Reconfigurable Frequency Selective Rasorber Covering Extremely Wide Transmission Frequency Range

Rasorbers with reconfigurable transmission frequency are of great interest over the past few years. However, existing devices exhibit a limited number of operation modes and cover a narrow transmission frequency range, making them unacceptable for wideband applications. In this work, a highperformance reconfigurable rasorber with mechanically-controlled frequency selective surface (FSS) and an electronically-tuned resistive layer is proposed. Theoretical analysis based on transmission line theory and numerical simulations is performed to investigate the performance of the proposed device. The rasorber were fabricated and free-space measurements were also carried out to validate the simulation results. The device can operate between single-, dualand triple-passband modes by mechanically tuning the FSS under a fixed external bias voltage. Under single-passband mode, the transmission frequency can also be tuned continuously through simultaneously controlling the mechanically-tunable FSS and electronically-tunable resistive layer. A minimum insertion loss as low as 0.16 dB at 4.03 GHz is achieved from the measurement results. The maximum measured fractional absorption bandwidth is 87.9%, comparable to the state-ofart maximum absorption bandwidth of existing reconfigurable rasorber. The measured overall fractional transmission frequency range under normal incidence wave is 63.9%, far beyond that of any other existing reconfigurable counterparts. INDEX TERMS reconfigurable, rasorber, mechanical, electronic


I. INTRODUCTION
E LECTROMAGNETIC (EM) frequency selective rasorber is an artificial material or device that allows EM waves propagate through nearly losslessly within its transmission band while absorbs EM waves at other frequency bands. The early attempt of utilizing periodic metamaterial structure to realize EM rasorbers have been conceptually introduced by B. A. Munk [1]. Later on, a couple of EM rasorbers with detailed theoretical analysis and experimental validation have been reported [2]- [25]. In terms of physical geometry, the existing rasorbers can be divide into lateral (or 2D) and 3D devices. Generally, a 2D EM rasorber consists of a 2D resistive layer on top and a 2D frequency selective surface (FSS) underneath. A quarter wavelength air gap between resistive layer and FSS is usually required in lateral devices [2]- [17] while their 3D counterparts are based on parallel-plate waveguide structures with subwavelength thickness [18]- [25]. The advantages of 3D rasorber compared with the lateral ones include broad transmission bandwidth, improved selectivity and stability under oblique incidence. However, due to the structural complexity of 3D rasorbers, most of the reported experimental investigations have been concentrated on 2D devices.
Experimental explorations of rasorbers with single absorption and transmission band were firstly reported in the early ages [2]- [7]. Those exhibit an absorption band at lower frequency and a transmission band at higher frequency are classified as A-T rasorbers, where A stands for absorption and T means transmission. The T-A rasorbers, on the contrary, hold a higher absorption band and lower transmission band. Later, a series of scenarios with dual absorption bands locating on both sides of the transmission window (i.e. A-T-A type) VOLUME 4, 2016 were also presented [8]- [17]. In addition, rasorbers with dual transmission bands were also experimentally investigated by M. Guo et al. [26], [27]. Compared with the A-T and T-A types with a single transition region between transmission and absorption bands, the A-T-A or dual-band rasorbers exhibit more transition bands with unacceptable absorption or transmission performance. To reduce the width of transition bands, X. Xiu et al. utilized high-order FSS to improve the selectivity of the transmission band for their A-T-A rasorber [15]. However, the resultant narrow transmission band is not favored in broadband applications. Although high-order FSS has also been used in rasorber design to achieve broad transmission bandwidth and fast roll-off characteristic [25], [28], the reciprocity of EM wave transmission within the passband degrades its stealth performance.
One potential solution to this constriction is to use PIN diodes as switches to turn off the transmission window when necessary [12], [29]. Hence, in the off-state both transmission and transition bands are eliminated and a single absorption band can be achieved over an ultra broad frequency of interest. However, the reported switchable rasorbers exhibit a relatively narrow transmission band compared with those exhibiting fixed absorption and transmission performance. Alternatively, the rasorbers with a narrow but tunable passband can also be used to extend the overall effective transmission frequency range. In addition, reconfigurable rasorbers are also suitable to be used as the radome materials for the radar cross section (RCS) reduction of reconfigurable antennas within its absorption band. In early 2019, L. Wu et al. reported a tunable A-T-A rasorber with fast roll-off characteristics on both sides of the passband [30]. However, the minimum insertion loss as high as 3.3 dB makes it unacceptable for practical applications. Later on, Y. Wang et al. proposed an A-T-A rasorber with varactor diodes as the active components to tune the transmission frequency [31]. To reduce the insertion loss caused by the series resistance of varactor, a unit cell structure with small capacitance in the resonant tanks of both resistive and FSS layers were used, resulting in the wide transition regions that significantly narrows the absorption bandwidth (≤ 63%). In addition, the device covers a narrow transmission frequency range that limits its implementation in broadband applications. Recently, S. C. Bakshi et.al experimentally demonstrated a reconfigurable rasorber whose passband can be switched among three discrete frequency ranges [32]. Hence, the device is able to operate among A-T, A-T-A and T-A modes. Although this rasorber exhibits over 80% fractional absorption bandwidth in A-T and T-A modes, the passband of the device cannot be tuned continuously, limiting its overall transmission frequency range.
Hence, in this work, a reconfigurable rasorber aiming at extending the overall transmission frequency range is proposed. It combines an electronically controlled resistive layer utilizing the equivalent tunable inductance and mechanically tuned FSS proposed in [33]. Due to the absence of varactors, the FSS exhibits ultra-low insertion loss and can be poten-tially used for high-performance tunable rasorber design. Although the low-frequency performance of the mechanically tuned FSS has been demonstrated experimentally in [33], the theoretical analysis of its operation principle and the corresponding equivalent circuit model are absent. In addition, its transmission and reflection characteristics at higher frequencies that are favorable for multi-passband rasorber design are not analyzed in detail either. Therefore, in section II of this paper, a detailed investigation of the mechanically tuned FSS, with both frequency and time-domain simulations, over a wide frequency range is presented. In addition, a resistive layer with a reconfigurable resonance tank and compact unitcell geometry for multi-passband rasorber design is also introduced in section III. Section IV illustrates the equivalent circuit analysis as well as the numerical simulation results of the proposed rasorber. The validation of the proposed reconfigurable rasorber is confirmed with experiment results in Section V. Section VI concludes the paper. The unit-cell structure of the mechanical reconfigurable FSS is presented in Fig. 1(a) and (b). It consists of a rectangular metallic frame with periodicity a and a metallic patch with thick d p . The thickness of the metallic wall is d and the gap between the metallic walls is s = a − d . d is the length of the metallic wall along the direction of wave prorogation. w p is the edge length of the square patch. Cylindrical shafts are inserted into the holes of metallic frame to connect the adjacent patches. The square shaft heads prevent the patches from skidding during the tuning procedure, which guarantees the normal vectors of patches in each row of FSS in parallel with each other at any time. Hence, one can easily tune the angle θ of each row from -90 • to +90 • by adjusting the knobs attached on the side of FSS, as shown in Fig. 1(c).

A. EQUIVALENT CIRCUIT MODEL OF MECHANICALLY TUNABLE FSS
To investigate the operation principle of this mechanically reconfigurable FSS, an equivalent circuit model (ECM) is derived in this section. The metallic frame can be divided into finite thick metallic gratings in parallel with the direction of the electric field and finite thick metallic gratings in parallel with the direction of the magnetic field, as shown in Fig.  2(a) and (c) respectively. Under normal incident y-polarized wave, the equivalent circuit model of the former gratings can be presented as Fig. 2(b), in which B a and B b are calculated as [34] FIGURE 2. Infinitely extended metallic gratings with finite thickness and axes parallel with (a) the magnetic of normal incidence and (b) the corresponding equivalent circuit. Infinitely extended metallic gratings with finite thickness and axes parallel with (c) the electric field of normal incidence and (d) the corresponding equivalent circuit. and Consequently, the equivalent circuit model of the later gratings can be presented as Fig. 2(d), in which X a and X b are calculated as [34] X a = aZ 0 λ ln a 2πr 0 where and e ≈ 2.718 is the base of the natural logarithm. When the metallic patch is placed in the center of the unit cell, additional capacitances are induced between the patch and walls. To calculate the capacitance between two vertically placed metallic plates with finite thickness, as shown in Fig. 3, the overall capacitance C t is divided into C L , C M and C R , in which C L and C R can be calculated as [35] where K(k) is the complete elliptic of the first kind and k is the complementary modulus of k. Consequently, the middle capacitance can be estimated with the classical equation used for a parallel plate capacitor as and the overall capacitance C tot is Hence, the corresponding capacitance in ECM is C t = C tot /2 due to the series connection of two identical C tot and the final equivalent circuit model of the metallic walls can be illustrated as Fig. 4 (a), in which To derive an analytical expression for the S parameters of proposed FSS, a conversion from T-to π-network is required, resulting in the ECM shown in Fig. 4 (b), where and are the impedance and admittance of the π-network. Hence, the ABCD matrix of proposed FSS is presented as and the |S 21 | of proposed FSS can be derived as At frequency one have Y af = 0 and (21) can be simplified as Therefore, EM waves can propagate through the device with a low insertion loss if |Z bf | Z 0 . In addition, With the help of the angle marks printed on the side of each row, the rotation angle θ can be controlled accurately. Therefore, C t can be varied and transmission frequency is tuned correspondingly.

OF FSS
Fig. 5(a) and (b) demonstrate the numerical simulation results and the theoretical results calculated with ECM from 1.5 to 12 GHz. As the frequency-domain solver of CST microwave studio provides excellent simulation accuracy for devices consisting of identical periodic unit cells, the corresponding simulation results achieved for frequency between 1.5 to 8 GHz are presented in the white area of Fig. 5(a) and (b). When f = 8 ∼ 12 GHz, the results achieved from the frequency-domain solver exhibit a lot of unexpected spikes that cannot be used to predict the device performance at all. Hence, the time-domain solver is used instead to achieve the required simulation results, as shown in the light blue area of Fig. 5(a) and (b). Although it does not provide accurate resonant frequency prediction compared with the frequencydomain counterpart, it is still a good tool for investigating the performance of a device when the results achieved from frequency-domain solver are not valid. It is worth mentioning that in the rest of the paper, the white areas illustrate the simulation results achieved from frequency-domain solver while the light blue area depicts the results achieved from time-domain solver for figures demonstrating numerical simulation results as well.   The geometrical parameters used in the simulations are as follows: a = 30 mm, w p = 20 mm, d p = 4 mm, d = 4 mm, d = 10 mm. The capacitance of θ = 0 • is achieved with (17) and those of non-zero angles, as shown in Fig. 6, are extracted by fitting the theoretical calculation to the numerical simulation results. It can be seen that results calculated with the proposed ECM agree excellently with the numerical simulation results below 5 GHz and a tunable passband with a center frequency between 3.9 to 5.6 GHz is observed. When the frequency increases so that a exceeds λ/2, the deviations between ECM and simulation results increase as the equations of ECM are valid for a ≤ λ/2 only [34]. For frequency above 8 GHz, the simulation results achieved with time-domain solver predicts an extra tunable transmission band centered around 9∼10 GHz.
To validate the results achieved from theoretical analysis, the FSS is fabricated with aluminum as shown in Fig. 7, in which the marks help to accurately control the rotation angle of FSS are also illustrated. Fig. 5(c) and (d) demonstrate the corresponding S parameters achieved with free space measurement. The device exhibits a tunable passband centered from 4.13 to 6.61 GHz when θ varies from 0 • to 90 • . The simulation results are well reproduced with laboratory measurements except at frequencies above 8 GHz due to the poor simulation accuracy of time-domain solver. However, the second tunable passband at higher frequencies for 45 • ≤ θ ≤ 90 • , as predicted by the numerical simulation, is also observed in the measurement results. Therefore, the proposed FSS is potentially suitable for the design of rasorbers with dual or multiple reconfigurable passbands.

III. TUNABLE RESISTIVE LAYER ANALYSIS
The frequency reconfiguration of the resistive layer is realized with varactor diodes. As well known, the capacitance of varactor diode can be tuned by varying the reverse bias voltage applied between its anode and cathode. In [30] and [31], the varactor diodes are connected in parallel with the geometrical capacitance, as shown in Fig. 8(a), resulting in a tunable capacitance to control the resonance frequency. The disadvantage of this design is that the bias voltage cannot be directly applied on the varactor as a DC short circuit is caused by the inductor, resulting in a complicated bias network distributed on multiple metallic layers of the device [30] [31].

A. RECONFIGURABLE RESISTIVE LAYER
In this work, the reconfigurable resistive layer is designed utilizing a resonant tank with two parallel-connected series LC, as shown in Fig. 8(b). L 2 , L 1 and C 1 are the geometrical inductors and capacitor, respectively. As jωL 2 is much greater than j/ωC var_max within the frequency of interest, the overall reactance of series L 2 -C var is inductive. Hence, by varying the reverse bias voltage, a tunable inductance is realized. Since the capacitance of varactor diode is maximized at V b = 0, the resultant tunable inductance branch exhibits its maximum inductance and hence introduces a relatively low insertion loss to the resultant rasorber. In addition, unlike the equivalent circuit illustrated in Fig. 8(a), the proposed approach allows the implementation of bias voltage directly on the ports of the varactors in Fig. 8(b). Hence, the bias network is embedded in the unit-cell configurations which greatly simplifies the design process. Fig. 9(a) and (b) illustrate the schematic and equivalent circuit model of a resistive layer designed utilizing the proposed resonant tank with reconfigurable inductance. The series R 0 -L 0 -C 0 is used to match the impedance of free space for wave absorption and the transmission band is generated with the resonant tank presented in Fig. 8(b).
Obviously, by varying the external bias voltage, C var is tuned and the transmission frequency of the resistive layer is varied as well.

Fig. 10(a) and (b) present the results achieved with numerical simulation and ECM calculation results of the resistive layer.
To match the passband of FSS presented in the last section, the following geometry parameters are used: h 1 = 0.254 mm, w 1 = 4 mm, w 2 = 3 mm, w 3 = 0.7 mm, w 4 = 0.3 mm, w 5 = 0.2 mm, w 6 = 0.3 mm, l 1 = 9.6 mm, l 2 = 0.3 mm, l 3 = 2 mm, l 4 = 2.65 mm, l 5 = 1.7 mm, l 6 = 1.3 mm, g 1 = 1.95 mm, g 2 = 0.5 mm, g 3 = 0.2 mm, g 4 = 0.4 mm. The substrate is Rogers 4350B with relative permittivity r = 3.48 and loss tangent tan δ = 0.0037. Since there is a parasitic resistance R s in series with the varactor that can cause significant insertion loss to the passband, the varactor diode SMV1405 manufactured by Skyworks with a low R s of 0.8 Ω and parasitic inductance of 0.45 nH was selected for this work. The capacitance of varactor can be tuned between approximately 2.7∼0.6 pF when V b varies within 0∼30 V.
Again, the ECM results agree well with the numerical simulation at frequency below 5 GHz only due to the increase of ratio a/λ (exceeding 1/2) when the frequency increases. Fig. 11 illustrates the photograph of the fabricated resistive layer. The corresponding measurement results are presented in Fig. 10(c) and (d). It can be seen that the measurement and simulation results agree well with each other over the whole frequency of interest and a continuously tunable passband centered from 4.03 to 4.61 GHz is observed, matching excellently with the lower passband of FSS. At higher frequencies, the resistive layer is nearly transparent to EM waves except for a narrow frequency range of 7.90∼8.76 GHz, making it suitable for rasorber design with multiple passbands as well.

IV. ECM VS. SIMULATION RESULTS OF RASORBER
The reconfigurable rasorber proposed in this work is designed utilizing the FSS and resistive layer presented above, whose unit-cell configuration is presented in Fig. 12 with an air spacer thickness of h 2 = 16 mm. The operation principle of rasorbers have been introduced in details in [1]- [25]. The basic idea behind a reconfigurable rasorber is to tune the passbands of FSS and resistive layer simultaneously, so that their transmission bands are always matched and a tunable transmission window can be created for the rasorber. As introduced above, it is obvious that the lower-frequency   transmission window of the proposed rasorber should be tuned by rotating the metallic patch in FSS (i.e. C t ) and adjusting the reverse bias voltage of varactor diode (i.e. C var ) simultaneously. At higher frequencies, as the resistive layer exhibits two wide bias-independent passbands, the passband(s) of rasorber is(are) controlled by just mechanically tuning the rotation angle (i.e. θ) of FSS.
Since the FSS exhibits a small resonant frequency variation between θ = 0 • to 10 • , the simulation results of proposed rasorber under θ = 0 • , 20 • , 30 • and 35 • are presented as solid lines in Fig. 13. It can be seen that when both θ and C var are tuned, the device demonstrates a low IL min between 0.11 to 0.49 dB while the IL min frequency varies from 3.94 to 4.52 GHz. Hence, the frequency range covered for |S 21 | ≥ -1.5 dB at lower frequency is between 3.74 to 4.69 GHz, covering a fractional transmission bandwidth of 22.5%. In addition, the device also exhibits low microwave VOLUME 4, 2016    reflection (i.e. |S 11 | < -10 dB) between approximately 2 to 6 GHz for all angles except θ = 0 • . Moreover, the ECM of the proposed raosrber is shown in Fig. 14 and the corresponding mathematical modelling results are also plotted as dashed lines in Fig. 13, demonstrating excellent agreement with the numerical simulation at frequency below 5 GHz. The detailed performance of proposed rasorber over 1.5∼12 GHz is investigated with numerical simulations and laboratory experiments in the following section. Fig. 15 demonstrates the photograph of proposed rasorber consisting of 8×8 unit cells with an overall dimension of 260×260×26 mm 3 . The free space measurement, as illustrated in Fig. 16, is carried out to investigate its transmission and reflection characteristics under normal and oblique incident waves as well.

A. NORMAL INCIDENCE
The device performance under normal incident waves is presented in this part. Fig. 17 to 21 illustrate the measured and simulated S parameters of the proposed device when V b is varied under fixed angles θ = 0 • , 10 • , 20 • , 30 • , 35 • . As can be seen, the measurement and simulation results agree excellently and the passband can be tuned from a center frequency of 3.91 GHz to 4.54 GHz while the device performance is nearly independent of the voltage variation above 6 GHz under a fixed θ. For θ = 0 • and 10 • , except the two absorption bands adjacent to the passband of rasorber, a third absorption band is also observed around the center frequency of 10.5 GHz, as predicted by the time-domain simulation with a frequency shift that caused by the inaccurate resonant frequency prediction of time-domain solver. Further increasing θ to 20 • , the highest absorption band disappears and the second absorption band on the right side of passband also narrows. For θ = 30 • and 35 • , the second absorption band is also eliminated and the device exhibits a transform from A-T-A to A-T type. When θ ≥ 35 • , since the high-frequency performance is independent of the external bias voltage and the device exhibits the widest absorption bandwidth under V b = 30 V, the rasorber is characterized with V b = 30 V in the following discussions.      As can be seen in Fig. 22, when θ increases to 45 • and 60 • , the device exhibits a single absorption band at lower frequencies and two passbands at higher frequencies. Further increasing θ to 80 • and 90 • , the device exhibit three passbands above the absorption band. The simulation results are well reproduced with measurement results and the performance of the proposed rasorber is summarized in Table 1. The device exhibits a fractional absorption bandwidth between 62.7% and 88% (57.4%+22.5%+8.1%) and a measured minimum IL min of 0.16 dB is achieved at θ = 20 • and V b = 0 V. The fractional transmission frequency range varies between 9.3% to 23% (6.6%+11.8%+14.6%) and the passband covers 3.68 ∼ 4.77 GHz (25.8%), 5.99 ∼ 6.4 GHz (6.6%), 6.85 ∼ 7.84 GHz (13.5%) and 8.52 ∼ 10.2 GHz (18.0%), corresponding to an overall fractional transmission frequency range of 25.8%+6.6%+13.5%+18.0%=63.9%.
To investigate the novelty of this work, the measurement results of proposed rasorber are compared with the measurement results achieved for other existing scenarios. Table 2 summarizes the comparison of the proposed device with other published reconfigurable rasorbers. The proposed device exhibits a maximum absorption bandwidth of 87.9%, comparable with the state-of-art maximum absorption bandwidth (88.09%) achieved in [28]. While the other reconfigurable rasorbers exhibit a single passband under all conditions, the proposed rasorber can be switched between modes with single, dual and triple transmission bands with at least one absorption band. Hence, the proposed device greatly outperforms the other counterparts in terms of the overall fractional transmission frequency range (63.9% vs. 26.7% in [30]), demonstrating excellent frequency reconfigurability of the proposed rasorber while maintaining a wide absorption bandwidth.

B. OBLIQUE INCIDENCE
To illustrate the device performance under oblique incident waves, the proposed rasorber is investigated experimentally under TM-and TE-polarized waves, as shown in Fig. 23, for various incident angles. As can be seen from the measured S parameters for TM-polarization under incident angles of ±15 • and ±30 • presented in Fig. 24 and 25, the rasorber is insensitive to the sign of incident angle. In addition, the device exhibits stable transmission and absorption performance at frequencies below 6 GHz while the lowest and highest transmission bands for θ = 90 • and V b = 30 V disappear under an incident angle of 30 • . However, the device still exhibits an overall fractional transmission frequency range of 47.6%, far beyond that achieved in [32]. The absorption bandwidths of the proposed rasorber under 30 • TM-polarized waves vary between 55.8% to 82.2%, demonstrating a slight degradation on the upper and lower bounds of absorption bandwidth.
Under TE-polarizations, the device demonstrates an overall transmission frequency range of 36.6% under an incident angle of 20 • . Further increasing the incident angle to 30 • , the transmission bands narrow to 20.1%. Hence,