Low RCS Dual-Polarized Crossed Dipole Antenna Co-Designed With Absorptive Frequency- Selective Reflection Structure

This article proposes a novel strategy for realizing a reduction in radar cross-section (RCS) of dual-polarized crossed dipole antenna. At first, a compact and polarization-insensitive absorptive frequency-selective reflection (AFSR) structure is proposed by incorporating a bandstop ring resonator within the circular-cross based broadband absorber. The bandstop ring resonator is designed on the backside of the resistive layer due to which a reflection window is realized at a frequency of 8.2 GHz between the two broadband absorptions (4.2–7.0 GHz and 9.2–11.5 GHz). A dual-polarized crossed dipole antenna is designed with operating frequency lying within the reflecting notch of the AFSR structure. A 6 × 6 AFSR structure array is truncated at the center where from which the crossed dipole is connected through a feed substrate. The AFSR structure enacts as a modified ground plane to the crossed dipole antenna. The proposed AFSR integrated antenna achieves an average mono-static RCS reduction of 12.51 dB and 12.62 dB for the TE and TM incident waves, respectively. Further, the AFSR based antenna is also measured for the bi-static RCS, wherefrom the average total RCS reduction of 80% for TE and TM incidence is attained in the frequency band of 4.2 to 11.5 GHz.


I. INTRODUCTION
The detectability reduction of the target from the radar system is a vital aspect of stealth technology. In recent years, studies involving the development of radiating systems exhibiting low radar cross-section (RCS) have gained a great deal of attention. RCS which quantifies the detectability parameter of the target should have the minimum value for enhancing the secrecy factor of the target in stealth technology [1]. Thus, the RCS of the radiating system employed in the stealth technology is of key consideration. In the literature, several approaches for the realization of low RCS radiating The associate editor coordinating the review of this manuscript and approving it for publication was Chinmoy Saha . systems have been reported. Some of these reported techniques include geometrical shaping of antennas [2], [4], applying radar absorbing materials [5], [7], utilization of polarization converter structures [8], [9], and usage of surfaces involving artificial magnetic conductor [10], [11]. However, in the reported techniques several drawbacks like poor radiation, narrowband RCS reduction with angle based low RCS are also accompanied.
The advent of absorptive frequency-selective reflection (AFSR) type structure has serve for a substantial application in the realization of a low RCS antenna system [12], [13]. The AFSR structure is a specialized category of frequencyselective surface (FSS), which features a notch shaped reflection band along with one-/two-sided absorption bands.
A suitable integration of the ASFR structure with the antenna can be effective in realizing a system with low out-of-band RCS. In the recent past several studies on the AFSR structures have been reported in the literature [14] and [19]. A band notched AFSR structure using circular slot resonator and metal strip resonator has been reported in [14]. A polarization-insensitive AFSR using multiple resonators has been proposed in [15]. A triple-layered polarization insensitive AFSR structure with reflectance band centered at 5 GHz has been proposed in [16].
Recently various studies on the RCS reduction techniques have been reported [20], [30], that includes realization of low RCS antenna using AFSR structure [23], [24]. In [23], a dipole antenna is studied with a single-polarization AFSR structure realizing a reduction in out-of-band RCS. Low RCS monopole and dipole antennas have been studied in [24], by integrating with a 3-D AFSR structure based on multimode resonators. The AFSR structure exhibiting polarization-insensitive behavior can have a suitable application for RCS reduction of the radiating system operating in dual polarization.
The objective aimed in this work is to study the integration strategy of a dual-polarized crossed dipole antenna with a polarization-insensitive AFSR structure for achieving RCS reduction. A brief preliminary study of this work has been presented in [31] in which a mono-static RCS reduction of a single polarized dipole antenna is discussed using the proposed ASFR structure. In comparison with the initial work [31], the study carried out for the extended work in this paper includes the experimental validation of the AFSR structure along with the corresponding equivalent circuit model. Further, the extended work in this paper in contrast to the initial work [31], employs a dual-polarized crossed dipole antenna integrated which is more suitable than a singlepolarization dipole antenna integrated with the polarizationinsensitive AFSR structure. Further, the mono-static RCS in this extended paper, unlike the initial work [31], is experimentally determined. Furthermore, in this extended work, the bi-static measurements are also performed. The novelties achieved in the proposed work are enlisted as: • Design of the 2-D polarization-insensitive AFSR structure by subtle idea of printing a circular ring resonator on the back of the front resistive substrate which achieves a reflection notch (at 8.2 GHz) with the adjacent side absorption bands (4.2-7.0 GHz and 9.2-11.5 GHz) on both side. The design complexity of the proposed AFSR is much lesser as compared to asymmetric AFSR design of [14], 3-layer design of [16] and 3D AFSR structures reported in [17] and [19].
• Integration of a crossed dipole antenna with a 6 x 6 proposed AFSR structure unit cells in a novel and proficient manner such that AFSR structure acts as a reflector at the antenna's operating frequency. The proposed work realizes a dual-polarized low RCS antenna in contrast to the single-polarized antennas reported in [23] and [28].  • The mono-static RCS of the AFSR integrated antenna achieves an average reduction of 12.51 dB and 12.62 dB for the TE and TM incidence, respectively in comparison with the conventional reflector back antenna.
• In contrast to the reported works [23], [29], the bi-static measurement on the proposed AFSR antenna is carried out at different angles for obtaining the normalized total scattered cross section. An average bi-static total RCS reduction of 80.2% is exhibited for both TE and TM incident wave as compared to the conventional reflector back antenna. The paper is organized in the sequence given. Section II discusses the AFSR structure including analysis and experimental validation. Section III presents the integration technique of crossed dipole with the AFSR structure for both mono-static and bi-static RCS reduction. Finally, conclusion is provided in section IV.

II. AFSR STRUCTURE A. DESIGN AND ANALYSIS
The geometrical discription of the proposed unit cell for the ASFR structure is depicted in Fig. 1, which is basically a two-layered combination comprising of resistive layer at top and ground layer at bottom and separated by distance H ( Fig. 1(a)). The design is printed on a 0.8 mm thick FR-4 substrate. The resistive layer comprises of a circular-cross shaped resonator on the front side in which the lumped chip resistors having resistance of 200 are mounted within the four gaps of the rectangular strip ( Fig. 1(b)). On the backside of the resistive layer a metallic circular ring resonator is printed. The CST Microwave studio is used for analyzing the structure.
The resistive layer with circular-cross resonator and lumped chip resistors placed from the ground layer at a distance of λ/4 provides a broadband absorption for the incident EM wave as shown in Fig. 2 and having 90% absorption bandwidth from 4.7 to 10.7 GHz. For obtaining a reflection notch in middle of the absorption band a metallic circular ring is printed on the backside of the resistive layer. This circular ring resonator acts as a bandstop FSS at the desired reflection notch of 8.2 GHz. It can be observed from Fig. 2 that due to insertion of ring resonator at the back side of resistive layer the broad absorption band gets bifurcated into two absorption bands from 4.2 to 7.0 GHz and 9.2 to 11.5 GHz with a notch shaped reflection band at the center frequency of 8.2 GHz.
For understanding the working related to the proposed AFSR structure, a corresponding equivalent circuit model (ECM) is designed as shown in Fig. 3. The ECM is a one-port network in which the corresponding models of front and back side resonators of the resistive layers are cascaded. For the circular-cross resonator at the front side, the lumped resistor and the associated gap capacitance is modelled by the parallel R − C 0 combination. The inductance of the rectangular strip is modelled by inductor L 0 while the parallel L 1 − C 1 models the circular ring. At the backside of resistive layer, the circular ring is modelled by the parallel L 2 − C 2 while the inter-capacitance is represented by C 3 . The thickness of the substrate is modelled by a transmission line having the characteristic impedance Z d = Z 0 / √ r (Z 0 denotes the impedance in free space) and length equivalent to the substrate thickness (t). The transmission line having characteristic impedance Z 0 and thickness H also models the air-gap between the resistive layer and the ground.  The Keysight ADS solver is used for analyzing the ECM of the proposed AFSR structure. The simulated reflection coefficient of the ECM compared with the full wave CST simulation is shown in Fig. 4. The response of the ECM without the backside ring resonator model is shown in Fig. 4(a), in which a broadband absorption is observed in close resemblence with the full wave simulation of the broadband absorber. The reflection of the ECM for AFSR is shown in Fig. 4(b) in which the reflection notch at the center is achieved by the addition of corresponding model of the backside ring resonator. The close concurrence between the ECM and CST responses explains the working of the AFSR design.
The proposed AFSR structure is analyzed in response to multiple polarization angles associated with the incident EM wave as depicted in Fig. 5(a). The consistent response of the design under various polarization angles determines the polarization-insensitive behavior of the AFSR structure. Furthermore, the AFSR structure is also analyzed corresponding to the oblique angle of incidence up to 50 o . The angular stability for the proposed structure exists up to 30 o beyond which the absorption bands gets degraded.

B. FABRICATION AND MEASUREMENTS
For obtaining the experimental validation of the AFSR structure, a prototype consisting of 21 x 21 proposed unit cells are fabricated using a 0.8 mm thick FR-4 substrate ( r = 4.4, tanδ = 0.02). The photograph of the fabricated prototype having overall size of 315 mm x 315 mm is given in Fig. 6. Lumped chip resistors with 200 resistance (CRCW0603200RFKEA from VISHAY ) are soldered within the gaps of the circular-cross resonators ( Fig. 6(a)). The metallic ring resonators are printed on the backside of the top resistive layer (Fig. 6(b)). The resistive substrate is separated from the ground layer using a plastic spacers as presented in Fig. 6(c).
The reflection measurements on the fabricated prototype is carried in an anechoic chamber using the free space technique with standard gain horn antennas of C, J and X bands in connection with the Keysight PNA N5224B. The measured reflection coefficient of the fabricated prototype in comparison with the simulated response is shown in Fig. 7(a). The close resemblance achieved between the experimental and simulated responses validates the proposed design experimentally. Further, the polarization-independence of the proposed structure is also experimentally validated by measuring the response at various polarization angles as shown in Fig. 7(b). The response of the proposed structure is measured under oblique incidence and the angular stability is experimentally validated up to 30 • , beyond which the absorptivity in the two bands gets degraded.

III. LOW RCS CROSSED DIPOLE ANTENNA
The design for the dual-polarized low RCS antenna utilizing the AFSR structure is proposed in this section. The AFSR VOLUME 10, 2022 structure exhibiting polarization-insensitive behavior is codesigned with the dual-polarized crossed dipole antenna. The crossed dipole antenna is designed having operating frequency around 8.2 GHz which corresponds to the reflection band associated with the proposed AFSR structure. A microstrip to broadside coupled stripline transition feeds the crossed dipole [32]. The reference structure consists of a crossed dipole backed by a metallic reflector as shown in Fig. 8(a). The dimension of the reflecting surface is taken to be equal with the size of 6 x 6 AFSR unit cell arrray (90 mm x 90 mm). In the design for AFSR integrated antenna, an array comprising of 6 x 6 unit cell is taken. On the resistive layer, a square portion with size of the order of the feed substrate is truncated at the center. The crossed dipole connected with the feed substrate as shown in Fig. 8(b), is inserted within this square portion and connected with the ground layer of the AFSR structure. In other words the grounded reflector of the antenna is modified with the 6 x 6 AFSR structure. The photographs showing the fabricated prototypes for the crossed dipole antenna with both metallic reflector and AFSR structure are depicted in Fig. 9.
The simulated and measured reflection coefficients (S 11 ) of the crossed dipole antenna with both reflector and AFSR structure are shown in Fig. 10. The reflection of less than −10 dB is observed at the operating frequency of 8.2 GHz for both the cases of reflector and ASFR backed antenna. At 8.2 GHz the gain of 5.52 dBi is observed for the AFSR backed antenna which is slightly less than the gain corresponding to reflector backed antenna (6.35 dBi). The simulated and measured radiation patterns of the crossed dipole antenna with both the reflector and AFSR backing in both the xz and yz planes are given in Fig. 11. It can be examined that the radiation pattern of the AFSR backed antenna is nearly similar as compare to the reflector backed antenna.

A. MONO-STATIC RCS MEASUREMENT
The radar cross section measurements are carried out for the fabricated structures involving both the reflector and AFSR backed crossed dipole antennas. The Keysight PNA N5224B featured with time domain gating application is employed for the mono-static RCS measurement using the same standard gain horn antenna for both transmitter and receiver. The RCS is defined from the general equation by (1). The terms P r and P t in (1) denote the received and transmitted powers, respectively. The gains associated with reflecting and transmitting antennas are represented by G r and G t , respectively. The range to target is given by R, whereas λ gives the associated wavelength.  The mono-static RCS is measured in reference to a standard square PEC having same size as that of the grounded dimensions associated with antenna. The comparitive monostatic RCS of the metallic reflector and AFSR backed crossed dipole antenna for the TE and TM incident waves are shown in Fig. 12. It is observable that as compared with the reflector backed antenna, the AFSR integrated antenna achieves a low RCS with the maximum reduction of 27.52 dB (at 10.6 GHz) and 38.01 dB (10.6 GHz) for the TE and TM incidence, respectively. The AFSR integrated antenna achieves an average reduction of 12.51 dB and 12.62 dB for the TE and TM incident waves, within the frequency band extending from 4.2 to 11.5 GHz, respectively as compared to the conventional metallic reflector backed antenna.

B. BI-STATIC MEASUREMENT
The bi-static RCS measurement involves the transmitter and receiver test antennas placed at different locations. In the bistatic measurement, as depicted by Fig. 13, the transmitting antenna is normal to the structure while the receiving horn antenna is moved from 0 o to 90 0 along the circular path at 10 o steps in either anticlockwise or clockwise direction. The E sca (f , φ) denoting the scattered field for each step angle is defined by (2) [33], [34].
where the transmission coefficients between the two horn antennas corresponding to the object and free space are represented by S 21,O and S 21,FS , respectively. The normalized total radar cross section RCS t,norm of the AFSR integrated antenna is calculated by the integrating scattered fields intensities as given in (3) [33]: where E sca,AFSRA and E sca,RA are the scattered fields of the AFSR based and conventional reflector antennas, respectively. The measured RCS t,norm for the AFSR combined crossed dipole antenna in reference with the metal reflector based antenna is provided in Fig. 14. It is noticed that with respect to the reflector based antenna, the AFSR structure based crossed dipole antenna achieves a significant reduction in the normalized total RCS. In the entire operating frequency band of 4.2 to 11.5 GHz, the average value of normalized RCS obtained is around 0.197 and 0.198 for the TE and TM incident wave, respectively which signifies a total RCS reduction of around 80.24% for both the TE and TM incidence, relative to the conventional reflector based crossed dipole antenna. Table 1 presents the proposed AFSR integrated crossed dipole antenna with the other low RCS antennas reported in the literature. The proposed study presents the RCS reduction of dual-polarized crossed dipole antenna for the first time in comparison with the single polarization antennas reported [23], [28]. The proposed design is compact in comparison with reported work in [23], [25], and [26]. Further, the design complexity of the proposed structure is less than [23], [24], [26], and [28], in terms of lesser quantity of lumped components utilized. Also, the -10 dB RCS reduction bandwidth of the proposed design is more than [23] and [27] in the lower band while the RCS reduction bandwidth for the upper band is higher than the work reported in [23], [25], and [26]. The average RCS reduction achieved in the proposed design significantly higher in comparison with the reported work [29]. The proposed study achieves the RCS reduction using a 2D AFSR structure in comparison with the 3D FSS structures used in [24] and [28]. Furthermore, in contrast to the reported works [23], [29], the scattered fields for the AFSR integrated antenna are measured in the proposed work at various angles and an average bi-static total RCS reduction of around 80% is experimentally demonstrated for both the TE and TM incident wave.

IV. CONCLUSION
In this study, a low RCS dual-polarized crossed dipole antenna is studied for the first time by judiciously integrating with a polarization-insensitive AFSR structure. The AFSR structure exhibiting a reflection notch (at 8.2 GHz) between the wide absorbing bands is designed by imprinting a bandstop ring shaped resonator on the back of the top resistive substrate. The crossed dipole antenna designed at the frequency concurrent with the reflecting notch of proposed AFSR structure, is adjusted within a small truncated area at the middle of the AFSR structure with 6 x 6 proposed unit cell array. The AFSR structure acts as a modified ground for the antenna. Both the mono-static and bi-static measurements performed on the dual-polarized antenna backed with the AFSR structure verify the RCS reduction while the other antenna parameters are observed to be maintained nearly the same. An average mono-static RCS reduction of 12.51 dB and 12.62 dB is acheived for the TE and TM incidence, respectively in the operating frequency band (4.2−11.5 GHz) for the AFSR based antenna compared with the conventional reflector comprising counterpart. Furthermore, an average total bi-static RCS reduction of around 80% is experimentally demonstrated in reference to the conventional reflector backed antenna. The proposed low RCS antenna is a potential candidate in the dual-polarized applications for stealth communication.