Claims:WE CLAIM:
1. A system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW), comprising:
a conductive layer comprising a top conductive layer and a bottom conductive layer;
a supportive substrate;
two or more distinct unit cell cavities [1, 2] between the top and bottom conducting layer;
FSS slots [4]; and
spatial filter with dual-band pass response,
wherein the cell cavities [1, 2] comprises of at least one wall of conductive material connected to said top conductive layer and said bottom conductive layer,
wherein the cavity [1] is shunted to the cavity [2] to yield dual band response multiple frequencies in a same unit cell, the cavity [1] and the cavity [2] are different in shapes and are merged to form the shunted SIW Cavity [3], and
wherein the vias of cavity [1] are shared with the vias of cavity [2] to form the unit cell, and the closed cavity [1] in the form of cross shape is modeled around the cross element by considering its shared vias along with the square cavity [2] resulting in increase of the parallel inductance of the equivalent circuit.

2. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the two distinct SIW cavities [3] are combined in one unit cell and the two distinct cavities are coupled together into a single unit cell, and the cavity design provides dual band responses with pass band characteristics between 10.9 GHz to 12.75 GHz in downlink centre frequency and 13.55 GHz to 14.55 GHz in uplink centre frequency.
3. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the cross cavity [1] is modeled to resonate lower frequency band and the square cavity [2] is modeled to resonate high frequency band.

4. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 3, wherein the vias used for the design of the square cavity [2] are used to design the cross cavity [1].

5. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the design of dual-band response is one band with narrow bandwidth characteristics and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies in satellite communication.

6. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the FSS slots [4] in two SIW cavities [2] are designed in such a way to resonate close to each other for an ultra-broadband response at two different frequencies.

7. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the frequency selective surface (FSS) is an electromagnetic band pass spatial filter.

8. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the broadband response comprises of 90 degree polarization conversion [5] (E-plane to H-plane and vice-versa) and linear-to-circular polarization conversion [6] (E-plane to circular, H-plane to circular and vice-versa) and the like, and the via forms an electrical contact between the two planes.

9. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the square cavity [2] comprises of four slots aligned near to the metallic via walls of cavity where the dominant modes are occurred.

10. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the tunability or shift of resonance frequency from the cross cavity [1] are attained by changing the dimensions (length and width of slot) to desirable value without disturbing the SIW cavity [3].

11. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the diameter of coupling vias in the cavity is 0.5 mm to 1.2 mm, and the diameter of tuning metallic post is 0.6 mm to 2.5 mm and the pitch of the metallic vias is 1.0 m to 1.5 mm.

12. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the period of FSS unit cell is preferably 20.0 mm and calculated based on the cavity mode of the SIW and is varied by changing the dielectric properties of substrate material.

13. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the dual cavity structure is created on the substrate material through metalization of cylindrical vias placed at equidistance with a defined diameter.

14. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the conductive layer are made of copper material with thickness 35 micrometer and conductivity 5.80×107 S/m at 20 °C and the conducting material is configured to create metallic vias.

15. The system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW) as claimed in claim 1, wherein the both sides of the unit cell comprises of identical slots with equal shape and size.

16. A method of designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW), comprising:
computing the size and mode of operations of the cavity analytically.
wherein the different substrate materials preferably dielectric substrate with dielectric constants varying from 2.0 to 6.0 is used to design the shunted cavity design,
designing a square cavity [2] to resonate at the desired frequency band based on its size;
modeling a cross cavity [1] in adjacent to the square cavity [2];
etching a cross slot from the top, and the bottom surface of the cross cavity [1];
aligning four symmetrical slots aligned near the edges of the square cavity [2]; and
equally spacing slots from the centre of the square cavity [2].
wherein the similar slots are etched on either side of the substrate material and the arrangement can generate the dual-band response in two distinct frequency bands,
placing an additional cylindrical vias at the centre of the square cavity [2]; and
wherein the additional cavity comprises the capability of tuning the operating frequency and bandwidth of second resonance,
estimating the resonance performance of the shunted substrate integrated waveguide cavity frequency selective surfaces,
wherein the plane-wave is exited from the z-axis to the top surface of FSS slots [4], and
wherein the apertures are created on both top and bottom metallic surfaces of the substrate material to form the shunted FSS slots [4] based on substrate integrated waveguide. , Description:FIELD OF THE INVENTION
The present invention relates to the field of field of microwave theory and techniques specifically a spatial filter with dual-bandpass response based on the high performance frequency selective surface with neighboring cell perturbation. More particularly the present invention relates to a system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW). Further the present invention relates to a method of designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW).

BACKGROUND OF THE INVENTION
Frequency Selective Surface (FSS) possesses a wide range of practical engineering applications. The proposed FSS is an electromagnetic bandpass spatial filter with very good selectivity and very low transmission loss in its passband. In the microwave field, the FSS frequency can be used as a multiplexer that can be used for communications satellite systems, the use of multi-feed configuration to expand the communication capacity. The SIW cavity based FSS designs found its spectacular applications in the design of polarization converters. Another interesting application of proposed design is the radome for base station and airborne to shield the stealth aviation radar antenna at targeted location. This can also be used to fabricate high performance monolithically integrated waveguide filters. The shape, size, dimensions of the FSS are crucial parameters for the design of FSS to attain the desired performance from FSS such as type of the filter, frequency selection, high selectively, polarization insensitivity, stable performance for higher incident angles, dual-band and multi-band. The conventional shapes and design techniques as well as fractal geometries of FSS require complex optimization techniques to obtain the aforementioned functionalities. Further, multilayer structures are modeled to achieve multiband and wide band structures.

However, these methods have poor performance and more complex structures which are more expensive and difficult to implement. These methods may take more time and efforts to implement the efficient designs. Particularly, for multi-band applications, the conventional method is difficult to achieve a narrow band gap.

Recently, the SIW based FSS are proved to be the efficient designs to overcome the above limitations. However, the intended cavity based FSS shows the broadband response at desired frequency band and does not exhibit multiband response. Dual band may achieved by cascading different sized cavities but that is results in complex design and inefficient designs. The satellite and military communication systems require multiband operations for downlink and uplink frequencies. Specifically, military communications-on-the-move (COTM) provides warfighters with mobile communication using satellites in order to connect multiple remote locations under the vital circumstances.

CN105069235B - The invention discloses a method for extracting an equivalent circuit parameter of a dual frequency-band frequency selective surface, and mainly solves the boundedness problem in the prior art that an empirical formula is depended to extract the equivalent circuit parameter. The method comprises the following implementation steps: 1) converting a scattering matrix of the frequency selective surface into a transmission matrix; 2) obtaining an admittance matrix and an impedance matrix of an equivalent circuit through the transmission matrix; 3) expressing the admittance matrix and the impedance matrix by lumped elements in the equivalent circuit; and 4) adopting a curve fitting optimization algorithm to extract the parameter in the equivalent circuit. The method can extract the equivalent circuit parameter of any dual-frequency-band frequency selective surface, improves the precision of the equivalent circuit parameter of the dual-frequency-band frequency selective surface and can be used for quickly and accurately analyzing the characteristics of the dual-frequency-band frequency selective surface. In the present invention is not dealing with the circuit parameter extraction. It is discussing about the design of Dualband Frequency Selective Surface based on Shunted Dual Cavity Substrate Integrated Waveguide.

CN102176543B- The invention discloses a cross spiral frequency selective surface (FSS) structure with dual band characteristics and a construction method thereof. The structure consists of an upper dielectric layer, a lower dielectric layer and an intermediate cross spiral FSS structure layer. The construction method comprises the following six steps of: 1, establishing structure units of the cross spiral FSS structure layer with the thickness of zero by using computer simulation technology (CST) software; 2, periodically extending the structure units towards directions X and Y to obtain the integrated 6*6 cross spiral FSS structure layer; 3, establishing the dielectric layers of the cross spiral FSS structure, and embedding the cross spiral FSS structure layer into a dielectric substrate by using the CST software; 4, capturing the cross spiral FSS structure unit from the CST software for incident wave transmittance analysis, regulating parameters of the structure unit, changing the resonance frequency of a stop band, and replacing an original structure by using the regulated structure unit; 5, setting the parameters such as boundaries and the like of the constructed cross spiral FSS structure, and testing the bandwidth stability and frequency selectivity of the structure; and 6, machining and manufacturing the cross spiral FSS structure by adopting a double-sided copper-clad plate according to an optimal size determined by the step 4. The present innovation discloses the design of dual-band response from the FSS based on SIW cavity design.

CN203085713U - A substrate integrated waveguide dual-mode filter comprises a dielectric substrate, wherein upper and lower surfaces of the dielectric substrate are respectively provided with an upper surface metal layer and a lower surface metal layer, and the dielectric substrate is further provided with a metal layer penetrating through the upper surface and a metalized via of the surface metal layer, wherein the metalized via, the upper metal layer and the lower metal layer form a square dual-mode cavity on the dielectric substrate, and the first cavity is provided with a first cavity a disturbing groove line and a second disturbing groove line, wherein the two sides adjacent to the dual-mode cavity are respectively provided with a first metal column inductive window and a second metal column inductive window, the first metal column inductive window and the second metal column The inductive window is provided with a coplanar waveguide input and a coplanar waveguide output. The present innovation consists of shunted substrate integrated waveguide cavity on a dielectric substrate. The top and bottom surfaces of the dielectric substrate material are coated with copper layer (perfect electric conductor) of thickness 35 microns. The dual cavity structure has been created on the substrate material through metalization of cylindrical vias. Furthermore, it is worth to mention that the invention presented in CN203085713U is a planar micro strip filter with micro strip feed. Whereas, the proposed invention deals with the spatial filter for free space application. The proposed FSS filters the free space electromagnetic wave.

US6670932 B1- An artificial magnetic conductor includes a frequency Selective Surface having a frequency dependent permeability a direction normal to the frequency dependent Surface, a conductive ground plane, and a rodded media disposed between the frequency Selective Surface and the conductive ground plane. the one or more arrays of artificial magnetic molecules is resonant in normal permeability at two or more frequencies and wherein the conductive posts of the array of conductive posts are electrically short relative to wavelength in the dielectric of the rodded media at the two or more frequencies. Proposed SIW cavity generates two resonant frequencies with wide spacing. The resonances are mainly due to the slots and cavities of the unit cell.
WO2009116934Al- substrate integrated waveguide comprises a top conductive layer and a bottom conductive layer provided on either side a substrate. At least one wall of conductive material is provided in the substrate to define, together with the top and bottom layers, the waveguide. The at least one wall comprise a multitude of thin conductive wires densely arranged close to each other in the substrate and having respective short ends connected to the top and bottom layers. The high number of wires per surface unit in the wall effectively prevent significant amount of power leakage through the wall during operation of the substrate integrated waveguide. In the present innovation, the SIW vias are created through the dielectric substrate. The substrate is placed in between the two metallic (conducting) layers.

The given reference patent file is about the SIW based waveguides and waveguide components. It comprises a design methodology of modeling waveguide and waveguide components. This contains a wired input and gives the output.

Whereas, the presented innovation is a spatial filter which used no wired input. The structure is an Electromagnetic filter which is designed to operate two frequencies to pass through it. There no similarities between the given patent file and proposed innovation except the SIW via concept.

In the proposed innovation and given patent file, only SIW is the common term. Whereas the design methodology and usage is completely different. Also, the claimed application is different.

There exists a need for a system for designing frequency selective surfaces (FSS) based on substrate integrated waveguide (SIW). Further there exists a need for a method of designing frequency selective surfaces (FSS) based on substrate integrated waveguide (SIW).

OBJECTS OF INVENTION
It is primary object of the present invention to provide a system for designing frequency selective surfaces (FSS) based on substrate integrated waveguide (SIW).

It is another object of the present invention to provide a system with high-quality factor (Q) of cavity-based structure, offers sharp roll-off performance characteristics.

It is another object of the present invention to provide a system with polarization conversion based on the arrangement of four slots in the square cavity.

It is another object of the present invention, wherein the substrate integrated waveguide is high-order dual-mode, and dual-cavity FSS is presented using substrate integrated waveguide technology and is applied on the upper and lower surfaces of the dielectric substrate.

It is another object of the present invention, wherein the designed slots based on the change in their orientation and position can exhibit multi-polarization behaviour.

It is another object of the present invention, wherein the FSS slots were shunted on a single planar surface to achieve the high-performance characteristics and two different shaped cavities are merged to form a single unit cell.

It is another object of the present invention, wherein the present system is multi-cavity, multi-slot, dual-band frequency selective surface with a distinct band of operations and it can also be used as a transmission type polarization converter by altering the slots in the cavity.

It is another object of the present invention, wherein the system can be used to convert the linearly polarized wave (e-plane or H-plane) to circularly polarize out going wave and vice versa.

It is another object of the present invention, wherein the proposed frequency selective surface is a high-performance design based on Substrate integrated waveguide cavity with dual-band operation such as narrow bandpass and wide bandpass with sharp roll-off performance characteristics at the edges of the passband.

SUMMARY OF THE INVENTION
One or more of the problems of the conventional prior art may be overcome by various embodiments of the present invention.

It is the primary aspect of the present invention to provide a system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW), comprising:
a conductive layer comprising a top conductive layer and a bottom conductive layer;
a supportive substrate;
two or more distinct unit cell cavities between the top and bottom conducting layer;
FSS slots; and
spatial filter with dual-band pass response,
wherein the cell cavities comprises of at least one wall of conductive material connected to said top conductive layer and said bottom conductive layer,
wherein the cavity is shunted to the cavity to yield dual band response multiple frequencies in a same unit cell, the cavity and the cavity are different in shapes and are merged to form the shunted SIW Cavity, and
wherein the vias of cavity are shared with the vias of cavity to form the unit cell, and the closed cavity in the form of cross shape is modeled around the cross element by considering its shared vias along with the square cavity resulting in increase of the parallel inductance of the equivalent circuit.

It is another aspect of the present invention, wherein the two distinct SIW cavities are combined in one unit cell and the two distinct cavities are coupled together into a single unit cell, and the cavity design provides dual band responses with pass band characteristics between 10.9 GHz to 12.75 GHz in downlink centre frequency and 13.55 GHz to 14.55 GHz in uplink centre frequency.

It is another aspect of the present invention, wherein the cross cavity is modeled to resonate lower frequency band and the square cavity is modeled to resonate high frequency band.

It is another aspect of the present invention, wherein the vias used for the design of the square cavity are used to design the cross cavity.

It is another aspect of the present invention, wherein the design of dual-band response is one band with narrow bandwidth characteristics and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies in satellite communication.

It is another aspect of the present invention, wherein the FSS slots in two SIW cavities are designed in such a way to resonate close to each other for an ultra-broadband response at two different frequencies.
It is another aspect of the present invention, wherein the frequency selective surface (FSS) is an electromagnetic band pass spatial filter.

It is another aspect of the present invention, wherein the broadband response comprises of 90 degree polarization conversion (E-plane to H-plane and vice-versa) and linear-to-circular polarization conversion (E-plane to circular, H-plane to circular and vice-versa) and the like, and the via forms an electrical contact between the two planes.

It is another aspect of the present invention, wherein the square cavity comprises of four slots aligned near to the metallic via walls of cavity where the dominant modes are occurred.

It is another aspect of the present invention, wherein the tunability or shift of resonance frequency from the cross cavity are attained by changing the dimensions (length and width of slot) to desirable value without disturbing the SIW cavity.

It is another aspect of the present invention, wherein the diameter of coupling vias in the cavity is 0.5 mm to 1.2 mm, and the diameter of tuning metallic post is 0.6 mm to 2.5 mm and the pitch of the metallic vias is 1.0 m to 1.5 mm.

It is another aspect of the present invention, wherein the period of FSS unit cell is preferably 20.0 mm and calculated based on the cavity mode of the SIW and is varied by changing the dielectric properties of substrate material.

It is another aspect of the present invention, wherein the dual cavity structure is created on the substrate material through metalization of cylindrical vias placed at equidistance with a defined diameter.
It is another aspect of the present invention, wherein the conductive layer are made of copper material with thickness 35 micrometer and conductivity 5.80×107 S/m at 20 °C and the conducting material is configured to create metallic vias.

It is another aspect of the present invention, wherein the both sides of the unit cell comprises of identical slots with equal shape and size.

It is another aspect of the present invention to provide a method of designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW), comprising:
computing the size and mode of operations of the cavity analytically;
wherein the different substrate materials preferably dielectric substrate with dielectric constants varying from 2.0 to 6.0 is used to design the shunted cavity design,
designing a square cavity to resonate at the desired frequency band based on its size;
modeling a cross cavity in adjacent to the square cavity;
etching a cross slot from the top, and the bottom surface of the cross cavity;
aligning four symmetrical slots aligned near the edges of the square cavity;
equally spacing slots from the centre of the square cavity;
wherein the similar slots are etched on either side of the substrate material and the arrangement can generate the dual-band response in two distinct frequency bands,
placing an additional cylindrical vias at the centre of the square cavity; and
wherein the additional cavity comprises the capability of tuning the operating frequency and bandwidth of second resonance,
estimating the resonance performance of the shunted substrate integrated waveguide cavity frequency selective surfaces,
wherein the plane-wave is exited from the z-axis to the top surface of FSS slots, and
wherein the apertures are created on both top and bottom metallic surfaces of the substrate material to form the shunted FSS slots based on substrate integrated waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, may be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments.
Figure 1 illustrates a cross sectional view of the FSS Cavity 1 based on SIW cavity according to one embodiment of the present invention.
Figure 2 illustrates a cross sectional view of the FSS Cavity 2 based on SIW cavity according to one embodiment of the present invention.
Figure 3 illustrates the design of proposed FSS based on SIW cavities according to the present invention.
Figure 4 illustrates a cross sectional view of the SIW Cavities in accordance with the present invention.
Figure 5 illustrates a cross sectional view of the frequency selective slots in accordance with the present invention.
Figure 6 illustrates the graphical representation of the EM response of the proposed FSS based on shunting the SIW cavities in accordance with the present invention.
Figure 7 illustrates the top and bottom view of the Polarization conversion of H-plane (X-polarized) (5) to circularly polarization in accordance with the present invention.
Figure 8 illustrates the top and bottom view of the Polarization conversion of E-plane (Y-polarized) (5) to circularly polarization in accordance with the present invention.
Figure 9 illustrates the graphical representation of the frequency response of the structure shown in Fig. 7 for X- and Y-polarized input wave in accordance with the present invention.
Figure 10 illustrates the graphical representation of the frequency response of the structure shown in Fig. 8 for X- and Y-polarized input wave in accordance with the present invention.
Figure 11 illustrates the photograph of the fabricated prototype in accordance with the present invention.
Figure 12 illustrates the graphical representation of the comparison of simulated and measured results in accordance with the present invention.
Figure 13a and 13b illustrates the Frequency response of FSS slots in conventional and SIW form. (a) Cross slot, (b) Fan shaped slot in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION
It is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The present disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

The present invention relates to the field of field of microwave theory and techniques specifically a spatial filter with dual-bandpass response based on the high performance frequency selective surface with neighboring cell perturbation. More particularly the present invention relates to a system for designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW). Further the present invention relates to a method of designing frequency selective surfaces (FSS) based on multi-cavity substrate integrated waveguide (SIW).

The configuration can be widely used in satellite communications, with one frequency dedicated to downlink (lower frequency band) and another frequency for uplink frequency and in radar weapon platform. Also, in the efficient wireless communication systems. The intended invention is mainly focused on the design of filter associated with military COTM KU Band frequencies. The design offers dual band responses with passband characteristics at 11.0 GHz (approximate downlink centre frequency) and 14.0 GHz (approximate uplink centre frequency).

The novelty of the proposed design lies in its cavity design. Two different cavities with different performance characteristics have been shunted together to obtain dual-band response. The concept shared vias have been employed here. The vias used for the design of Cavity-2 has been used to design the Cavity-1. This shunting mechanism helps in combining different FSS unit cells based on SIW cavities together to exhibit unique performance from this kind of structures. In this work, we mainly focused on the design of dual-band response: one and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies of military COTM (Communications-On-The-Move) application. The slots in two cavities can also design in such a way to resonate close to each other, so that an ultra-broadband response can be achieved.

The invention will give the new dimensions to the design of FSS for multiband application based on shunted cavity with shared vias. The invention is most suitable for military COTM application to facilitate uplink and downlink frequencies. Also, the structure can exhibit polarization conversion capabilities with band pass response. The sharp roll-off behavior will help in avoiding the interference with other frequency band signals.
The proposed design is based on the substrate integrated waveguide cavity filter. The structure is designed to operate at two distinct frequency bands with sharp roll-off performance characteristics to suppress the out-of-band antenna interference. In the proposed concept, each unit cell contains two distinct FSS slots separated by metallic via cavity on a dielectric substrate. The designed slots based on the change in their orientation and position can exhibit multi-polarization behavior.

The invention relates to the superior performance frequency selective surface based on shunted dual substrate integrated waveguide cavity model with multi-functional characteristics based on neighboring unity cell perturbations. Conventional frequency selective surfaces suffer from large volume, poor performance stability and low selectivity. The frequency selective surfaces based on substrate integrated waveguide have developed to overcome the aforementioned problems. In the present invention, two distinct unit cell cavities (namely cavity-1 and cavity-2) are shunted in such a way to provide two different frequency bands with different characteristics. The cavity-1 is instituted for narrowband response whereas the cavity-2 is modeled for broadband response. Cavity-1 consists of fixed design on top and bottom surfaces with high selectivity. The cavity-2 is designed in such a way that it can offer multi-functionalities by altering its slots on top and bottom surface. It can show the broadband response, 90 degree polarization conversion (E-plane to H-plane and vice-versa) and linear-to-circular polarization conversion (E-plane to circular, H-plane to circular and vice-versa). In addition, the design also exhibits improved selected passband and can easily achieve the passbands with independent controllable properties with stable performance for incident and polarization angles.

Substrate: Rogers RT Duroid 5870 with dielectric constant er = 2.33 and tand = 0.0012 having thickness, h = 1.57 mm has been used as a dielectric substrate. Conducting layers: A copper material with conductivity 5.96×107 (S/m) and resistivity 1.68×10-8 (?.m) with thickness 0.035 mm is as a conducting layer for the design. Same conducting material is used for creating the metallic vias. Cavities: Cavity is created on the substrate material. Which comprises series of metallic vias placed at equidistance with a defined diameter. Vias and dimension: C1 and C2 are two separate cavities shown in above figure. The vias between the cavities is shared two forms the proposed unit cell. Dielectric material: Rogers RT Duroid 5870 with dielectric constant er = 2.33 and tand = 0.0012 having thickness, h = 1.57 mm has been used as a dielectric substrate.

An frequency selective surface based substrate integrated waveguide cavity is proposed. Top and bottom metallic planes are connected with metallic vias. Two different shaped cavities are merged to form the proposed shunted SIW Cavity. Two distinct SIW cavities consist different slot elements with different dimensions. Used to resonate at two different frequencies. The closed SIW cavity connects the top and bottom metallic planes. The via forms an electrical contact between the two planes. Proposed SIW cavity generates two resonant frequencies with wide spacing. The resonances are mainly due to the slots and cavities of the unit cell. The FSS based on SIW contains conductive posts of the array of conductive posts comprise plated through holes in a printed circuit board, each plated through hole being in electrical contact with the conductive ground plane. In the proposed model, both sides of the unit cell contain identical slots with equal shape and size. No foam material is used in this proposed work. The vias are created through the dielectric substrate and in contact with the bottom surface and makes the electrical contact. A method of designing shunted SIW cavity based frequency selective surface is presented in the innovation. Two different SIW cavities shunted to generate multiple frequencies in a same unit cell. Further, there is a provision to generate multi-polarization through the slots on the top and bottom surfaces of the unit cell. The SIW cavity is formed using series of metallic vias with equal spacing. In the present innovation, the SIW vias are created through the dielectric substrate. The substrate is placed in between the two metallic (conducting) layers.
Two distinct SIW cavities namely cavity 1 and Cavity 2 which possesses different characteristics and have diversity in their design procedure. The both cavity models are shown in Fig. 1 and 2. The Cavity-1 intended for high-selectivity application with narrow band characteristics and Cavity-2 will exhibit wider bandwidth with flat-top response. In addition, the Cavity-2 can possess different characteristics in different alignments of it slots on top and bottom surfaces.

This two distinct cavities were coupled together into a single unit cell to operate in two different frequency bands to facilitate both downlink and uplink frequencies in satellite communication. The proposed coupled dual cavity formed based on shunting two SIW cavities is shown in Fig. 3. Since, the filter allows specific frequency signals through it and all the external/unwanted signals in the out-of-operating band will be suppressed. The alignment of cavity vias and slots position is clearly shown in Fig. 4 and 5. The frequency response of the proposed dual cavity FSS based on SIW is shown in Fig. 6.

The design of FSS based on SIW is quite different from conventional planar periodic FSS. Despite of printing the metallic pattern on one side the substrate, a cavity based discrete unit cells will be modelled on the substrate. To design a FSS based on SIW: First, a metallic cavity with vias has to prepare to resonate (fr, SIW) at its dominant mode [3]. Next, the FSS slots will be printed on either side of the cavity to resonate at desired frequency (fr, FSS). By adjusting the both fr, SIW and fr, FSS resonate close to each other, the wideband frequency response can be achieved with flat top response. Also, the vias connected both top and bottom surface of the FSS will act as parallel inductance to the equivalent circuit of FSS. This will increase the overall inductance of the circuit meanwhile the quality factor (Q) will increase. As a result, a sharp roll-off response will be achieved in the frequency response of the structure.

In the proposed design, two different cavity models have been designed. At first, the cavity holes have been drilled as per the designed shape and size on the copper coated dielectric substrate. Then the proposed FSS slots have etched on the either sides of cavity. A conventional low cost PCB process combined with electroplating has been employed to design the structure. The design parameters of the proposed structure have been given in Table 2.

Table 2: Design dimensions of the Proposed FSS based on shunted SIW cavities.
Design Parameter Design Value Design Parameter Design Value
W 20.0 mm dv1 0.80 mm
L1 10.8 mm dv2 1.00 mm
L2 8.40 mm dp 1.25 mm
L3 L2-S2 D 15.0 mm
S1 1.20 mm h 1.57 mm
S2 2.80 mm er 2.33

The main object of the present invention is designing dual band FSS based on shunting two distinct SIW cavities namely cavity-1 and cavity-2 for individual performance. Each cavity has its own significance.

The system for designing frequency selective surfaces (FSS) based on substrate integrated waveguide (SIW), comprises a conductive layer comprising a top conductive layer and a bottom conductive layer, a supportive substrate, two or more distinct unit cell cavities [1, 2] between the top and bottom conducting layer, FSS slots [4] and spatial filter with dual-band pass response. The cell cavities [1, 2] comprises of at least one wall of conductive material connected to said top conductive layer and said bottom conductive layer. The cavity [1] is shunted to the cavity [2] to yield dual band response multiple frequencies in a same unit cell, the cavity [1] and the cavity [2] are different in shapes and are merged to form the shunted SIW Cavity [3]. The vias of cavity [1] are shared with the vias of cavity [2] to form the unit cell, and the closed cavity [1] in the form of cross shape is modeled around the cross element by considering its shared vias along with the square cavity [2] resulting in increase of the parallel inductance of the equivalent circuit.

The two distinct SIW cavities [3] are combined in one unit cell and the two distinct cavities are coupled together into a single unit cell, and the cavity design provides dual band responses with pass band characteristics between 10.9 GHz to 12.75 GHz in downlink centre frequency and 13.55 GHz to 14.55 GHz in uplink centre frequency. The cross cavity [1] is modeled to resonate lower frequency band and the square cavity [2] is modeled to resonate high frequency band. The vias used for the design of the square cavity [2] are used to design the cross cavity [1]. The design of dual-band response is one band with narrow bandwidth characteristics and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies in satellite communication.

The FSS slots [4] in two SIW cavities [2] are designed in such a way to resonate close to each other for an ultra-broadband response at two different frequencies. The frequency selective surface (FSS) is an electromagnetic band pass spatial filter. The broadband response comprises of 90 degree polarization conversion [5] (E-plane to H-plane and vice-versa) and linear-to-circular polarization conversion [6] (E-plane to circular, H-plane to circular and vice-versa) and the like, and the via forms an electrical contact between the two planes.

A method of designing frequency selective surfaces (FSS) based on substrate integrated waveguide (SIW), comprising the following steps:
computing the size and mode of operations of the cavity analytically.
the different substrate materials preferably dielectric substrate with dielectric constants varying from 2.0 to 6.0 is used to design the shunted cavity design,
designing a square cavity [2] to resonate at the desired frequency band based on its size;
modeling a cross cavity [1] in adjacent to the square cavity [2];
etching a cross slot from the top, and the bottom surface of the cross cavity [1];
aligning four symmetrical slots aligned near the edges of the square cavity [2]; and
equally spacing slots from the centre of the square cavity [2].
the similar slots are etched on either side of the substrate material and the arrangement can generate the dual-band response in two distinct frequency bands,
placing an additional cylindrical vias at the centre of the square cavity [2]; and
the additional cavity comprises the capability of tuning the operating frequency and bandwidth of second resonance,
estimating the resonance performance of the shunted substrate integrated waveguide cavity frequency selective surfaces,
the plane-wave is exited from the z-axis to the top surface of FSS slots [4], and
the apertures are created on both top and bottom metallic surfaces of the substrate material to form the shunted FSS slots [4] based on substrate integrated waveguide.
The present innovation discloses the design of dual-band response from the FSS based on SIW cavity design. The design consists of an FSS based on Shunted SIW cavity design. This includes two different shaped SIW cavities such as square shape and cross shape. Two different FSS slots were etched in the cavities to exhibit dual frequency bands.

In the present invention, the design is as follows: Step 1: Initially, the size and mode of operations of the cavity are computed analytically. A dielectric substrate with er = 2.2 has been used to design the proposed shunted cavity design.
Step 2: Design a square cavity to resonate at the desired frequency band based on its size (Cavity -2). Next, a cross cavity has been modeled in adjacent to the square cavity (Cavity -1).
Step 3: A cross slot has been etched from the top, and the bottom surface of the cross cavity and four symmetrical slots have been aligned near the edges of the square cavity. These slots are equally spaced from the centre of the square cavity. The similar slots were etched on either side of the substrate material. This arrangement can generate the dual-band response in two distinct frequency bands.
Step 4: An additional cylindrical vias has been kept at the centre of the square cavity. This additional cavity has the capability of tuning the operating frequency and bandwidth of second resonance.
Step 5: The resonance performance of the proposed shunted SIW cavity FSS is estimated using CST full-wave simulation. The plane-wave is exited from the z-axis to the top surface of FSS. The unit cell boundary conditions were assigned along the x- and y- directions whereas the Floquet boundaries were assigned along the z-direction.

Shunted Cavity Design: The novelty of the proposed design lie in its cavity design. Two different cavities with different performance characteristics have been shunted together to obtain dual-band response. The concept shared vias have been employed here. The vias used for the design of Cavity-2 has been used to design the Cavity-1. The sharing of vias can be seen in the Fig. 3. This shunting mechanism helps in combining different FSS unit cells based on SIW cavities together to exhibit unique performance from this kind of structures. In this work, we mainly focused on the design of dual-band response: one band with narrow band characteristics and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies of military COTM (Communications-On-The-Move) application. The slots in two cavities can also design in such a way to resonate close to each other, so that an ultra-broadband response can be achieved.

Miniaturized Performance: Next notable design metrics of the proposed structure is the size of the unit cell. The size of the unit cell is 20 mm which corresponds to 0.733?. Which consist of two cavities to offer double band response. Based on the perturbation phenomenon and shared cavity the miniaturized performance has been achieved in single unit cell. The miniaturized design will be more effective for using onto the conformal surfaces. In practice, many practical EM structures such as transmit arrays and radomes are in conformal shape. Generally, the airborne radomes are in conical form. Hence, at the tip of the radome the curvature bend radius is small. The miniaturized FSS element can be easily incorporated at these small dimensions for optimal performance.

Ultra-thin Design: The thickness of the substrate used for the design is 1.57 mm which is 0.057? (? at lower operating frequency). The ultra-thin design can be more useful to incorporate the proposed structure into the walls of dielectric layers and other substrate materials easily. Especially for the design of hybrid radomes for Military COTM application this design may found more suitable candidate.

Cavity-1: The cavity-1 consist of cross shaped FSS slot whose length and width will decide the resonance frequency of the bandpass filter. A closed cavity in the form of cross shape has been modeled around the cross element by considering its shared vias along with the Cavity-2. The closed cavity results in increase the parallel inductance of the equivalent circuit thus increases overall inductance of the circuit. This results in improvement of quality factor (Q) and reduction in its bandwidth. The relation between the Q-factor and frequency can be expressed as:
(1)
(2)
where L, R and C represents the inductance, resistance and capacitance, respectively.

The reduced bandwidth has its significant application in higher selectivity which is most desirable for tracking applications. The tunability or shift of resonance frequency can be achieved by changing the dimensions (length and width of slot) to desirable value. As per the given dimensions of the cavity a tunability of GHz can be achieved.

Cavity-2: Cavity-2 is the square cavity comprising of four slots arranged at the four edges of the cavity as shown in Fig. 3. Similar slots in same pattern have been printed on either side of the substrate. The slots in its presented form will act as a bandpass filter with wideband characteristics. The cavity-II has designed in such a way to resonate in Ku band with a bandwidth of 1.0 GHz from 13.55 GHz to 14.55 GHz with an insertion loss (IL) of less than -0.4 dB in entire operating band at its designed dimensions shown in Table 1. In addition the minimum insertion loss has been maintained in entire operating band which is evident of flat-top response. Also, a sharp roll-off performance has achieved at the lower frequency of the operating band. The sharp roll-off will helps in suppressing the out-of-band signals to pass through the structure. Further, the bandwidth and insertion loss can be improved by changing its design dimensions.

The square cavity will generate a resonance frequency (fr, SIW) and the slot (fr, S) will generate second resonance frequency. By designing the cavity and slots resonate close to each other will generate dual resonance response with wideband characteristics. The resonance frequency and size of the SIW cavity can be calculated by []:
(3)

(4)
the above equation is valid for . Dxeff and Dyeff are the effective length of the SIW cavity along the x- and y- axis respectively. Whereas, dv1 and dp are the diameter via and pitch distance between the vias respectively. This conditions of via alignment is applicable for both cavity models as the same vias have been utilized and shared the vias to create cavity models. The size of the square SIW cavity for different operating modes have been computed and given in the Table 1.

Proposed Shunted dualband SIW Cavity:
The novelty of the proposed design lies in its cavity design. Two different cavities with different performance characteristics have been shunted together to obtain dual-band response. The concept shared vias have been employed here. The vias used for the design of Cavity-2 has been used to design the Cavity-1. The sharing of vias can be seen in the Fig. 3. This shunting mechanism helps in combining different FSS unit cells based on SIW cavities together to exhibit unique performance from this kind of structures. In this work, we mainly focused on the design of dual-band response: one band with narrow band characteristics and another band with wide bandwidth characteristics to facilitate the uplink and downlink frequencies of military COTM (Communications-On-The-Move) application. The slots in two cavities can also design in such a way to resonate close to each other, so that an ultra-broadband response can be achieved.

The frequency response of cross slot and fan shaped slot in its conventional form as well as in the SIW cavity form is shown in Fig. 13a and 13brespectively. As discussed, the cross slot has shown narrow bandwidth than its conventional form. Whereas, the fan shaped slot has shown broad bandwidth with the effect of SIW Cavity. The proposed combined shunted SIW cavities have been shown in Fig. 4 along with the design notations. Further, the alignment of cavities and slot positions were shown in Fig. 4 and 5 respectively.

The design parameters of the proposed structure have been given in Table 1. Rogers RT Duroid 5870 with dielectric constant er = 2.33 and tand = 0.0012 having thickness, h ¬= 1.57 mm has been used as a dielectric substrate. The proposed FSS based on SIW has been designed on the substrate in two steps.
Step-1: At first the metallic cavity is created on the substrate material by creating a series of cylindrical vias on the substrate in such a way to touch the top and bottom metallic coating of the substrate.

Step-2: Next, the cross and fan shaped apertures were created in the cross and square shaped cavities. The apertures were created on both top and bottom metallic surfaces of the substrate material to form the proposed shunted FSS based on SIW as shown in Fig. 4.

FSS element design:
In the proposed model, two different kinds of slots have been used to generate the dual resonance characteristics. In the first case, the cavity-1 consist of a cross slot which is modeled to resonate lower frequency band (fr1). The resonance frequency of the cross slot can be determined as [vordoxglou book]:

(1)

Where, L1 is the length of cross element. c0 ¬and er are the velocity of light in free-space and dielectric constant of the substrate material used.

In the second case, the cavity-2 consists of four slots aligned near to the metallic via walls of cavity where the dominant modes are occurred. The slots are designed to resonate at higher frequency band. The resonance frequency of four slots (fr2) aligned in a cyclic order can de expresses as []:
(2)

where, S2 and L2 are the width and length of each slot in the cavity-2.

The comparison of frequency response of proposed Cavity-1 and Cavity-2 based on SIW and its conventional form has been shown in Fig. 6 respectively. The cavity-1 is designed to resonate at X-band. However, the cavity is nothing to do with the shape of the slot. Hence, the resonance frequency is remains unchanged. From the observations, it is corroborated that the fractional bandwidth (FBW) of cross slot is reduced (%) compared with its established single layer form. It is mainly because of the alignment of SIW cavity created in the cross shape. The alignment will enables the unit cell as a virtually thick screen. Also, the SIW vias will introduce the parallel inductance to the equivalent circuit of cross slot. This will increase the total inductance and results in reduction of the bandwidth as per the equation ().

The tunability or shift of resonance frequency from the cavity-1 can be attained by changing the dimensions (length and width of slot) to desirable value without disturbing the SIW cavity. As per the given dimensions of the cavity and cross slot the tunability of GHz can be achieved.

Next, the cavity-2 has designed in such a way to resonate in Ku-band with a bandwidth of 1.0 GHz from 13.55 GHz to 14.55 GHz with an insertion loss (IL) of less than -0.4 dB in entire operating band at its designed dimensions shown in Table 3. In addition the minimum insertion loss has been maintained in entire operating band which is evident of flat-top response. Also, a sharp roll-off performance has achieved at the lower frequency of the operating band. The sharp roll-off will helps in suppressing the out-of-band signals to pass through the structure.

As a combined effect three zeros have been formed in the frequency response of the proposed structure. The zeros and poles have been marked in the Fig. 6. The zeros marked as Zal, Zah and Zak whereas the poles have marked as Zbl and Zbh.

Further, the bandwidth and insertion loss can be improved by changing its design dimensions.

Effect of Centre Post: At the centre of the square cavity (cavity-II), a via post has been placed with a diameter of ‘dv2’. The resonance frequency and bandwidth of the structure can be improved by increasing the diameter of the centre via post. The larger bandwidths can be achieved by this post.

Polarization Conversion: The FSS based on SIW designed in Cavity-II can be reconfigured for polarization conversion applications such as x-polarized (H-plane)/y-polarized (E-plane) incoming wave to the circularly polarized outgoing wave through the structure and vice versa by manipulating the slots on top and bottom surfaces of the cavity-II. The detailed structural alignment of the polarization converter from E-plane to H-plane and vice versa has been shown in Fig. 7 and 8 respectively. Also, the EM performance of the same has been given in Fig. 9 and 10. Further, the H-plane (X-polarized) to E-plane (Y-polarized) conversion is possible by keeping vertical slots (Fig.7a) on top surface and horizontal slots (Fig.8a) on bottom surface of the substrate. Similarly, the vice-versa is possible by changing the top and bottom surfaces.

Experimental verification:
The proposed dual cavity FSS shown in Fig. 3 has been fabricated using convention PCB process combined with electroplating technique. An array 10×10 element has been fabricated on a 200 mm × 200 mm substrate. A Rogers RT duroid 5870 with dielectric constant 2.33 and loss tangent 0.0012 has been used as dielectric substrate. Further, the EM performance of the fabricated prototype has been experimentally verified in a fully shielded anechoic chamber using standard free space measurement setup. The comparison of simulated results with measured results has been shown in Fig. 11. A very good matching has been found between simulated and measured results and followed the nature of simulated results. The bandwidth and insertion loss of measured results are exactly matching with simulated results. However, the slight shift is found in its lower frequency band due to the fabrication limitations of the prototype and measurement setup.

In this invention, a dual band FSS based on dual cavity SIW has been proposed. A dual cavity SIW has been modeled based on shared vias concept. Two different cavities with different performance characteristics have been implemented in a single unit cell. Cavity-I has performance of narrow bandwidth response. Whereas, Cavity-II has the broadband response characteristics. The cavity-II can exhibits different polarizations conversion properties like linear (E-plane/H-plane) input to circularly polarized out going wave and vice-versa. Also, E-plane wave to H-plane conversion and vice-versa. The structure can exhibit multiple performance characteristics. This performance will help in facilitate both uplink and downlink frequencies in one filter structure. The work can be extended for all Military COTM application for different frequency bands used for different systems like MILSTAR, COTM KA Band, COTM KU band and X bands.

The prototype of the intended invention has been fabricated and tested using a standard fully shielded anechoic chamber available at CSIR-National Aerospace Laboratories (NAL), Bangalore with the help of lab technicians and under the supervision of Senior Scientists. (The experimental results are supplied in the enclosed document).

The invention is a spatial filter with dual band frequency response having unique features for each band of frequency based on the design topology. Airborne radomes, ballistic radomes, sub-reflectors, transmit arrays, spatial filters aerospace and satellite communications are the few of its intended applications.

The intended application of the proposed design is for Military COTM application. Military COTM provide warfighters with mobile communication using satellites in order to connect to the several remote locations quickly and under critical conditions. Like other spacecraft, communicates with Earth through the use of radio waves. Radio waves are sent back and forth between Earth and spacecraft. Communications can take many forms. They can be commands for what the spacecraft should do, information about the basic health of the spacecraft, or information collected during space flight. Also, plays a crucial role in the design of spatial filters to facilitate both uplink and downlink frequencies in two different frequency bands respectively in one structure. The proposed dual band structure found its application not only in military but also in aerospace, commercial and industrial applications. The independent polarization response of each slot makes it different from the conventional FSS and earlier designed FSS based on SIW filters. The performance of the filter can effectively improve based on the perturbations occurred inside the cavity. In addition, the geometry corresponds a physical meaning, easy to design, simple structure, easy processing, fabrication and low cost. The proposed cavity further extended to different polarization conversion either at lower band or higher band based on altering its cavity designs. Multiband design with different polarization conversion characteristics can also be implemented by using the proposed concept.

The technology is most useful for the aerospace, defense and commercial sectors. Most of the defense and aerospace, aeronautical companies in India (like DRDO laboratories, CSIR laboratories, BEL, HAL, ISRO centers including many private defense companies) as well as around the world (like COBHAM, IAI, BOEING, GE Aviation, NASA, etc.) needs this technology for the design of efficient filtering structures for strategic applications. The most crucial application of the invention involves in the strategic warfare components like stealth radomes, aircraft fuselage. Also, it places a crucial role in the aerospace and satellite communication.

Despite, the invention also found its efficient usage as an EMI shielding structure to avoid the unwanted radiation in the sophisticated environments like automated control systems, microwave measurements, data-processing equipment, communication networks and electronic medical equipment. Hence, the technology found it usage in defense, industrial, aerospace, medical and commercial sectors. Frequency selective surfaces have many applications in different sectors including defense, aerospace and aeronautical. Hence, many aerospace, defense industries may interest in this technology.
Advantages:
• Combines two distinct SIW cavities in one unit cell and show a dual band response one with narrow band and another with wide band response.
• Useful of satellite communication, Ballistic radomes. Multi-resonance aerospace filters, etc.

Although, the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention.