Polarization Reconfigurable and Beam-Switchable Array Antenna Using Switchable Feed Network

A novel array antenna with reconfigurable polarizations and radiation pattern agility is presented in this paper. The digital-controllable array antenna has the functional capabilities of reconfigurable polarization and switching beam, which consists of cavity-backed antenna element based on substrate integrated waveguide (SIW) technique, parasitic patch array, power dividers and phase shifters. Polarization modes and radiation patterns can be dynamically controlled by means of Microcontroller Unit (MCU) with the different feeding ports and phase difference. To validate this proposal, the performance of the fabricated device was measured at the operating frequency of 5.4 GHz. Measured results indicate that the 10 dB impedance bandwidths and 3 dB axial ratio (AR) bandwidths cover from 5.25 to 5.6 GHz, and the measured gains range from 7.9 to 10.1 dBic. The experimentation and simulation verify that the proposed antenna can achieve two polarization states and five radiation beams of each polarization state, which indicate a good performance of the polarization reconfigurable and beam switching antenna array.

The second solution is to introduce controllable perturbation segments in the antenna's structure [13], [33]. In [13], two switchable shorting vias are adopted in radiation layer for introducing the perturbation, and the LP, RHCP and LHCP modes can be achieved, but the basing circuit and PIN diodes of the radiation layer will distort the radiation pattern of the antenna. Finally, the radiation pattern and polarization reconfigurable antenna can be achieved by using dielectric liquid [3], [31], but the operating features cannot be changed dynamically comparing with PIN diodes and MEMS switches.
Beam switchable antennas [14]- [19] can be realized by antenna structure, phased arrays and the different feeding ports. In [19], a polarization reconfigurable and pattern switchable antenna was achieved by CMA, and the operating feature can be switched by the feeding network. More recently, some digital coding Metamaterial antenna [26] were proposed to dynamically manipulate beams by PIN diodes, and the coding technology is more effective in switching radiation states with low cost and low weight.
For overcoming the difficulty mentioned above and taking advantage of digital coding technology, a CP reconfigurable and beam switchable antenna array is proposed. The operating states can be dynamical controlled by MCU. The remaining text of the paper is organized as follows. In the Section II, the design and operational principle of the proposed antenna is discussed, and then the performance of the antenna element and antenna array is revealed. In the section III, the prototype is fabricated and measured. At the same time the simulated and measured results are presented and analyzed. Finally, the conclusions are provided in the Section IV.

II. ANTENNA CONFIGURATION AND DESIGN A. ANTENNA ELEMENT AND THE FEEDING NETWORK
The geometry of the proposed polarization reconfigurable antenna element is shown in Fig. 1, and its detail dimensions are illustrated in Table 1. The antenna element consists of three parts, which include the parasitic patch layer, a square substrate integrated waveguide (SIW) cavity layer and the feeding network layer, and all structures are designed on two 0.508-mm and a 0.762-mm thick RO4350B substrates (ε r = 3.66 and tanδ = 0.004 at 5.4 GHz). The parasitic patch layer is composed of four-square patches. The square SIW cavity layer includes a circular split ring as a radiation slot and two grounded coplanar waveguide feeding (GCPW) structure [23], [24]. The feeding network layer consists of a polarization switching network, and the parasitic patch layer is introduced for improving the operating frequency band [27], [28].
The square SIW cavity is formed by metallic vias and its length is W 5 . The metallic via has the diameter of d and the space between two adjacent metallic vias is p, which  is designed at operating frequency. The square SIW cavity consists of two switchable feeding ports, GCPW line and annular slot. The GCPW lines are designed to have a 50-ohm characteristic impendence with a width of w 50 , and a length of lf and a gap of gap 2 , which are used to feed the SIW cavity at the port 1 and the port 2. At the same time, the width of 50-ohm input microstrip line is equal to w 50 for having a smooth transition from microstrip line to GCPW line. A good VOLUME 10, 2022 return loss can be achieved by tuning the length of lf and the deflection angle α of the GCPW line, as shown in Fig. 1(c).
The traditional annular slot radiates linearly polarized waves, when the SIW cavity with annular slot is fed individually by the port1 and port2. It can support two resonating modes, i.e., TE 210 and TE 120 modes that similar to a square patch antenna resonating TM 10 and TM 01 . To obtain the circular polarization character, two square slots are introduced to generate perturbation and separate two orthogonal modes forming a 90-degree phase difference between the two orthogonal modes. In order to excite the two orthogonal modes, the initial dimension of the square SIW cavity can be calculated by the following formulates [29]: where ε and µ are the permittivity and permeability of the substrate. L eff and W eff are the equivalent length and width of the SIW cavity. L is the length of the cavity, m = 2, n = 1, p = 0. d and d P are the via diameter and spacing between neighboring vias, respectively. The inner and outer radius of the annular slot can be determined by the formula [30]: where c 0 is the velocity of light in free space, and f mnp is the operating frequency. The initial size of the parasitic patches can be determined by the following formula: where λ 0 is the wavelength of the operation frequency in free space. For implementing polarization reconfigurable and beamswitchable, the single-pole double-throw (SPDT) switches are employed to switch polarization states, i.e., LHCP and RHCP, and produce two phase states, i.e., 0 degree and −130 degrees. PIN diodes of four branches are denoted as D 1 ∼ D 4 , as shown in Fig. 1(c). Hence, the polarization and phase states can be considered as two-digital units. The first bit represents the polarization states, i.e., LHCP ('0') and RHCP ('1'), and the second bit represents the phase states, i.e., 0 degree ('0') and −130 degrees ('1'). i.e., '01' are selected, the antenna will radiate LCHP wave with −130 degrees phase when D1 and D4 are ON state, D2 and D3 are OFF state. If '10' are selected, the antenna will radiate RCHP wave with 0 degree phase when D2 and D3 are ON state, D1 and D4 are OFF state (the '00' represents state 1 or Path 1, the '01' represents state 2 or Path 2, the '10' represents state 3 or Path 3 and the '11' represents state 4 or Path 4).
In the implementation, a 2-bit digital programmable antenna element is developed with PIN diodes. Each state has good properties on insertion loss and shift phase difference. In order to get the better insertion loss, the SPDT switch adds another PIN diodes in each branch of the SPDT switch. The simulated results are presented in Fig. 2 and demonstrate that the design SPDT switches are capable of switching each state, where the insertion loss of S12 and S13 represent straightthrough port and isolation port.
The Direct Current (DC) bias circuits are introduced to control the PIN diodes (BAR-64L). The Multi-layer Ceramic Capacitors and the Chip inductors with 0402 package are used for DC and Radio Frequency (RF) blocking. The DC power is supplied by the General-purpose input/output (GPIO) of the MCU. The Surface Mounted Devices (SMD) resistors with 0402 package play an important role in limiting the maximum current for matching the drive capability of the GPIO and promise antenna radiation performance. In order to make the feeding structure more compact and improve the effect on radiation performance, polarization switching network, phase shifter and the bias circuits are designed on the feeding network layer as shown in Fig. 1(c).
As a demonstration, the antenna element including DC bias circuits is designed with the help of the full-wave simulator CST, and the final design parametric values are listed in Table 1. As expected, two polarization states are obtained by properly choosing the different excited states of the proposed antenna element, i.e., RHCP state and LHCP state. Fig. 4(a) shows the return loss bandwidth and Axial Radio (AR) bandwidth, where the 10 dB return loss bandwidth are 4.9-5.7 GHz, and the 3dB AR bandwidth are 5.2-5.65 GHz. Fig. 4(b) shows simulated boresight gains with different states polarization of polarization. The simulated radiation patterns at xz-plane and yz-plane are presented in Fig. 4(c) and (d), it can be found that the proposed antenna possesses good far field properties. The surface current distribution of the parasitic patches at different time phases are shown in Fig. 3, which clearly verifies that the antenna operates at CP mode.

B. 2 × 2 ARRAY ANTENNA
Based on the antenna element and the feeding network, a 2 × 2 array antenna with circular polarizationreconfigurable and beam-switchable is developed. In this design, a 4-way power divider which is composed of three 3 dB Wilkinson power dividers is used to feed the different antenna elements. The layout of the array antenna is displayed in Fig. 5.  The polarization states of antenna element and the radiation beams of the array antenna can be dynamically  controlled by different PIN diodes with different coding sequences of '0' and '1', i.e., LHCP and RHCP, and different phases, i.e., 0 degree and −130 degrees. For example, when all antenna elements operate in LHCP and are fed with −130 degrees, a directional beam (phi = 0, theta = 0) will be produced. The coding sequence is set as '0-1111', where the first bit and the rest of bits represent that all antenna elements operate in LHCP state and phase state of antenna elements E1 ∼ E4, respectively, as shown in Fig. 5.
All the radiation states including the corresponding polarization state, phase distribution, coding sequence and the simulated maximum direction are listed in Table 2. In order to clearly observe the radiation states, the simulated far-field 3D radiation patterns of different radiation states are illustrated in Fig. 6 under different coding sequences at LHCP state.

III. EXPERIMENTAL RESULTS
The simulated and measured results of the proposed array antenna under different radiation states are compared and discussed in this section. The prototype has been fabricated by using the low-cost printed circuit board (PCB) technology, and the metals are made of 0.035 mm thick copper layers,  as shown in Fig. 7. A MCU is used to control the radiation states with different coding sequences.
The reflection coefficients of the array antenna were measured under different radiation states by Agilent 8721ET Network Analyzer, and simulated and measured results are plotted in Fig. 8. The measured 10 dB reflection bandwidths are wider than the simulated result for the LHCP, RHCP modes respectively, which are caused by the small tolerance of the fabrication of process and the modeling residue for the actual capacitances that caused by Surface Mounted Technology. At the same time, the simulated resonance point is deviated and another resonant point is introduced.
For measuring the CP properties, circularly polarized horn antenna servers as source antenna. The measured gain and AR of all operating states at the max radiation direction are depicted in Fig. 9 and Fig. 10. The measured gain for  LHCP and RHCP mode range from 7.9 dBic to 10 dBic and 7.8 dBic to 10.1 dBic, respectively, and the measured 3 dB AR bandwidths range from 5.25 to 5.6 GHz for all operating states. It can be observed the measured gains are about 1.4-2 dB smaller than simulated gains, and this discrepancy is caused by the additional loss, i.e., SMA connector, PIN diodes and copper. According to the data sheet of the BAR-64 PIN diode, ON state is equivalent to a 2 ohm resistance, however the actual equivalent circuit model is more complicated which will cause additional heat loss and reduction of the radiation gain. At same time, the substrate, copper, SMA connector and dc bias circuits contribute to corresponding loss which are not taken into consideration in the full-wave simulator. Ignoring errors which caused  by equipment and measurement, the reconfigurable antenna array has stable realized gain over the whole 3 dB AR bandwidth.
To evaluate the measured radiation efficiency of the array antenna, the measured gain and measured radiation efficiency for B1 at LHCP state is depicted in Fig. 11. Due to the symmetry of the array antenna structure, the other operating states have similar result, so the representative operating state, i.e., B1 at LHCP state, is taken into consider for simplicity. The measured radiation efficiency, which is obtained by calculating the ratio of the measured gain to the simulated gain, is about 53% of the entire operating frequency band.
Due to the symmetry array antenna structure, the proposed array antenna possesses similar radiation patterns with different polarization states. As shown in Fig. 12, the measured and simulated normalized radiation patterns with different beams are compared in the LHCP state of 5.4 GHz. The measured results of the radiation patterns have small errors, and the pattern of each state has mild sidelobe. Because the slots of the GCPW feeding lines have serious difference for the measured radiation patterns that bring about the sidelobes deviate the simulated radiation patterns and larger back-lobe. The dc bias circuit is designed on the bottom layer, which has little effect on the main lobe.
Finally, the performance of some up-to-date polarization reconfigurable antennas and this work are summarized in Table 3 for comparison. From Table 3, it is evident that the proposed reconfigurable array antenna has advantages of wide impendence bandwidth, a high enough gain with two polarization state. At the same time, it has the ability to control the radiation beams by MCU, including B1, B2, B3, B4, B5 radiation beams.

IV. CONCLUSION
To conclude, a polarization reconfigurable array antenna with radiation pattern agility is proposed, which is combined using annular slot antenna element based on SIW technique in this paper. Then the radiation patterns and polarization states can be dynamically altered by different coding sequences using MCU. A prototype of the array antenna is fabricated, debugged and tested under different coding sequences. The measured results are in good agreement with the simulated results considering the various actual errors. The proposed array antenna has the advantages of low-profile structure, the polarization reconfigurable and pattern flexible ability. It can be potentially used for low-cost, polarization reconfigurable antenna in the wireless systems, and this work provides significant guidelines for designing new high-performance reconfigurable antenna.