Wideband Reconfigurable Reflectarray Based on Reflector-Backed Second-Order Bandpass Frequency Selective Surface

In this communication, a wideband reconfigurable reflectarray based on reflector-backed active second-order bandpass frequency selective surface (FSS) is presented. The reflector is composed of periodic short-circuited parallel plate waveguide (PPW) and the FSS is composed of stacked non-resonant metallic elements separated by thin dielectric substrates. By integrating microwave varactors in the capacitive layers of FSS, more than 270° continuous phase tunability is achieved within a fractional bandwidth of 14%. A 1-D reflectarray prototype operating at C-band is fabricated and measured. The experimental results show that it can achieve ±55° beam scanning coverage. Symmetric beam steering is observed due to the center-fed configuration. With advantages of low cost and simple structure, the proposed reflectarray can be potentially used in wideband wireless communication and radar systems.

Employing spatial feeding architecture, reflectarrays can facilitate the design of large-aperture antennas by eliminating the complicated feeding networks and the associated loss. To enable beam steering, several mechanically controlled reflectarrays have been developed. Based on rotatable elements, a wideband reconfigurable reflectarray with 360 • continuous phase coverage has been proposed [16]. Using true-time-delay elements, ultra-wideband reflectarray antennas have also been proposed [18], [19] and the beam steering was achieved by adjusting the position of the feeding antennas. The advantages of mechanically reconfigurable reflectarrays are their low element loss and large phase tunability. Manuscript  On the other hand, the integration of tunable components into the elements allows the design of electrically reconfigurable reflectarrays. By integrating p-i-n diodes in microstrip patch resonators, several 1-bit reconfigurable reflectarrays have been proposed [13], [14], [15]. The 1-bit phase correction scheme (0 • /180 • ) has advantage of simplified bias circuit, but it is at the expense of antenna efficiency and sidelobe level (SLL) due to the large phase quantization [22]. The continuous phase tunability can been achieved by introducing microwave varactors in reflectarray elements. In [12], a varactortuned reflectarray using aperture-coupled patch has been proposed and around 320 • continuous phase agility was obtained within a fractional bandwidth of 5.3%. Based on single-layer dual-resonance structure, a reconfigurable reflectarray with full-phase tunability has been developed [20], [21]. In addition, barium strontium titanate (BST) thin film [17] and liquid crystal [5], [6], [7] have been used to design continuously tunable reflectarrays. In contrast to the reflectarrays with mechanical reconfigurability, implementation of wideband electrically reconfigurable reflectarrays with large phase tunability is more challenging.
In this communication, a wideband reconfigurable reflectarray is proposed and experimentally demonstrated at C-band. The unit cell is composed of an active second-order bandpass FSS and a shortcircuited parallel plate waveguide (PPW)-based reflector. To obtain optimal phase response, the reflector is located around 1/4 wavelength behind the FSS. By loading varactors in the top layer and bottom layer of FSS, the designed reflectarray element can function as a reflection-type spatial phase shifter. The reflection response of the fabricated reflectarray is investigated by the free-space measurement method. Based on the measured phase tunability, a wideband reconfigurable reflectarray is implemented. Experimental results show that the fabricated reflectarray prototype can achieve ±55 • beam steering within a bandwidth of 1 GHz.
This communication is organized as follows. In Section II, the beam steering principle of the reflectarray antenna is introduced. The design and the simulation of the proposed reflectarray element are also given in this section. In Section III, the measured results of the fabricated reflectarray prototype are presented. Finally, a conclusion is drawn in Section IV.

II. DESIGN OF WIDEBAND REFLECTARRAY
An active second-order bandpass FSS backed by a short-circuited PPW is employed to construct the wideband tunable element. The schematic architecture of the proposed 1-D reflectarray antenna is shown in Fig. 1(a), in which a cylindrical wave incidence is considered. The element period p is kept to less than a half wavelength, which denotes that only transverse electromagnetic (TEM) wave can be excited inside the PPW. In [23], it was pointed out that the PPW can transform the incident TM wave into TEM wave inside the PPW. In this way, the FSS would sense the obliquely incident wave as a normally incident wave, which can enable the designed reflectarray element insensitive to the angle of the incidence wave. Since the 0018-926X © 2022 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See https://www.ieee.org/publications/rights/index.html for more information. short-circuited PPW structure has been used, the proposed unit cell is only effective for TM polarization. The beam steering principle of reflectarray antenna can be analyzed by the geometric-optics method [1], [3]. In order to collimate the incident wave to the desired direction, the reflection phase of each element is assumed to be individually controllable, i.e., each element behaves as a reflection-type spatial phase shifter. The required phase shift value of each element can be calculated by where k 0 is the wavenumber in free space, ϕ i and ϕ j are the reflection phases of unit cells. The S i , S j , P i , and P j denote the length of different rays. Based on these tunable spatial phase shifters, the phase difference between different rays can be compensated and the cylindrical wavefront can be transformed into a plane wavefront. The FSS used in this work is based on a previously designed lens unit cell in [3]. As shown in Fig. 1(b)-(d), the FSS unit cell is composed of four metallic layers that are separated by two thin dielectric substrates and one bonding sheet. The dielectric substrate is WL-CT350 material with the relative permittivity of 3.5 and the loss tangent of 0.004. The top layer and the bottom layer are identical structure, each consisting of parallel strips etched with a slot. Two middle layers, designed as crossing strips, are also in the same form. With this design, the top layer and the middle layer can be characterized as capacitance and an inductance [24], [25], respectively. In order to obtain tunable capacitance, two flip-chippackaged varactors (MA46h120) are embedded in the capacitive layers. The typical capacitance-voltage curve can be found in the official datasheet of Ma46h120 or [3]. Instead of using one inductive layer (middle layer), two inductive layers separated by a thin bonding sheet (Rogers 4450F, 0.2 mm thickness) are employed to maintain the symmetry of the FSS structure. With this arrangement, two inductive layers in the middle act as a single composite inductive layer [26]. Since the FSS is composed of two dielectric substrates that need to be bonded together, introduction of the bonding layer creates an asymmetry in the FSS. This asymmetry would change the response of FSS [26]. With two-inductive-layers design, this asymmetry can be eliminated and the impact of the bonding layer can be minimized [26]. Such design would not increase the fabrication complexity compared to the single-inductive-layer design that is used in [3]. The geometric parameters of the designed element are given in Table I. By further optimizing the geometric parameters and the thickness of dielectric substrate, the length of the unit cell (l 1 ) can be further increased. The simplified equivalent circuit model of the reflectarray element is shown in Fig. 1(e), in which the characteristic impedances Z 0 and Z 1 denote the wave impedances in free space and dielectric substrate, respectively.
To investigate the performance of the proposed reflectarray element, full-wave simulations are performed with the periodic boundary in CST Microwave Studio. 1 In the simulation, the embedded varactor is modeled as a discrete capacitor in series with a 2 resistance [27]. The influence of the distance between the reflector and the FSS is shown in Fig. 2. From 6.5 to 8.3 GHz, it is observed that the reflection phase can maintain a relatively good linearity when the distance h 3 is around 1/4 wavelength.
To facilitate the practical fabrication and assembling, a small gap (q 2 /2 − q 1 /2) has been introduced between the inductive layers and the PPW, as shown in Fig. 1(d). The simulated reflection response for reflectarray unit cell with and without the gap is shown in Fig. 3. It can be observed that introduction of a small gap would slightly change the reflection curves, but it has little impact on the primary second-order response of the designed element.   The simulated reflection phase at normal incidence and the oblique incidence is shown in Fig. 4. It is seen that the reflection phase would change remarkably at oblique incidence if PPW was removed (without PPW). In contrast, the PPW-based reflectarray exhibits excellent angular stability. Therefore, the PPW is employed to obtain an angle-insensitive reflection response in this work. This angleinsensitive feature makes the proposed element a good candidate for constructing the reconfigurable reflectarray antennas. Different from [3], an asymmetrical capacitive strip is employed in this work. Such design can be used to reduce the element loss, as indicated in Fig. 5. On the other hand, it was found in the simulation that the reduced e 1 would reduce the range of phase tunability. In this work, we have chosen the proper e 1 to ensure the  phase tunability is larger than 270 • within 1 GHz bandwidth while maintaining relatively low loss.
At normal incidence, the simulated reflection amplitude and phase with different capacitance values are shown in Fig. 6(a) and (b), respectively. It is seen that the reflection phase can be tuned by changing the capacitance value, meaning the designed element can function as a reflection-type spatial phase shifter. With the capacitance value tuned from 0.14 to 0.85 pF, the phase tunability ranges between 274 • and 337 • within the band 6.65-7.65 GHz. Meanwhile, the reflection amplitude varies between −4.4 and −0.3 dB, in which the insertion loss introduced by the varactor is around 0.2-3.6 dB for different capacitance values. The reduced reflection amplitude is primarily caused by the dielectric loss and the ohmic loss of varactor.  These losses will decrease the gain and the aperture efficiency of the reflectarray antenna.

A. Fabrication
Based on the above-simulated results, a 1-D reflectarray prototype is fabricated. As shown in Fig. 7, adjacent elements in the vertical direction are electrically connected with each other while isolated in the horizontal direction. In this way, the varactors mounted in the same column can be biased by the same voltage. The reflectarray consists of 20 columns and each column can be independently biased. Fig. 8 shows the implemented control circuit board. It was designed with one micro-controller unit (MCU, STM8S105K4) and eight digital-to-analog converters (DACs, DAC61416). An MCU circuit developed in previous work [3] was employed to facilitate the experimental work. The MCU is used to communicate with the laptop through the SPI interface, and send the control voltage command to DACs. With 12-bit resolution, the output voltage precision of the DAC61416 chip can reach 0.01 V. The reflectarray and the control circuit can be connected through flat cables.

B. Reflection Measurement
The reflection response of the fabricated reflectarray prototype was investigated with the free-space measurement method [3], [28], as shown in Fig. 9(a). The experiment was performed with a vector network analyzer and two horn antennas acting as the transmitting antenna and the receiving antenna, respectively. The distance between the horn antennas and the reflectarray is around 1.2 m. A piece of the absorber was used to suppress the potential direct coupling between two horn antennas. Considering the PPW-based reflectarray is angle-insensitive to the incident wave, a small-angle incidence has been used to investigate the relationship between the reflection phase and the bias voltage.
The measured reflection amplitude and phase are shown in Fig. 9(b) and (c), respectively. Similar to the simulated results, the reflectarray prototype behaves as a tunable phase shifter. Compared to Fig. 6, the measured results in Fig. 9 are similar to the simulated results in terms of the trend of wideband tunability. When the bias voltage is increased from 1.5 to 15 V, the phase tunability ranges between 279 • and 315 • within the band 6.65-7.65 GHz. Note that this insufficient phase tunability will slightly decrease the efficiency of reflectarray antennas [22]. Considering the possible capacitance variation indicated in varactor datasheet and the potential fabrication error, the expected capacitance values (in brackets) generally agree with the ones that used in simulation. The corresponding reflection amplitude varies between −6 and −0.4 dB. In the future, the influence of limited tuning range and element loss need to be further investigated. Based on the measured relationship between the reflection phase and the bias voltage, a wideband beam steerable reflectarray can be implemented.

C. Antenna Measurement
The beam steering performance of the reflectarray antenna was measured in an anechoic chamber and the experimental setup is shown in Fig. 10. A wideband horn antenna, acting as the feeding  antenna, was placed 200 mm in front of the reflectarray. The feeding horn is placed in the center, along the boresight of reflectarray. The aperture size of the feeding horn antenna is 34 × 94 mm. With a 200 mm distance, the field taper from the center illumination to the edge illumination is −4.6 dB at 7.65 GHz. The control circuit board was arranged behind the reflectarray. Fig. 11 shows the measured radiation patterns at different frequencies. It is seen that the reflectarray antenna can achieve symmetric beam steering due to the center-fed configuration. For the 30 • beam deflection angle, the measured SLLs at 6.65, 7.0, 7.3, and 7.65 GHz are −12.9, −13.1, −9.0, and −11.1 dB, respectively. For 45 • beam deflection angle, the measured SLLs are −12.1, −13.2, −13.5, and −13.5 dB, respectively. On the other hand, relatively high SLLs are observed for a 0 • deflection angle because of the feed blockage. For 0 • beam deflection angle, the measured SLLs are −8.7, −9.8, −9.3, and −9.2 dB, respectively. Considering the SLL of a reflectarray is closely related to its size, a reflectarray with larger aperture could be implemented to improve the SLL [22]. At 7.0 GHz, the measured maximum gain is 16.1 dB, corresponding to a 2-D aperture efficiency [29], [30] of 29.2%. Higher-quality varactors and dielectric substrate can be used to further reduce the element loss and improve the antenna efficiency.
The performances of the proposed reflectarray are compared with that of other continuously tunable reflectarrays, as summarized in Table II. Previous reflectarray works were designed with a singlelayer tunable surface. In our work, the designed structure is imple- mented by stacking multiple-layer tunable surfaces inside a shortcircuited PPW. It can be seen that the proposed design can achieve a wider bandwidth. Similar to the wideband phased array antennas, the beam squint effect is observed in Fig. 11. Within a bandwidth of 1 GHz, the beam squinting at 30 • deflection angle is around 3 • and it increases to 8 • at 45 • beam deflection angle. To reduce beam squint effect, electrically tunable true-time-delay elements should be employed to construct the wideband reconfigurable reflectarrays, which may be potentially achieved in the future.

IV. CONCLUSION
A C-band reconfigurable reflectarray antenna is proposed and experimentally demonstrated. The reflectarray element is designed with an active second-order bandpass FSS and a short-circuited PPW based reflector. It is found that the reflection phase can achieve a relatively good linearity when the distance between the reflector and the FSS is around 1/4 wavelength. Wideband phase tunability is achieved by loading the varactors in the capacitive layers of FSS. From 6.65 to 7.65 GHz, more than 270 • phase tunability is obtained. Using center-fed configuration, symmetric beam steering within ±55 • is observed. The proposed design can be a useful candidate for wideband wireless systems.