Wirelessly Pattern Reconfigurable Yagi Antenna Based on Radio Frequency Identification

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Introduction
Pattern reconfgurable antennas have gained considerable attention due to their potential to enhance transmission signal quality and coverage in modern communication systems [1][2][3][4].Tese antennas can dynamically adjust their radiation patterns to meet specifc requirements while operating at a single frequency band.Te switching of radiation patterns makes pattern reconfgurable antennas highly desirable for use in a wide range of applications, including cognitive radio, indoor wireless networks, base stations, and multiple-input multiple-output (MIMO) systems [5].
Pattern reconfgurable Yagi antennas typically utilize printed circuit board (PCB) technology and PIN diodes for implementation [1][2][3][4].However, the long direct current (DC) control cables used to maintain the on and of states of the PIN diodes can couple radiation current and signifcantly impact pattern stability and return loss [6].In addition, the installation of these antennas in outdoor, rooftop, or base station environments further increases the overall cost of the communication system due to the need for extended control cables.
References [7,8] present optically controlled and bluetooth-controlled reconfgurable antennas, respectively.However, these methods have limitations such as the high cost and power consumption of optical control and limited control range of Bluetooth.In contrast, reference [9] introduced an RFID wirelessly controlled reconfgurable antenna but only provided simulated results.Te RFIDcontrolled method ofers advantages in terms of low power consumption, long control range, and low cost.Commercial RFID tags with −18 dBm sensitivity can achieve communication ranges of tens of meters, and the cost of tags is relatively low.Tis work represents the frst application of RFID technology in the feld of smart antenna control, addressing the challenges of long control range and low power consumption.Measurement results demonstrate that our proposed antenna achieves a wireless control range of up to 25 m, while the entire system consumes only 12 μW of power in the common work mode.
Tis paper starts with an overview of our design, highlighting the problems it aims to solve.Te subsequent section provides a detailed description of the antenna's structure, as well as the pattern reconfgurable control circuit.Te paper then presents the simulation and measurement results.Finally, a conclusion and discussion are provided in the last part.

Proposed Antenna Structure
Te proposed antenna is composed of two main parts: the Yagi resonator unit and the RFID tag unit, as illustrated in Figure 1(a).Te Yagi resonator unit comprises a rectangular active printed monopole antenna (PMA) resonator and two passive resonators that function as director or refector elements.Each passive resonator includes two SPDT switches that control the length of the resonator, enabling it to function as a director or refector.As noted in reference [10], SPDT switches ofer superior antenna performance compared to PIN diodes.Te control function is executed by the RFID tag, which receives commands from the reader connected to a personal computer (PC).In this section, we provide a detailed description of both the Yagi unit and the RFID unit.
2.1.Yagi Antenna Design.Classic Yagi antennas typically use a dipole structure as the active resonator [3,11,12].However, in modern communication systems, coaxial cable is often used as the transmission line between the antenna and the transceiver, which can cause pattern distortion problem with the diferential dipole resonator, resulting in rugged pattern test results [13].While adding a balun can efectively solve this problem [13], microstrip baluns increase the antenna size, and lumped ferrite and wire baluns induce excessive insertion loss [14].To address this issue, we use a PMA structure as the active resonator [15,16], and the size of the PMA is designed following the method in reference [17,18].Based on these criteria, the geometry of the proposed Yagi unit is designed in Figure 1(b).
Te theory of Yagi antennas, as discussed in reference [12], states that the passive resonator acts as either a refector or a director based on its efective length compared to the active resonator.When the passive resonator is longer, it exhibits inductive characteristics and functions as the refector.Conversely, when the passive resonator is shorter, it behaves capacitively and acts as the director.Te SPDT switches enable the switching between refection and direction roles by connecting the center metal patch line to the stub or the open end.Te behavior of the SPDT element is controlled by the RFID reader, as shown in Figure 2. Te Yagi unit was cosimulated using the electromagnetic and circuit methods with the S3P fle [19,20] in Ansys TM HFSS full-wave simulation software.

RFID Antenna Design.
Our work primarily focuses on the RFID-controlled pattern reconfgurability of the Yagi antenna.As RFID tag antenna design is a relatively mature technology, we provide only a brief description of the tag antenna structure in this paper.Interested readers can refer to books like [21] for more detailed information on tag antenna design.
Two ultra-high frequency (UHF) band RFID tags were implemented on the left and right sides of the Yagi antenna to ensure pattern symmetry.Te chosen RFID chip, EM4325, is a low-power consumption Class-3 Generation-2 (Gen2) IC with 4 general input/output (IO) ports.Te tag antenna was designed using a folded dipole structure, following the ofcial recommendation in [22,23], which aims to minimize its impact on the Yagi antenna pattern.Te geometry of the folded dipole used in the RFID tag is depicted in Figure 1(b).To simulate the coin battery's efect on the antenna, a circularmetal electrode was incorporated on the backside of the RFID tag, as shown in Figure 1(a).Te RFID tag operates in the battery-assisted passive (BAP) mode.One side of the battery provides energy from the EM4325's four general IO ports to the SPDT switches, while the other side improves the tag's sensitivity from −7 dBm to −28 dBm [22].

Simulation and Measurement Results
To validate the proposed design, a PCB was designed with the dimensions shown in Figure 1.Te substrate material is FR4 (ε r � 4.0 ∼ 4.2, tan δ � 0.012 ∼ 0.014), with a thickness of 1.2 mm.Te top and bottom views of the PCB are shown in Figure 3(a).Te antenna's port impedance matching characteristics were assessed through simulated and measured S-parameter analyses, using HFSS and Keysight TM vector network analyzer (VNA) E5071C, respectively.Figure 4(a) presents the simulated and measured S-parameters of the proposed Yagi antenna.It can be observed that the simulated S-parameters for left radiation (state I) and right radiation (state II) are not identical (blue and black lines in Figure 4(a)).Tis is due to the S-parameter diference between the RF1 and RF2 channels of the SPDT switch [19].Figures 2 and 1(a) illustrate that the two passive resonators are center symmetric.However, the active resonator is left-right symmetric.Consequently, there is a slight diference in the current distribution of the passive resonators between State I and State II, which can infuence the agreement of the S 11 simulations.
Te simulated and measured −10 dB impedance bandwidth could cover the frequency range of 2.36 ∼ 2.46 GHz.Simulation and measurement diferences are mainly due to manufacturing tolerances such as the dielectric constant, which ranges from 4.0 to 4.2.Furthermore, the SPDT chip was manually mounted on the PCB board using surfacemount technology (SMT) and refow soldering, leading to the inevitable coupling of the SPDT pads with the passive resonator via parasitic capacitors and inductance.
To demonstrate the antenna's operation property at the measured center frequency of 2.41 GHz, we conducted  International Journal of Antennas and Propagation simulations and measurements of the antenna's electrical feld for both states I and II, as depicted in Figure 1(a).Te radiation patterns were measured in a professional microwave anechoic chamber, as shown in Figure 3(b).Figures 4(b)-4(e) present the radiation patterns obtained from both measurement and simulation for the two directional radiation states.Our analysis revealed that states I and II have similar radiation patterns, and the results from both simulation and experiment agreed well.Te discrepancies in the radiation patterns can be attributed to various measurement accessories such as the cables, packaged components, and SMA connectors in close proximity to the antenna.We achieved peak gains of 7 dBi and a frontto-back ratio of 10 dB, and the measured radiation efciencies exceeded 59% for both states at 2.41 GHz.
In Figures 5(a) and 5(b), the simulated port impedance matching characteristics of the RFID tag antenna are shown, which reveal an impedance of 7.1 + j114.2Ω at 915 MHz.Since the port impedance of the EM4325 varies at diferent frequencies, measuring the S 11 of the tag antenna may have less signifcance than directly measuring the tag's read range.Following the industrial standard, we measured the pattern and read range of the tag antenna in a semianechoic chamber using professional equipment from Voyantic TM [24] (Figures 3(c) and 3(d)).Te performance of the antenna was evaluated using a horizontally polarized antenna with a gain of 6 dBiL.Te transmitting power was set to 33 dBm, and the reader sensitivity was −90 dBm.Normalized radiation pattern of the tag antenna is presented in Figure 5(c).
Our implemented antenna has a much higher radiation efciency (61.6%) than the ofcial recommended design (13.7%).Furthermore, we present the measured read range and a comparison of the forward power and sensitivity of the RFID tag in Figure 5(d).When the tag was placed in the semianechoic chamber, the read range was measured using horizontal polarized antennas and Tagperformance TM software provided by Voyantic TM .From Figure 5(d), we can see that the control range of the antenna is 25 m at 915 MHz.
As a summary, a comparison between our work and related pattern reconfgurable schemes is presented in Table 1.

International Journal of Antennas and Propagation
Conventional pattern reconfgurable methods using diodes and DC control lines sufer from issues related to pattern and return loss stability.When the antenna is built remotely, the consumption of the control cable cannot be ignored.Optically controlled methods can address the pattern stability problem but are limited by the number of antennas that a single laser equipment can serve, as well as their high power consumption and cost.Bluetooth methods, especially Bluetooth low energy (BLE) schemes, have lower power consumption but limited control range, typically not exceeding 10 m.In contrast, RFID technology ofers advantages in both power consumption and control range.Te cost of RFID readers can be averaged across thousands of tags, making it a feasible option for implementing reconfgurable antenna arrays and phase change array antennas.

Conclusion
Tis paper presents a novel RFID-controlled pattern reconfgurable Yagi antenna designed for wireless communication systems.Te antenna operates at a center frequency of 2.41 GHz and achieves a gain of 7 dBi.Measurement results validate the wireless control capability of the antenna at a distance of 25 m, with a power consumption of only 12 μW in the work mode and 36 μW in the dynamic reconfguration mode.Tese results confrm the antenna's suitability for remote wireless communication applications, making it a promising candidate for practical use in radio direction fnders, digital terrestrial television, cell phone outdoor relays, and other related felds.

Figure 4 :
Figure 4: Simulation and measurement results of the Yagi antenna: (a) input refection coefcient; (b) E(yz)-plane in state I; (c) H(xy)-plane in state I; (d) E(yz)-plane in state II; (e) H(xy)-plane in state II.

Figure 5 :
Figure 5: Simulation results of the RFID tag antenna: (a) input refection coefcient; (b) impedance of the antenna; (c) normalized pattern of the E(yz) plane; (d) read range and forward power on sensitivity of the RFID tag.