Introduction
Stealth technology plays a key role in military warfare. It reduces the visibility of aircraft, ships, and ground missiles in RADAR. RADAR absorbing materials [Reference Mouritz1–Reference Carmo, Batalha, Ribeiro and Resende5] are widely used for stealth applications. These absorbers have attracted more consideration due to their low profile, simple fabrication process, and high adaptability in defense applications that require electromagnetic (EM) waves to be absorbed in a specific frequency band. Several researchers have used various techniques [Reference Debidas, Akhilesh and Ajay6–Reference Ghosh and Srivastava12] to design dual-band absorbers for stealth applications. A frequency-selective absorber (FSA) based on a tetra-arrow structure is proposed in reference [Reference Munaga, Gosh, Bhattacharyya, Chaurasiya and Srivastava13]. The design of an asymmetric dual-arm spiral incorporated with a resistor for stealth technology is detailed in reference [Reference Agrawal, Misra and Singh14]. The broadband property of both resonances is obtained by placing the chip resistor in the middle of the unit cell FSA. Multilayer FSAs exploiting the use of lumped elements are described in references [Reference Zhang, Jin and Shen15–Reference Hamid, Karnbach, Shakhtour and Heberling17]. Single-layer, wideband microwave absorbers using active elements with 3-dimensional profiles are illustrated in references [Reference Tennant and Chambers18–Reference Omar and Shen20]. Multiband absorbers in references [Reference Dong21, Reference He-Xiu22] are demonstrated by utilizing a large number of resonator structures. The reported absorber is designed in an asymmetric configuration [Reference Kato, Morita, Shiomi and Sanada23] where it offers polarization sensitivity. An absorber for Ka-band sensing applications is designed in reference [Reference Hakim, Alam, Almutairi, Mansor and Tariqul Islam24] with a footprint of 0.712 λ0 × 0.712 λ0, which achieves a minimum absorptivity of 90% with the composition of a higher profile. The dual-frequency response is realized by combining the single resonance, where the f L and f H are dependent on each other. To control the resonant frequencies and to improve the impedance performance, the P-type-Intrinsic-N-type (PIN)diodes are incorporated in reference [Reference Ghosh and Srivastava25] which increases the fabrication complexity. The narrow-band absorbers are designed using octa sharp ship configuration, and modified square ring resonator in references [Reference Dincer, Karaaslan, Unal and Sabah26, Reference Singh, Abegaonkar and Koul27], respectively. Dual-band metamaterial absorbers with wide angular stability are reported in references [Reference Bagci and Medina28, Reference Chaluvadi and Rao29]. However, the absorptivity slightly drops at 80% and 85% by increasing the angle of incidence. A high profile of 3 mm is composed in reference [Reference Musa30]to realize the broadband characteristics and to improve the impedance matching. A metamaterial absorber is proposed with a footprint of 1 λ0 × 1 λ0 in reference [Reference Musa30] for microwave applications. A co-polarized microwave absorber is presented in reference [Reference Sen31]. This absorber is characterized by using a narrow bandwidth, with a minimum absorptivity of 90% at 3.5 and 5.8 GHz. A triple-band absorber is designed using closed loop resonators at reference [Reference Genikala32].
The aforementioned methods result in the development of a complex FSA design with a bulkier configuration [Reference Musa30, Reference Sen31], low absorptivity [Reference Hakim, Alam, Almutairi, Mansor and Tariqul Islam24, Reference Musa30], narrow band [Reference Kato, Morita, Shiomi and Sanada23–Reference Wu33], and polarization sensitivity [Reference Kato, Morita, Shiomi and Sanada23]. In addition, impedance matching using lumped resistors increases the manufacturing difficulty of the FSA. Thus, a low-profile FSA unit cell with desired impedance matching using distributed elements is proposed to reduce the manufacturing complexity with improved performance. A linearly polarized dual-wide band FSA in the millimeter-wave (Ka-band) regime exploiting only the distributed elements is presented in this paper. The proposed dual-band absorber achieves a wider absorption band from 25.5 GHz (1 GHz) (f L), and 31.8–32.5 GHz (0.7 GHz) (f H) with the minimum absorptivity of 96% and 92% by integrating multiple resonators in a single Printed Circuit Board (PCB)layer. This FSA operates within the RADAR frequency range of 31–32.5 GHz as prescribed by the Federal Communications Commission. Thus, the proposed dual-band absorber can be used for RADAR applications. Further, it can also be used for stealth aspects due to its wider bandwidth properties. The proposed dual-band absorber can be directly implemented in electronic warfare systems. The absorber-loaded systems will not reflect the incident EM waves from the intruders, thereby reducing the RADAR signature and improving the stealth aspects. The organization of the article is as follows: the “Performance analysis of the FSA” section of this paper presents the absorber design, construction, evolution, and analysis. The simulated and measured results of the proposed dual-band FSA are discussed in the “Fabrication and measurement” section followed by conclusions in the “Conclusion” section.
FSA realization
Dual-band millimeter-wave absorber design
The geometry of the proposed FSA is depicted in Fig. 1. The unit cell achieves dual-band resonance characteristics by tightly packing multiple resonant structures as described in Fig. 1(a). Figure 1(b–d) represent the side, rear, and perspective views of the proposed FSA unit cell, respectively. The FSA is developed on a FR-4 substrate (
$\varepsilon$r = 4.3, tan δ = 0.023 and h = 1.6 mm). The proposed dual-band absorber is constructed by utilizing distributed elements to achieve desired impedance matching without the use of additional lumped elements as in references [Reference Agrawal, Misra and Singh14–Reference Rashid, Shen and Aditya16, Reference Tran34].
Proposed dual-band absorber. (a) Front view, (b) Side view, (c) Rear view, and (d) Perspective view. p = 7 × 7 mm, l 1 = 6.95 mm, Sl 1 = 6 mm, l 2 = 5 mm, l 3 = 2 mm, Sl 2 = 5.6 mm, w 1 = 0.42 mm, w 2 = 0.3 mm, Sw 1 = 0.5 mm, w 3 = 0.2 mm, w 4 = 0.25 mm, h = 1.6 mm, 
$\varepsilon$r = 4.3 (red: copper, yellow: FR-4 lossy substrate).
The evolution of the proposed absorber is shown in Fig. 2. The unit cell of the FSA has evolved from a concentric square loop resonator enclosing a Jerusalem cross resonator as illustrated in Fig. 2(a) and (b). The Jerusalem cross is widely used for realizing higher absorptivity characteristics [Reference Landy8, Reference Munk9] and it is adopted in this research to improve bandwidth and absorptivity performance. The reported combination in Fig. 2(b) offers 800 and 250 MHz bandwidth with a peak absorptivity of 94% and 95%, and reflectance of 32% and 25% at f L and f H, respectively. The bandwidth at f H is limited by the capacitance effect. Hence, a suitable impedance compensation network is required. To accomplish this, an inductive cross grid is introduced as shown in Fig. 2(c). However, the absorptivity is as low as 75%, and reflectance is 28% at f H. In order to improve the absorptivity and bandwidth performance, the inductance is increased by adding the corrugations to the cross grid as shown in Fig. 2(d). Thus, the proposed absorber achieves the minimum absorptivity of 92% with the reflectance of 15%, and 27% at f L and f H, respectively. The wavelength equation is used to optimize the resonant parameters of the proposed absorber. It is inferred in Fig. 3(a), that the absorptivity of the single-layered millimeter-wave absorber is achieved by an optimized inductive element in the appropriate position. Hence, the impedance is matched to obtain a −10 dB reflection coefficient.
Evolution of the proposed unit cell using Jerusalem cross enclosed within (a) Outer square loop, (b) Outer and inner square loop, (c) Square loops embedded with cross grid, and (d) Square loops with corrugated cross grids.
(a) Evolution of the proposed unit cell [solid line: absorptivity, dotted line: reflectance], (b) Normalized impedance of the proposed absorber.
Equivalent circuit model
The approximate equivalent circuit model (ECM) of the FSA is shown in Fig. 4(a). The metallic loops, corrugated cross grid, and Jerusalem cross are equivalent to a series LC circuit [Reference Munk9]. In the equivalent circuit of the proposed absorber, “
${z_{{\text{Sub}}}}$” is the impedance offered by the dielectric with a thickness of 1.6 mm. Thus, the dielectric impedance “
${z_{{\text{Sub}}}}$” is given by [Reference Singh, Abegaonkar and Koul27],
\begin{equation}{Z_{{\text{Sub}}}} = \frac{{{Z_0}}}{{\sqrt {{\varepsilon _r}} }}\tan \left( {\beta h} \right)\end{equation}(a) The ECM of the proposed FSA. The approximate value of the circuit parameters Ais Cgs (substrate) = 0.408 pF, L 1 = 0.856 nH, C 1 = 0.268 pF, CC 1 =1.5 pF, L 2 = 0.534 nH, C 2 =1.5 pF, CC 2 =1.5 pF, L 3 =0.142 nH, C 3 = 1.5 pF, CC 3 = 1.5 pF, L 4 = 0.436 nH, C 4 = 0.1 pF, and CC = 0.366 pF, h = 1.6 mm, (b) Comparison of the ECM and simulated performance.
In the equation, β is the propagation constant, Z 0 is the characteristic impedance and h is the profile. Z FSS is the impedance of the proposed absorber. The effective inductance and capacitance of the square loop are represented as “L 1, L 2” and “C 1, C 2.” Similarly, the Jerusalem cross and corrugated cross grid resonators are described as “L 3, C 3” and “L 4, C 4.” CC 1 and CC 2 are modelled as coupling capacitors. Model CC represents the gap between the unit cells and Cgs is the value of the substrate. The total input impedance is given by,
\begin{equation}{Z_{{\text{in}}}} = {Z_{{\text{FSS }}}}||{ }{Z_{{\text{Sub}}}}\end{equation} The input reflection coefficient “
$\Gamma $” is given by,
\begin{equation}\Gamma = \frac{{\left( {{Z_{{\text{in}}}} - {Z_0}} \right)}}{{\left( {{Z_{{\text{in}}}} + {Z_0}} \right)}}\end{equation}To optimize the absorption of an incident EM wave, it’s essential to ensure that the real part of the impedance of the absorber closely matches the characteristic impedance of free space, which is approximately 377 Ω and its imaginary part should be zero for minimum reflection which results in the maximum absorption. The free space impedance is given by,
\begin{equation}{\text{ }}{Z_0} = \sqrt {{\mu _0}/{\varepsilon _0}} \end{equation} Therefore, the absorption of the incident EM wave is maximized by controlling the resonance of 
$\mu$ and 
$\varepsilon$. This resonant frequency is governed by the equivalent capacitance (
$C$) and inductance (
$L$) of the absorber’s metallic structure and metal surface as given by equation (5),
\begin{equation}{\omega _0} = \frac{1}{{\sqrt {LC} }}\end{equation} When the equivalent series circuit is at resonance, the total reactance, the total reactance of “Z FSS” and “Z Sub” becomes equal and opposite. The component values of the LC resonators are calculated using the model explained in reference [Reference Dong21]. The ECM is developed and simulated using Advanced Design System (ADS) . From Fig. 4(b), it is inferred that the ECM simulation results are consistent with the layout simulation results in Computer Simulation Technology (CST). This confirms the accuracy of the proposed ECM for the proposed FSA configuration. The normalized impedance of the proposed FSA is shown in Fig. 3(b). The relative impedance (
${z_r}$) is calculated using the equation in reference [Reference Sen31],
\begin{equation}{z_r} = \pm \sqrt {\frac{{{{\left( {1 + {S_{11}}} \right)}^2}}}{{{{\left( {1 - {S_{11}}} \right)}^2}}}} \end{equation}It is inferred that, at the dual absorption peak frequencies, the real and imaginary parts of the impedance are close to unity and zero respectively, which validates the perfect impedance matching under free space conditions (377 + j0). Thus, the proposed FSA realizes the minimum reflection and absorptivity close to unity as depicted in Fig. 3(b). The proposed FSA is simulated using CST Microwave Studio 2020. The simulation is carried out using a Frequency domain solver in the tetrahedral mesh with the no. of cells per wavelength of 15. Further, the Floquet port with the periodic boundary conditions is used to simulate the proposed FSA.
Performance analysis of the FSA
Angular stability
The absorption spectra of the proposed FSA under TE and TM modes with different incidence angles are shown in Fig. 5(a, b). At higher oblique incidences, parasitic resonance is induced between the two resonant frequencies. This parasitic resonance is attributed to the corner effects of the induced slots and stubs in the FSA. The corner effect deteriorates the magnetic flux between the metallic resonant layers. This deterioration increases when the oblique incidence (θ) increases and leads to an electric resonance predominant and the associated impedance mismatch [Reference He-Xiu22]. Therefore, the reflection coefficient performance changes at a higher frequency, when the angle of incidence increases [Reference Ye, Liu and Lin35]. Here, the lower absorption depreciates with the minimum absorptivity of 90% when the angle of the incident is increased to 60°. Similarly, the bandwidth is reduced to 0.3 GHz at the frequency range of 32.2–32.5 GHz with a minimum absorptivity of 85%. Nevertheless, the proposed absorber is stable up to 45° without any bandwidth and absorptivity variations.
Angular stability (a) TE mode and (b) TM mode.
The variation of Φ angles at both TE and TM modes is shown in Fig. 6(a, b). It is inferred that, owing to the symmetric structure, the proposed FSA exhibits a stable response with altered bandwidth and absorptivity at f L and f H.
Polarization stability (a) TE mode and (b) TM mode.
Parametric analysis
The effect of the length l 1 of the square loop is plotted in Fig. 7(a). If l 1 is reduced from 6.95 to 6.85 mm, the resonant frequency shifts from 26.0 to 26.7 GHz with declining absorptivity from 90% to 70% at f L. When the l 1 is increased to 6.98 mm, it produces a narrow bandwidth at f H with a frequency shift from 25.3 to 26.3 GHz at f L. Thus, l 1 is optimized and fixed at 6.95 mm. The higher frequency (f H) is controlled by the width of the stub, w 3. From Fig. 7(b), it is evident that there is a trade-off between the operating bandwidth and absorptivity. With the reasonable reduction in w 3 attains the bandwidth of 0.7 GHz at f H. When w 3 increases from 0.2 to 0.3 mm the absorptivity reduces from 92% to 75%. Thus, the value of w 3 is optimized to 0.2 mm for the consideration of high absorptivity and desired bandwidth.
(a) Optimization of the length of the outer square loop (l 1), (b) Optimization of the width of the stub (w 3).
Surface current distribution
The surface current distribution of the proposed unit cell at resonant frequencies is shown in Fig. 8. Strong electric current density is induced over the metallic loops and Jerusalem cross at 26 GHz as shown in Fig. 8(a) validating the resonant behavior at f L. Similarly, at 32 GHz, the corrugated cross grid has more current concentration thus validating the resonant behavior at f H as shown in Fig. 8(b). Further, it can be inferred that the frequency response of f L and f H can be independently controlled in the proposed FSA.
Surface current distribution at contour representation (a) 26 GHz and (b) 32 GHz.
Fabrication and measurement
The proposed FSA is fabricated and tested in the free space using the microwave analyzer. The prototype of the FSA is fabricated using the conventional photolithographic process. The measurement setup of the proposed dual-band FSA is illustrated in Fig. 9(a). The performance of the proposed FSA is measured using a Vector Network Analyzer (VNA), and Transmit/Receive (T/R) horn antennas operating at 18–40 GHz with an aperture dimension of 50 × 50 mm and an average gain of 14 dBi. These antennas are mounted along the line of sight and positioned at a distance of 1 mm 
$\left( {\frac{{2{D^2}}}{\lambda }} \right)$. The analyzer is calibrated by normalizing the operating frequency range and power level. The measured and simulated results of the dual-band absorber are shown in Fig. 9(b). The inset of Fig. 9(b) depicts an array of 28 × 28 cells fabricated on the FR-4 substrate (h = 1.6 mm). The measured bandwidth of 1.5 and 1 GHz with 100% absorptivity is realized at f L and f H. The discrepancy between the measured and simulated results at f L occurs due to the higher losses of the substrate. Table 1 shows the comparison of the proposed FSA with the state-of-the-art.
(a) Measurement setup, (b) Absorptivity of the simulated and measured result (inset: fabricated phototype with a magnified view).
Quantitative comparison of the proposed absorber with the state-of-art
(UCS-unit cell size, f L- lower frequency, f M-middle frequency, f H-higher frequency, NR-not mentioned, *-approximate values, θ-angular stability).
The research on the development of dual-band millimeter-wave absorbers at the intended RADAR frequency range (31.4–33 GHz) is limited. The higher absorptivity and enhanced bandwidth are highly essential to satisfy the requirements of RADAR or stealth applications [Reference Munk9]. Thus, the work of the proposed absorber is compared with other relevant works (Table 1) with an absorption rate of 90%,
1) The proposed FSA offers a minimum absorptivity of 92% and bandwidth response of 1 GHz at f L and 0.7 GHz at f H which is wider than the bandwidth realized in references [Reference Kato, Morita, Shiomi and Sanada23–Reference Wu33].
2) The unit cell dimension in references [Reference Hakim, Alam, Almutairi, Mansor and Tariqul Islam24] and [Reference Musa30] is 20% and 69% large than the proposed design.
3) The proposed unit cell has exploited the distributed elements to realize higher absorptivity with wider bandwidth and to reduce the complexity of the design. This is unlike the research in references [Reference Ghosh and Srivastava25] and [Reference Wu33], where the active and passive elements are incorporated to achieve the necessary impedance matching and bandwidth broadening.
4) The frequency shift is more than 0.5 and 0.75 GHz in references [Reference Dincer, Karaaslan, Unal and Sabah26] and [Reference Musa30] for incidence angles 15° and 30°, respectively. Furthermore, the absorptivity drastically decreases (<70%) in references [Reference Ghosh and Srivastava25–Reference Chaluvadi and Rao29] and [Reference Musa30] at higher oblique incidences. This is unlike the proposed FSA maintains a stable frequency response and absorptivity of 92% up to 45° in both TE and TM modes.
5) The FSA reported in refrences [Reference Hakim, Alam, Almutairi, Mansor and Tariqul Islam24] and [Reference Musa30, Reference Wu33] has poor band separation with intermediate absorptivity maintained at 50%. However, in the presented research, the absorptivity between both bands is as low as 8%.
6) The reported absorber in refrerence [Reference Kato, Morita, Shiomi and Sanada23] is designed in an asymmetric configuration. Similarly, in reference [Reference Wu33] the spiral resonator is rotated to 90° and combined with the original spiral in an orthotropic position to achieve polarization insensitivity and broadband.
7) A high discrepancy between the measured and simulated result is realized in references [Reference Hakim, Alam, Almutairi, Mansor and Tariqul Islam24, Reference Dincer, Karaaslan, Unal and Sabah26] and [Reference Sen31, Reference Genikala32], whereas the proposed absorber achieves the measured bandwidth of 1.5 and 1 GHz with 100% absorptivity at f L and f H.
Conclusion
A novel design of a dual-band absorber is presented for RADAR applications. The proposed absorber is constructed using integrated elements such as Jerusalem cross, and cross grid with corrugations. It is developed by utilizing distributed elements to achieve desired impedance matching without the use of additional lumped elements, and high profile. The minimum absorptivity of 95% and 92% is achieved in the frequency range of 25.5–26.5 GHz (1 GHz) and 31.8–32.5 GHz (0.7 GHz), respectively. The parametric analysis and the surface current distribution for the bandwidth are analyzed and the results are presented in this work. The simulated results are verified by experimental results and show a good agreement which validates the suitability of the proposed absorber for stealth aspects. In future, the rasorber will be designed and developed with the composition of bandpass FSS.
Competing interests
There is no conflict of interest in the submission
Geethanjali Govindarajan received her BE degree from Sri Sairam Institute of Technology, West Tambaram in the year 2020. She received her ME degree from Sri Sivasubramaniya Nadar College of Engineering in the year of 2022. She is currently pursuing her Ph. D. degree at Anna University, Chennai. She is a research scholar in the Department of Electronics and Communication Engineering, College of Engineering Guindy (CEG) Campus, Anna University, Chennai. Her research interests are Metamaterial Absorbers, Frequency Selective Rasorber (FSR) for RADAR and defense applications.
Gulam Nabi Alsath Mohammed received his BE, ME, and Ph.D. degrees from Anna University Chennai in the years 2009, 2012, and 2015, respectively. He is currently serving as an Associate Professor in the Department of Electronics and Communication Engineering, Anna University, Chennai, India. His research interests include Microwave Components and Circuits, Antenna Engineering, Signal integrity Analysis, and Solutions to EMI problems. To his credit, he has 2 US patents and 18 Indian patents. He has published several research articles on antennas and microwave components in leading International Journals. He has also presented and published his research papers in the proceedings of International and National conferences. He is currently serving as an Associate Editor in IET Microwaves Antennas and Propagation and Microwave and Optical Technology Letters.
Kirubaveni Savarimuthu obtained her BE and ME degrees from Anna University, Chennai. She currently serves as an Associate Professor in the Department of Electronics and Communication Engineering, Anna University, Chennai. She has 13 years of teaching and research experience. Her research interests include MEMS and NEMS device design and VLSI design. She is currently involved in the growth of ZnO nanorods for piezoelectric energy harvester and gas sensor applications. She is an active life member of IETE and a senior member of IEEE.
Malathi Kanagasabai completed her Bachelor of Engineering in Electronics and Communication Engineering and Master of Engineering in Microwave and Optical Engineering from Madurai Kamarajar University. She received her PhD on “Analysis of Rectangular Shielded Strip Line Enclosures” from Anna University, Chennai. She currently serves as a Professor in the Department of Electronics and Communication Engineering (ECE), Anna University, Chennai. Her research interests are antennas, UWB microwave components, electromagnetic shielding, and signal integrity analysis in RF-printed circuit boards. She has more than 25 granted patents including two US patents. She has authored more than 130+ research articles in leading international journals and guest edited a special section on THz components for Microwave and Optical Technology Letters.
