B]------------Cassegrain Type: This is a Dual ?Reflector antenna with feed near or at the vertex of the parabola rather than is at its focus. The larger reflector at or near the vertex has a parabolic contour and the sub reflector has a hyperbolic contor. The chief advantage of the Cassegrain type is that the feed at or near the apex of the parabola does away with the need for long transmission out to a feed at the normal focus of the parabola , further more it allows grater flexibility in size of the feed system , it is very popular for mono pulse tracking radars and a very , very effective type also. -------------Parabolic Cylinder: Another method of achieving asymmetric radiation pattern is to use a parabolic cylinder . This antenna surface is generated by moving the parabolic contour parallel to itself . A line source such as a linear array , located at the focus of the cylinder is used to illuminate the parabolic cylinder reflector . The beam shape and the beam width in the plane containing the linear feed can be determined by the illumination of the line-source feed , while the beamwidth in the perpendicular plane is determined by the illumination across the parabolic profile. An advantage of the parabolic cylinder is that large number of individual radiators on its linear array feed provides more control of the aperture illumination than does a single point source feeding a parabolid The parabolic cylinder can also generate an asymmetrical fan beam with a much larger ratio of two orthogonal beam widths than a section of the parabolid . Aspect Ratios more than 8:1 are practical here Example : The US Marine AN/TPS-63 air surveillance radar. Now lets move on to Jets [ yes now we are taking ] PATCHED/Slotted Array antenna: This type is based on the Micro Wave law which that states that ?any hole in a conductor which is negligible when compared to the dimensions of the conductor radiates a Micro Wave Signal? ,thus these kinds of antennas will contain arrays of microscopic slots (slotted waveguide: remember used in lca) arranged in form of a neat array (the slots are precise to a nm) is used to transmit in Micro Wave Spectrum and the receiver is also of the same design (in form of a mesh) thus they are also called as meshed antennas there are three types in this arrangement a) inter digitized Overlay c) Matrix PHASED ARRAY ANTENNA [PA] : IS The phased array is the future? Why, what?s the big fuss about it anyway? Well here are the answer?s the PA(phased array ) is similar to the patched antenna but with a major difference,. For simplicity reasons let?s consider a 5*5 array ,if it were a PA the rows (1,0-1,5) will be connected to phase shifter of say 0 degrees the next (2,0-2,5) to PS of 30 deg ..and so on .now during actual working the different the arrays can transmit in different phase?s thus causing it to,, well a phenomenon described as ?steering? thus what it does is without the jet having to turn ,the beam can itself turn to almost +- 60 deg with respect to the nose in all dimensions and axis now I hope you guys can understand the significance of the PA. In general the advantages are ---Agile , Rapid Beam steering ---Potential for large peak and large average power , since each element can have its own transmitter the power aperture product can be very large at low frequencies ---Multiple Target Tracking ---Convenient method to employ solid state transmitters in case of Series-Series feed developed by the Americans [ more on these to follow] ---Convenient shape for flush mounting or blast hardening. ---Control of aperture illumination ---Lower RCS [ in case of AESA] ---Multifunction Capacity Comparison of Russian and American Phased Array with a detailed look on AESA Vs PESA debate. Contrary to popular beliefs here that Russians can develop and AESA antenna, we as signal processing engg seem to think otherwise, why? since a detailed discussion in the comparison of Russian Vs American radars must cover the following topics -------Radiation Patterns of Phased Array -------Beam Steering and Array Feed Network -------Phase Shifters, in which we have a) Diode Phase Shifter Ferrite Phase Shifter Under Ferrite we have Reggia-Spencer, Latching Ferrite, Flux Drive, Dual-Mode, Polarization-insensitive shifters --------Frequency Scan Arrays --------Forms of Frequency Scan Array --------Radiators for Phased Array --------Architecture for Phased Array -------Finally we have Radiation pattern synthesis For our discussion let me limit with the Architecture for Phased Array Architecture for Phased Array The Term "Architecture" is often used in the discussion of Phased Array radars but it has not been officially defined and there does not seem to be universal agreement as to what it encompasses, thus I am using the Term to include the various ways the Phased Array Radars can be configured or Structured. An extremely important part of any practical phased array system architecture is the means by which the power from the Transmitter is efficiently divided and distributed to the Radiating Elements and the reciprocal problem of combining the signals received after the elements and providing them to the receiver and signal processor. The structure that performs this function is called the ARRAY FEED. Two major methods for these were developed which were -------Constrained Feed developed by United States -------Space Feed developed by the Evil Empire We also have -------Multiple Beam forming arrays, Active aperture arrays and Digital beam forming array which were basically offshoots of the constrained feed array. Lets bring it on then, but before this both side tried their hands on each other feeds but neither had much of any success in each other but Russians had the edge with their space feed arrays till of course 1987 when things stared crumbing apart and now their plight is pathetic Constrained feed: The constrained feed is basically a 1 * N power divider where N= Total number of elements in an Array , now each of the element has its own Phase Shift .The required phase shift to steer the beam in two dimensions can be determined for each element by the Beam Steering computer and then distributed to each phase shifter .Alternatively a small computation chip is to be placed at each element to computer the Phase required at the particular element , based on being told the azimuth and elevation angles to which the beam is being steered. Feeds consisting of co-axial or Waveguide transmission lines can handle higher power with low loss and can be constructed with excellent precision. they can on the other hand be expensive , the most excellent region of these types of feeds is the L-Band where strip line scan be precisely fabricated when used with very,very,very precise and accurate manufacturing skills of course which then again push up the cost. Now the power distribution in each column is distributed with a series feed to he vertical elements, this is called Parallel-Series feed. If the columns were fed with a Parallel feed then it would be a Parallel-Parallel feed, there can also be Series-Series feed, Series feed technique is used in Active Aperture array where the Transmitters and receivers are located between the feed and the Radiators but they have narrower bandwidth than parallel feed. Brick and Tile assemblies :Brief mention should be made of two Types of array architecture , one is called as the Brick and the other is called as the Tile , These terms relate to the manner in which the array is constructed in relatively larger sections to make assembly easier .These are used with a corporate feed .The array is often grouped into sub arrays of rows , columns or areas with each sub array fed separately , Brick Construction: In Brick construction the array is assembled with circuits on boards that are mounted perpendicular to the Array face Tile Construction: In tile construction which is also called as monolithic array construction , the array is assembled with one or more layers parallel to the array fee. Brick uses greater depth than Tile , so it allows more room for circuits , better thermal dissipation and more easier maintenance. Tile has the advantage of being thin so that it can be made to conform on Aircraft or Missiles . It can also be folded and stowed for erection in space and it can be compatible with Robotic and other automatic means of fabrication , generally these have more interest at Higher frequencies than at lower frequencies. Monopulse Beams: When multiple beams are need for monoplule angle measurement with a constrained feed phased array , the output of each receiving element can be split into three separate outputs that connect to three separate outputs that connect three separate beam forming networks . One output is used with a beam-forming system to provide the sum beam and the other output are used with separate beam formers to generate angle beams . Two angle beams can have different aperture illuminations than the sum beam so as to produce desirable difference patterns with low side lobes and a good error-signal slope. Lets Go to Mother Russia Space Feeds: A space feed is similar to the feed of a reflector antenna .It enjoys the advantage of the relative simplicity that characterizes feed for reflectors .There are two types of space feeds depending on whether the array is analogous to lens or to a reflector and also called the Space Feed Array as a Phased array is also not doing justice since Space feed arrays have the same construction setup as the slotted array and must always be termed as Pseudo Phased Array but slotted array can be easily converted to a Phased Array by using this technique , thus an Active Aperture can never be implemented by this technique Lens Arrays: Although I speak about lens , much of what I say is applicable to reflect array as well. The lens is fed just as would as a lens antenna. The primary feed might be a collection of single Horns or a monopulse cluster of horns. An array of antenna elements collects the energy radiated by the feed and passes it through phase shifters which provide a phase correction to convert the incident spherical wave to a Plane wave .The Phase shifters also apply the Phase shifts required to steer the plane wave to some angle off the broadside .The antenna elements on the opposite side of the lens array then radiate the array into space .The Feed Illumination the space feed array provides natural amplitude taper to produce lower side lobes than would be a uniform illumination. The feed may be placed off-axis to avoid reflections than the lens returning to the feed and producing a large VSWR. Reflect arrays: The energy from the feed enters the antenna elements , passes through the phase shifters, is reflected and passes back through the Phase Shifters to be radiated as a plane wave in the desired direction . Because the energy passes through each phase shifter twice, a phase shifter need only be capable of half the Phase Shift needed for a lens array or a conventional array .The Phase shifters , however must be reciprocal .This can be a limitation since some ferrite phase shifters with excellent properties for use in Phased Arrays are non reciprocal and therefore cannot be sued in reflect array. Comparison: Spill over radiation from the feed of a space fed array an result in higher side lobes than for broadside than would be for constrained feed , unless some means are taken to minimize the spill over . Both sets of antenna elements at the front and back of the lens array require matching , this increases the matching probability of the lens array and can result in very low antenna efficiency , however it is relatively straight forward for space feed horns to those used for monopulse angle tracking with a reflector antenna , compared to constrained feed space feed has lower loss but space feed is not a True Phased array either. Subarrays: It is sometimes convenient to divide the array into sub arrays . These can simplify manufacture and assembly of the array , provide broader signal-bandwith and allow multiple transmitters to be used to obtain greater power . Each sub array could have its own transmitter and receiver , but it is not necessary to do so to utilize the sub arrays . It is also possible to give identical Phase steering commands to similar elements in each sub array to allow simplification of the beam steering unit. Becasue of the discrete nature of the sub arrays , the phase distribution across the aperture has the character of a stair-step, with one step over each sub aperture .This can result in what are called as quantization lobes . Such lobes can be reduced by either overlapping the sub apertures or inserting a small amount of randomization to the phases of the sub apertures .the term sun array also in past applied to the constrained array feed networks to produce the sum and difference for monopulse angle measurement. Sub arrays can achieve wide signal bandwith by a variable time -delay element at each sub array.The original Aegis (AN/SPY-1) phased array radar system for ship air defense employed 32 transmitters and 68 receiving sub arrays of different sizes.Each of the 32 transmit sun arrays has its own CFA ( Cross Field Amplifiers). Active Aperture Phased Array: ( AESA as you guys will know) The constrained feed technique can only be used with Active Aperture Phased array, the cost of the array depends on ------------Whether the amplitude Taper is applied in the beam former network or in the T/R modules ------------ Degree of reliability ------------Array is wide or narrow band Also we can see that the distributed architecture of the active aperture can smooth the effect of Pulse-To-Pulse amplitude and phase variations introduced by the RF power source and therefore increase the MTI improvement factor and obtain better detection of moving targets in the clutter also a large power supply is not required in Active arrays. The proponents of Passive array will argue that the passive array is very cheap [ When used with a Space fed network since for these fabrication cost is very,very less] and also the high cost of T/R modules , require very high signal processing capacity .........etc will make the Passive array a better option but as the cost of the platform shooting to more than $100 million and for stealth jets the only option is active or no Radar people have no option but to go for the Active Array anyway lets see the active array and compare it with Russian ones ACTIVE ARRAY: The first "modern" Phased array was the AN/FPS-85 satellite surveillance radar , which became operational at Eglin air force base, Florida in 1969, in some respects it can be termed as the First active aperture phased array in that it employed 5184 individual transmitter units , one at each of the radiating elements .it operates at UHF (centered at 442 MHZ) , separate transmitting and receiving arrays were used since it was cheaper to employ two arrays rather than one array with duplexer . The receiving aperture was larger than the Transmitting aperture and employed 19,500 receiving elements . Only 4660 of the elements in the receiving array were active ( had the receivers connected to them ) the rest were inactive and were Terminated . The receiving elements were arranged in a thinned , space-taper manner to reduce the number of receivers required while maintaining suitable side lobe level. The first all solid-state active aperture phased array was the AN/FPS-115 , more commonly known as Pave Paws . It also operated at UHF and was designed to detect submarines-launched ballistic missiles fired at USA. It has a secondary mission to perform space surveillance .Pave Paws employed 1792 active elements arranged in a circular aperture of 72.5 ft in dia , plus 885 dummy elements . The peak power per T/R module was 335 W , which produced a total peak power of 600 KW and an average power of 150 KW per face . A Pave Paws radar consisted of two faces to cover 240 degree in azimuth .Its range was estimated at 3000 nmi for a 7 m2 target. There were four operational Pave Paws radars in the United States one of these in Georgia was expected to be increased in capacity by 10 dB to replace the AN/FPS-85 , a larger version of the Pave Paws has also replaced the parabolic torus reflector antennas in the Ballistic Early Warning System ( BMEWS ) The THAAD radar is an Active Aperture radar designed for ballistic missile defense. It has also been known ass the GBR . Radars for Ballistic missile defense have to perform target Detection, acquisition, track , identification , discrimination and assessment if target kill. The THAAD GBR is an X-Band radar with 25,344 radiating elements each with its own gallium arsenide T/R modules . In Order to be able to operate with a wide-bandwith signal , the aperture is divided into 72 sun apertures , each containing 352 active elements . There is Time Delay steering element at each sub array to permit the use of wide band wave forms without distortion . The Array Aperture is 100 ft2 which is quite large aperture for an X-band Phased Array . Because there are so many of them , the T/R Modules are very important part of this radar . It is said that if $100n was saved in he T/R cost compounds cost corresponds to $2.5 million less for the entire array, The array alone weighs 46,000 pounds. A different solid-state active array for TMD is the Israeli L-band EL/M-2080 though the architecture is same as the Pave Paws it was developed specifically for Israeli requirements. The Active Aperture Array has also been considered for airborne radar and for ship borne applications though the radiating elements is between 1000-2000 for Air and 4000 to 8000 modules for sea based but a serious limitation of the Phased array is limited by the antenna coverage when kept in the nose of the nose of the aircraft , although a Phased Array is able to steer +or - 60 degrees , the main beam gain decreases , the beam broadens ,and the side lobes raise significantly [ trust you do not want the side lobes to increase in any jet let also a stealth jet] and this is practical to steer with mechanical steering but not with Phased Array , thus the Raptor two side arrays were installed on each side with fixed electronic steering but then again cost increases [ well around $5-$10 million for the AN/APG-77] , we shall see bit more about these in the finale of the essay. Comparison with a Russian System : As I had said before Russians employ different Technique than their American counter parts namely the Space Fed Array which has their own advantages and limitations but was their principle and policy true and what is the significance of their research Today ? Is Russian radars Comparable with its American counterparts ? What is their present state ? In Todays AWACS and Active Aperture Environment can the Russian system still potent as it was a Two Decades ago ? Well friends The answer to the questions was given by Mr David Barton who we all know is the First person to carry a full scale analysis of the Russian radar systems called as the Barton papers , Of course the Full Barton papers goes for about 2000 pages , thus I am Quoting exerpts from him , then again three radars caught my interest N001,N011,N011M and the Grill Pan , Mr barton considers the Grill Pan as the most advanced Russian radar and since N011 M has indian processors and his comparison was for the Actual Russian N011 series let me stick with the Grill Pan. Now the Russians have always adopted a different approach to radars since their requirements were different ,While Western countries focused on AWACS and AEW'S to establish Air Superiority over the Soviet air space , the Soviet Doctrine was to safe guard their Air Space and prevent intrusion?s rather than to establish Air-Superiority over NATO air space and as I said in my abstract erstwhile Soviet Union did have the lead in Air Defence systems when compared with NATO till they broke apart , now a Decade later Russians are at the cross roads since as seen by Gulf War and the Kosovo conflict the concept of Air Defence System is fat eroding and getting replaced by Counter Air Offensives since while their entire Radar industry is tuned to making air defence radars and systems they had never focused on Aircraft based systems and one of their biggest drawback is that the Russian Brain Child , the highly effective Space Fed Array is incapable of supporting an Active Aperture and Russians must start their research from scratch to focus on constrain feed arrays , still the Grill Pan is testimony of Russian Achievement in Russian Air Defence system which I will be covering in depth. The US Army?s C-band patriot and the US Navy?s S-band Aegis systems use multi function phased array radars that perform the various radar functions required for Air Defence within a single system operating with a single frequency band, but this represents a compromise since the optimum bands required for search and Track are different and to counter these effects the system must be clubbed with a very powerful signal processor which increases the cost of the systems exponentially , like the case of an Aegis equipped Ticondera class cruiser could cost anything upward of $ 1 billion but the Russians were not able to afford such high cost devices since Russia is a vast country and require many Air Defense systems and such an option will never have been feasible , thus they use separate radars for each function and the Radars operate at frequencies more suited for thier particular function and the main emphasis here was on Low cost approach and easy maintenance rather than on heavy performance , but this approach lead to the degradation of the research in Constrained feed array and even the Fighter plane radars started using the Space Feed Arrays which were ill suited for these jobs since they offered no advantage what so ever when compared with the Conventional Slotted array except that scanning is faster. The Russian Air defense system S300V ( NATO Designation SA-12 ) uses a 10,000 element X-Band Lens array for multiple tracking and weapon guidance . The NATO designation for this X-Band lens array is the Grill Pan , the low cost and low RF loss experienced by this system was due to , in part, to the separation of surveillance and tracking functions rather than being combined in one multifunction array, since Space Feed uses multimode monopulse horn feeds , so it did not experience the larger loss which a constrained feed would have .Faraday rotation dual mode ferrite phase shifters were used which operated to circular polarization .Rather than converting the normal linear polarization [ as done by constrained feeds] to circular before transmitting and back to linear while reception , individual antennas transmit and receive circular polarizations thus reducing the need for the dual mode phase shifters and enormous cost involved by supporting it with a powerful processors and for reception the array received orthogonal circular polarization , and to reduce cost further the phase shifters were non-receprocal and the Phase Shifters need not be reset since different antenna's is used for different purpose and the receive signal was orthognal to the Transmitted and this feature eliminated the need for a fast circular further reducing loss and cost nad also the receiver was partially isolated from the Transmitter due to orthogonal polarizations and the rugged electrostatic amplifiers further eliminated the need for duplexer or solid state receiver protector, further reducing the loss and more importantly the cost of the systems , though this approach works brilliantly for the Air Defence systems it is actually a non starter for Air Borne applications as will be discussed later. The Russian Phase shifters has two sections in series to provide 720 degree out of phase shift [ insted of the normal usual 360 phase shift ] . Each Section had its own control coil .One coil was for setting the phase required to steer in elevation and the other was for the phase to steer in Azimuth . All the row coils were in series to provide elevation steering . The 10,000 element array [ 100*100] required only 100 row drivers and 100 column drivers also as a cost cutting measure the above set up did not require logic circuits ,data buses or dc power buses needed in the aperture for determining the combined phase shift for steering in two angles , thus the total two way RF loss from the Transmitter to Receiver , excluding the propagation loss was 5 dB when compared with 10 dB found in Patriot [ the first version] and 12 dB for the Aegis [ the first version]. Coming to the disadvantages of the Russian space feed array technique , the first obvious dis-advantage is the lack of up gradation potential in such a system namely since the world is moving towards Active Aperture when un-fortunately the Space feed does not support and the Russian radar development has reached their end of development cycle , although Space Feed array has almost half the loss of the comparable western system , a decade later it still stays at 5 dB where as the PAC2 Patriot has a loss of 4 dB , the L band EL/M-2080 has less than 1 dB and the AN/APG-77 has a loss of less than .25 dB , this is primarily due to the fact that very low power is transmitted in the first place and MMIC can do all the signal processing jobs right at the Transmission and Reception end as discussed earlier , also loss of a single driver can cause the loss of entire row or column of elements , also due to the reason that low cost and simplicity is the principle behind the development of these Space Feed technique , low side lobes can never be achieved and as discussed in previous section can be about 10 times higher when compared with a Similar constrained feed which essentially made it a sitting duck for a ARM launch but then again the S-300V has a very high range [ which only recently has been offset by the ARROW of Israel ] and this practically makes any SEAD operation against the Grill pan a near impossibility which forced the development of Stealth Jets like the F-177 A Night hawk whose primary mission is to fire a ARM at the Grill Pan and the B-1B stealth bomber , though any offensive SEAD operations in the Future will be carried out by the Joint Strike Fighter , but these high lobal characteristics make the Space Feed arrays extremely vulnerable to easy hostile detection and Jamming especially when operating in Enemy Air space but the biggest draw back to the Space feed technique comes from the Constrained feed's Active aperture array, also since Space feeds are inherently a very efficient system , Russians have never invested in power signal processors , in which field the United States has a Twenty year lead to say the Least , which can be great problem when investigating constrained feed since the current Russian signal processors are incapable of handling such work load but on the Flip side the Slotted array of existing systems can be converted to Passive Aperture array for about a few hundred thousand dollars where as to go from Active Aperture V2 From Slotted V1 in the Eagle upgrade program costs upward of $ 1.5 Million [ But who said constrained feed was cheap ? ] The X-Band Grill Pan described above was the Target Tracking and guidance radar for the SA-12 air defense system . Air surveillance was performed with the S-Band Bill Board radar which is a scanning beam 3D radar using a phase-scanned array with slotted waveguide radiators . The Array could be stowed for transport in a short time of one minute .The SA-12 also employed a separate sector search Radar for Detection of Ballistic missiles .The Russian SA-10 , a similar Air defense system also deployed on tower a horizon search Radar called as Clam Shell for detection of Low-Altitude targets . Thus according to Barton , " The Russian radar systems is in a cross roads , in one hand all their slotted array waveguide radiators could be easily converted to Passive Aperture Phased Array like the N011M but this is the extent to which Space Feed could be developed , thus the Russians can either start research on Constrained Feed from scratch or can develop their Space Feed Array Further , either way the Russians do seem to have made up their mind and their outstanding and in many cases ingenious Techniques are getting overtook by the Active Aperture in Air Defense systems which the Former Soviet union had the edge in" Finally the AESA Vs PESA Antenna debate What is a AESA Antenna ? AESA stands for Active Electronically Scanned Array , in other words Active Elements are used in the construction of the Antenna , ie active elements are the elements which can induce gain , for example in the AN/APG-77 Radar consists of Total elements and T/R?s as 600,000 , note that only Active elements can induce gain , in other words a AESA antenna is in all aspects a ?Smart Antenna? What is a PESA Antenna ? PESA stands for Passive Electronically Scanned Array , in other words just like other antenna elements are made up of passive elements with the Phase Shift Being provided by the Phase Shifters [ like Ferrite Phase Shifters] , note that Passive elements cannot produce gain and thus it is up to the Tubes to provide the Required Amplification. What is a tube ? Tubes are MW amplifiers ,in these tubes electrons recieve potential energy from the DC beam voltage ,before they arrive in MW interaction region and this energy is converted to kinetic energy(velocity modulation) in the interaction region electrons are either accelerated or de-celearated by MW field and they drift down the tube,the bunched electrons inturn induce current in the output structure. Thus what do I mean by this ---the pesa has two amplifiers thus T?xg more power than AESA (it has no tubes) thus providing a huge power /energy o/p thus creating the perfect target for the AMRAAM coming in radar homming mode ,coupled with the fact that it has a limited freq of operation and no air modes for ECCM (PESA)you got one dead duck jet by using the PESA in an AMRAAM environment. Ex of tubes:twa,klystron,magnetron..etc. Advantages of AESA over PESA 1] Usage Of MMIC: The AESA ps consist of MMIC?s (microwave monolithic IC?s) ,so what is the beauty of this ?well unlike passive elements whose phase shift must be mechanically varied the MMIC varies it electronically thus increasing the steering capacity dramatically and also because the active elements need no separate external power supply they can operate in about 5v(compare that with 500v for twa) produce no to less heat ,are feather light but this does not end here if in PESA one of two of the tubes fail you got a non working radar also the tubes generate a lot of noise (~5 db of noise is induced in the input signal ) coupled with the fact that their effieciency is max 58.2%(eff=.582(beta)v2/v1) one can easily realise the significance of the MMIC based AESA ,but then again it does not stop here all the initial processing is carried out un the MMIC itself and feeding the digital signal for processing allowing wide range of freq of operation (PESA?s FEERITE PS inputs analog signal with noise) also the frequency spectrum of the entire radar varies along with it?s power o/p thus providing the least possible chance for anyone to detect it let alone fire a homing missile on it , but then again it does not stop here the radar entire characteristics can varied dramatically(remember the EMCON modes of raptor) ,it supports Active Echo Cancellation, every MMIC could do multi tasking at different freq ,also the AESA can illuminate on specifically on one target on a narrow pattern thus guiding the AMRAAM on the target without getting detected from other sources ???etc. 2] AESA cannot be jammed PESA is very easy to JAM TWT or any tubes for that matter they suffer from a major problem,due to the high power involved the performance degrades over a period of time unlike a MMIC and the tubes break down [ ie not that they will physically be broken but the efficiency drops below 30% at which point the distance of the radar coverage decreases rapidly thus they require removal after every period of time (it varies on many parameters) and replacement made ] , and if during actual war due to unforeseen circumstance if a tube gets damaged then you can kiss the radar good by ,also the tubes require very high power ~800 watts for each tube thus providing a great burden to the electrical systems of the aircraft. And they are easy to jam I since because of the low freq range of the ferrite PS in PESA and because it does not apply ss(spread spectrum) t?xn it is very easy to jam when compared AESA though it is less prone to jamming than the slotted array radars(what you generally refer as the pulse doppler radar-----the fact is even the PA is also a pulse Doppler radar and people wrongly call it as a PA radar where as it a radar with a PA antenna and nothing more in fact even the LIDAR though contrary to popular mis conceptions, cannot be used to detect moving targets or anything like that also uses Doppler shift (light in this case) )b/w now I hope you agree that PESA are far more easier to jam than AESA since the RADARS which Employ TWT?s and also in presence of large amount of signals ( ex from a jammer pod ) the tube can be driven to saturation using a property of TWT called as INTERMODULATION distortion. 3] AESA has the ability to find the altitude by its operation / whereas PESA and slotted arrays do this by guessing business , lets see how [ Not the most accurate working , but this was written taking into account that people who do not know the basics of EM can understand and to give them a general idea ] shall try to as simple as possible . The first thing you got know is something ( PA in a minute , trust me ) called as EM waves which stands for ELECTROMAGNETIC WAVES , now there are two kinds of waves EM waves and mechanical waves ( sound waves ex :SONAR ) , EM waves are refered to as 3D waves actually no , now consider a ELECTRICAL wave in X-axis AND MAGNETIC waves in Y- axis now a wave is produced perpendicular to both X and Y axis (in Z-axis in this case )which is called as EM waves , all the waves from LF ( low freq ) ,MF ,HF, VHF, UHF,SHF,MW,LIGHT,IR,X-rays,ALPHA,BETA, GAMA ..etc follow the order of frequency lower the freq higher and lower the band with ( info carrying capacity ) now our radar uses the FREQUENCY of 3-30 Ghz called as MICRO waves . Now how is the AESA antenna ( read the def form pervious post ) better ? For this I need to go to the concepts of TE and TM waves TE waves : these stands for Transverse Electric waves ( what the hell ? ) , what is means is that all the Electrical components in the EM wave is made parallel to the Y-axis and Perpendicular to the X-axis TM waves : these stands for Transverse Magnetic waves ( what the hell ? ) , what is means is that all the Magnetic components in the EM wave is made parallel to the Y-axis and Perpendicular to the X-axis like shown in the below figure ( drawing in bit map , will upload in my homepage shortly ) Now we come to modes in these waves , generally on TE modes are used in Radar , so what are the modes and what is the application ? well using these ?modes? we can Find the altitude of the target without the computer GUSSING the height HOW ? Now consider that the target is in the same height as the interceptor ( using a PA antenna) now assume that TE 10 mode is txd ( ie all waves are parallel to x-axis and are in only one direction ie from say X to Y then it is called as 1 and 0 in Y-axis thus comes TE 10 mode ) because the target is at same height the received wave will be in TE 10 in opposite polarity (ie lets assume txd is + then received will be -) thus giving the height as zero or equal to the interceptor altitude , now assume that the target is in same height but in left of the JET then the received wave will be of TE 20 mode in positive ( in right will be TE 20 but -ve) now assume that the target is below the jet then in this case the mode will be in TE 11 -ve ( same point but below ) and vice versa(ie TE 11 + ve ) if in upwards and to the right (of antenna) then the mode will be TE 21 +ve for 2 and +ve for 1 thus giving us the altitude and the orientation of the Target when compared to sheer guessing in PESA/Slotted array antenna( note that the ability to find the altitude is the key for a BVR hit) ?bit tough kindly adjust , of-course since the slotted array/PESA does not have an MMIC to do the T?xg and receiving operation , the central computer can never calculate the modes and all the info is lost in case of the Russian Junk technology. 4] AESA can identify and track jets and types by comparing it with the signature bank unlike a PESA/Slotted where all the crucial and sensitive info is lost because of Passive components. Remember signature in my previous post ? in slotted array the computer again guesses upon the plane depending on the signal amplitude ( highly prone to error )but not here any signal incident will get a mode change right ? if the phase is varied for the signal incident on the Target the and corresponding signal is received there will be phase shift in the signal which will correspond to the jet classification just a finger print ( the only draw back is that the actual phase shift must be known for all these jets just like a finger print must be known before any classification can take place that is why the joint exercises ( with MKI) could prove to a vital info gathering for the Americans so that their AESA database could be updated to counter any eventuality ) note that one tell the difference b/w SU-27 trainer from a fighter Su-27 ( using AESA antenna ie ) such is the sensitivity of the shift , difference b/w the versions of the MIG-29 like A,U..etc the radar could tell the difference which even the human eye might find tough to spot but this does not stop here does , ( only for AESA :my beauty AESA( used in RAPTOR,E-3 SENETARY ?american versions ?though uses a slotted array still can use this concept because of the sheer amount of brute processing power and you cannot carry processors like these in Jets) could actually draw the jet with its nose pitched up, banking?etc and more crucially can even find the type of missiles carried , ordinance carrying?..etc ( now you must be afraid of AESA ) thus providing vital info which could actually crush the SU even before it has a chance to do its fancy manuvares and actually fire a shot because all the info is already known to point where he even launches a missile can be SEEN get me , there is a joke going around circles is that using AESA every thing till the pilots under pants can visualized !!!! 5] AESA says to PESA/Slotted , ?I see You ,Can jam you , You do not see me , neither can you jam me? or in other words AESA in EMCON 2-3 Modes is Zero Probability of Intercept for any present RWR. AESA employs what we comm. Engg call as SPREAD SPECTRUM modulation (read def from previous post )well in lay mans terms the power txd is only one percent of what is txd in PESA and the sinal is spread over a wide area thus Jamming is almost impossible to perform also because only a faint recidual trace is left this RADAR. AESA employs what we comm. Engg call as SPREAD SPECTRUM modulation (read def from previous post )well in lay mans terms the power txd is only one percent of what is txd in PESA and the sinal is spread over a wide area thus Jamming is almost impossible to perform also because only a faint recidual trace is left this RADAR what is called as LPI Jamming Spread spectrum modulation have several advantages when compared with conventional systems . A properly designed Spread spectrum system can operate reliably in various types of RFI [ radio frequency interference] including multi path interference , multiple access interference and hostile jamming .In addition the signal is spread over a wide large bandwith , thus the average power in any slot is low .At any rate the density of Spread spectrum signal is far less than that of conventional signals . this means Spread spectrum systems can share a frequency band with several narrow band signals by NB equipment, thus it very difficult to detect a Spread spectrum signal even by using a NB equipment Process gain and Jam margin: The properties of Spread spectrum are characterized by two parameters ------process gain ------Jam margin in general process gain=[bandwith of t?xd Spread spectrum signal]/ data rate in base band channel It is a well known fact that comm. System operates with a required prob of error, if the ratio of energy per bit to the spectral density of interference exceeds some threshold value , the received signal power is product of energy per bit and bit rate so Pi[watts]=No[watts/Hz]*Wss[Hz] Thus at the input of the receiver , the signal interference power ratio is SIR=Pr/Pi=Eb*R/No*Wss But in spread spectrum signal Wss is much greater Than R and so Eb/No could be in acceptable range [~10] even when SIR is small . foe ex if Eb/No=10 and Wss/R=1000 ,SIR=.01 or -20 dB .Thus Wss/R is called as process gain of the system .It is possible for a spread spectrum receiver even when its input is buried in noise.From the above ex it is clear that even if [Wss/r]/[Eb/No]=100 interferences present at the same band in same time receiver could still operate. Jamming Margin: It expresses the capability to perform in hostile environment .Jm takes into account requirement for a useful system output signal to noise ratio and allows for internal losses J/S and Antijam margin: The ratio of [J/s] is a figure of merit that provides a mesure of how invulnerable a system is to interference .The larger the [J/S] ,the greater is the systems is to interference but forces to employ a greater processing gain thus used only in AESA. Anti jam margin:[ j/s] required-[j/s] , in other words one must jam the entire MW spectrum to jam SS signals. Finally the American AESA employs a technique called Frequency Hopped Spread Spectrum with Poly Phase Encoding ,though a detailed discussion about this is long due I will speak on the way by which it works since it will be beyond the scope of many who do not have a Signal Processing background. The F-15 AESA the APG 63 (V-2) range is around 80- 95 nm in LPI for 1m2 target and with out LPI can reach till 120-130 nm Which will be less than N011 , but the 350 km range cannot be with for 1m2 target , maximum estimates put the radar at 250 kms [ 147 nm ] against a 2 m2 target which is still less than the F-22 , but assuming that all jets are >2m2 this generally does not matter unless of course one is tracking AAM?s at a very long range , but does this mean a tie b/w F-15 and MKI I cannot agree because in LPI mode the AESA radar cannot be DEDECTED and even if LPI limitations are removed no Current RWR can track the source of an AESA antenna which means essentially a F-15 can have a Free ride in BVR also did I ever mention that there is no need to lock in a AESA antenna I guess so , now comparing this with the 60 nm [ 1m2] range of Apg-63 V1 and all these for just a Million extra it is a win win situation for the USAF. From the above advantages it is very clear the benefits and potential offered by the Active ESA for a fighter plane , but this is just part of the story since an AESA antenna is both a nightmare to manufacture and also the cost involved in a AESA is too high especially in fighter based one , a good example is the time it took for USA to fabricate a fighter based AESA when they had X-Band modules for some time now , the reason behind this that just because a T/R module is designed is fabricated does not mean that the Radar will be operational tomorrow and will be atleast a decade from the point at which the Modules were developed such is the complexity and the Russian insistence on their Space Feed Concepts by totally neglecting the Constrained feed for Three Decades , European reluctance to invest much in this field made the US to have quite possibly the most decisive tactical weapon ever designed since as I said above its significance in the BVR environment is truly devastating for any opponent , but then things are not so bleak either since the European AESA for the Rafale and the EuroFighter will be ready by 2012 , but other countries projects like Indian , Russian ventures is anybody?s guess because they will have to start from scratch.  |
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