Title of Invention

HIGH GAIN WIDEBAND PLANAR MICROSTRIP ANTENNA FOR SPACE BORNE APPLICATIONS

Abstract

A HIGH GAIN WIDEBAND PLANAR MICROSTRIP ARRAY ANTENNA AT C BAND FOR SPACE BORNE APPLICATION The invention relates to a high gain wide band planar microstrip antenna at C band with dual polarization for space borne applications comprising of planar array of tiles, wherein each tile is a multilayer printed antenna comprising an upper patch (1), a lower patch (2), a thick dielectric foam (3), radome (4), substrate(5) for lower patch ,ground plane (8), adhesive layers (9) a composite base plate (10) ,two buried feedlines (7), wherein the said tile is characterized by 480 electromagnetically coupled radiating elements arranged in 24 linear arrays of 20 radiating elements (16) and corporate feed network (17) of hybrid type to feed the radiating elements (16).The antenna is adapted to be installed in a Radar Imaging Satellite (RISAT) fitted with a Synthetic Aperture Radar (SAR) and to operate at frequency 5.35GHz with bandwidth of 225 MHz , gain of 44.5 dBi, beamwidth of 0.5°(AZ) xl.5°(EL), cross-polarization of -23dB and return loss of -15dB.

Full Text

Field of Invention The present invention relates to a large size printed planar array microstrip antenna at C-band for space borne applications, particularly for high resolution multimode SAR (Synthetic Aperture Radar) systems, catering the requirements of dual polarization, wide bandwidth, beam agility, high gain, and accommodating the feed networks for dual polarization. The invention has particular use in the SAR system developed for RISAT (Radar Imaging Satellite), the first satellite-imaging mission of ISRO. Background of invention Extensive investigation and literature survey carried out in the field of large planar array advanced SAR antenna for space-borne application revealed that very few developments have taken place in the past few years to develop a very large size planar array antenna at C-band for space-borne SAR application. One such antenna is the RADARSAT SAR antenna developed by the Canadian Space Agency (CSA) to meet the requirements of Canada's advanced 5.3 GHz space based synthetic aperture radar having the dimension of 15 m by 1.5 m. It was developed in four panels using slotted planar array antenna. It has a narrow bandwidth of 32 MHz. It is difficult to achieve bandwidth of 225 MHz as desired for RISAT antenna. Hence this configuration can not be used for RISAT antenna. Moreover the gain specification is also less although the antenna area utilized is more than the area allotted for RISAT antenna. Under ESA program another planar array antenna was developed for ENVISAT. It carries an advanced SAR with an active phased array antenna as instrument sensor (ASAR). It has the capability to operate in different modes like that of RISAT SAR antenna and is much more flexible than the ERS SAR. The dimension of SAR antenna for ENVISAT is 10 m by 1.3 m and consists of 5 structural panels. Each panel has four electrical panels (tiles), each tile being provided with 384 radiating elements arranged in 16 rows of 24 radiating elements each. In total 320 T/R modules are connected to the 320 rows of dual polarized printed microstrip annular slot elements, using two feed networks, one for each polarization. The printed annular slot array antenna was designed with lesser bandwidth (100 MHz) than required for RISAT antenna. In this antenna annular slot radiators were fed from feed lines on another layer. This antenna besides being too long for installation in SAR for RISAT has the limitation of relatively narrow bandwidth of 100 MHz. With a view to overcome the limitations of the conventional planar array antenna, in the present invention a new configuration is chosen to develop the SAR antenna for RISAT which has not been reported earlier for space-borne advanced SAR application. This antenna is developed to meet stringent specifications of bandwidth, gain, cross-polarization and beamwidth. The object of the present invention is to provide a planar microstrip antenna for space borne applications having thin profile, low weight and having reduced length of 6m and surface area of 12 sq. m., suitable for operating in the C band of microwaves. The other object is to provide a planar microstrip antenna for space borne applications at C band with dual linear polarization, having wide bandwidth and high gain. Another object of the present invention is to provide a planar microstrip antenna for space borne applications at C band wherein cross-polar suppression technique used in patch feeding to get highly suppressed cross-polar radiation. Summary of the invention: With the above and other objects, which will become apparent from the description that follows hereinafter, in view, the invention provide high gain wide band planar microstrip antenna at C band for space borne applications comprising of planar array of tiles, wherein each tile is a multilayer printed antenna comprising an upper microstrip radiating patch , a lower microstrip radiating patch, a thick dielectric foam of low permittivity separating the upper patch and the lower patch, radome formed by the substrate of the inverted upper patch, substrate for lower patch ,a composite base plate, two buried feedlines to feed the stacked microstrip patches for dual linear polarization operation wherein the said tile is characterized by: 480 electromagnetically coupled radiating elements arranged in 24 linear arrays of 20 radiating each; - corporate feed network of hybrid type to feed the radiating elements in which the central 8 radiating elements are fed in corporate network, 8 of the remaining outer radiating elements are fed in corporate and the other four of the radiating elements with series feeding network. In general, a resonant patch antenna radiates at frequency where length of patch is half wavelength i.e. A/2. The narrow bandwidth of patch is attributed to the high Q factor which can be reduced for increasing the bandwidth by increasing the volume of patch or reducing the permittivity of the substrate material. In order to enhance the bandwidth several approaches have been proposed such as stacked patches, thick substrate with low dielectric constant and parasitic patch close to radiation patch. In the present invention a new configuration is chosen to develop the SAR antenna for RISAT, which has not been reported earlier for space-borne advanced SAR application. This antenna is developed to meet stringent specifications of bandwidth, gain, cross-polarization and Beamwidth. The RISAT SAR antenna is 6 m long and 2 m wide having a surface area of 12 sq.m. and is made up of three deployable panels of size 2 m by 2 m. The panel of 2 m by 2 m further comprises 4 tiles of size 1 m by 1 m. Each tile of 1 meter by 1 meter consists of 24 rows of 20 radiating elements each. Each tile further comprises three subtiles each of size lm x 0.33 m. The numbers of radiating elements in each row of tile have been chosen 20 as compared to 24 in ENVISAT. The top and the bottom patches each constitute a resonant circuit at the frequency of 5.35 GHz in the C-band of microwaves, because the length of each patch is made equal to the wavelength corresponding to this frequency. The radiating element is an electro-magnetically coupled stacked patch antenna in which square stacked microstrip patches are fed with two buried feed line to cater dual linear polarization operation. Multilayer, stacked electromagnetically coupled printed antenna is selected, which overcomes the bandwidth limitation of the conventional microstrip antenna. Aperture coupled antenna although offers the advantage of optimizing feeder network and patch independently, but the impedance matching is poor due to improper grounding when coaxial feeding below the ground is required for the array. In the present configuration, the dual polarization is achieved by choosing top and bottom patches as square shaped with orthogonal feeding. Since, this configuration does not utilize any slot on ground plane to couple power from feed lines to patch, the front to back ratio of EMC patch antenna is better as compared to the aperture coupled microstrip patch antenna. The lower and upper patches are separated by a thick layer of foam of low permittivity (dielectric constant equivalent to air) to enhance the radiation efficiency and bandwidth performance. Upper patch is inverted and its substrate acts as radome. The antenna is designed for wideband operation to meet stringent requirements of 44.5 dBi gain, minimum 23 dB cross-polarization over 225 MHz bandwidth. In total 288 T/R (Transmit /Receive) module pairs are connected to the 288 rows of dual polarized printed microstrip square patch radiating elements, using two feed networks of novel configuration, one for each polarization. Due to close connection of the T/R modules to the radiating elements, the phase shifter loses in both cases i.e. transmit and receive, are low, compared to passive array systems. This leads to low noise figure and high transmit efficiency. New type of feed network is designed in which wideband performance of a corporate feed network is realized by using hybrid network in which central 8 elements are fed in corporate network and out of remaining 12 outer elements, 8 are fed in corporate and other four with series feeding network. This feeder network is optimized within the constrained space available within the two linear array in vertical configuration. Feed network design is innovative not only in terms of layout but also the feed line circuit's orientation while feeding the patches. The network is designed to achieve highly suppressed cross-polar radiation performance in the array environment. Thus the antenna is realized in multilayer microstrip antenna configuration using thin profile materials and new fabrication processes to meet stringent electrical specifications for space-borne application. In the invented planar antenna the amplitude and phase of the network are nearly uniform over 10 % bandwidth. The cross polarization of single element is improved by using a cross-polarization compensation technique. Very thin profile substrate is used for making the feed network. The substrate used for feed network is R T Duroid 6002 of 15 mm thickness and having the dielectric constant (e) of 2.92. Each tile is provided with 480 radiating elements arranged in 24 rows each containing 20 radiating elements and is fed by the invented feed network. The mode transducer is being optimized in multilayer configuration. Brief Description of the Drawings: For better understanding an illustrative embodiment of the invention will now be described with reference to the accompanying drawings. It will however be appreciated that the embodiment exemplified in the drawings is merely illustrative and not limitative to the scope of the invention. In the accompanying drawings: Figure 1 shows the configuration of the microstrip stacked radiating element used for realizing an antenna tile with sub-element layers separated from one another; Figure 2 is a perspective view of an antenna tile with different layers thereof put together face by face; Figure 3 is a schematic view of the arrangement of 480 radiating elements with feeding points and feeding lines for dual-polarization of 24 linear arrays each containing 20 radiating elements in a tile of lm x lm size; Figure 4 is a photograph of the antenna tile of size lm x lm; Figure 5 is a photograph of an antenna sub-tile of size lm x 0.33m showing (a) front view with one radiating aperture and (b) back view with 16 connectors for feeding 8 dual linear arrays of radiating elements. Detailed Description of Drawings Referring to figures 1,2 & 4, Stacked Electromagnetically coupled patch(EMCP) antenna for RISAT SAR comprises an upper patch (1), a lower patch (2), a thick dielectric foam (3) of low permittivity separating the upper patch(l) and the lower patch(2), a radome (4) for housing the RADAR equipment, formed by the substrate of the inverted upper patch(l), substrate (5)for the lower patch, feed lines (6) to excite the patches ( 1 & 2), substrate for the feed lines(7), ground plane (8) ,adhesive layers (9) and a composite base plate (10). The upper patch (1) and the lower patch (2) are square in shape with orthogonal feeding in order to achieve dual polarization and formed by printing. The front to back ratio of EMC patch antenna is better than the aperture coupled microstrip antenna because this configuration does not utilize any slot on ground plane to couple power from feed lines(7) to patches(l& 2). Each multilayer tile, having the dimension of 1 m X 1 m is further made up of three subtiles tiles (11) of size lm X 0.3 m, namely the first subtile (12), the second subtile (13) and the third subtile (14). Each tile is provided with SMA connectors (15) for external connection. Referring to figures 3, each square tile (1 m x 1 m) consists of 24 rows of 20 radiating elements (16) each fed by the invented feed network (17) at the mid positions (18) thereof. The feed network (17) is of hybrid type in which central 8 elements are fed in corporate network and out of remaining 12 outer elements, 8 are fed in corporate and other four with series feeding network. This feeder network (17) is optimized within the constrained space available within the two linear arrays in vertical configuration. Very thin profile substrate (7) is used for making the feed network. In a preferred embodiment the substrate (7) for feed network is R T Duroid 6002 of 15 mm thickness and dielectric constant ( s ) of 2.92. Referring to Fig. 5, the front side of each sub-tile (Fig. 5 (a)) is provided with a linear radiating aperture (19) and the backside of each sub-tile (Fig. 5 (b)) is provided with sixteen connectors (20) for feeding eight dual linear arrays of radiating elements in a sub-tile. The antenna of the present invention is adapted to be installed in a Radar Imaging Satellite (RISAT) fitted with a Synthetic Aperture Radar (SAR) for observing objects on the Earth's surface and to operate at frequency 5.35 GHz with bandwidth 225 MHz, gain 44.5 dBi, beamwidth 0.5° (AZ) x 1.5° (EL), cross polarization -23 dB and return loss-15dB. The invented antenna has been assembled within close tolerance limits of the dimensions and thickness is individual layers thereof using specially selected materials for the substrates and adhesives, and following improved techniques of vacuum bagging and curing. The constructional and operational features of the invented tile and antenna are presented in Table I and Table II respectively , from which it is noted that the invented antenna is of size 6m x 2m and comprises three structural panels of size 2m x 2m each, twelve antenna tiles of size lm x lm each and thirty six antenna sub-tiles of size lm x 0.33 m each, The invented full (6m x2m ) antenna is operable at 5.35 GHz frequency with bandwidth 225 MHz, cross polarization-23 dB and return loss -15 db. The gain and beamwidth specifications of full antenna using 12 tiles of lm x lm are 44.5 dBi and 0.5° (AZ) X 1.5° (EL) respectively. Other electrical specifications of full antenna are same as that of single tile antenna of I m x lm size. Measured electrical results closely match with the simulated results and electrical specifications. Excellent cross-polar suppression is achieved in planar array environment. The measured value of cross-polar radiation of a linear array is of the order of -23 dB and for the planar array of subtiles and full tile it is of the order of -36 dB due to cross-polar suppression technique used in the feeding mechanism of patch radiators. The invented antenna has a number of advantageous features over the prior art antennas, such as, (i) Decreased length, surface area, volume and weight; (ii), Increased gain and band width; (iii) Reduced cross polarization; and (iv) dual-polarization operation. Field of Application: The invented multilayer printed antenna of size lm x 1 m at C-band can be successfully utilized as an active phased array antenna for multimode synthetic aperture radar antenna for space-borne and air-borne remote sensing applications and wilfenable monitoring in a wide filed of applications such as; vegetation, agriculture, forestry, soil moisture, geology, sea ice, coastal processes, and man-made object identification. In addition it can also be used for disaster monitoring services. We claim: 1) A high gain wide band planar microstrip antenna at C band for space borne applications comprising of planar array of tiles, wherein each tile is a multilayer printed antenna comprising an upper microstrip radiating patch (1) , a lower microstrip radiating patch (2), a thick dielectric foam (3) of low permittivity separating the upper patch(l) and the lower patch (2), radome (4) formed by the substrate of the inverted upper patch(l), substrate (5) for lower patch ,ground plane (8), adhesive layers (9) a composite base plate (10) ,two buried feedlines (7) to feed the stacked microstrip patches for dual linear polarization operation, wherein the said tile is characterized by: 480 electromagnetically coupled radiating elements arranged in 24 linear arrays of 20 radiating elements (16) each; - corporate feed network (17) of hybrid type to feed the radiating elements (16) in which the central 8 radiating elements are fed in corporate network, 8 of the remaining outer radiating elements are fed in corporate and the other four of the radiating elements with series feeding network. 2) A planar microstrip antenna as claimed in claim 1 wherein the stacked microstrip patches are square in shape for orthogonal feeding. 3) A planar microstrip antenna as claimed in claim 1 wherein the feed lines are arranged to feed two orthogonally polarized signals at the mid positions (18) of each row of radiating elements. 4) A planar microstrip antenna as claimed in claim 1 wherein 288 T/R (Transmit /Receive) module pairs are connected to the 288 rows of dual polarized printed microstrip square patch radiating elements. 5) A planar microstrip antenna as claimed in claim 1 wherein each tile is of size lm X lm. 6) A planar microstrip antenna as claimed in claim 5 wherein each tile of 1 m x 1 m further comprises of an array of three subtiles tiles, each having the dimension of 0.33m X lm. 7) A planar microstrip antenna as claimed in claim 1 wherein very thin profile substrate is used for making the feed network. 8) A planar microstrip antenna as claimed in claim 7 wherein the substrate for the feed network is RT Duroid of 15 mm thickness and having dielectric constant 2.92. 9) A planar microstrip antenna as claimed in claim 1 wherein the antenna is of length 6m and width 2 m and comprises of three deployable panels having the dimension of 2m X 2m. 10) A planar microstrip antenna as claimed in claim 9 wherein each panel further comprises 4 tiles having the dimension of lmXlm. 11) A planar microstrip antenna as claimed in any preceding claim, wherein each sub-tile is provided with a linear radiating aperture (19) and sixteen connectors (20) for feeding eight dual linear arrays of radiating elements on the back side thereof. 12) A planar microstrip antenna as claimed in an preceding claim wherein each tile is to operate at a frequency of 5.35 GHz with beam width of 2.8° (AZ) X 2.8° (EL), having gain of 33.8 dBi, gain bandwidth of 1 dB, antenna bandwidth of 255 MHz, return loss of -15 dB and cross polarization of -23 dB. 13) A planar microstrip antenna as claimed in any preceding claim, which is adapted to be installed in a Radar Imaging Satellite (RISAT) fitted with a Synthetic Aperture Radar (SAR) for observing objects on the Earth's surface and to operate at frequency 5.35 GHz with bandwidth of 225 MHz , gain of 44.5 dBi, beam width of 0.5° (AZ) xl.5°(EL), cross polarization of -23dB and return loss of -15dB. 14) A high gain wide band planar microstrip antenna for dual polarization low-cross polarization operation at C-band, substantially as herein described and illustrated in the accompanying drawings.

Documents:

2139-CHE-2007 AMENDED PAGES OF SPECIFICATION 29-02-2012.pdf

2139-CHE-2007 AMENDED CLAIMS 29-02-2012.pdf

2139-CHE-2007 CORRESPONDENCE OTHERS 18-11-2011.pdf

2139-CHE-2007 FORM-1 29-02-2012.pdf

2139-CHE-2007 FORM-3 29-02-2012.pdf

2139-CHE-2007 OTHER PATENT DOCUMENT 29-02-2012.pdf

2139-CHE-2007 AMENDED CLAIMS 30-03-2012.pdf

2139-CHE-2007 CORRESPONDENCE OTHERS 30-03-2012.pdf

2139-CHE-2007 EXAMINATION REPORT REPLY RECEIVED 29-02-2012.pdf

2139-che-2007-abstract.pdf

2139-che-2007-claims.pdf

2139-che-2007-correspondnece-others.pdf

2139-che-2007-description(complete).pdf

2139-che-2007-drawings.pdf

2139-che-2007-form 1.pdf

2139-che-2007-form 26.pdf

2139-che-2007-form 3.pdf

2139-che-2007-form 8.pdf