# Design of Pattern Reconfigurable Patch Antenna Array Based on Reflective Phase-Shifter

Authors: Yuqiu Shang; Qingsheng Zeng; Wanzhao Cui; Xinwei Wang; Gengqi Zheng
Journal: International Journal of Antennas and Propagation
 (2022)

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## Abstract

A circularly polarized (CP) broadside antenna array with a continuously adjustable beam is proposed and implemented. First, a reflection phase shifter with varactor diodes loaded on two output terminals of a branch directional coupler is realized. By controlling the voltage across both ends of the varactor diodes in the terminal load circuit, the reactance of the load is changed, allowing control of port phase shift. Following that, the microstrip patch antenna array is combined with the reflection phase shifter to achieve the pattern reconstruction for the array. For verification, a prototype of the designed pattern reconfigurable antenna (PRA) is fabricated and measured. The measurement results indicate that the beam direction of the antenna can be continuously adjusted with a change in the voltage across the varactor diodes. The maximum beam direction of the antenna reaches ±21° when the biggest DC bias voltage across the varactor diode is chosen as 15 V. Besides, the antenna shows good CP performance in different beam directions.


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## Body

## 1. Introduction

In recent years, reconfigurable antennas have been widely studied for they may alter their performance and function drastically without changing their inherent structures. Besides, since this type of antenna enables easier adjustment of frequencies, polarization, and radiation patterns, it allows reconfigurable antennas to be applied to both satellite and wireless communications [1]. Among reconfigurable antennas, pattern reconfigurable antennas (PRA), which are able to change patterns while keeping their inherent structures unchanged, are the most popular ones, since they may provide wider radiation coverage and an increase in transmitting power and wireless channel capacity [2–8].With the development and popularization of fifth-generation mobile communication, access points have become one of the hottest research topics in 5G in recent years [9]. The antenna beams for wireless access networks need to be adjusted as required. The traditional method of adjusting antenna beams is to change the phase of the phase shifter or the phase-shifted feed network at the back end of the antenna elements of an antenna array. As a result, such a method has seriously increased the complexity and volume of the system. As shown in [8, 10–15], PIN diodes were employed as key reconfigurable physical devices for flexible control of forward and backward beam switching, and then a feed network had to be introduced for spurring the PIN diodes. These reconstructed patterns that are either discrete or complementary to each other were created by the on-off effect of diodes or by the discontinuous effect of impedance. Up to now, few documents have been conducted on reconfigurable antennas that are able to continuously change beam pointing and patterns.Today, the application of new materials in the antenna field has helped to improve reconfigurable antenna design and thus enriched relevant theory [16, 17]. Mechanical methods and phase shifting techniques can be employed to realize reconfigurable antenna designs [18–20]. When phase shifting techniques are adopted, variations in reflection phase shift may be continuous, and insertion loss is usually caused by reactive components [21].In this study, a reflective phase shifter is designed along with a quaternary microstrip antenna array so that a pattern reconfigurable antenna with a continuously adjustable beam can be realized. The beam direction is implemented by adjusting and controlling the voltage loaded on the reactance element. The phase difference between the input and output ports can be continuously changed with the continuous variation of the voltage loaded on the reactance element. In comparison with several of the antennas mentioned above, this antenna could change the beam staring continuously. Although the scanning angle is not big enough, the designed antenna could work as an indoor access antenna installed on the roof of a building, which does not need a bigger scanning angle. And the center frequency of this study is 2.6 GHz, which is called channel n41 assigned by China Mobile. And, the maximum beam offset is ±21°, which is sufficient in practical applications.


## 2. Antenna Design and Principle of Operation

### 2.1. Configuration

As can be seen, the 3-D structure and geometric configuration of the reconfigurable antenna are shown in Figure1. The reconfigurable antenna array is realized by combining four microstrips circularly polarized (CP) patch elements with their feeding networks. Each radiating element is fed through a reflection phase shifter, and four reflection phase shifters are cascaded with a Wilkinson power divider to form the antenna feeding network. The four patches are etched in four substrate F4BM with a thickness of 4.0 mm, a permittivity of 3.5, and a loss tangent of 0.0035. The feeding network is also etched on an F4BM substrate with a permittivity of 2.65 and a thickness of 1 mm. Two layers of substrates are laminated together, and the patch antenna and the feeding network are etched in different substrates, respectively. By controlling the voltage at both ends of the varactor diode in the reflection phase shifter, the phase can be continuously adjusted so that the feeding phase of each antenna element, and the beam direction can be continuously changed accordingly. It should be noted that, in the design of the network, the widths of the four sides of the 3 dB impedance transformation branch directional coupler vary greatly, which is caused by the choice of different impedance ratios.Figure 1
Geometrical configuration of the proposed pattern reconfigurable antenna array.


### 2.2. Basic Theory of the Reflective Phase Shifter

The reflective phase shifter [22], as shown in Figure 2, is composed of a branch directional coupler and two varactors. Here, a 3 dB impedance transformation branch directional coupler (ITBC) terminates with two identical loads. When power is fed to port 1, the same reflective loads connected to port 3 and port 4 reflect power to port 2. ZA, ZB1, and ZB2 are characteristic impedance of the four branches of ITBC and decide the maximum phase shift of each phase shifter where the ZA , ZB1, and ZB2 are expressed as (1).Figure 2
Construction of the reflective phase shifter.(1)ZB1=Z0,ZB2=Z0rZ,ZA=Z02rZ.The terminal reflection coefficient may be expressed as [22](2)ΓL=ΓLejΦ21,=rZRsRp−RsZ0−RpZ0+jXLrZRp−Z0rZRsRp+RsZ0+RpZ0+jXLrZRp+Z0,where rZ is the impedance ratio of ZB1 to ZB2, Rs is parasitic resistance of the varactor diode. In Figure 2, Cv is the junction capacitance of varactor diode, while LS is the parasitic inductance of varactor diode. Different varactor diodes have different parasitic parameters. The type of chosen varactor is SMV2020-079LF with RS being 2.5 Ω and LS being 0.7 nH. CV changes with the reverse voltage and ranges from 0.35 to 3.2 pF. L0 in the circuits is used for matching. XL in (2) is represented as follows:(3)XL=ωLS+L0−1ωCV.With the change of the voltage at both ends of the varactor diode, the reactance value of the circuit changes too. Accordingly, the reflection coefficient and the phase between port 1 and port 2 vary so that the design of the pattern reconfigurable antenna can be realized.


### 2.3. Design of the Pattern Reconfigurable Antenna Array

The impedance ratiorZ and the voltage across the varactor diode are the key parameters to determine the maximum relative phase shift between port 1 and port 2. The relationship between the maximum phase shift and rZ is shown in Figure 3.Figure 3
The relation curve of phase shift and therZ.The optimal value ofrZ can be selected by the curve in Figure 3. When rZ ranges from about 0 to 1.5, the maximum value of phase shift increases. When rZ is greater than 1.5, the phase shift decreases. Furthermore, when rZ becomes larger, the difference in the microstrip line width between ZB1 and ZB2 will also become larger, which makes it difficult to manufacture the printed circuits. Therefore, it is very important to select the optimal value of rZ in practice. In this study, choosing rZ  = 1.1 leads to the maximum phase shift of 191.7° which can be obtained with a reversed voltage of 15 V.The phase difference is controlled not only byrZ but also by the voltage applied across the varactor. As the reversed junction voltage increases, the PN junction becomes thicker, so the junction capacitance becomes smaller. Meanwhile, the phase shift becomes bigger. When the reverse DC bias is set to 0 V, the junction capacitance is the maximum value, and the phase shift still exists. Hence, for the entire circuit, the difference between the maximum phase shift and the minimum phase shift is about 190°. Therefore, the maximum difference of the feeding phase between the adjacent antenna elements is 63°. The distance between the elements is 56 mm, which is 0.48λ of the center frequency. According to (4), it is found that the maximum direction of the beam points in the direction of θM = 68.62°, which biases the direction of vertical to the surface of antenna array by 21.38°.(4)kdcosθM+β=0±2πm,where k=2π/λ is the wavenumber, θM is the maximum direction which biases from the axial of the antenna array, and β is the phase difference between adjacent antenna elements. It is assumed here that the leftmost antenna element is the reference element, and its feeding phase is zero degrees. (4) also shows that when the “intraarray phase difference” and “spatial phase difference” between antenna elements are balanced, that is, when the phase provided by the intraarray phase shifter can completely compensate for the phase difference caused by spatial propagation (the two phase values are the same, but the combination is opposite), the maximum value of the antenna pattern is obtained.


## 2.1. Configuration

As can be seen, the 3-D structure and geometric configuration of the reconfigurable antenna are shown in Figure1. The reconfigurable antenna array is realized by combining four microstrips circularly polarized (CP) patch elements with their feeding networks. Each radiating element is fed through a reflection phase shifter, and four reflection phase shifters are cascaded with a Wilkinson power divider to form the antenna feeding network. The four patches are etched in four substrate F4BM with a thickness of 4.0 mm, a permittivity of 3.5, and a loss tangent of 0.0035. The feeding network is also etched on an F4BM substrate with a permittivity of 2.65 and a thickness of 1 mm. Two layers of substrates are laminated together, and the patch antenna and the feeding network are etched in different substrates, respectively. By controlling the voltage at both ends of the varactor diode in the reflection phase shifter, the phase can be continuously adjusted so that the feeding phase of each antenna element, and the beam direction can be continuously changed accordingly. It should be noted that, in the design of the network, the widths of the four sides of the 3 dB impedance transformation branch directional coupler vary greatly, which is caused by the choice of different impedance ratios.Figure 1
Geometrical configuration of the proposed pattern reconfigurable antenna array.


## 2.2. Basic Theory of the Reflective Phase Shifter

The reflective phase shifter [22], as shown in Figure 2, is composed of a branch directional coupler and two varactors. Here, a 3 dB impedance transformation branch directional coupler (ITBC) terminates with two identical loads. When power is fed to port 1, the same reflective loads connected to port 3 and port 4 reflect power to port 2. ZA, ZB1, and ZB2 are characteristic impedance of the four branches of ITBC and decide the maximum phase shift of each phase shifter where the ZA , ZB1, and ZB2 are expressed as (1).Figure 2
Construction of the reflective phase shifter.(1)ZB1=Z0,ZB2=Z0rZ,ZA=Z02rZ.The terminal reflection coefficient may be expressed as [22](2)ΓL=ΓLejΦ21,=rZRsRp−RsZ0−RpZ0+jXLrZRp−Z0rZRsRp+RsZ0+RpZ0+jXLrZRp+Z0,where rZ is the impedance ratio of ZB1 to ZB2, Rs is parasitic resistance of the varactor diode. In Figure 2, Cv is the junction capacitance of varactor diode, while LS is the parasitic inductance of varactor diode. Different varactor diodes have different parasitic parameters. The type of chosen varactor is SMV2020-079LF with RS being 2.5 Ω and LS being 0.7 nH. CV changes with the reverse voltage and ranges from 0.35 to 3.2 pF. L0 in the circuits is used for matching. XL in (2) is represented as follows:(3)XL=ωLS+L0−1ωCV.With the change of the voltage at both ends of the varactor diode, the reactance value of the circuit changes too. Accordingly, the reflection coefficient and the phase between port 1 and port 2 vary so that the design of the pattern reconfigurable antenna can be realized.


## 2.3. Design of the Pattern Reconfigurable Antenna Array

The impedance ratiorZ and the voltage across the varactor diode are the key parameters to determine the maximum relative phase shift between port 1 and port 2. The relationship between the maximum phase shift and rZ is shown in Figure 3.Figure 3
The relation curve of phase shift and therZ.The optimal value ofrZ can be selected by the curve in Figure 3. When rZ ranges from about 0 to 1.5, the maximum value of phase shift increases. When rZ is greater than 1.5, the phase shift decreases. Furthermore, when rZ becomes larger, the difference in the microstrip line width between ZB1 and ZB2 will also become larger, which makes it difficult to manufacture the printed circuits. Therefore, it is very important to select the optimal value of rZ in practice. In this study, choosing rZ  = 1.1 leads to the maximum phase shift of 191.7° which can be obtained with a reversed voltage of 15 V.The phase difference is controlled not only byrZ but also by the voltage applied across the varactor. As the reversed junction voltage increases, the PN junction becomes thicker, so the junction capacitance becomes smaller. Meanwhile, the phase shift becomes bigger. When the reverse DC bias is set to 0 V, the junction capacitance is the maximum value, and the phase shift still exists. Hence, for the entire circuit, the difference between the maximum phase shift and the minimum phase shift is about 190°. Therefore, the maximum difference of the feeding phase between the adjacent antenna elements is 63°. The distance between the elements is 56 mm, which is 0.48λ of the center frequency. According to (4), it is found that the maximum direction of the beam points in the direction of θM = 68.62°, which biases the direction of vertical to the surface of antenna array by 21.38°.(4)kdcosθM+β=0±2πm,where k=2π/λ is the wavenumber, θM is the maximum direction which biases from the axial of the antenna array, and β is the phase difference between adjacent antenna elements. It is assumed here that the leftmost antenna element is the reference element, and its feeding phase is zero degrees. (4) also shows that when the “intraarray phase difference” and “spatial phase difference” between antenna elements are balanced, that is, when the phase provided by the intraarray phase shifter can completely compensate for the phase difference caused by spatial propagation (the two phase values are the same, but the combination is opposite), the maximum value of the antenna pattern is obtained.


## 3. Results and Discussion

### 3.1. Reconfigurable Antenna Element

A microstrip patch antenna is used for the antenna element, which can radiate circularly polarized waves and has a pair of tangent angles on the patch, as shown in Figure4. This patch is fed by one probe, which can lead to two radiation modes, TM01 and TM10. The two modes form a circular polarization.Figure 4
Fabrication of microstrip CP patch antenna element.At the same time, the reflection phase shifter is also designed and fabricated, as shown in Figure5. The values of the components are shown in Table 1.Figure 5
Fabrication of reflection phase shifter.Table 1
The values of the components.
C1 (pF)L1 (nH)L0 (nH)R0 (Ω)RP (Ω)0.5362.4510002000Figure6 indicates that the curve of the phase difference between the two ports of the reflection phase shifter varies with the voltage across the varactor diode. It can be seen from Figure 6 that when the applied voltage is greater than 15 V, the rate of phase shift increase slows down, and the maximum phase shift reaches 200°. In this paper, 15 V is chosen as the highest voltage, and the corresponding phase shift is 190°, while the phase difference between elements has to be chosen as 63°.Figure 6
Phase shift of reflection phase shifter with loading voltage.In the design of reflective load, capacitor C1 is connected bypass to block DC voltage from being loaded on other varactor diodes through the microstrip line. Inductor L1 is connected between DC voltage and a varactor diode, which can prevent RF energy from radiating through the power line. At the designed center frequency of 2.6 GHz, the relative phase shift can reach 190° and change with the variation of voltage. The power dissipation is so small that the electric current is not able to be measured by the meter. The datasheet of the selected varactor diode mentioned that the electric current is only 50 nA under the loaded voltage of 16 V. Its insertion loss changes from 0.8 to 2.1 dB in the frequency band as shown in Figure 7.Figure 7
Measured insert loss curve.Figure8 plots the phase curve for the reflection phase shifter with different loading voltages. Figure 9 illustrates the simulated and measured reflection coefficient and radiation efficiency of the antenna element. It can be seen from Figure 9 that the antenna unit has good matching performances between 2.5 GHz and 2.7 GHz, and its radiation efficiency exceeds 80% in this interval. Figure 10 shows the axial ratio pattern of the antenna element at 2.6 GHz.Figure 8
The measured phase shift of the reflection phase shifter under different loading voltages.Figure 9
Measured and simulated |S11| and efficiency of the reconfigurable antenna element.Figure 10
Measured and simulated results of axial ratio atϕ = 0° for the reconfigurable antenna element (at 2.6 GHz).


### 3.2. Reconfigurable Antenna Array

By cascading four reconfigurable units with a Wilkinson power divider of 1 : 4, a four-element pattern reconfigurable linear array can be obtained, and the fabricated prototype is shown in Figure11. The dimension size of the entire antenna array is 255 × 140 × 5 (mm)3; meanwhile, the size of the antenna element is 30 × 30 × 4 (mm)3. The measured insertion loss of the feeding network is about 1.9–2.1 dB in the entire frequency band.Figure 11
Photograph of the fabricated prototype.Figure12 shows the measured and simulated reflection coefficient and radiation efficiency of the proposed antenna. The overlapping impedance bandwidth of the antenna under beam scanning is less than −10 dB, which is 7.7% (2.5–2.7). The measured radiation efficiency is nearly 80% at the central frequency of 2.6 GHz (without scanning) and over 70% under other scanning conditions.Figure 12
Measured and simulated results of the fabricated antenna array: (a) reflection coefficient |S11|; (b) radiation efficiencies.
(a)(b)The maximum beam direction of the antenna is continuously adjusted by the different voltages applied across the varactor diode, as shown in Figure13. When the phase of the reflection phase shifter is 0°, ±30°, or ±63°, the maximum gain beams of the array point to five directions: ±0°, ±10°, or ±21°, respectively. In Figure 13, the bracketed numbers are direction (degree) and gain (dB). Figure 14 shows the measured axial ratio pattern of the antenna array in two orthogonal main planes at 2.6 GHz. It can be seen that the antenna has excellent axial ratio characteristics at a 2.6 GHz frequency.Figure 13
Measured normalized radiation patterns of the pattern reconfigurable antenna array at 2.6 GHz on theyoz-plane.Figure 14
Measured and simulated axial ratio of proposed antenna array at 2.6 GHz: (a)xoz-plane; (b) yoz-plane.
(a)(b)Table2 provides a comparison between the proposed work and some previously reported PRA, which shows that the proposed antenna has achieved continuous variation of the direction in comparison with the other works. Moreover, CP is achieved for all operating directions.Table 2
Comparison between proposed and other reported PRA.
Ref.Ant. typePol.Direction changeBeamwidth (°)Peak gain (dBi)No. of diodes[2]Planar monopoleCP4 statusN/A1.02/0.858[4]End-firedLP3 status120°5.864[5]Slot ant.LP4 status80°7.0–9.314[6]Microstrip ant.LP11 status60°1.8–2.94[11]Folded dipolesLP2 status150°/75°54Prop.Microstrip ant.CPContinuous variation30°6.9–8.18Ref.: reference; Pol.: polarization; and Prop.: proposed antenna


## 3.1. Reconfigurable Antenna Element

A microstrip patch antenna is used for the antenna element, which can radiate circularly polarized waves and has a pair of tangent angles on the patch, as shown in Figure4. This patch is fed by one probe, which can lead to two radiation modes, TM01 and TM10. The two modes form a circular polarization.Figure 4
Fabrication of microstrip CP patch antenna element.At the same time, the reflection phase shifter is also designed and fabricated, as shown in Figure5. The values of the components are shown in Table 1.Figure 5
Fabrication of reflection phase shifter.Table 1
The values of the components.
C1 (pF)L1 (nH)L0 (nH)R0 (Ω)RP (Ω)0.5362.4510002000Figure6 indicates that the curve of the phase difference between the two ports of the reflection phase shifter varies with the voltage across the varactor diode. It can be seen from Figure 6 that when the applied voltage is greater than 15 V, the rate of phase shift increase slows down, and the maximum phase shift reaches 200°. In this paper, 15 V is chosen as the highest voltage, and the corresponding phase shift is 190°, while the phase difference between elements has to be chosen as 63°.Figure 6
Phase shift of reflection phase shifter with loading voltage.In the design of reflective load, capacitor C1 is connected bypass to block DC voltage from being loaded on other varactor diodes through the microstrip line. Inductor L1 is connected between DC voltage and a varactor diode, which can prevent RF energy from radiating through the power line. At the designed center frequency of 2.6 GHz, the relative phase shift can reach 190° and change with the variation of voltage. The power dissipation is so small that the electric current is not able to be measured by the meter. The datasheet of the selected varactor diode mentioned that the electric current is only 50 nA under the loaded voltage of 16 V. Its insertion loss changes from 0.8 to 2.1 dB in the frequency band as shown in Figure 7.Figure 7
Measured insert loss curve.Figure8 plots the phase curve for the reflection phase shifter with different loading voltages. Figure 9 illustrates the simulated and measured reflection coefficient and radiation efficiency of the antenna element. It can be seen from Figure 9 that the antenna unit has good matching performances between 2.5 GHz and 2.7 GHz, and its radiation efficiency exceeds 80% in this interval. Figure 10 shows the axial ratio pattern of the antenna element at 2.6 GHz.Figure 8
The measured phase shift of the reflection phase shifter under different loading voltages.Figure 9
Measured and simulated |S11| and efficiency of the reconfigurable antenna element.Figure 10
Measured and simulated results of axial ratio atϕ = 0° for the reconfigurable antenna element (at 2.6 GHz).


## 3.2. Reconfigurable Antenna Array

By cascading four reconfigurable units with a Wilkinson power divider of 1 : 4, a four-element pattern reconfigurable linear array can be obtained, and the fabricated prototype is shown in Figure11. The dimension size of the entire antenna array is 255 × 140 × 5 (mm)3; meanwhile, the size of the antenna element is 30 × 30 × 4 (mm)3. The measured insertion loss of the feeding network is about 1.9–2.1 dB in the entire frequency band.Figure 11
Photograph of the fabricated prototype.Figure12 shows the measured and simulated reflection coefficient and radiation efficiency of the proposed antenna. The overlapping impedance bandwidth of the antenna under beam scanning is less than −10 dB, which is 7.7% (2.5–2.7). The measured radiation efficiency is nearly 80% at the central frequency of 2.6 GHz (without scanning) and over 70% under other scanning conditions.Figure 12
Measured and simulated results of the fabricated antenna array: (a) reflection coefficient |S11|; (b) radiation efficiencies.
(a)(b)The maximum beam direction of the antenna is continuously adjusted by the different voltages applied across the varactor diode, as shown in Figure13. When the phase of the reflection phase shifter is 0°, ±30°, or ±63°, the maximum gain beams of the array point to five directions: ±0°, ±10°, or ±21°, respectively. In Figure 13, the bracketed numbers are direction (degree) and gain (dB). Figure 14 shows the measured axial ratio pattern of the antenna array in two orthogonal main planes at 2.6 GHz. It can be seen that the antenna has excellent axial ratio characteristics at a 2.6 GHz frequency.Figure 13
Measured normalized radiation patterns of the pattern reconfigurable antenna array at 2.6 GHz on theyoz-plane.Figure 14
Measured and simulated axial ratio of proposed antenna array at 2.6 GHz: (a)xoz-plane; (b) yoz-plane.
(a)(b)Table2 provides a comparison between the proposed work and some previously reported PRA, which shows that the proposed antenna has achieved continuous variation of the direction in comparison with the other works. Moreover, CP is achieved for all operating directions.Table 2
Comparison between proposed and other reported PRA.
Ref.Ant. typePol.Direction changeBeamwidth (°)Peak gain (dBi)No. of diodes[2]Planar monopoleCP4 statusN/A1.02/0.858[4]End-firedLP3 status120°5.864[5]Slot ant.LP4 status80°7.0–9.314[6]Microstrip ant.LP11 status60°1.8–2.94[11]Folded dipolesLP2 status150°/75°54Prop.Microstrip ant.CPContinuous variation30°6.9–8.18Ref.: reference; Pol.: polarization; and Prop.: proposed antenna


## 4. Conclusion

This paper proposes a new type of reconfigurable antenna with a continuously adjustable beam pattern that is composed of a reflection phase shifter and a microstrip patch antenna array. To begin with, the phase variation of the two ports of the reflection phase shifter is carefully analyzed, the results of which show that the reactance can be changed by adjusting the voltage loaded on the varactor diode. As a result, the reflection phase of the two ports can be changed accordingly. It is right based on such an idea, a pattern reconfigurable antenna is designed by combining the reflection phase shifter with the microstrip patch antenna array. Measurements show that, with the change of the voltage applied to the two ports of the varactor diode, the beam direction of the antenna can be continuously adjusted, thus realizing the reconfiguration of a pattern. Then, the prototype of the proposed antenna is fabricated, assembled, and tested. Results indicate that when the diode is loaded with five different voltages, the antenna beam can achieve five different directions. Therefore, it can be concluded that the antenna has good pattern reconfiguration performances and good CP performances in different beam directions.



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*Source: 2803285-2022-02-11.xml*