Akhil simulation and optimisation has been carried out

Akhil Gupta-Sr Engineer, Bal Mukund Jha- Dy Manager, Sanjay Choube- Sr Dy General Manager Development & Engineering-AntennaBharat Electronics Limited, Bharat Nagar, Ghaziabad (U.P.) 201010                                                                               [email protected] The paper presents the design of Single Ridge Slotted waveguide planar array Antenna in X Band for semi-active phased array radar applications.At higher frequencies, Slotted waveguide array antennas are preferred due to their inherent compactness and high power handling capability. Semi Active and Active Phased Array Antenna need to be calibrated for their operational requirement. Coupling in slotted waveguide array is very critical as coupling slot significantly affects power distribution across the aperture. In this paper, coupling is achieved via non-offset longitudinal slots from ridge waveguide to rectangular waveguide. To meet a particular form fit requirement in X-Band, ridge waveguide with customized dimensions is selected for intended application. Prototype design of antenna array consisting of 5 linear rows is presented. Each linear row of antenna is consisting of 40 slots divided into two sub arrays (of 20 slots each) for bandwidth enhancement and monopulse requirement. Coupling of -40±3 dB is achieved at the Calibration Port with each of the left and right ports of linear rows. The coupling waveguides are integrated with two way equal waveguide power combiner so as to have a single calibration port. Other end of the coupling waveguide is terminated with a matched load.The Design, modelling, simulation and optimisation has been carried out using HFSS RF simulation tool. Taylor (SLL 30dB, nbar4) distribution is used for Azimuth radiation pattern. However in Elevation, distribution is uniform. Coupling values simulated are within specifications.   Keywords- Ridge waveguide, slotted array, Calibration.I.   INTRODUCTIONRadar systems have undergone rapid transitions over the years like high data rate and multifunction capabilities. This necessitated the use of phased array antennas which employs electronic steering. Phased Array Antenna is State of the Art technology where Antenna Beam is steered electronically at a very high speed. Phased Array technology has been evolved from Passive Phased Array to Active Phased Array. An active Phased Array Antenna typically requires as many Transmit/Receive modules as total number of radiating elements in the Array Antenna. In Semi Active Phased Array Antenna, a group of Radiating Elements are fed by one T/R module. Semi Active and Active Phased Array Antenna need to be calibrated for their operational requirement. Over the years, Slotted Waveguide array antennas have found wide applications in radars and communication systems due to many significant advantages such as high power handling capability, simple feeding, inherent compactness, low loss, etc. Due to mechanical constraints, sometimes there is a need to operate a rectangular waveguide at a lower cut off frequency. Considering this, ridge waveguide is found useful in various applications as it can be operated at a lower frequency. In addition to above criteria, ridge waveguide also have a wider bandwidth free from higher mode interference and lower impedance than a simple rectangular waveguide with the same internal dimensions at the expense of higher attenuation and lower power handling capability.Fig-1:Block Diagram of Semi-Active Phased array Antenna (MxN Elements)Coupling in slotted waveguide array is very critical as coupling slot significantly affects power distribution across the aperture. The paper presents the Prototype design consisting of 5 linear rows (40 slots in each rows), configured into 2 sub arrays. Array is designed for Low Side Lobe Level using Taylor Distribution (SLL 30dB, nbar4) for Azimuth radiation pattern. Coupling of -40±3 dB is achieved at the Calibration Port with each of the left and right ports. In Elevation, distribution is uniform. The Design, modelling, simulation and optimisation has been carried out using HFSS.II.   RIDGE WAVEGUIDE THEORY                                                       b2   b1                                                              a2                                         a1 Fig-2: Cross sectional view of single ridge waveguideCut off frequency of the TE10 mode is lower from that of the rectangular waveguide, which depends on width alone. At cut off there is no longitudinal propagation down the guide. The waves simply travel back and forth between the side walls of the guide. It is similar to parallel plate waveguide of infinite width where the width corresponds to the direction of propagation of the normal guide. The TE10 mode cut off occurs at the lowest order resonant frequency, where there is only one E field maximum across the guide and it occurs at the centre for a symmetrical ridge. Due to the reduced height of the guide under the ridge, the effective TE10 mode resonator is heavily loaded as if a shunt capacitor is placed across it. The cut off frequency is thus lowered but at the expense of higher attenuation and lower power handling capability. III. DESIGN CALCULATIONSA.WAVEGUIDE DIMENSIONSTask was to design a X-Band slotted waveguide planar array antenna with specified dimensions and pitch between two linear arrays placed vertically over each other. Pitch requirement necessitated the waveguide broader dimensions to be 12.7mm. Dimensions were verified for given specifications of Antenna parameters like gain and Beam width.     B. RIDGE DIMENSIONSSingle symmetrical ridge waveguide was selected for linear slotted arrays. Ridge dimensions were optimized to meet the cut off frequency requirement. With the finalized dimensions, the waveguide cut off frequency is 6.6 GHz. C. SLOT DIMENSIONS CALCULATIONSlot dimensions calculation involves calculation of parameters like slot length, slot width, offset and slot spacing. The slot parameter calculation procedure starts with the Taylor function where antenna characteristics such as the SLL, the number of elements and the number of side lobes are the inputs, from which individual voltage Ratio (an) for each slot is calculated. The next step is the evaluation of the required admittances. The required normalized conductance’s (gn) are derived using following equation.                    gn =      n =1, 2, 3……….N                      and for End-Feed  Thereby for each sub array i.e. 20 elements , summation of gn is equal to unity. Next step is to find Slot resonant length and offset for each slot. RF Simulation tool Ansoft HFSS has been used to generate these parameters. A longitudinal slot of    width and  length is created on the broad wall of the ridge waveguide and admittance is measured at a distance of one-half the guided wavelength from the centre of slot; the waveguide being terminated in a short circuit placed at a distance of quarter of guided wavelength from the centre of slot.  The length of slot is varied and at each point conductance and susceptance is measured. The real and imaginary parts of admittance i.e. conductance and susceptance are denoted as h1(y) and h2(y) respectively, where y is the slot length normalized by the resonant length, l/lr can be written as                     = (h1(y) +j h2(y)) g()                 is the offset of slot from the centre of waveguideNext, the slot offset is varied and at each value of offset, the resonant slot length is recorded i.e. where the imaginary component of admittance is zero. Normalized resonant conductance g(x) = of the longitudinal slot is given as follows:                       g() = g1                         Where g1 = 2.09   .where a and b are waveguide width and height respectively.D. CALIBRATION NETWORK Calibration network consists of coupling waveguides (WR-90), waveguide bends, matched load and two way equal power divider. Coupling is achieved via non-offset longitudinal slots from ridge waveguide to rectangular waveguide. The centre of coupling slot is at a distance of multiple of one-fourth of guided wavelength from the short placed at the centre. The Coupling slot of    width and  length is created which is then optimised to achieve the coupling specifications of -40 ± 3 dB. The coupling waveguides are integrated with two way equal waveguide power combiner via waveguide bends  so as to have a single calibration port BIT (Built in Test) Port is also incorporated to detect the health status of other ports. Coupling between BIT and CAL Port is -10± 3 dB and with Antenna ports is   -50 ± 3 dB.IV. DESIGN & SIMULATIONHFSS Model of Array of 5 rows3D-ViewFront ViewRear ViewSimulation Results:                       V.   SUMMARISATION OF RESULTS ParameterProto Array of 5 rowsFrequency  (GHz)10.210.310.4Realized gain28.4228.1527.44Az Bw2.51º2.50º2.73ºAz SLL-26.68 dB-26.09 dB-23.1 dBReturn Loss  (worst)-10.6 dB-16.9 dB-14.6 dBCoupling values-44.5 dB-42.3 dB-39.2 dB   TABLE 1 : Summary of Results.                                VI. CONCLUSIONDesign of slotted ridge waveguide antenna for semi active phased array radar is presented. Conventionally, for phased array radars, radiating element like Micro-strip patch is used. The calibration in those cases is achieved by a coupled line on the same substrate. In this paper ,coupling is achieved via non-offset longitudinal slots. Design presented can find various applications in Radars and communication systems where advantages such as compactness, low loss, high power handling are of prime importance.REFERENCES                            1 John D. Kraus, “Antennas”2 Wiley – ELLIOT Antenna Theory and Design”3 A Broadband Slotted Ridge Waveguide Antenna array, “IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION VOL.54.NO.8AUGUST 2006″4 MARCUVITZ (N) ED. “WAVEGUIDE HANDBOOK”5 Calculation of the parameters of Ridge Waveguides “IRE TRANSACTIONS OF MICROWAVE THEORY AND TECHNIQUES”.