Direction Finding (DF) Antenna Systems
Rotary DF Antenna Systems
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 (PÄL band), where size and weight is premium, a novel
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
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 discriminator (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 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 360ø in azimuth. Good DF accuracy has been realised. Low-loss
radomes have also been developed for protecting the arrays from severe environmental
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.