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Vol . 8      No. 2    April  2000

ESM Antennas

Cavity Backed Archimedean Spiral Antennas

Spiral antennas represent a class of antennas which works over multi octave frequency bands. They are compact in size, light-weight, can be flush-mounted without any protrusion from the surface on which they are mounted and are iDEALly suited for airborne platforms. They are circularly polarised which takes into account the pitch, roll and yaw variations of airborne vehicle during maneuvers. Most of the airborne systems use this type of antennas. These antennas are also adopted in systems for ship borne, submarine, and vehicle-mounted platforms and fixed installations.

Appropriate competence has been established in this critical area by developing these antennas over a wide frequency range of 0.5 to 40 GHz in split bands. The performance ha optimally adjusted by developing printed circuit techniques to achieve high degree of repeatability antenna consists of a broad-band raDIATing element, balun, cavity and low-loss radome and has been made smaller in size than the conventional one by incorporating size compression techniques, sine modulation of the expanded spiral arms. A broad-band tapered line, microstrip balun, has been developed using Klopfenstein Tchebycheff optimum impedance transformer. Stray raDIATions from the balun have been suppressed by loading the cavity of the balun with ferrite absorbing materials. This ensures perfect transformation with acceptable level of squint in the raDIATion patterns. Back cavity has been loaded with honeycomb absorbers to suppress higher order modes and back raDIATions. These techniques resulted in high quality performance with excellent phase and amplitude tracking characteristics covering various frequency bands, like 0.5-2, 2-18, 8-18, 8-40 and 1-18 GHz. These antennas find various applications in sector surveillance, as feed to reflectors for high accuracy rotary direction finder (RDF), as an antenna element for wide open amplitude comparison direction finder (DF) system and in high accuracy baseline interferometer (BLI) DF system. Suitable radomes have also been designed and developed for protecting these antennas from severe environmental extremities. All these antennas have been subjected to rigorous electrical and environmental qualification tests. Production lines have been established to meet the bulk requirements.

Log Periodic Antennas

tfapr3.jpg (28247 bytes)Log periodic antennas are frequency independent antennas which theoretically have no limitation on bandwidths. In these antennas, the linear dimension depends on frequency bands and antennas performance repeat periodically as a function of logarithm of frequency resulting in extremely broad-band antennas. Wide range of log periodic antennas, like coplanar log periodic, pyramidal log periodic, cross-polarised log periodic, and cavity-backed log periodic, have been designed and developed covering high frequency to microwave frequency range. State-of-the-art fabricational techniques like chemical milling and chemical etching have been used to realise the frequency of operation up to 26 GHz.

These antennas find extensive use in sector surveillance, RDF, amplitude comparison direction finder, base line interferometry, communication links, etc. and have been configured to suit all the platforms of the Services.

 

 

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Biconical/Discone Antennas

tfapr6.jpg (31336 bytes)These antennas with omnidirectional raDIATion patterns are tfapr7.jpg (15948 bytes)used with frequency receiver sub-system for instantaneous signal interception. Basically, biconical horn antennas are linearly polarised. Broad-band slant (45 ) and printed circuit polarisers have been developed and integrated with these antennas enabling them to respond to any type of signal polarisation. Antennas covering frequency range from VHF/UHF to MW have been developed and productionised for ship borne a vehicle-based platforms. These antennas have also been configured for DF applications based on time difference of arrival technique airborne platforms resulting in very high DF accuracy.

 

 

 

 

 

Conical Log Spiral Antennas (CLSA)

tfapr9.jpg (15297 bytes)Conical log spiral antennas (CLSA) belong to a class of frequency independent antennas. Various variants of this antenna with desirable raDIATion pattern, such as directional, hemispherical, and omnidirection have been developed along with low-loss radomes for use on different platforms. CLSA with tear drop raDIATion pattern find use as broad-band feed for reflectors in DF systems, airborne tail-mounted early warning radar system, etc. CLSA with hemispherical and omnidirectional raDIATion pattern are used where 100 per cent probability of signal intercept is required.

 

Blade Antennas

tfapr8.jpg (17689 bytes)Blade antennas are compact, light-weight omnidirectional antennas which are aerodynamics shaped for high performance airborne applications. Th antennas are designed to work over large frequency bands of the order of 5 :1 covering VHF/UHF S-band. A special radome has also been developed protect the blade antennas against severe environment experienced on airborne platforms.


Stripline/Microstrip Antennas

Tapered Slotline RaDIATor

Tapered slot line antenna (TSA) belongs to a class of a periodic, continuously scaled,tfapr10.jpg (18646 bytes) slow leaky end fire travelling wave antennas. It has a wide frequency bandwidth of operation with significant gain and linear polarisation characteristics. TSA consists of symmetrically etched slots in the metalisation on the dielectric substrate and fed by wide-band strip line to slot line transition. This antenna has been developed with reduced axial length using strip line to slot line transition for impedance matching, and the concept of serrations on the edges incorporated to reduce the effect of surface currents and diffraction effects on raDIATion characteristics. This is considered a breakthrough in achieving compact slot line raDIATor meeting the geometrical constraints of the circular arrays used in high accuracy direction finding system called digital bearing discriminator. TSA can be used as a high gain antenna, a feed for reflector or lens. Arrays of slot line raDIATors have a number of potential applications like high resolution millimeter wave imaging as well as light-weight alternatives for focal plane application in satellite communication antennas.

Printed Folded Dipole Antenna

It is a novel type printed circuit dipole antenna which exhibits low profile and broader frequency bandwidth compared to conventional dipole. The antenna is folded and energised by a microstrip feed with capacitive coupling and offers very good impedance matching with spacing of one-tenth of wavelength from ground plane that makes it very efficient. The antenna has been designed and developed in L-band for IFF phased array system.

Element Microstrip Patch Array Antennas

In recent years microstrip antennas have gained considerable popularity. These are low profile, easily conformable to non-planar surfaces, inexpensive to manufacture in large quantities using modern printed circuit techniques, flexible to produce a wide variety of pattern and polarisation, and easy to integrate with active components. The microstrip antenna in its simplest configuration consists of a raDIATing patch on one side of dielectric substrate which has a ground plane on the other. The dielectric constant of the substrate should be low so as to enhance the fringing fields which account for the raDIATion.

The microstrip 2 x 2 element patch array has been developed for radio altimeter applications. It provides symmetrical directional beam with low side lobes with excellent cross-polarisation characteristics. These antennas, used in pair one for transmitting and the other for receiving, have salient features of low cross-coupling of the order of -77 to -80 dB separated by a distance of 40 mm Other applications include feed for reflector, satellite communication, telemetry, weapon fusing satellite navigation receivers, etc.

Electronic warfare (EW) is a silent and invisible war as it uses the full spectrum electromagnetic energy to accurately intercept, identify, locate and counter the enemy radar modern war, weapon systems depend very heavily on this silent and invisible mode of conflict. In EW, antennas play extremely important role since they are ears and eyes of an electronic system. EW antennas are special class of antennas as they require raDIATion pattern capable of receiving signal from all directions and responding to all polarisations over multi octave frequency band. Such antennas are generally not available off-the-shelf from foreign sources. Hence, they have to be tailor-made to meet the specific requirements of our Services.

DRDO has designed, developed and productionised a large variety of state-of-the-art EW antennas and radomes covering frequency range from high frequency to millimeter 1.5 MHz to 40 GHz to meet the most stringent requirements of the Armed Forces. For the characterisation of these antennas and radomes, advanced antenna test facilities like rectangular anechoic chamber, tapered anechoic chamber, open test range, and ground reflection have also been set up with the latest instrumentation. DRDO has developed core competence design of EW antennas. As a result, total self-sufficiency has been achieved in this critical fi EW antennas and radomes.