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Vol . 8      No. 2    April  2000

Direction Finding (DF) Antenna Systems

Rotary DF Antenna Systems

tfapr11.jpg (14035 bytes)Rotary DF antenna systems have been designed and developed for locating the enemy radar accurately. The antenna system is composed of a cylindrical parabolic reflector with prime focussed broad-band co-planar log periodic feed and generates a narrow beam in azimuth and broad beam in elevation. The antenna is mechanically scanned in azimuth to obtain 360 azimuth coverage with elevation coverage obtained by the wide elevation beam, which maintains a constant amount of signal on the target at a given altitude. To receive the signals of any polarisation, the antenna is made slant linear Polarised at 45.

For low frequency bands (PL band), where size and weight is premium, a novel tfapr12.jpg (13250 bytes)compact monopulse antenna system has been developed with improved angular accuracy. This RDF antenna works on the principle of generation of sum and difference pattern. The antenna system consists of two-amplitude and phase-matched co-planar printed log periodic dipole array antenna enclosed in very low-loss thin wall radome. The sum and difference modes are formed in azimuth at microwave frequencies by using hybrid junctions and phase-matched interconnections. The size reduction of 10:1 and corresponding reduction in weight has been achieved.

A light-weight MM wave rotary DF antenna covering Ku-Ka frequency band has also been developed. Fabrication Al techniques like CNC machining for reflector and wire cut EDM techniques for feed were used because of small size and tight dimensional and surface accuracy requirements at millimeter wave frequencies. The antenna is currently under production.

These antennas work over very wide frequency band and can be used on vehicles, ships, and aircraft with suitable radomes. A very low-loss 'A' sandwich radome has also been designed and developed for using the antenna on land-based platform.
Amplitude Comparison DF Antenna System

tfapr13.jpg (12182 bytes)tfapr14.jpg (11887 bytes)Amplitude comparison DF antenna system uses multiple directional antennas displaced symmetrically in azimuth plane. A broad-band 8 antenna amplitude comparison DF system has been developed covering a frequency range of 1 to 18 GHz for ship borne applications. This system is wide open in frequency and bearing with 100 per cent probability of intercept. Since the system works on amplitude comparison techniques all pyramidal log periodic raDIATing elements are developed with amplitude matching within +1.5 dB over 1:18 frequency bandwidth. These antenna elements are slant polarised at 45 with very low cross-polarisation characteristics. The antenna elements are covered with specially designed individual low-loss radomes to protect them from hostile environment and also reduce the mutual coupling between antennas. The amplitude comparison DF antenna system has been productionised and inducted into the Services.

Antenna DBD Circular Array

Circular array of a wide open DF receiver called digital bearing discriminatortfapr15.jpg (19806 bytes) (DBD) is small in size and has low RCS. The DF system works on phase comparison technique with a field of view of 360 and 100 per cent probability to intercept/detect and locate enemy targets in a high dense signal environment. It is wide open both in frequency and bearing. It gives angle information on a single pulse basis with bearing accuracy better than 2.5 RMS. The circular array consists of raDIATing elements equally spaced around a circle to provide a field  of view of 360 with each raDIATing element connected to an input port of the feed network. Power applied to each output port of the feed network provides separate excitation of spatial phase modes. The nth phase mode consists of an excitation which is constant in amplitude with phase varying linearly with angle so that there are n complete cycles of phase change around 360 of angle. The higher order modes are used for accuracy and lower order modes are used for resolving the ambiguity. Phase comparison between these output ports directly gives the angle of arrival.

The circular array consists of specially designed 16 raDIATing elements of reduced size printed circuit tapered slot line antennas working over 1:4 frequency bandwidth (P and L band) incorporating innovative techniques. This system working over P and L bands is not available off-the-shelf from world market for import. DRDO's technology is simple and cost-effective.

Base Lie lnterferometric Array

Phase comparison base line interferometric (BLI) arrays are used where tfapr16.jpg (10266 bytes)high DF accuracy is required. The angle of arrival is measured in the interferometer by measuring the phase difference of signals received by two antennas separated in space. The measured phase difference is directly related to the angle of arrival. The interferometer is the optimum solution for best DF accuracy over a maximum field of view. The maximum distance between two antennas without causing ambiguity is half the wavelength. If accuracy without ambiguity over large band-width is required a multiple antenna array with non-uniform spacings is used. DRDO has developed 4-element interferometric arrays covering 2-8 and 8-18 GHz bands for ship borne and vehicle based platforms. This technique can also be adopted for airborne applications. Four such arrays are used to cover 360 in azimuth. Good DF accuracy has been realised. Low-loss radomes have also been developed for protecting the arrays from severe environmental conditions.

TDOA DF Antenna System

Time difference of arrival (TDOA) is a high accuracy large base line DF system. TDOA technique uses time measurement as a parameter for evaluating the angular location of the target. This technique is also wide open both in frequency and bearing with 100 per cent probability of intercept. It works with compact antenna system even at low frequencies without antenna rotation and the time measurement is not sensitive to antenna perturbations.

A four antenna TDOA DF system has been developed covering a wide frequency bandwidth for airborne platform using gain and phase-tracked biconical antennas. These antennas are compact in size, light in weight, rugged in construction and work with slant linear 45 polarisation covering 360 in azimuth. These antennas are covered with low-loss radomes to protect them from environmental effects. With the present day advances in speed and accuracy of time measurement TDOA technique is used for short base line platform, like aircraft, helicopters, ships, etc. with good DF accuracy.