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Abstract
A low loss wideband active reflectarray element (ARE) is designed in this paper. One PIN diode has been loaded into microstrip metal patch, which alters an element resonant property when the PIN diode is ON or OFF. Thus a 1-bit unit cell is generated by controlling the working states of the PIN diode. Besides, two ingenious designs have been employed to reduce the insertion loss of ARE, one is loading a bridging capacitor in parallel with the PIN diode, the other is using an asymmetric design of the position of electronic device and slot. These technologies together enable the ARE to operate from 8.45 to 12.60 GHz with 180°±30° phase difference (the relative bandwidth is up to 40%). The losses of the ARE with ON and OFF states are less than 0.75 dB in aforementioned frequency range. The experimental results are in line with the simulated ones. Compared with other designs, the proposed ARE has advantages of low insertion loss and wide working band, which is very suitable to achieve high performance multifunctional metasurface.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a new generation of high-gain beam scanning antenna, reconfigurable reflectarray antennas (RRAs) have attracted much research attention since their first presentation [1]. In recent years, some developments of satellite communication and radar detection have resulted in the immense demands for novel beam scanning antennas. Therefore, among these systems, employing RRAs is one of the optimal choices. The design process of RRAs is divided into two steps—the active element design and the reflectarray antennas(RAs) design.
From [2], it can be seen that there are three general approaches employed in the design of ARE. In particular, it is one of the most effective methods to adjust operating phase resolutions (1-bit, 2-bit, or continuously tunable phase) through applying tunable electronic devices [3–6], which have been loaded into the metal structure. In [3,7], PIN diodes are adopted to achieve the dynamic control of reflection phase (1bit). It is effective since RF-MEMS (Micro Electro Mechanical Systems) technology has been applied in digital [8] or analog [9] to achieve low loss of ARE. Some continuous Scanning Beams are shown by applying varactors [10,11,12]. Many tunable materials can also be employed to design a continuous operating phase resolutions of ARE, such as liquid crystals [13], graphene [14], and metal microfluidic [15].
For many years, Bandwidth and loss of ARE have always been the vital problems of the research on RRAs. Some technologies have been considered in several literatures [16–18] to expand operating band and reduce loss of ARE. In literature [16], the concept of dual-band antenna is proposed, which ingeniously utilizes multiple electromagnetic wave (EW) modes in the same ARE. Another dual-band RRA is presented in [18]. The element, which integrates one varactor across the gap between the ring and the square patch, achieves continuous phase adjustability at S and F bands (3.38 and 6.12 GHz). Although these dual-band AREs are successfully designed, the high insertion loss still exists. The element of the electronically beam scanning transmitarray working in circular polarization at Ka-band [19] provides 12% operating band within the loss lower than 1.65 dB. A 2-bit active element is proposed for both linear and circular polarizations [20]. By controlling the states of the 5 PIN diodes electronically, the active element achieves an insertion loss lower than 1.1 dB within 20% relative bandwidth. The reflectarray made from sub-wavelength elements can achieve a higher gain in comparison with its counterparts with half wavelength elements. Simulations demonstrated that the gain bandwidth can be improved to around 20% by applying a single-layer patch element with a cell size of λ/3 [21]. In [22], a reflectarray whose element size is one-sixth of a wavelength is proposed. It can achieve a 19% 1-dB gain bandwidth. It is interesting to note that, due to the low loss advantages of 1-bit unit, the satisfactory scanning beams can be obtained even with 1-bit RRAs [23]. Several theses [24–27] have demonstrated that the 1-bit design is a good tradeoff, especially in large-aperture RRAs implementations.
Despite all the above-mentioned contents are demonstrated successfully in the design of ARE, narrow operating bandwidth and low loss of ARE are ongoing challenges in the reflectarray antenna design. The cause lies at the fact that it is difficult to obtain an applicable ARE that meets phase and amplitude requirements simultaneously in a wide frequency band.
In this paper, a novel 1-bit ARE loading one PIN diode and one capacitor is designed for this challenge. The ARE allows a reconfigurable phase response by controlling the state of PIN diode. A bridging capacitor has been loaded into the microstrip patch to reduce the induced current passing through the diode, which achieves low loss. In addition, the asymmetry of electronic devices position and patch shape are also designed to obtain appropriate phase distribution and low loss. On the basis of these designs, the proposed ARE shows strong abilities to control the reflection phase and amplitude. In this way, the challenge of achieving low loss within a broadband range in previous works can be successfully solved.
2. The active element structure
Due to many advantages, such as simple design, easy processing, and cheap price, the microstrip patch is popular to be used as ARE. The ARE with biasing circuit is presented in Fig. 1, which consists of 3 layers in total. The top layer consists of asymmetric metal patches, which are connected together by a PIN diode and a capacitor, as shown in Fig. 1(b). The substrate is microwave composite material with the thickness h = 3 mm, dielectric constant εr=2.65, and loss tangent tanδ=0.001. Metal ground is located at the bottom of the superstrate. Other parameters of the ARE structure are: p = 10 mm, px = 6.0 mm, py = 7.5 mm.
The simulation of ARE has been accomplished using full-wave electromagnetic simulation software Ansoft HFSS. The states of diode are equivalent to different RLC circuit. According to the datasheet of SMP1340-040LF, the PIN diode can be equivalent to a series of capacitor, resistor and inductor for off-state(R = 10Ω, C = 0.086pF, L = 450 pH), and a resistor in series with an inductor for on-state(R = 1Ω, L = 450 pH). Figure 2 gives the reflection characteristics of the proposed ARE, under x-polarized incident wave, it is revealed that the 180°±30° phase difference is obtained in the frequency range from 8.45 to 12.60GHz, and the loss is less than 0.75 dB, as shown in the shaded area of Fig. 2 (Drawing the shaded area is to more directly observe the reflection characteristics of the proposed ARE in operating band). However, there is no phase change under y-polarized incident wave, the reason is that the PIN diode along x-direction is isolated from y-polarized wave. So the ARE is only capable to work in x-polarization mode. And why the ARE can achieve 40% low loss bandwidth under x-polarized incident wave can be discussed in the following designs.
Fig. 2. The reflection characteristics of ARE (the meaning of the shaded area is to indicate the bandwidth of operation): (a) reflection amplitude; (b) reflection phase.
3. Analysis and optimization of active element
The main cause for generating 1-bit phase difference comes from loading PIN diode into the microstrip patch. So in order to analyze the proposed ARE, the ARE only loading PIN diode (ARE-OLP) is discussed first(See Fig. 3(a).). The two-dimensional (2-D) reflection characteristics vary with the patch size at 10GHz in Fig. 4. When the PIN diode is changed from OFF to ON, the reflection amplitudes stay within −2dB, as shown in Figs. 4(a) and (b). Apparently, the 180° phase difference between OFF and ON can be obtained by optimizing the size of the patch, which is verified by the results shown in Fig. 4(c). The patch size corresponding on phase difference within 180°±30° is marked with red area in Fig. 4(c). It is noteworthy that there is a potential broadband design of ARE-OLP, and the phase difference of other frequencies can be designed through applying the size in red area. When the patch sizes are px = 6 and py = 7.3, Fig. 5 shows that ARE-OLP obtains phase difference in the 4dB loss bandwidth from 8.70 to 12.01GHz.
Fig. 3. (a) The ARE-OLP structure; (b) the induced current distribution in on-state; (c) the induced current in off-state; (d) the ARE loading PIN diode and capacitor (ARE-LPC); (e) the induced current of ARE-LPC in on-state; (f) the induced current of ARE-LPC in off-state.
Fig. 4. The 2-D reflection characteristics of ARE-OLP vary with the size of the patch.(the reflection amplitudes with (a) off-state, (b) on-state, and (c) phase difference).
This way is effective to obtain appropriate reflection phase information, However, the loss is more than half. For the microstrip patch with low dielectric constant, it has no function for absorbing the energy of electromagnetic wave. The equivalent resistance of PIN diode is a main source of energy loss. As shown in Fig. 3(a), in order to make the PIN diode work properly, metal patches at both ends of the diode must be completely separated by the slot (the PIN diode will not be short-circuited). This design has brought the results that induced currents completely pass through the PIN diode, which is shown in Figs. 3(b) and (c). Comparing Figs. 3(a) and (b), when the PIN diode is changed from ON to OFF, the color of the area near PIN diode becomes redder, but the color of edge of the metal patch is unchanged. This means that the states of PIN diode have strong effect on the induced current distribution of ARE. Particularly, the induced current near PIN diode is significantly enhanced in off-state. When induced current passes through the PIN diode, the 10Ω equivalent resistance of PIN diode will convert the electromagnetic energy into thermal energy, which results in large energy loss in Fig. 5(a). As mentioned in literature [28], it is obvious that RF-MEMS (Micro Electro Mechanical Systems) technology is applied to achieve low loss of ARE while accurately modulating phase accurately. However, due to the complicated process as well as high price, there are more restrictions in the application range. For high-gain large aperture, such as the precision approach radars, PIN diode is more rational. In addition, we have proposed two ingenious designs to reduce the loss of ARE-OLP.
The first method is to load capacitor. In order to achieve low loss, a bridging capacitor has been loaded on the slot to reduce the induced currents passing through the PIN diode, which is shown in Fig. 3(d). After loading the bridging capacitor, it is obvious that the difference of currents distribution between the Figs. 3(e) and (f) becomes larger. And 1-bit phase difference is obtained in wider frequency band in Fig. 6. The reason can be understood from the element resonance. Comparing Figs. 3(b) and (e), the induced current near the PIN diode is becoming weak. A part of the induced current passes through the bridging capacitor in Fig. 3(f), which lightens the PIN diode load and weakens the thermal energy from equivalent resistance of the PIN diode. So the loss is reduced. As the capacitance changes, the loss is reduced from 4dB to 1.3dB, as shown in Figs. 6(a) and (b). The cause is rooted in the value of capacitor, capacitor can not only affect the equivalent parameter of microstrip patch, but also achieve shunting effect. From Fig. 6(a), as the capacitance increases, the resonance point shifts to the lower frequency, and when the capacitance is higher than 0.3pF, the resonance point is lower than 7.0GHz. Thus the capacitor mainly plays capacitive-shunting effect role in the X-band, and the reflection amplitude is to −1.3dB. Figure 6(c) also verifies that the low capacitance can change the ARE resonant frequency and the high capacitance can achieve capacitive-shunting effect. The discovery is an advance in the regulation of EW. In particular, for the ARE loading PIN diode and capacitor (ARE-LPC), besides changing the patch size, loading capacitor provides additional degrees of freedom to modulate EW.
Fig. 6. The reflection amplitudes of ARE-LPC at different capacitances in (a) off-state and (b) on-state. (c) The phase differences of ARE-LPC at different capacitances.
The second method to design asymmetric ARE(A-ARE). There are two main ways of asymmetric design in ARE, one is the asymmetric loading position of the electronic device, and the other is the asymmetric design of the slot position and the slot width.
We have established the coordinate system with the center of the patch as the origin, as shown in Fig. 7(a). In the A-ARE, the PIN diode is set to change the phase state, but the capacitor is arranged to obtain the low loss. The coordinate (x1, y1) indicates the diode position and (x2, y2) is the loading position of the bridging capacitor. We obtain the phase differences of all PIN diode loading position coordinates in the third quadrant. As y1 varies from −1.5 to −0.5, the phase difference can be obtained in a wide frequency band, but the bandwidth will become narrower and narrower with |x1| increases. For amplitude, the capacitor can achieve low loss by carrying a part of currents from PIN diode. The capacitive-shunting capacity depends not only on the value of capacitor but also on the position of the capacitor. In order to obtain better amplitude while getting 180° phase difference, we continue to optimize the position of capacitor in the first quadrant. Figure 8(b) reveals that the amplitude changes with the value of y2, it is obvious that the −1 dB reflected amplitude can be obtained with y2<0.5 or y2>1.5 in the off-state. And in Fig. 8(c), −0.8 dB reflected amplitude can be obtained with x2<1.0 in the on-state.
Fig. 7. (a) The electronic devices position in coordinate system. (b) Four length parameters (s1, s2, s3, s4) of A-ARE
Fig. 8. (a) The phase differences of PIN diode at the different potions. The reflected amplitude varies with capacitor potions of (b) off-state and (c) on-state.
To illustrate the mechanism of mode generation for different PIN diode and capacitor positions, taking 10 GHz for example, we monitor the current distributions of A-ARE with ((x1=−1.5, y1=−3)(x2=1.8, y2=2.5)), ((x1=−0.5, y1=−0.5)(x2=0.7, y2=0.7)) and((x1=−0.2, y1=−0.4)(x2=1.4, y2=2)), as seen in Fig. 9. When both the diode and the capacitor are loaded into the edge of the patch, the first working mode of A-ARE is higher-order mode. The current distributions of higher-order mode are sketched in Fig. 9(a) and (b). The induced current passing though the diode is weak, and the loss is reduced to a lower degree. However, the weak current narrows the frequency band of 1-bit phase difference in Fig. 8(a). When both the diode and the capacitor are loaded into the center of the patch, the first working mode of A-ARE is fundamental mode. As shown in Figs. 9(c) and (d), the induced current intensity increases, and the difference of current distribution between ON and OFF becomes larger. In this case, the bandwidth of phase difference is available, but capacitive-shunting effect is not obvious. So the loss is high. Therefore, it is worth that making the effects of fundamental and higher-order mode occur simultaneously. Through optimization, as shown in Fig. 9(e) and (f), the diode should be loaded into the area(−0.2,−0.4)near origin to control the induced current ON and OFF effectively. Moreover, the capacitor should be loaded into edge (1.4, 2) to shunt loading while reducing the effect on the phase.
Fig. 9. The surface current distributions at ((x1=−1.5, y1=−3)(x2=1.8, y2=2.5))in (a) off-state and (b) on-state, ((x1=−0.5, y1=−0.5)(x2=0.7, y2=0.7)) in (c) off-state and (d) on-state, ((x1=−0.2, y1=−0.4)(x2=1.4, y2=2)) in (e) off-state and (f) on-state.
The way of asymmetric loading has successfully achieved low loss of ARE. However, there also have some influences on the phase shift. According to the circuit theory, slitting on patch also can change the equivalent circuit parameters of the patch. More ingeniously, the width of slot provides additional degrees of freedom to modulate EW. As shown in Fig. 7(b), four length parameters(s1, s2, s3, s4) are introduced to adjust the position and width of the shot, respectively. s1 and s3 are the width of slot. Figure 10(a) indicates that the phase difference in the low frequency can be modulated by changing s1. On the contrary, the change of phase difference in the high frequency varies with the size of s3 in Fig. 10(b). The s2 and s4 indicate the slot position which is perpendicular to the x-polarized incident wave. Therefore, the phase does not deflect. However, the indicated current distribution will be changed with the value of s2 and s4. Hence, the amplitude can be optimized again (See Fig. 10(c) and (d)).
4. Element prototype and verification
On the basis of the above investigation, Fig. 11 shows the final design result for oblique incidence at X-bands. It is observed that some shifts occur in terms of both amplitude and phase difference when the incident angle increases. However, the effective phase difference and the acceptable loss are still achieved, even for 30° oblique angle of incidence.
Fig. 11. (a) The phase differences vary with incident angle. (b) The amplitudes of on-state vary with incident angle. (c) The amplitudes of off-state vary with incident angle.
To validate the proposed design experimentally, as shown in Figs. 12(a) and (b), we have fabricated consisting of 16×16 unit cells with the size of 160×160 mm2 by applying conventional printed circuit board (PCB) technique. On the back of the practical model in Fig. 12(a), four DC input boards each of which controls 64 elements are designed to receive DC signals from the biasing circuit control board. Thus all elements can be regulated independently. Furthermore, the biasing circuit control board is elaborately designed to output 256 DC channels. Figure 12(c) displays the schematic illustration of the measurement setup. One horn antenna is used as the source (transmitting antenna) and the other one is the receiving antenna, both of which apply two 2–18GHz wideband horn antennas. In order to carry out the normal incidence measurement in the experiment, the separation angle between the two antennas is set to be 6°. The practical model is characterized experimentally in an anechoic chamber with an Agilent 5230C network analyzer and a Programmable direct current (DC) Power Supply RIGOL DP831 which supplies a 10mA DC to each column.
Fig. 12. (a) and (b) show the fabricated physical model. (c) the schematic illustration of the measurement setup. (d) the detail of element model.
All elements can be set to OFF by applying a field-programmable gate array (FPGA), so that the measured result of off-state can be obtained. On the contrary, the measured result of on-state can also be obtained in this way. The comparison between simulation and measurement results is presented in Figs. 13(a) and (b). Obviously, the measurement results are in good agreement with the simulation results. However, some observed discrepancies between simulations and measurements are mainly attributed to the deviation of the equivalent circuit parameters of the PIN diode. In addition, other factors, such as test method, imperfect soldering, measurement accuracy, fabrication and assembling errors, contribute to the discrepancy as well.
5. Conclusion
In summary, we have designed and fabricated a low loss wideband active element. Some effective methods, such as loading PIN diode, loading capacitor and asymmetric design, are employed to make the active element have wider band phase adjustment range and lower insertion loss. The simulation results demonstrated that the insertion losses under both ON and OFF are less than 0.75 dB in the frequency range of 8.45–12.60GHz. In this frequency range, the 180°±30° phase difference is obtained. The measured results are in line with the simulated ones. In addition, oblique incidence is also discussed in the paper. Even for 30° angle of incidence, the effective phase difference and the acceptable insertion loss are still achieved. Based on the ARE, scattering, focusing and OAM(orbital angular momentum) high performance multifunctional metasurface will be generated. Moreover, these devices have a broad application prospect in wireless communications, cognitive radars, MIMO(multiple input multiple output) systems, adaptive beam forming, and holographic imaging.
Funding
National Natural Science Foundation of China (No. 61801508, No.61471389, No.61671464, No.61701523); Natural Science Basic Research Program of Shaanxi Province (No. 2018JM6040, No.2019JQ-103); Postdoctoral Innovative Talents Support Program of China (No. BX20180375).
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