Vol . 8 No. 1 February 2000
Reflector Antenna Technology
A singly-curved parabolic reflector and a doubly-curved shaped reflector have been designed and developed for use in CW radar INDRA I and INDRA PC radars, respectively. These radars have been inducted into the Services. A cassegrain parabolic reflector with a dielectric plate has been designed and developed for naval tracking radar application for simultaneous operation at X and Ku-bands with monopulse capability. This antenna has worked well at both the frequency bands of operation. A parabolic reflector for instrumentation radar has also been designed, tested and integrated with other sub-systems and is working satisfactorily.
Phased Array Technology
DRDO has taken up design and development of search/track radar required to handle multiple targets and provide air defence against low flying aircraft. This radar utilises phased array technology to handle all aspects of radar sensing. The inhe~ent flexibility offered by the phased array antenna in steering the beam by means of electronic control is of great interest. This has an application in those radars where it is necessary to shift the beam rapidly from one position in space to another to get multitarget information at a flexible and rapid data rate.
The phased array system is a planar phased array working at C-band frequency. The monopulse space feed, two-plane scanning array consisting of dual mode ferrite phase control modules, the beam steering computer for digital support and associated power supply units have been designed, developed and evaluated. The assembly wiring and initial testing of the C-band array comprising nearly 4446 PCMs is a task which involves coordinated effort and proper material planning and management of large data, deft handling of delicate and costly components like PCMs, adopting the right assembly and wiring techniques and procedures, etc.
Slotted Waveguide Array Technology
Slotted waveguide arrays are required for several airborne applications and for various seeker heads. High gain and efficient antennas are required for these radar applications. Since waveguides provide exceptionally low path loss and accurately repeatable array elements, waveguide slotted arrays have been attractive candidates for high efficiency planar antennas. The rugged and compact structure, high raDIATion efficiency and high power handling capacity of the mechanically scanned slotted waveguide antenna array makes it the workhorse of the most tactical aircraft radars and missile systems deployed in the field today, apart from a number of nonmilitary radar applications.
A design methodology has been developed based on extensive electromagnetic modelling using method of moments and finite element techniques which offers high degree of accuracy. A computer aided design tool has also been developed followed by very good accuracy in machining the slots and fabrication of various parts of the antenna. The dip brazing process to join up to four layers of the antenna together has also been developed which is a very critical process. Based on these design methodologies a number of software have been developed.
All the antennas developed meet all technical requirements and are comparable with the best in the world. Presently, DRDO has come to a stage of maturity where state-of-the-art slotted array antennas can be designed, developed and produced for any frequency band. Brief descriptions on these antennas are given here.
Flat Plate Antenna
A high performance multi-function flat plate antenna has been developed. This type of state-of-the-art antenna with stringent electrical, mechanical and environmental specifications has been developed for the first time in the country.
Low side lobe levels and high gain
Integrated IFF and guard channels
Low profile and light weight
The planar waveguide slotted array used in this case is divided into four quadrants to get two-plane monopulse, and each quadrant is further divided into a number of sub-arrays to get the required bandwidth performance. A complex, multilayer feeding network is required to get the signal at the four outputs which is further connected to a two-plane monopulse comparator network. This array uses longitudinal offset slots as raDIATing elements and centerinclined slots as feeding elements. The performance of very low side lobe array depends on the accuracy with which individual raDIATing and coupling slots are characterised. Due to the complex nature of the boundary value problem encountered in the analysis of raDIATing and coupling slots, efficient theoretical and numerical analyses are required to achieve high degree of accuracy.
To meet these requirements, a design methodology was developed based on extensive electromagnetic modelling using method of moments and finite element techniques, which offers high degree of accuracy. Based on the moment method analysis, with great amount of mathematical and computational effort, several software packages, namely, COBRAS, COBRASDC, COBICS, COBICSNO, LADSOFF and ANSA were developed for raDIATing and coupling slot characterisations and linear array designs and analysis for slotted arrays. Designing novel test jigs has validated these software theoretically against limited published literature and also experimentally. These software are menu-driven, user-friendly? and valid for wide frequency band, take care of any standard/non-standard waveguide dimensions, have the availability of single and double precision, and are mostly not available in the world market.
Novel design methodologies were developed to design planar slotted array consisting of raDIATing slots and various levels of coupling slots to get the required antenna raDIATion pattern requirements like side lobe levels, beam widths and gain etc. As these antennas have to have a two-plane monopulse capability, a technology to design high power unequal power divider network and two-plane planar monopulse comparators was developed based on method of moments and finite element technique for the first time in the country.
As this antenna is required to have in-built IFF and guard channels, design and development of IFF antenna consisting of array of dipoles fed with stripline monopulse comparator and guard antenna was also carried out successfully. The complicated three-dimensional structure of the slotted arrays have been of great concern mechanically as it requires perfect electrical contact at joining and high accuracy of alignment in assembling the various parts of the antenna. All the four layers of the antenna integrated with guard antenna and IFF monopulse comparator were joined by dip brazing process.
Ku-Band Slotted Monopulse Antenna
Ku-Band Slotted Monopulse Antenna has been successfully designed, fabricated and tested, meeting all electrical, mechanical and environmental specifications required by the system. The antenna is a circular flat plate array with multilayer feeding network to get sum and two orthogonal difference pattern. The design philosophy of the present slotted array is quite different from the conventional design methodologies. First, the entire raDIATing aperture has been divided into four quadrants diagonally to get two-plane monopulse capabilities and thus reducing the mechanical complexities in terms of avoiding the joining of one more layer of folded shorts. Secondly, it uses a nonorthogonal waveguide-to-waveguide coupling through a centerinclined slot to connect the monopulse comparator with feed network. The third and most critical part is the design and development of a compact two-plane monopulse comparator network based on the new type of planar magic tees developed. Further, to make the antenna ultra thin and light-weight, nonstandard reduced height waveguide dimensions are used for all the layers. And finally, all the layers (including monopulse comparator) are joined together using adhesive bonding to meet the required electrical specifications and to qualify the environmental specifications.
Slotted Waveguide Array Antenna for Maritime Patrol Radar
Based on method of moments analysis and finite elements method techniques, a computer aided design technique has been utilised in designing a horizontally polarised light-weight and high performance slotted waveguide array antenna. The four-layer antenna of dimensions 800 mm x 250 mm x 20 mm, weighing about 2.7 kg, is elliptical in shape for maximum utilisation of the available area and has been designed using nonstandard, reduced height waveguides. The method of construction has been simplified to make it cost-effective without making any compromise on electrical and environmental specifications.
Slotted Waveguide Array Antenna
The antenna designed and developed is an ultra low side lobe planar slotted waveguide array antenna. The raDIATing layer of the antenna consists of 16 linear waveguide slotted arrays stacked to form the planar array. To achieve maximum bandwidth and good return loss, the nonresonant slotted array is chosen. As a result, the power that reaches the load end of the array will get dissipated in high power matched loads. The longitudinal offset raDIATing slots are cut on broadwall of the rectangular waveguide to reduce the total thickness of planar array. The linear arrays are fed from a compact high power waveguide unequal power divider mounted on reverse of the planar array. Due to the elliptical contour of the array, a phase compensating waveguide network is used to achieve the desired phase distribution at the array inputs.
BEL Flycatcher Radar Antenna
The Flycatcher radar is a ground-based radar being productionised by Bharat Electronics Limited (BEL) for the Indian Army. The radar consists of search and track facilities on a moving platform. The search radar is in the X-band and uses a slotted raDIATor enclosed in a flare. The antenna works in horizontal and circular polarisation. BEL has produced low-band and high-band radars under an MoU with HSA, Holland. Currently, the mid-band radar is being designed and developed by BEL. The slotted raDIATors for low-band and high-band radar antennas are being imported by BEL from HSA, Holland. The slotted raDIATor for antenna used in the mid-band radar has been designed and developed by DRDO. The technology has been transferred to BEL to undertake production of this antenna.
Dual Polarised VSAT Antenna Feed
VSAT (very small aperture terminal) provides two-way communication through satellite. The network comprises three networking elements: the remote terminal, the satellite and the hub station. A remote VSAT terminal comprises three subsystems: (i) antenna system, (ii) outdoor electronics like solid-state power amplifier, low noise block converter and IF, and (iii) indoor electronics like modem cards, base band processors, data ports, voice ports, and video ports. The most attractive feature of a remote VSAT system is its small antenna size (1.8-2.4 m in diameter) enabling the user terminal to operate without any compromise. The antenna sub-system consists of a parabolic reflector fed by dual polarised feed.
DRDO has designed and developed C-band dual polarised feed system consisting of corrugated conical horn and orthomode transducer and frequency filter. Dual polarisation with frequency filter is required because antenna has to work in two different modes, i.e., uplink (5.925 6.425 GHz) mode and downlink (3.74.2 GHz) mode with an isolation of the order of 60 dB or less. More than 100 (Nos) feeds have been supplied to ITI Ltd., Bangalore, on commercial basis for integration with their reflector and other electronics.
Dual Polarised Scalar Feed for Doppler Weather Radar Antenna
Considering the emerging trends in radar meteorology worldover and recognising the importance of Doppler information in characterising severe weather systems like cyclones, India Meteorological Department (IMD) is modernising the existing cyclone detection radar network by using Doppler Weather Radars (DWR) being developed by ISRO. The antenna for DWR is a parabolic reflector fed by a corrugated conical horn (scalar feed) with orthomode transducer. The design and development of high performance corrugated conical horn along with orthomode transducer has been done successfully meeting all electrical and mechanical specifications. To use pulse DWR as a polarisation diversity meteorological radar, it must have (i) simultaneous coherent reception of two orthogonal polarisations and (ii) rapid switching of the transmitted signal between two orthogonal polarisations. Thus, the design of the feed system has been taken care so that it can support orthogonal polarisation operation. Orthomode transducer was identified for the discrimination of two signals of the orthogonal dominant modes present at the horn port (which is a corrugated conical horn) of the feed system. The corrugated conical horn, also called scalar feeds, was chosen for its good pattern symmetry, low cross-polarisation, and low spillover.