|Title of Invention||
ULTRA LOW SIDE LOBE ANTENNA ARRAY.
|Abstract||This invention relates to an ultra low side lobe antenna array comprising board band microstrip radiator and three level corporate feed network (6) wherein the transmission lines of the feed network have rectangular centre conductors (9) housed inside rectangular outer conductors, characterised by power handling capability of the order of 150 kW peak power, said antenna array (1) being divided into four quadrants (3) and each quadrant (3) being divided into four sub panels (4) and each sub panel has 64 radiating elements arranged in 8 x 8 matrix form.|
|Full Text||FIELD OF INVENTION
This invention relates to an ultra low side lobe antenna array for airborne applications.
Any antenna array if it is larger in size than its wavelength is expected to form unwanted sidelobes apart from the required main-lobe. Theoretically, these sidelobes can be totally eliminated only by having an array of infinite length and breadth where the sidelobes are merged with the main lobe. Under this condition the beam will have 0° beam width. In fact it is required to have a finite degree of beam width, which depend on the requirements. For the elements in the array being excited uniformly, the peak sidelobe level will be of the order of-13 dB below the main lobe. Theoretically it is possible t achieve low sidelobes through proper weights (amplitude distribution) for the excitation of the elements. However, it is implementation at design, development and most importantly at fabrication levels that practically controls the sidelobe levels. One such attempt has been successfully made for the airborne radar, which is based on the microstrip antenna element and rectangular coaxial line based feed network.
REQUIREMENT FOR ULTRA LOW SIDELOBE ANTENNA ARRAY FOR AIRBORNE RADAR
In the airborne surveillance radar applications, the radar has to operate in look down mode. In this case the detection performance has to be carried out under severe clutter environment. The clutter not only gives strong unwanted returns but also spreads in the entire Doppler domain and thereby causing the operation of the radar to be clutter limited rather than noise limited. Hence reduction of sidelobe clutter for better performance of the radar is possible only through ultra low sidelobes for the antenna array.
In addition the radar has to operate in frequency agility mode for Electronic Counter Counter measures (ECCM) purposes, the bandwidth of the radar has to be large, of the order of ±5% or above. Further as the transmitted power has to be very high for long range detection, the antenna should be capable to operate at very high average and peak power. The antenna has to be light in weight, since it has to be mounted in the aircraft.
In a nutshell an antenna for the airborne surveillance radar should give ultra low sidelobe levels over large bandwidth, operable in high power and should be lightweight.
Low sidelobe antenna arrays with sidelobe levels of the order of 35db and below has been developed and used .elsewhere in the wpjid, for surveillance radars. These antennas are mainly slotted waveguide arrays, microstrip antennas using stripline/microstripline feed and reflector antennas. However, these antennas do not satisfy all the requirement of the airborne radar. The slotted waveguide array is capable of providing ultra low sidelobe levels of the order of 40dB and below but lacks in providing large bandwidth. These antennas are limited to ±3% bandwidth.
Microstrip .antennas based on stripline/microstripline feeds provide large bandwidth and low sidelobe levels but cannot be used for high power applications. Reflector-antennas present space constraints for mounting in the aircraft due to its extension in size in the third dimension and hence the required gain could not be achieved.
The ultra low sidelobe,microstrip patch antenna can be used for several applications like Spacebome surveillance, spacebome SAR (Synthetic Aperture Radar), spaceborne microwave remote sensing, multiple beam generation from space platform, Airborne surveillance radar, Airborne SAR, Shipborne surveillance radar, Conformal surface radar antennas mounted on building walls, Conformal high gain receive antennas mounted in rooftop or building wall for strategic surveillance / communication.
OBJECTS OF PRESENT INVENTION
The primary object of the present invention is to provide ultra low sidelobe antenna array with corporate feed network based on rectangular coaxial lines (RCL). The feed network was optimally designed to meet the all the requirements namely, the amplitude and phase distribution, power and the spatial requirements of the array.
An important feature of this feed network is the low insertion loss due to the closed nature of the transmission line thereby getting better antenna gain. Corporate feed network is used as it has the advantage of maintaining uniform phase over a large bandwidth.
Another object of the present invention is to provide ultra low side lobe antenna array which utilizes the broadband nature of the microstrip radiator, capable of providing broad bandwidth of around 400 MHz (>12%).
Another object of the present invention is to provide an ultra low side lobe antenna array with high power handling capability of the order of 150 kW peak power.
Further object of the present invention is to provide an ultra low side lobe antenna array which is very light weight.
BRIEF DESCRIPTION OF THE INVENTION
According to this invention there is provided an ultra low side lobe antenna array comprising board band microstrip radiator and three level corporate feed network wherein the transmission lines of the feed network have rectangular centre conductors housed inside rectangular outer conductors, characterised by power handling capability of the order of 150 kW peak power, said antenna array being divided into four quadrants and each quadrant being divided into four sub panels and each sub panel has 64 radiating elements arranged in 8 x 8 matrix form.
The Ultra Low Side Lobe Antenna of present invention is a light weight antenna using broadband microstrip radiator with RCL based feed network. The total feed network of this array is corporate in nature in the sense that it consists of three levels of feed network, first being the rectangular coaxial line have planer feed network optimally designed to handle the power and distributed in the available space. The second level is linear feed network based on rectangular coaxial line with larger cross section and the third level is designed using waveguides.
The antenna system developed with the nominal specifications indicated in Table 1 on page 9, consists of an array of 16 x 64 elements fed by a complex feed network to achieve the desired pattern. The total antenna array was divided into four quadrants and each
quadrant was fed by a waveguide network which has provision for generating both sum and difference patterns in the elevation plane incorporating a magic tec at the input. A quadrant panel is further divided into four panels each containing 64 radiating element patches. These four panels of a quadrant arc fed by a second level squarcax feed network. All the 16 radiating panels are fed by squareax feed network of a smaller cross section compared to the second level squareax network. With this arrangement of feed network an overall insertion loss of less than 0.5 dB was achieved.
The feed network based on the rectangular coaxial line consists of two components, the outer conductor being fabricated in the form groove and the inner conductor. The groove of the outer conductor has a constant cross section of 10mm x 9mm. The cross section of inner conductor varies in width and height depending on the requirements of the impedance values. The coaxial nature of the inner and outer conductor is maintained throughout the feed network through the use of locators at different locations.
The sixteen panels have been grouped into eight pairs of vertical panels. Thus each pair consists of two panels which are lap jointed with staggered fastening. The radiating panel is then bonded with the vertical pair to form the complete radiating sub assembly. These eight pairs are butted together along their breadth using two T sections, which not only holds the array intact but also forms the mounting structure for the array with the aircraft. The second level and the third level feed network were assembled to form the full-fledged array.
The feed network has been fabricated using sophisticated numerically controlled machines. The feed network panels were fabricated using aluminium alloy and machining was done to a flatness of 0.5 micron on both sides. To reduce weight, unwanted portions were scooped out from outer conductor. The inner conductor of the squareax feed network was fabricated with Al alloy using CNC milling, Electric Discharge Machining (EDM) and wire-cut EDM in order to maintain the required accuracy. This fabrication was extremely critical and special care was taken to realize the dimensional tolerances.
Special teflon locators are used to locate inner conductors in the outer conductor for maintaining the coaxial nature within ± 20 micron. Error analysis for the required accuracy of fabrication showed that an accuracy of ± 20 micron is required for both inner and outer conductor. A thin aluminium alloy sheet is bonded to the outer conductor panel using suitable adhesive after the assembly of inner conductor.
DESCRIPTION OF FIGURES
The construction of the antenna array of the present invention will now he more particularly described with reference to the accompanying drawings where-in
Fig 1 : Shows the overall assembly of Ultra low Sidelobe Antenna Array Fig 2 : Shows the planar microstrip array configuration wherein
Fig 2(a) : Shows Front-View radiating elements layout
Fig 2(b) : Shows back view Feed network layout
_Fig 2(c) : Expanded view of one radiating patch element
Fig 3 Shows the quadrant panel
Fig 4 Shows the Radiation Panel
Fig 5 Shows the first level Feed network (Inner and Outer Conductor)
Fig 6 Shows the second level Feed network (Inner and Outer Conductor)
Fig 7 Shows the components of Third Level Feed Network
Fig 8 Shows First Level Outer Conductor
Fig 9 Shows First Level Inner Conductor
Fig 10 : Shows the assembled Radiation panel
Fig 11 : Shows the Back view of the outer conductor (Assembled with the Radiation
Fig 12 : Shows the Measured Azimuth Pattern of the Antenna Array
Fig 13 : Shows the Measured Elevation Pattern of the Antenna Array
Fig 14 : Shows the Measured Return Loss in the E- Plane.
Fig 15 : Shows the Measured Return Loss measured in the H-Plane/
DESCRIPTION WITH REFERENCE TO DRAWINGS
Fig. 1 shows the ultra low side lobe antenna array (1) mounted on the mast.
Referring to Fig. 1, the planer microstrip array configuration of ultra low side lobe antenna array (1) is subdivided into four quadrants 3. One such quadrant 3 is shown in fig (3). Each quadrant 3 is further divided into four sub panels. Thus the array is divided into 16 radiating panels as is illustrated in fig. 4. The configuration of 16 sub radiation panels is exactly like (6). Each sub panel, as illustrated in (4), comprises of 64 radiating element patched arranged as 8x8 matrix. The expanded view of one of such radiating microstrip patch element is illustrated at 2 (c). The sixteen panels have been grouped into eight pairs of vertical panels. Each vertical panel consists of two sub panels assembled by an overlapping configuration and staggered fastening.
Fig 2 (b) shows the back view of the antenna array depicting the layout of the antenna array depicting the layout of the feed network. Each quadrant panel (3) is fed by a second level squareax feed network (6). All the 16 radiation panels are fed by first level squareax feed network of a smaller cross section compared to the second panel squareax network.
Fig 3 shows the first level squareax feednetwork panel. The squareax feed network based on the rectangular coaxial lines consists of two components, the outer being fabricated in the form of groove (8) and the inner conductor (9). Fig 6 shows the complete layout of second level feed network.
Fig 4 shows the Radiation panel consisting of 64 radiating elements arranged in 8x 8 matrix form. It is made up of three layers, the lower aluminum sheet, the middle honeycomb core and the top radiating element layers. The microstrip radiating elements were etched out from thin Teflon Fibre Glass (TFG) printed sheets. These sheets were then bonded to 0.5 inch honeycomb core and the 0.3 mm aluminium sheet with proper adhesives using vacuum bonding techinques. The radiating element is designed to
perform over a large bandwidth (> 20%). The radiation panel bonded with In si level feed network is shown in fig (10).
The final antenna array was subjected to various eleclrieal. mechanical and environmental tests to ascertain the performance. As the expected averaj'.r side lobe level is around -50 dB, elaborate arrangements were made to make such measurements possible.
The antenna was tested in both the near field and far field measurement set up. Fig 12 and Fig 13 show the azimuth and elevation patterns obtained respectively. The antenna was also tested for gain and return loss. A typical value of ,34 dBi rain and 15 dB return loss were obtained.
Fig 14 and Fig 15 show the return loss plots measured at Sum and Difference ports of the Antenna respectively.
The power handling tests were conducted in a sub-panel level and detailed analysis was carried out to confirm that the antenna meets required power handling capability.
Frequency : S-Band
Bandwidth : 12%
Azimuth : 2 Degrees
Elevation : 8 Degrees
Side lobe level
Azimuth : -35 dB
Elevation : -30 dB
Gain : 34 dBi
Polarization : Vertical
Return loss : -13 dB and better
Peak : 150 kW
Average : 8 kW
Temperature -20° to +55° C
Altitude 30.000 feet (Max)
Acceleration 3 G
Vibration level 1.5Gto3G
Shock 10 G
Humidity 75% at -20° C, 50% at +55° C
Pressure 14.75 to 4.3 PSI
Size : 4.2* 1.2 m* 0.025m
Weight : 80 Kgs
1. An ultra low side lobe antenna array comprising board band
microstrip radiator and three level corporate feed network (6)
wherein the transmission lines of the feed network have
rectangular centre conductors (9) housed inside rectangular
outer conductors, characterised by power handling capability of
the order of 150 kW peak power, said antenna array (1) being
divided into four quadrants (3) and each quadrant (3) being
divided into four sub panels (4) and each sub panel has 64
radiating elements arranged in 8 x 8 matrix form.
2. An ultra low side lobe antenna array as claimed in claim 1
wherein said corporate feed network (6) comprises two levels of
squareax lines and a third level waveguide feed network.
3. An ultra low side lobe antenna array as claimed in preceeding
claims wherein the feed network (6) has the side lobe levels in
both the azimuth and elevation planes.
4. An ultra low side lobe antenna array as claimed in preceeding
claims is capable to perform over the large bandwidth of around
400MHz (>12%) in comparison to microstrip antennas.
5. An ultra low side lobe antenna array as substantially herein described and illustrated in the accompanying drawings.
|Indian Patent Application Number||2144/DEL/1998|
|PG Journal Number||40/2010|
|Date of Filing||24-Jul-1998|
|Name of Patentee||THE CHIEF CONTROLLER, RESEARCH AND DEVELOPMENT,MINISTRY OF DEFENCE|
|Applicant Address||MINSTRY OF DEFENCE, GOVERNMENT OF INDIA NEW DELHI, INDIA.|
|PCT International Classification Number||H01Q 21/22|
|PCT International Application Number||N/A|
|PCT International Filing date|