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Vol . 8      No. 2    April  2000

Electronic Countermeasure (ECM) Antennas

Horn Antennas

The conventional horn antennas are linearly polarised operating over limited frequency tfapr19.jpg (15984 bytes)bandwidth. DRDO has developed horns with linear, circular, slant linear (45) and dual polarisation. Suitably designed ridges have been incorporated for achieving broad bandwidth. Dielectric phase shifters/metallic phase shifters have been developed to achieve circular polarisation with minimal ellipticity ratios over large bandwidth. Unique external dielectric decollimating lenses are used for achieving required beam width aspect ratio. These antennas have high power handling capability.

DRDO has developed state-of-the-art broadband horn antennas for airborne and ship borne jammer applications over the frequency range of 0.540 GHz in split bands. These antennas also find application as feed to reflectors and as auto track horns. All these antennas have been subjected to rigorous electrical and environmental qualification tests. Production lines have been established for meeting the bulk requirement.

High Power ECM Reflector Antennas

tfapr18.jpg (15242 bytes)tfapr20.jpg (24385 bytes)The ECM system requires high effective raDIATed power (ERP) for effective jamming. High ERP which is the product of transmitter power and antenna gain can be obtained by increasing either the antenna gain or the transmitter power. Power output of the transmitter is limited by many important considerations, like size, weight, cooling, etc. principal contributor the antenna gain. Increase in antenna gain is accompanied by narrow beam-width thereby decreasing the coverage substantially. The narrow beam creates its own problem of missing the target or ineffective jamming. The high gain is obtained by using prime focussed paraboloidal reflector fed by high power ridged horn feed. To direct the narrow beam of this jammer antenna accurately to the enemy radar, two pairs of amplitude matched auto-track horns are integrated to the reflector. These auto-track horns work on amplitude comparison monopulse technique with their crossover axis aligned accurately to main jammer antenna axis for directing the jammer beam to enemy radar both in azimuth as well as elevation plane for effective jamming. Such ECM antenna has been developed capable of working over L, S, C, X-Ku and millimeter frequency bands for the Services.

Microwave Rotman Lens Fed Multiple Beam Array Antenna

Microwave Rotman lens fed multiple beam array antenna system providestfapr21.jpg (16081 bytes)simultaneous high gain multiple beams with each beam possessing full gain of array aperture. The microwave Rotman lens is a power distribution-cum-phasing network. It is considered as a microwave analogue of optical lens. It offers the possibility of variable refractive index which provides many degrees of freedom to design lenses with multiple foci as compared to optical lenses which are made of dielectric with fixed refractive index and hence single on axis focal point.

The Rotman lens consists of TEM mode parallel plate transmission region with input and output ports called beam and array ports, respectively. Coaxial transmission lines of unequal lengths connect the array ports to the raDIATing elements of the array. These variable length transmission lines which constitute a transmission delay provide variable refractive index to achieve the path length compensation. The length of these transmission lines and array port positions are designed to provide perfect focussing along the input circular arc. The focussing is a consequence of providing equal electrical path length from a given focal point out to the corresponding raDIATed wave front to each element of the array. These lens-fed array systems provide optimum utilisation of system resources in time, space and frequency. Microwave Rotman lens fed array system is cost-effective with inertia-less scanning capability and finds extensive application in: (i) ECM against simultaneous multiple threats, (ii) surveillance, (iii) point to multi-point communication, (iv) remotely piloted vehicle system for multiple drone control and broad-band data retrieval links, (v) integrated multipurpose system accomplishing the functions of radar, ECM, surveillance and communication, (vi) satellite communication with spot beam coverage capability, and (vii) low side lobe pattern generation.

A microwave Rotman lens fed array antenna system which is a state-of-the-art concept has been designed and developed, covering the frequency band more than 2:1 capable of generating 15 simultaneous beams with ERP 100 KW nominal.