Circularly Polarized 2 Bit Reconfigurable Beam-Steering

2416 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 68, NO. 3, MARCH 2020 Communication Circularly Polarized 2 Bit Reconﬁgurable Beam-Steering Antenna Array Peiqin Liu ,YueLi , and Zhijun Zhang

Abstract—In this communication, a circularly polarized (CP) reconfigurable 2 bit antenna array is proposed for beam-steering applications. The proposed antenna array utilizes a novel CP reconfigurable 2 bit patch antenna as radiation element. For a reconfigurable 2 bit element, there should be four phase states (i.e., 0◦,90◦, 180◦, and 270◦).Inthe proposed radiation element, a corner truncated microstrip patch antenna is presented for CP radiation. Owing to the symmetry of the patch antenna, the 0◦ and 180◦ phase states are achieved by selecting feeding points of the patch instead of using complex feeding network. A 90◦ digital phase shifter is cascaded to realize the 90◦ and 270◦ phase states for the 2 bit reconfigurable element. Based on the novel 2 bit CP element, an eight-element CP reconfigurable 2 bit antenna array is fabricated and tested to verify the design strategy at 3.65 GHz. The measured results match well with simulation. By appropriately controlling the ON and OFF states of p-i-n diodes, the proposed CP array can be steered from −49◦ to +49◦. Index Terms—2 bit radiation element, beam scanning, circular polarization, reconfigurable array.

I. INTRODUCTION Circularly polarized (CP) beam-steering antennas have attracted special attention in various applications, such as satellite communication system, wireless communication system, and radar system [1]. In the past decade, many researchers have been concen- trating on beam steering with circular polarization. Switched-beam antenna array is a typical method to achieve beam steering [2]–[5]. In switched-beam array, Butler matrix and delay line feeding network are used to control phase and beam direction. However, as array size increases, the complexity of feeding network also increases. To solve the problem, different kinds of CP antenna arrays are proposed. In [6]–[8], researches mechanically rotate CP elements to manipulate the phase of each radiation element. By periodically locating CP elements along a transmission line, leaky-wave antennas can steer CP beam at different frequencies [9], [10]. Another electronically steering method is loading tunable varactors to continuously change the phase of each CP element at fixed frequency [11], [12]. Unfortunately, continuous phase control suffers from problems, such as temperature instability, analog control, and variable insertion loss [13], [14]. Fig. 1. Configuration of the CP 2 bit reconfigurable antenna element. In fixed-frequency beam-steering array, reconfigurable bit array can (a) Exploded view. (b) Cross-sectional topology. discretize phase distribution of the antenna array, which simplifies the biasing network and control logic [15]–[17]. Reconfigurable bit has advantages of low profile and ease of integration with other arrays can be categorized into 3-D structure and planar structure. components [21], [22]. Phased array is a typical method to achieve Reconfigurable reflectarray [18] and transmitarray [19], [20] are beam-steering property with planar structure [23], [24]. However, typical antenna arrays using 3-D structure. However, the 3-D struc- conventional phased arrays suffer from large size due to the existence tures, such as illuminating feed and antenna supporter, restrict their of phase shifters [25]. applications in some space-limited systems. Planar antenna array In this communication, a novel CP 2 bit reconfigurable antenna element is proposed for beam-steering application. The CP radiation Manuscript received October 24, 2018; revised July 18, 2019; accepted element is presented based on a corner truncated patch antenna. August 25, 2019. Date of publication September 11, 2019; date of current By using p-i-n diodes, the CP patch can be excited with reconfig- version March 3, 2020. This work was supported by the National Natural urable 2 bit phase states. In conventional 2 bit phase shift module, Science Foundation of China under Contract 61525104. (Corresponding ◦ author: Zhijun Zhang.) the 180 phase delay circuit requires the largest area. In the proposed ◦ ◦ The authors are with the Beijing National Research Center for Information antenna element, the 0 and 180 phase states are realized by utilizing ◦ Science and Technology (BNRist), Tsinghua University, Beijing 100084, the symmetry of the CP patch antenna. As a result, the 180 phase China (e-mail: [email protected]). shifter is integrated with the CP patch, which effectively reduces Color versions of one or more of the figures in this communication are ◦ available online at http://ieeexplore.ieee.org. the dimension of the phase shift circuit. Besides the 180 phase ◦ Digital Object Identifier 10.1109/TAP.2019.2939669 shift, a 90 digital phase shifter is cascaded to the antenna element 0018-926X © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See https://www.ieee.org/publications/rights/index.html for more information.

Fig. 3. Surface current distribution of the proposed CP patch antenna element at 3.6 GHz. (a) Excited through feeding point V1. (b) Excited through feeding Fig. 2. Geometry of the proposed CP reconfigurable 2 bit element. (a) Front point V2. view. (b) Back view. Detailed dimensions: L1 = 20, L2 = 4.8, L3 = 9.6, and L4 = 15.4 (unit: mm). ◦ ◦ for the 90 and 270 phase states. Based on the discretized phase, the phase distribution of an antenna array can be quantized and the beam-steering scheme can be simplified [26]. An eight-element antenna array is fabricated and measured to validate the beam-steering strategy. The simulated and measured results show that the proposed ◦ ◦ CP reconfigurable 2 bit array can steer from −49 to 49 at 3.65 GHz. The measured peak gain varies from 10.1 to 11.8 dBi in the scanning range. It should be noted that using reconfigurable feeding network to realize bit array is a common method in beam-steering applications [19]. Based on the idea, this communication proposes a new way to design antenna array with 2 bit reconfigurable states and achieve beam scanning property.

II. CP 2 BIT ELEMENT DESIGN The configuration of the proposed CP reconfigurable 2 bit radiation element is illustrated in Fig. 1. Fig. 1(a) is the exploded view of the element. The proposed CP element consists of two layers of F4b substrate (εr = 2.47, tan δ = 0.002). The CP patch element locates on the top layer and the reconfigurable phase shift circuit locates on the bottom layer. The two layers are bonded together and share the same ground plane. The thickness of the top and bottom layers is 3 and 1 mm, respectively. Fig. 1(b) depicts the cross-sectional topology of the CP element. As shown in Fig. 1(b), the ground plane is defected ◦ and two vias are utilized to excite the CP patch. It should be noted that Fig. 4. (a) Geometry of the 90 digital phase shifter. Detailed dimensions: = . = . = . = = . = . = . all other vias in the proposed antenna element are directly connected W1 1 35, W2 2 1, W3 0 7, W4 2, W5 5 5, W6 6 5, W7 13 6, and W8 = 7.3 (unit: mm). (b) Phase and magnitude S21 for ON-state and to the ground plane. In this communication, the full-wave simulations OFF-state, respectively. are carried out by commercial software ANSYS High Frequency Structure Simulator (HFSS) version 18 [28]. element. Fig. 2 illustrates the geometry of the proposed CP patch The motivation of the proposed antenna element is using a recon- antenna. As shown in Fig. 2(a), the CP radiation is realized by figurable circuit to provide 2 bit phase states for a CP antenna a corner truncated patch antenna. There are two feeding points

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TABLE I RECONFIGURABLE STATES OF THE 2BIT ANTENNA

Fig. 6. Radiation properties of the CP element. (a) Radiation patterns in the xoz plane at 3.6 GHz. (b) Gain and radiation efficiency. ◦ the antenna element. The 180 phase shifter can provide phase states ◦ ◦ ◦ ◦ of 0 and 180 . The 90 phase shifter can provide phase states of 0 ◦ and 90 . As a result, combing the two phase shifters can provide ◦ ◦ ◦ ◦ four phase states, i.e., 0 ,90, 180 , and 270 , which satisfy the 2 bit phase states. However, the phase shifters also increase the size ◦ Fig. 5. Influence of via V3. (a) Simulated |S11|. (b) Simulated AR. of phased array. Among the phase shifters, the 180 phase shifter provides the largest phase shift and occupies the largest size. In the ◦ (i.e., V1 and V2) in the patch antenna. The ground plane under proposed antenna element, the 180 phase shift is realized without ◦ V1 and V2 is defected for feeding. Via V3 is directly connected using the conventional 180 phase shifter. Owing to the symmetry ◦ to the ground plane. Fig. 2(b) shows the back view of the element. of the patch antenna, the 180 phase shift can be achieved by Four p-i-n diodes (i.e., PIN1–PIN4) are utilized for reconfiguring. selecting feeding point. Fig. 3 depicts the surface current distribution Due to the limitation of full-wave simulation software, p-i-n diodes of the proposed CP element at 3.6 GHz. Fig. 3(a) shows the current cannot be simulated in HFSS. So, equivalent circuits with passive distribution excited through feeding point V1, and Fig. 3(b) is components are utilized to simulate the effects of p-i-n diodes. In the the current distribution excited through feeding point V2. One can ◦ circuits, ON-state of the p-i-n diode is modeled as a 5 resistor observe that the current distributions are out of phase, and the 180 ◦ and OFF-state is modeled as a 0.046 pF capacitor. Because of the phase shift is achieved. So, as shown in Fig. 2, the 180 phase shift resistance, the p-i-n diode would introduce some insertion loss into can be controlled by PIN1 and PIN2. When PIN1 is in ON-state and the antenna. PIN2 is in OFF-state, the CP patch is excited through via V1. When DC bias circuits of the p-i-n diodes are also illustrated in Fig. 2. PIN1 is in OFF-state and PIN2 is in ON-state, the CP patch is excited As shown in Fig. 2(b), PIN 1 and PIN 2 are controlled by the right through via V2. ◦ ◦ bias line, and PIN3 and PIN 4 are controlled by the left bias line. RLC Besides the 180 phase shift, a 90 digital phase shifter [26] is low pass filters are utilized to isolate the dc and RF sections of the CP cascaded to achieve the 2 bit phase states in the proposed antenna ◦ reconfigurable element. In the low pass filter circuit, the inductance element. The detailed dimensions of the 90 phase shifter are shown and capacitance are 100 nH and 100 pF, respectively. According to in Fig. 4. As shown in Fig. 4(a), the digital phase shifter is controlled filter theory, the cutoff frequency of the low pass filter is 50 MHz, by PIN3 and PIN4. When PIN3 and PIN4 are in OFF-sate, the phase which is much lower than the operating frequency of the proposed shifter operates in OFF-state. When PIN3 and PIN4 are in ON-state, CP element. As a result, the dc bias circuit has little effect on the the phase shifter operates in ON-state. The phase difference between ◦ ◦ CP reconfigurable radiation element. Beside the 90 phase shifter, the phase shift of ON-state and OFF-state is 90 . The simulated results the microstrip line is shorted to ground through a 100 nH inductor. of the phase shifter are depicted in Fig. 4(b). One can observe that ◦ This circuit completes the dc loop for PIN3 and PIN4. the phase shifter successfully provides 90 phase shift in the desired ◦ In conventional phased array, the 2 bit phase states are achieved band around 3.6 GHz. Based on the 180 phase shift circuit and ◦ ◦ ◦ by utilizing a 180 phase shifter and a 90 phase shifter to excite the 90 phase shifter, the reconfigurable states of the 2 bit element

Fig. 7. (a) Geometry of the 1:8 power divider. (b) Simulated results of the power divider. (c) Configuration of the eight-element CP reconfigurable 2 bit array. Detailed dimensions: A1 = 2.5, A2 = 5.2, A3 = 3.4, A4 = 2.2, A5 = 1, A6 = 0.5, A7 = 0.5, A8 = 1.3, B1 = 15, B2 = 42.2, B3 = 15, and B4 = 15 (unit: mm). ◦ are listed in Table I. It should be noted that the 90 digital phase shifter would introduce some insertion loss into the feeding network. At 3.6 GHz, the magnitude of S21 is −0.20 and −0.73 dB for ON-state and OFF-state, respectively. The insertion loss is relatively low and acceptable in practical applications. As shown in Fig. 2, there are three vias (i.e., V1, V2, and V3) in the proposed CP patch element. Via V1 and Via V2 are utilized to excite the patch antenna. Via V3 is utilized for biasing on PIN1 and PIN2. Considering the electric field distribution is null in the center of the CP patch antenna, V3 is located in the center to avoid affecting the performance of CP patch antenna. The influence of V3 is studied as shown in Fig. 5. Fig. 5(a) shows after adding via V3, the input impedance matches well at the desired band of 3.6 GHz. Fig. 5(b) is the simulated axial ratio (AR) results. One can observe that adding via V3 makes the AR shift only 10 MHz to the upper band. As a conclusion, adding via V3 has slightly influenced the performance of the proposed CP patch antenna. The radiation properties of the CP element are illustrated in Fig. 6. As shown in Fig. 6(a), the CP element radiates in broadside direction. The co-polarization is left-handed CP (LHCP) and gain is 5.2 dBic. The cross-polarization Fig. 8. Photograph of the proposed CP eight-element reconfigurable 2 bit is right-handed CP (RHCP) and the cross-polarization level is less array. (a) Front view. (b) Back view with the controller. than −18 dBic in the broadside direction. Fig. 6(b) depicts the results of LHCP gain and radiation efficiency. At the operating frequency coefficients are omitted for simplicity. From the results, one can of the CP element, i.e., 3.6 GHz, the gain of RHCP is 5.2 dBic and observe that the magnitude distribution of the power divider varies − − radiation efficiency is 0.86. from 9.42 to 9.06 dB at 3.6 GHz. There is a nonuniform amplitude distribution between the output ports. Further simulation shows that the nonuniform distribution would make the directivity of the array III. CP 2 BIT ARRAY DESIGN decrease 0.2 dB. The phase distribution of the power divider is Based on the CP 2 bit patch antenna, an eight-element reconfig- shown in Fig. 7(b). For the convenience of plotting, the angles urable 2 bit array is proposed in this section. A 1:8 power divider in the phase distribution are not normalized to the range from ◦ ◦ is proposed as shown in Fig. 7(a). In the power divider, the power −180 to +180 . At 3.65 GHz, the phase angles in port 2, port 3, ◦ ◦ ◦ ◦ is equally transmitted from input port 1 to output ports 2–9. The port4, and port 5 are −224.7 , −306.1 , −389.8 ,and−467.8 , distance between each output port is λ0/2 (λ0 is the wavelength respectively. Although there are some phase differences between in free space). Microstrip lines with different values of width are the output ports, the differences can be adjusted by reconfiguring utilized for impedance variation. Fig. 7(b) depicts the simulated the radiation elements. In the proposed antenna array, because each magnitude and phase distribution in the output ports of the power patch element can individually provide 2 bit phase states, the phase divider. Due to symmetry of the power divider, a half of transmission distribution of the antenna array can be optimized to achieve desired

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TABLE II ARRAY CONFIGURATIONS FOR DIFFERENT STEERING ANGLES

Fig. 9. Simulated and measured results of the proposed CP eight-element reconﬁgurable 2 bit array. (a) |S11|. (b) Axial ration. (c) radiation patterns in the xoz plane.

radiation direction with acceptable phase errors. Based on the power IV. EXPERIMENTAL RESULTS divider, an eight-element antenna array is proposed as shown in Fig. 7(c). Eight CP configurable 2 bit patch element are excited by A prototype of the proposed CP eight-element reconfigurable 2 bit the 1:8 power divider. The full-wave simulation results show that the array is fabricated and tested to verify the design strategy. The pho- isolation between the CP elements is higher than −19.1 dB, which tograph of the proposed array is shown in Fig. 8. Fig. 8(a) shows the is acceptable for the design of beam-steering array. front view and Fig. 8(b) shows the back view. As shown in Fig. 8(b), Table II shows the array configurations for different steering angles. four batteries are utilized to provide +3and−3 V dc. In each ◦ ◦ The proposed array can scan from −49 to +49 by selecting antenna element, four PIN diodes are utilized for reconfiguring. The different phase states of the antenna elements. Cases 1–4 show the PIN diode chip (MA4SPS402) is a surface mount device, which ◦ ◦ array configurations for scanning from 0 to +49 . Due to the is fabricated by M/A-COM Technology Solution Inc, Lowell, MA, symmetry of the array geometry, the configurations for scanning USA. By using Dupont wires, the dc voltage is biased to the antenna ◦ ◦ from 0 to −49 are omitted for simplicity. In case 1, the patch array to control the phase states of each CP patch element. elements are reconfigured to compensate the phase distribution of the The simulated and measured results of the proposed eight-element power divider. As a result, the array radiates in broadside direction. array are illustrated in Fig. 9. The S-parameters are measured by vec- To make the array scan at different angles, phase difference should be tor network analyzer E5071B, and the AR and radiation patterns are introduced into the array elements. For example, in a linear array with measured in a far-field anechoic chamber. To verify the beam-steering ◦ λ0/2 spacing between elements, the phase difference should be 90 to property, the proposed eight-element array is reconfigured to different ◦ make the array steers at the angle of 30 . So, in case 2, the elements configurations (i.e., cases 1–4). Fig. 9(a) shows the simulated and ◦ are reconfigured to achieve the phase difference for scanning at 30 . measured results of |S11| from 3.0 to 4.0 GHz. The simulated However, the quantized phase distribution in 2 bit array would results match well with the measurement. In the desired band introduce errors in scanning angles [22]. So the realized scanning around 3.65 GHz, the reflection coefficients are lower than −10 dB. ◦ angle of case 2 is 28 . Similarly, the scanning angle of case 3 and Compared with the bandwidth of the CP patch antenna in Fig. 5, ◦ ◦ case 4 is 37 and 49 , respectively. one can observe that the bandwidth of the proposed array is larger

Authorized licensed use limited to: Tsinghua University. Downloaded on July 21,2020 at 09:00:08 UTC from IEEE Xplore. Restrictions apply. IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 68, NO. 3, MARCH 2020 2421 than the CP patch element. When the CP elements are excited by the [6] M. Euler and V. F. Fusco, “Frequency selective surface using nested 1:8 power divider circuit, the reflection wave of each element would split ring slot elements as a lens with mechanically reconfigurable beam add at the input port of the power divider circuit. When the reflection steering capability,” IEEE Trans. Antennas Propag., vol. 58, no. 10, pp. 3417–3421, Oct. 2010. waves are out of phase, the impedance bandwidth of the antenna array [7] Y.-L. Yao, F.-S. Zhang, and F. Zhang, “A new approach to design is improved. Fig. 9(b) depicts the AR results of each case. 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