A Simultaneous Beam Steering and Polarization Converting S-Band Transmitarray Antenna

In this paper, a multi-functional transmitarray antenna consisting of miniaturized unit cells for S-band radar applications is proposed. The unit cell size is <inline-formula> <tex-math notation="LaTeX">$0.4\lambda _{0}\,\,\times \,\,0.4\lambda _{0}$ </tex-math></inline-formula>, and it is composed of a receiving (Rx) antenna, a miniaturized inductively-loaded reflection-type phase shifter, a PIN diode switch for polarization conversion, and a dual-polarization transmitting (Tx) antenna. The types of Rx and Tx antennas are coupled-fed stacked patch antennas. The designed reflection-type analog phase shifter has a 36 % smaller area than the conventional reflection-type hybrid coupler-based phase shifter. It has a 3-bit phase-shifting capability realized with a 0 ~ 330° analog phase shifting. The designed SPDT PIN diode switch can select the polarization of the transmitted wave and accept a high input RF power of 23 dBm. The proposed 8 <inline-formula> <tex-math notation="LaTeX">$\times8$ </tex-math></inline-formula> transmitarray antenna with F/D of 0.33, made from its unit cell, has a gain of about 7.9 dBi and performs 90° 2D beam steering for vertical-to-horizontal and vertical-to-vertical polarization conversions at the center frequency.


I. INTRODUCTION
There has been a growing interest in remote electromagnetic (EM) wave detection for vehicle radar or next-generation satellite communications in recent years. Numerous research papers on array antennas capable of multi-functional beam scanning have been reported because of their various applications [1], [2], [3], [4], [5], [6], [7]. The widely used beam scanning antennas for remote sensing are mechanically rotating dish reflectors, active phased arrays, and transmitarray or reflectarray antennas. The conventional active phased array antennas are most widely used for beam steering but require complex feeding networks, high power, high-cost transceiver module, etc. Therefore, transmitarray and reflectarray antennas have attracted huge attention as alternatives to active phased array antennas, especially in the military and satellite communication industries that use extremely high or The associate editor coordinating the review of this manuscript and approving it for publication was Luyu Zhao . limited power. Since transmitarray and reflectarray antennas are very similar in structure and are classified according to the direction of the feeding antenna and the radiation direction from the array antenna, an appropriate array antenna among these two types is usually selected according to the design environment.
There are three main approaches to unit cell design of a transmitarray [4]: (a) multi-layer frequency selective surface (M-FSS) [8], [9], [10], (b) metamaterial/transformation [11], [12], [13], and (c) receiver-transmitter design [14], [15], [16] approaches. Since the multi-layer FSS structure is separated by either an air gap or a thick substrate, the M-FSS approach is difficult in transmitarrays with a low profile. On the other hand, the configuration having metamaterial characteristics is designed to vary the effective permittivity (ε eff ) and permeability (µ eff ) of the substrate or a specific material in the metamaterial/ transformation approach. Since this approach uses a simple metal pattern on a substrate or uses a specific material, it has a complexity issue in implementing a 2-bit VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ or more phase shifting performance. This research uses the receiver-transmitter design approach because of the required unit cell's ease of fabrication (compatible with multi-layer PCB and SMT processes), low profile property, and broad phase-shifting range (2-bit or more). Generally, it is necessary to phase control each unit cell for beam steering. In addition, each unit cell has a phase shifter using a PIN diode or a varactor. The PIN diode-based phase shifting controls the current flow using two or more PIN diodes to achieve discrete (or digital) phase shifting of 1 or 2-bit states. However, the PIN diode-based phase shifting has limited beam steering because of rough phase compensation. Hence, a reflection-type phase shifter having a hybrid coupler is widely used for continuous analog phase shifting in a wide range due to its simple and practical structure. However, the typical reflection-type phase shifter requires a large space due to its hybrid coupler's quarter-wavelength (λ g /4) transmission line structure. Therefore, it is critical to realize a miniaturized unit cell by reducing the size of its reflection-type phase shifter to avoid a larger transmitarray.
Many reported reconfigurable transmitarray designs have focused only on beamforming or polarization conversion. In addition, polarization conversion in most studies was restricted to converting a linearly polarized (LP) wave into a circularly polarized (CP) wave using the element rotation technique [16], [17], [18]. Only a few reported research papers presented LP-to-LP conversion and beam steering capability using a transmitarray structure. Likewise, the proposed transmitarray antenna of this research multifunctionally performs beam steering and linear polarization conversion (vertical-to-vertical or vertical-to-horizontal). In particular, linear polarization conversion is achieved by integrating a dual-polarized Tx square patch antenna and a single pole double throw (SPDT) PIN diode switch. The phase shift function is implemented using a miniaturized broad range reflection-type phase shifter. Notably, a 3-bit phase shifting is implemented using an analog phase shifter in this research to take advantage of both digital and analog phase shifting methods. This 3-bit phase shifter is capable of fine phase tuning at each state.
The organization of this paper is as follows. The structure and measurement of receiving (Rx) and transmitting (Tx) antennas, the miniaturized reflection-type phase shifter, and the SPDT PIN diode switch in a unit cell are studied briefly in Section II. This section also discusses the unit cell's performance using a 1 × 2 transmitarray antenna. Subsequently, Section III experimentally studies the beam steering and polarization conversion of the proposed 8 × 8 transmitarray antenna. Finally, Section IV concludes the research.  it is received by a vertically polarized (V-pol) Rx antenna. The phase of the Rx signal is adjusted to the desired phase value by a reflection-type phase shifter using a miniaturized hybrid coupler. The polarization of the transmission wave to be transmitted through a dual-polarized transmitter (Tx) patch antenna is converted to horizontal (called a V-to-H-pol conversion) or maintained (called a V-to-V-pol conversion) by the SPDT PIN diode switch. The proposed unit cell was fabricated on an FR4 (ε r = 4.4 and tan δ = 0.023) substrate and designed in a miniaturized size (0.4λ 0 × 0.4λ 0 ) to avoid grating lobe and scanning blinds. The unit cell of the proposed transmitarray is designed to cover a 3.3 % fractional frequency bandwidth (BW = 100 MHz) in S-band. ANSYS HFSS 2021 R1, a full-wave FEM (Finite Element Method) EM simulation tool, and Keysight ADS 2021 were used to evaluate this design.

II. DESIGN AND VERIFICATION OF A MINIATURIZED UNIT CELL
The Rx and the Tx antennas operating in the S-band are designed in this research as coupled-fed stacked patch antennas because of their high gain and low-profile properties, as shown in Fig. 1(b). Fig. 2 shows the measured radiation patterns of Rx and Tx antennas. The Rx antenna has the measured 3.8 dBi peak gain and 80 • beamwidth and the Tx antenna has the measured 4.2 dBi peak gain and 80 • beamwidth, respectively. The respective measured fractional bandwidth of Rx and Tx antennas was 3.3 % at the operating frequency. Fig. 3 shows the circuit diagram of the proposed reflection-type phase shifter, relative phase shift, and insertion loss (IL) according to a DC bias voltage. The reflection-type phase shifter has a 36 % smaller area (0.08λ 0 × 0.09λ 0 ) than conventional hybrid couplers (0.14λ 0 × 0.14λ 0 ) by replacing a quarter-wavelength (λ g /4) branch line of the conventional coupler with a transmission line having slow-wave characteristics [19]. The reactive loads consisting of two varactors and a λ g /4 transmission line are connected in series to widen the phase shift range. The fabricated phase shifter has a continuous (analog) phase shifting range of 0 ∼ 330 degrees. It corresponds to a 3-bit digital phase shifting range (45 degrees phase quantization). The measured IL was 1.8 ∼ 5.5 dB depending on the DC voltage. This IL variation is due to the resonance of varactors and the loaded λ g /4 transmission line. Fig. 4 shows the circuit diagram of the proposed SPDT PIN diode switch, IL (S 21 ) and isolation (S 32 ) between two output ports according to the input power when D1 is in the ON state and D2 is in the OFF state. The proposed SPDT PIN diode switch is arranged as follows. Unlike the conventional SPDT diode switch where the anode of one diode and the cathode of the other diode are connected, the anodes of both diodes are connected in common. Both PIN diodes share a common DC ground terminal at the anode, and the bias of each diode is controlled by two DC bias lines at the cathode. For instance, the proposed SPDT PIN diode switch is able to handle 23 dBm of RF power (from P1 to P2) when   15 dB, and the IL increased as the input power increased and was found to be 1.2dB at 22dBm input power.

B. UNIT CELL VERIFICATION
A 1 × 2 array antenna was designed and fabricated based on the proposed unit cell structure to verify its polarization conversion and beam steering capability. As shown in Fig. 5(a), the radiation patterns of the 1 × 2 array antenna were measured in an anechoic chamber. The applied phased difference between antenna elements were −90 (green lines), 0 (red lines), +90 (blue lines) degrees, and their measured normalized radiation patterns were shown in Fig. 5(b) and (c). The measured gain at 0 • phase difference was −0.33 dBi for maintaining the wave polarization (V-to-V-pol conversion) and −0.9 dBi for polarization conversion (V-to-H-pol conversion). The measured beam steering angle range was −27 • to 27 • for V-to-V-pol and −33 • to 13 • for V-to-H-pol conversions. This asymmetrical beam steering angle of 1 × 2 array antenna was caused by a rectangular ground plane which has an asymmetrical ground plane for the V-to-H conversion mode. It should be noted that the polarization conversion and phase shifting capability of the proposed unit cell structure are clearly observed. It also shows a good agreement between   and polarization reconfigurable transmitarray using an analog phase shifter and a PIN diode switch of miniaturized unit cell at S-band for radar applications. Fig. 7 shows the nearfield measurement on the fabricated 8 × 8 transmitarray antenna in an anechoic chamber. The far-field measurement is widely used to test the antenna performance, but it is challenging to build an anechoic chamber for a large antenna array system due to its huge far-field range. On the other hand, the near-field measurement is an efficient way to measure the performance of a large array structure because of its compact measurement setup [20]. In particular, it can also measure the performance of each unit cell because of the small size of the probe. 2D plane Near-field pattern is measured by a rectangular waveguide probe and is transformed into far-field radiation pattern. Moreover, the phase of each array element was calculated by back-projection, projecting the measured near-field electromagnetic wave energy onto the antenna aperture surface [21], [22]. This research used a 10 dBi horn antenna (LB-284-10-C-NF, A-info [23]) as a feeding source horn antenna. The distance between the horn and 8 × 8 transmitarray antennas was set to 150 mm, resulting in a focal length ratio (F/D ratio) of 0.33. The DC voltage control board applies a +1.4 V forward bias to the activated polarization control line and a −5 V reverse bias to VOLUME 10, 2022 another deactivated polarization control line for polarization conversion. It also applies a voltage of 0 to 10 V to obtain the required phase compensation of each unit cell for beam steering at a specific angle. Fig. 8 shows the required phase compensation of each unit cell for (0, 0), (15, 0), (30, 0), and (45, 0) beam steering angles expressed in the azimuthover-elevation, (Az, El), coordinate system. In addition, each negative azimuth or elevation beam steering angles can be obtained by the flipped phase compensation of Fig. 8 because the 8 × 8 transmitarray is symmetric.

B. EXPERIMENTAL RESULTS
The radiation patterns in the V-to-H-pol and V-to-V-pol conversions were measured with the required phase compensation of the unit cell mentioned above. Fig. 9 shows the beam steering of the 8 × 8 transmitarray antenna in terms of radiation patterns at the center frequency for different azimuth (Az) and elevation (El) angles. The measured gain at the azimuth angle was 7.9 dBi and 7.4 dBi for V-to-V-pol and V-to-H-pol conversions, respectively, at the center frequency. Likewise, the measured gain at elevation angle was 7.3 dBi for V-to-V-pol and V-to-H pol conversions, respectively, at the center frequency. The beamwidths and the steerable beam angles at both azimuth and elevation angles were 19 • and +/-45 • , respectively. The 8 × 8 transmitarray antenna's 2D radiation patterns for V-to-V-pol and V-to-H-pol conversions under different (Az, El) angles are shown in Fig. 10. In particular, the 8 × 8 transmitarray antenna's measured efficiency was 8.1 %, which was in good agreement with its 8.4 % theoretical efficiency. Table 1 shows the loss analysis results of the 8 × 8 transmitarray antenna, including coupling loss (spillover loss) between the horn and transmitarray antennas. Notably, most of the 8 × 8 transmitarray antenna's losses are attributable to the substrate (FR4 board). These losses can be reduced significantly using a low-loss substrate, such as a Teflon-based one.
This study on the 8 × 8 transmitarray antenna shows that this antenna has been successfully designed for multifunction performance, such as simultaneous polarization conversion and beam steering. A performance comparison between the measured performances of the proposed 8 × 8 transmitarray antenna and the literature-reported ones is summarized in Table 2. It shows that the proposed 8 × 8 transmitarray antenna has high phase resolution performance compared to other literature-reported transmitarray antennas. In addition, this antenna realizes beam steering and polarization conversion with the smallest unit cell size reported so far in the literature. The miniaturization techniques presented in this paper is scalable to other applications that sidelobes need to be suppressed or space is limited, such as small satellite communication for next-generation communications. The beam steering angle of this antenna is much larger than +/−45 • , but the measurement setup has limited it. Theoretically, the proposed 8 × 8 transmitarray  antenna performs a +/−60 • beam steering in a 2D (Az, El) plane.

IV. CONCLUSION
This research proposed a multi-functional transmitarray antenna using a miniaturized unit cell for S-band radar applications. The miniaturized unit cell consists of an Rx antenna, an inductively-loaded miniaturized reflection-type phase shifter, an SPDT PIN diode switch for polarization conversion, and a dual-polarization Tx antenna on its 0.4λ 0 × 0.4λ 0 structure. The Rx/Tx antennas are coupledfed stacked patch antennas. The reflection-type phase shifter has a 36 % smaller area than conventional hybrid couplers, and it also has a 3-bit phase-shifting capability realized with a 0 ∼ 330 • analog phase shifting. This unit cell's SPDT PIN diode switch uses +1.4 V forward and −5 V reverse bias voltages to handle a high input RF power of 23 dBm. Finally, an 8 × 8 transmitarray antenna with F/D of 0.33 made from the proposed unit cell has a gain value of 7.9 dBi and more than a 90 • 2D beam steering capability for V-to-H-pol and V-to-V-pol conversions. Notably, its measured efficiency of 8.1 % is due to the high loss of the FR4 substrate, and this loss can be reduced using a low-loss substrate, such as a Teflonbased one. The proposed transmitarray is scalable design that has broad application range, such as radar, satellite, and next generation communications.