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Review

RF Multifunctional Components with Integrated Filtering Characteristics: A Review

by
Weiyu He
1 and
Kaida Xu
1,2,*
1
School of Information and Communications Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2
Department of Signal Theory and Communications, Polytechnic School, University of Alcalá, 28871 Alcalá de Henares, Spain
*
Author to whom correspondence should be addressed.
Microwave 2025, 1(3), 11; https://doi.org/10.3390/microwave1030011
Submission received: 22 September 2025 / Revised: 17 October 2025 / Accepted: 30 October 2025 / Published: 5 November 2025
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Abstract

This paper provides a comprehensive review of recent advancements in radio-frequency (RF) multifunctional components with integrated filtering characteristics, including tunable filtering attenuators, filtering power dividers, filtering couplers, and filtering Butler matrices, all of which play critical roles in wireless communication systems. With the increasing demand for miniaturization, integration, and low-loss performance in RF front-ends, multifunctional components with filtering characteristics have become essential. This review first introduces tunable attenuators and filtering attenuators based on various technologies such as PIN diodes, graphene-based structures, and RF-MEMS switches, and also analyzes their advantages, limitations, and performance. Then, we discuss filtering power dividers developed from Wilkinson structures, three-line coupled structures, resonator-based coupling matrix methods, and SSPP-waveguide hybrids. Furthermore, filtering couplers and filtering Butler matrices are reviewed, highlighting their capability to simultaneously achieve amplitude and phase control, making them suitable for multi-beam antenna feeding networks. Finally, a brief conclusion is summarized. Future research directions, such as hybrid technologies, novel materials, broadband and multi-band designs, and antenna-matrix co-design, are suggested to further enhance the performance and practicality of multifunctional RF components for next-generation wireless communication systems.

1. Introduction

With the rapid development of wireless communication technologies shown in Figure 1, various wireless communication products have grown significantly. In recent years, research attention has already shifted toward the sixth-generation (6G) wireless era, which aims to achieve data rates beyond terabits per second, ultra-low latency, and seamless integration of communication, sensing, and intelligence. The International Telecommunication Union (ITU), through its Radiocommunication Sector (ITU-R), initiated the development process for the 5G standards. In 2015, ITU-R defined frequency bands and performance specifications for 5G systems. Since 2018, several countries and telecommunications companies have begun commercial deployment of 5G networks [1]. Massive Multiple Input Multiple Output (MIMO) technology is one of the critical technologies for 5G communications, resulting in an increased demand for radio-frequency (RF) components within wireless communication systems. Consequently, this trend has imposed higher requirements on RF devices in terms of miniaturization, integration, and low-loss performance. The forthcoming 6G systems will further extend these requirements toward even higher frequencies in the sub-THz and THz ranges, calling for innovative multifunctional RF components with enhanced bandwidth, reconfigurability, and energy efficiency.
In 5G wireless communication systems, the number of integrated RF transceivers has significantly increased compared to 4G systems. Therefore, multifunctional, high-performance components are attracting more and more attention. As a critical component of the RF front-end of wireless communication systems, filters play a crucial role in enhancing system performance [2]. Filters are used to improve the signal-to-noise ratio of the systems, whose performance and size directly influence the overall performance and compactness of wireless communication systems. Tunable attenuators play a crucial role in wireless communication systems by precisely controlling signal levels [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. With the advent of new fabrication techniques and the improvement of manufacturing precision, many kinds of filters with miniaturization, low insertion loss, low cost, and high frequency selectivity are proposed. Multifunctional microwave and millimeter-wave devices integrating filtering characteristics are attracting more and more attention, including filtering attenuators [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40], filtering power dividers [41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66], filtering couplers [67,68,69,70,71,72,73,74,75,76], and filtering Butler matrices [77,78,79,80,81,82,83]. The applications of these multifunctional devices can effectively reduce the number of individual components in the RF front-end, thus reducing the size of the system and reducing extra losses caused by cascading.

2. Tunable Attenuators and Tunable Filtering Attenuators

Tunable attenuators can dynamically adjust the signal level in a wireless communication system, which has been widely employed in RF systems. For instance, phased-array antennas are utilized in various radar and base station systems, where tunable attenuators can be employed to control the amplitude of feeding signals to partial antenna elements, thus improving the radiation performance of the phased-array antenna. Many researchers have conducted research on tunable attenuator designs. PIN diodes [3,4,5], varactor diodes [6], graphene [8,9,10,11,12,13,14,15,16], RF-MEMS switches [17,18], and field-effect transistors (FET) [19] have been employed to realize tunable attenuators.
In [4], a voltage-controlled tunable attenuator based on PIN diodes operating in the Ku-band is proposed. The circuit diagram of the PIN diode attenuator is designed and analyzed, and the attenuation versus DC bias voltage over the temperature range is measured. The attenuation increases with the bias voltage increase at any temperature and can be adjusted within a range of 0.5 dB to 15 dB.
In [6], Muhammad Yasir et al. proposed a graphene-based tunable microstrip attenuator. Graphene pads are loaded between a microstrip line and a pair of grounded metal vias. The attenuation is controlled by applying a DC bias voltage across the graphene pads. When the DC bias voltage is 0 V, the graphene exhibits high impedance, effectively creating an open circuit between the microstrip line and the grounded vias. Therefore, the signal passes from Port 1 to Port 2 with minimal attenuation. As the bias voltage increases, the graphene’s resistance decreases, allowing part of the signal to leak through the graphene into the ground plane; thus, the attenuation increases with the increase in the DC bias voltage.
Based on the tunable microstrip attenuator proposed in [9], Muhammad Yasir et al. proposed a two-pair posts tunable attenuator, a three-pair posts tunable attenuator, and a four-pair posts tunable attenuator, respectively. The four-pair posts graphene attenuator is fabricated and measured. The measured results show that the four-pair configuration exceeds 60 dB maximum insertion loss in the 1–10 GHz range while maintaining good input matching, which has the highest tunable attenuation range.
In [8,9], few-layer graphene flakes are used to realize tunable attenuation characteristics. Graphene sandwich structures (GSSs) can be used as well. In [8], a dynamically tunable attenuator based on GSSs is presented, where two GSSs are symmetrically integrated on both sides of a spoof surface plasmon polariton (SSPP) waveguide. By adjusting the DC bias voltage to control the graphene surface impedance, tunable attenuation is achieved. The measured attenuation ranges from 0.3 to 15 dB at 3 GHz, with a bias voltage ranging from 0 to 6.5 V. The multilayer GSS (graphene-PVC-paper) enhances electric field coupling, offering a novel approach to high-performance tunable microwave devices.
In [18], an 8-bit (256-state) reconfigurable power attenuator fabricated using RF-MEMS technology in the CMM-FBK process is presented. By employing electrostatic MEMS ohmic switches to selectively engage series/shunt resistive networks, the attenuator achieves dynamic attenuation tuning from 10 MHz to 110 GHz. With a compact size of 3 mm × 1.95 mm, it demonstrates an attenuation range of 10 to 45 dB (flatness: 5–6 dB below 50 GHz) and VSWR < 4. Measured results align well with FEM simulations, highlighting the capability of MEMS technology for high-frequency tunable devices and offering a low-loss, miniaturized solution for 5G RF front-end systems.
In summary, tunable RF attenuators can be broadly categorized into continuously tunable and digitally switched types. Continuously tunable attenuators offer fine-grained control, making them suitable for analog signal conditioning applications. These attenuators are typically implemented using active components like PIN diodes or graphene, whose impedance can be smoothly varied through bias voltage control [4,5,6,7,8,9,10,11,12,13,14,15,16,19]. Digitally switched attenuators provide stepwise attenuation with high precision, commonly realized through MEMS switches or transistor arrays. Each switching element contributes a fixed attenuation value, and their binary combinations enable programmable attenuation ranges [17,18].
With the increasing attention to multifunctional microwave and millimeter-wave components integrating filtering characteristics in wireless communication systems, various researchers have recently proposed filtering attenuators that integrate filtering characteristics and attenuation characteristics [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40].
In 2005, S. M. Daoud et al. first reported an integrated filtering attenuator design [20]. The tunable filtering attenuator is fabricated using a 0.15 μm GaAs pHEMT technology, achieving an attenuation range from 2.5 dB to 15 dB within the frequency range from 24 GHz to 32 GHz. Since then, tunable attenuators integrating filtering characteristics have attracted increasing attention and investigation.
In [22], B. Wu et al. employed microstrip structures for filtering characteristics, incorporating GSSs whose resistances vary with DC bias voltages. Three tunable filtering attenuators with different bandwidths are designed. One of the tunable filtering attenuators is fabricated and measured. The simulated and measured results demonstrate that the tunable attenuation range of the tunable filtering attenuator is 1.7 dB to 7.6 dB in the passband.
Based on the above-mentioned research, the same research group further developed two dual-band tunable filtering attenuators with similar performance [23]. The dual-band tunable filtering attenuator achieves attenuation ranges of 1.5–7.1 dB and 1.3–6.7 dB in two passbands, respectively. Although this approach results in a compact tunable filtering attenuator, its attenuation range is relatively limited, and the graphene fabrication process is complicated and costly.
The GSSs are complicated to fabricate, and the cost is relatively high. Therefore, some low-cost alternative methods have been proposed, such as variable resistors [29,30] and lumped-element components [31]. In [29], a compact microstrip broadband filtering attenuator based on variable resistors with high-frequency selectivity is proposed. The ideal circuit model of the tunable filtering attenuator shown in Figure 2a is analyzed through odd- and even-mode methods. The corresponding layout is designed based on the above ideal circuit model, as shown in Figure 2b. Figure 2c shows the attenuation of the filtering attenuator at the center frequency of the passband when the value of R0 is tuned from 300 to 2000 Ω. It can be seen that the attenuation decreases with the increase of R0, which realizes tunable attenuation from 3.26 to 15.61 dB.
PIN diodes can be used to design a filtering attenuator to achieve voltage-controlled tunable attenuation within the passband [32,33,34,35,36,37,38]. In [32], PIN diodes are utilized to design a tunable filtering attenuator. The filtering structure consisted of microstrip resonant circuits, each with a PIN diode placed in the center. In microwave frequency ranges, the PIN diode serves as an equivalent resistor, whose resistance could be adjusted by varying the DC bias voltage, thus controlling the attenuation. Simulated and measured results of the fabricated prototype show that the tunable attenuation range is 3.8–30.4 dB within the frequency range of 2.19–2.6 GHz. Compared to graphene-based designs, the PIN diode method is more cost-effective. However, the return loss of this filtering attenuator deteriorates significantly as in-band attenuation increases.
To improve the in-band return loss performance, a voltage-controlled tunable filtering attenuator using PIN diodes is designed in [33]. The ideal circuit model of the tunable filtering attenuator and the corresponding layout are shown in Figure 3a and Figure 3b, respectively. And then the layout is fabricated and measured. The measured S-parameters of the filtering attenuator are shown in Figure 3c. By changing the forward DC bias loaded on the PIN diodes and the resistance value of the PIN diodes, the tunable filtering attenuator can achieve the attenuation of 2.76 dB to 16.99 dB from 1.34 GHz to 2.65 GHz. In the meantime, |S11| does not show significant deterioration and remains below −10 dB.
The comparison of tunable attenuators and filtering attenuators is summarized in Table 1. Based on the above analysis, it can be seen that many kinds of filtering attenuators have been designed by different technologies, including IC technology [11,12], graphene [13,14,15,16], variable resistors [17,18], lumped-element components [19], and PIN diodes [32,33,34,35,36,37,38]. These tunable and filtering attenuators are particularly useful for amplitude control and dynamic gain adjustment in phased-array antennas, adaptive transmitters, and power calibration modules of RF front-ends. Furthermore, researchers have designed multifunctional devices that integrate additional functionalities, including both filtering and attenuation characteristics. For example, Zhang et al. proposed a multifunctional bandpass filter integrating tunable attenuator and reflectionless phase shifter functionalities, providing greater flexibility in RF system design [84].

3. Filtering Power Dividers

Power dividers are essential components in wireless communication systems, as they can split an input signal into equal or unequal outputs and thus play a significant role in antenna array design and MIMO systems. In recent years, many researchers have put more and more attention on filtering power dividers, which integrate filtering characteristics and power division characteristics.
The Wilkinson power divider is one of the most commonly used power dividers [85], and filtering power dividers can be designed based on the concept of the Wilkinson power divider [41,42,43]. In [41], the impedance transformer lines of a conventional Wilkinson power divider are replaced by filtering structures, thereby achieving the integration of the filtering characteristics and power division characteristics. The ideal circuit model of the filtering power divider is designed, and the corresponding layout is obtained. The measured results show that the proposed filtering power divider achieves equal power division within the frequency band from 0.81 GHz to 1.23 GHz, and the in-band isolation of the two output ports is greater than 23 dB.
The three-line coupled structure can be employed in the design of filtering power dividers as well [44,45]. In [44], a filtering power divider is realized using three-line coupled structures, whose ideal circuit model and layout are designed and analyzed. By adjusting the parameters of the three-line coupled structure, both the bandwidth and the center frequency of the filtering power divider can be tuned. The measured S-parameters show that the center frequency of the proposed filtering power divider is 2.42 GHz with a 3 dB fractional bandwidth of 82.6%. The isolation between the two output ports is greater than 23 dB at the center frequency.
Filtering power dividers designed based on Wilkinson power dividers or three-line coupled structure are typically fabricated using PCB technology. However, the dielectric losses of the dielectric substrate are relatively high at the millimeter-wave frequency band, which makes the Wilkinson power dividers and three-line coupled structure unsuitable for the design of the millimeter-wave filtering power dividers. An alternative approach is to employ the coupling matrix method, where microstrip resonators, substrate-integrated waveguide (SIW) cavities, or waveguide resonators can be used to design filtering power dividers [46,47,48,49,50,51,52]. In the millimeter-wave frequency band, waveguides exhibit much lower loss, making them more suitable for low-loss filtering power divider design.
In [46], a Ka-band filtering power divider is designed using rectangular waveguide resonators based on the coupling matrix method. According to the air model structure, it can be observed that the filtering power divider adopts a fully rectangular resonant-cavity configuration, and the power division is achieved through the coupling between the rectangular waveguide resonators. The corresponding S-parameters are given. As can be seen, the Ka-band filtering power divider exhibits a fifth-order Chebyshev filtering response, which is further used as the feeding network of the filtering antenna array. Nevertheless, the bandwidth of filtering power dividers designed via the coupling matrix method is constrained by the filter order.
Spoof surface plasmon polariton (SSPP) structures are derived by introducing optical concepts into the design of microwave and millimeter-wave devices [86]. Because of the high-frequency cutoff characteristics, SSPP structures can be employed as the unit cell to design low-pass filters [53,54,55]. SSPP structures can also be utilized to design power dividers [56,57,58,59]. Accounting for the high-frequency cutoff characteristics of the SSPP unit cell, power dividers based on SSPP structures typically exhibit low-pass filtering characteristics. In [56], an H-shaped SSPP unit cell is used to design a planar Y-shaped SSPP power divider. The S-parameters of the Y-shaped SSPP power divider indicate that the power divider operates below 0.75 THz, with both |S21| and |S31| higher than −4.5 dB.
SIW structures and waveguide structures inherently exhibit low-frequency cutoff characteristics, and the cutoff frequency is determined by the dimensions of the structure. Therefore, combining SSPP structures with SIW or waveguide structures enables the design of power dividers with bandpass characteristics [60,61,62,63,64,65,66,67]. In [60], a filtering power divider is designed by the SIW-SSPP structure. The corresponding S-parameters confirm its bandpass characteristics, where the lower cutoff frequency is determined by the width of the SIW structure, and the higher cutoff frequency is determined by the depth of the SSPP slot structure. In [66], a millimeter-wave waveguide filtering power divider based on an SSPP-rectangular waveguide (RWG) structure is proposed, as shown in Figure 4. The design employs both double-sided and single-sided SSPP-RWG unit cells with identical cutoff frequencies, and the simulated results demonstrate a wide passband from 22.65 GHz to 27.32 GHz with excellent frequency selectivity, equal magnitude outputs, and a phase imbalance within 1°. This validates the effectiveness of SSPP-waveguide integration for achieving compact, low-loss filtering power dividers in the mmWave band.
From the above analysis, it can be observed that the cutoff frequency of filtering power dividers based on SSPP structures can be flexibly controlled by adjusting the dimensions of the unit cells. This provides higher design flexibility and facilitates the realization of broadband filtering power dividers.
The comparison of filtering power dividers is summarized in Table 2. Based on the above analysis, various filtering power dividers have been developed using different technologies, including Wilkinson structures [41,42,43], three-line coupled structures [44,45], microstrip or waveguide resonators based on the coupling matrix method [46,47,48,49,50,51,52], and SSPP-based structures [60,61,62,63,64,65,66]. Each of these approaches exhibits distinct advantages and limitations. Wilkinson and three-line coupled structures are simple to implement on PCB substrates and are widely used at relatively low frequencies, but they suffer from significant dielectric losses when working at mmWave frequencies. Resonator-based designs using the coupling matrix method offer high selectivity and can achieve advanced filtering responses, though the bandwidth is usually limited by the filter order. SSPP-based structures provide adjustable cutoff frequencies through adjusting the dimensions of unit cells, enabling bandpass characteristics when combined with SIW or waveguide structures. This section introduces several methods to integrate filtering and power division characteristics through diverse structural technologies, providing enhanced flexibility, low loss, and wideband performance for next-generation wireless systems, which are suited for feeding networks of multi-port antennas and beamforming systems, where compactness and in-band isolation are essential.

4. Filtering Couplers and Filtering Butler Matrices

Couplers are key components of Butler matrices. Therefore, designing filtering couplers that integrate both coupling characteristics and filtering characteristics is one of the important methods for realizing filtering Butler matrices. In recent years, many researchers have proposed filtering couplers based on the coupling matrix method [67,68,69,70,71,72,73,74,75,76].
In [67], C.K. Lin et al. designed a filtering coupler using four microstrip resonators, which achieves a second-order Chebyshev filtering response. The filtering coupler has two input ports and two output ports. When Port 1 is excited, the signal is equally divided into in-phase outputs at Ports 2 and 3. Conversely, when Port 4 is excited, the signal is equally divided into out-of-phase outputs at the two output ports. The measured results further validate this design method for a 180° filtering coupler, which corresponds well with the ideal results. At present, a variety of filtering couplers have been developed using microstrip resonators and the coupling matrix method to meet different design requirements [70,71,72,73,74,75,76,77].
Substrate-integrated waveguide (SIW) resonators can also be employed in the design of filtering couplers based on the coupling matrix method. In [71], a filtering coupler operating at 11 GHz is designed using single-layer SIW structures. The filtering coupler consists of four resonators, among which one resonator operates at the TE201 mode and the other three resonators operate at the TE101 mode. Each resonator is coupled to one port and two other resonators. By employing the TE201 mode, isolation between Port 1 and Port 4 is achieved, while simultaneously realizing equal-amplitude, out-of-phase outputs at Ports 2 and 3 when Port 4 is excited. Similarly, in [72], one resonator working at the TE201 mode and the other three resonators working at the TE101 mode are used to design an H-plane waveguide filtering coupler with a second-order filtering response. In [73], Chen et al. proposed a ring-shaped E-plane waveguide filtering coupler operating at D-band, which also employs the TE201 and TE101 modes to achieve the desired coupling and filtering characteristics.
Dielectric resonators are also commonly employed in the design of filtering couplers [74,75,76]. However, in the current design of filtering Butler matrices, dielectric resonators are relatively difficult to integrate with other components to build matrices. As a result, microstrip resonators and waveguide resonators are more widely used in the design of Butler matrices [77,78,79,80,81,82,83].
In [77], V.T. Crestvolant et al. applied the 180° filtering coupler design method introduced in [71] to design a waveguide filtering coupler with a second-order filtering response. Based on this filtering coupler, a filtering Butler matrix composed of four such filtering couplers is further developed. When input ports are excited, the S-parameter results indicate a fourth-order filtering response. However, the output phase differences in this filtering Butler matrix are not suitable for multi-beam antenna design.
In [78], Q. Shao et al. develop a 2 × 4 filtering Butler matrix based on a 180° filtering coupler using microstrip resonators. The fabricated prototype and simulated and measured results are shown. When the signal is input from Port 1, the outputs of the four output ports are excited with the same amplitude and in-phase. When the energy is input from Port 2, the output of the four output ports is excited with the same amplitude and a 180° phase difference. Therefore, this 2 × 4 filtering Butler matrix can serve as a feeding network for filtering multi-beam antennas. In the following works, Q. Shao et al. further design 4 × 6 and 2 × 8 filtering Butler matrices [78,79], as well as 4 × 4 and 8 × 8 filtering Butler matrices using double-layer PCB technology [80].
In [82], W. He et al. proposed a 4 × 4 mm-wave filtering Butler matrix composed of two 180° filtering couplers designed using the coupling-matrix method, two 90° couplers, and three −90° phase shifters, as shown in Figure 5. Full-wave simulations confirm that the 4 × 4 filtering Butler matrix provides stable phase differences of 0°, 180°, and ±90° across the 29.7–30.3 GHz band, with excellent return loss and compact integration.
The filtering Butler matrices in [77,78,79,80,81,82] operate in a single frequency band. In [83], S. Qi et al. proposed a dual-band filtering Butler matrix. Firstly, a dual-band 180° filtering coupler is designed by employing branch-loaded resonators, which provide both odd-mode and even-mode resonant frequencies. Then, by combining the proposed dual-band filtering coupler with a broadband 90° coupler and a broadband 90° phase shifter, a dual-band filtering Butler matrix is realized. The S-parameters indicate that the Butler matrix operates at 12.25 GHz and 17.25 GHz.
The comparison of filtering couplers is summarized in Table 3. Based on the above analysis, various types of filtering couplers have been developed using different resonator technologies, including microstrip resonators [67,68,69,70], SIW resonators [71], waveguide resonators [72,73], and dielectric resonators [74,75,76]. Building on filtering couplers, researchers have proposed various filtering Butler matrices ranging from small-scale 2 × 4 structures to larger 8 × 8 structures, demonstrating their potential as feeding networks for multi-beam antennas, which are key functions in phased-array and MIMO systems for 5G/6G wireless communication [77,78,79,80,81,82,83]. The comparison of filtering Butler matrices is summarized in Table 4. Nevertheless, challenges remain in achieving regular output port spacing and in realizing integrated antenna-matrix architectures, which are crucial for compact and high-performance multi-beam antenna systems.

5. Conclusions

This paper has reviewed recently reported works in multifunctional RF components, including tunable filtering attenuators, filtering power dividers, filtering couplers, and filtering Butler matrices. These components integrate filtering characteristics with additional characteristics such as attenuation control, power division, and beamforming, which enable compact size, lower insertion loss, and improved frequency selectivity, making them highly attractive for compact and efficient RF front-ends. Future research should focus on broadband and multi-band designs, cost-effective fabrication, and antenna-matrix co-design to further enhance performance and practicality in advanced wireless communication systems. In addition, when selecting among different fabrication technologies, designers must consider the trade-offs between fabrication cost, integration complexity, and achievable RF performance. Low-cost PCB- and PIN-diode-based implementations remain attractive for low-cost and large-scale manufacturing of reconfigurable RF devices, while intermediate-cost techniques such as CNC machining and 3D printing offer rapid prototyping and flexible geometries for compact or customized designs. In contrast, graphene- and MEMS-based devices, though offering superior tunability and precision, involve higher fabrication complexity and cost. Therefore, the choice of technology should be guided by both performance requirements and economic feasibility.

Author Contributions

Conceptualization, W.H. and K.X.; investigation, W.H.; resources, K.X.; data curation, W.H. and K.X.; writing—original draft preparation, W.H. and K.X.; writing—review and editing, K.X.; supervision, K.X.; project administration, K.X.; funding acquisition, K.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China under Grant 62471374.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Osio, J.; Keith, E. 5G Tracker: 94 Markets Worldwide Have Commercial 5G Services. S&P Global Market Intelligence. 2023. Available online: https://www.spglobal.com/market-intelligence/en/news-insights/research/5g-tracker-94-markets-worldwide-have-commercial-5g-services (accessed on 6 April 2025).
  2. Cruz, P.; Gomes, H.; Carvalho, N. Receiver Front-End Architectures—Analysis and Evaluation. In Advanced Microwave and Millimeter Wave Technologies Semiconductor Devices Circuits and Systems; InTech: Huston, TX, USA, 2010; Volume 1. [Google Scholar]
  3. Ananasso, F.G. A low phase shift step attenuator using p-i-n diodes switch. IEEE Trans. Microw. Theory Tech. 1980, 28, 774–776. [Google Scholar] [CrossRef]
  4. Jang, B.-J. Voltage-controlled PIN diode attenuator with a temperature-compensation circuit. IEEE Microw. Wirel. Compon. Lett. 2003, 13, 7–9. [Google Scholar] [CrossRef]
  5. Chen, J.; Peng, H.; Chen, L.; Sun, T.; Liu, Y.; Tatu, S.O.; Yang, T. Voltage-controlled attenuator based on PIN diode and compensated gold wire inductors. Microw. Opt. Technol. Lett. 2024, 66, e34295. [Google Scholar] [CrossRef]
  6. Yuan, Y.; Chen, S.J.; Fumeaux, C. Transmission-Type Varactor-Based Tunable Attenuator. IEEE Trans. Microw. Theory Tech. 2024, 72, 5082–5094. [Google Scholar] [CrossRef]
  7. Han, B.-Y.; Xu, J.; Su, J.-H.; Zhao, M.; Liu, F.; Wan, H. A Broadband Attenuator Using Dual-Branch Resistors and Microstrip-Line-Loaded Slotline Structure with Improved Attenuation Slope. IEEE Microw. Wirel. Technol. Lett. 2024, 34, 1227–1230. [Google Scholar] [CrossRef]
  8. Yasir, M.; Bistarelli, S.; Cataldo, A.; Bozzi, M.; Perregrini, L.; Bellucci, S. Enhanced tunable microstrip attenuator based on few layer graphene fakes. IEEE Microw. Wirel. Compon. Lett. 2017, 27, 332–334. [Google Scholar] [CrossRef]
  9. Yasir, M.; Bistarelli, S.; Cataldo, A.; Bozzi, M.; Perregrini, L.; Bellucci, S. Voltage-Controlled and Input-Matched Tunable Microstrip Attenuators Based on Few-Layer Graphene. IEEE Trans. Microw. Theory Tech. 2020, 68, 701–710. [Google Scholar] [CrossRef]
  10. Zhang, A.-Q.; Lu, W.-B.; Liu, Z.-G.; Chen, H.; Huang, B.-H. Dynamically tunable substrate-integrated-waveguide attenuator using graphene. IEEE Trans. Microw. Theory Tech. 2018, 66, 3081–3089. [Google Scholar] [CrossRef]
  11. Tian, D.; Kianinejad, A.; Zhang, A.X.; Chen, Z.N. Graphene-based dynamically tunable attenuator on spoof surface plasmon polaritons waveguide. IEEE Microw. Wirel. Compon. Lett. 2019, 29, 388–390. [Google Scholar] [CrossRef]
  12. Yasir, M.; Savi, P. Commercial graphene nanoplatelets-based tunable attenuator. Electron. Lett. 2020, 56, 184–187. [Google Scholar] [CrossRef]
  13. Tang, J.; Chen, P.; Zhang, F.; Ni, H. The Structure Design of a Microstrip Tunable Attenuator Based on Few-Layer Graphene. J. Phys. Conf. Ser. 2023, 2625, 012035. [Google Scholar]
  14. Zhang, A.-Q.; Liu, Z.-G.; Wei-Bing, L.; Chen, H. Graphene-Based Dynamically Tunable Attenuator on a Coplanar Waveguide or a Slotline. IEEE Trans. Microw. Theory Tech. 2019, 67, 70–77. [Google Scholar] [CrossRef]
  15. Zhang, A.-Q.; Liu, Z.-G.; Yi, Y.; Lu, W.-B. Graphene-Based Tunable Attenuator on Coplanar Waveguide. In Proceedings of the 2018 IEEE International Conference on Computational Electromagnetics (ICCEM), Chengdu, China, 26–28 March 2018; pp. 1–3. [Google Scholar]
  16. Wu, B.; Zhang, Y.; Zu, H.; Fan, C.; Lu, W. Tunable Grounded Coplanar Waveguide Attenuator Based on Graphene Nanoplates. IEEE Microw. Wirel. Compon. Lett. 2019, 29, 330–332. [Google Scholar] [CrossRef]
  17. Iannacci, J.; Huhn, M.; Tschoban, C.; Pötter, H. RF-MEMS technology for future mobile and high-frequency applications: Reconfigurable 8-bit power attenuator rested up to 110 GHz. IEEE Electron. Device Lett. 2016, 37, 1646–1649. [Google Scholar] [CrossRef]
  18. Hah, D. RF MEMS variable attenuators with improved dB-linearity. Microsyst. Technol. 2023, 29, 311–320. [Google Scholar] [CrossRef]
  19. Saavedra, C.E.; Zheng, Y. Ring-hybrid microwave voltage-variable attenuator using HFET transistors. IEEE Trans. Microw. Theory Tech. 2005, 53, 2430–2434. [Google Scholar] [CrossRef]
  20. Daoud, S.M.; Shastry, P.N. A novel wideband MMIC voltage controlled attenuator with a bandpass filter topology. IEEE Trans. Microw. Theory Tech. 2005, 54, 629–632. [Google Scholar]
  21. Bae, J.; Nguyen, C. A 44 GHz CMOS RFIC dual-function attenuator with band-pass-filter response. IEEE Microw. Wirel. Comp. Lett. 2015, 25, 241–243. [Google Scholar] [CrossRef]
  22. Wu, B.; Fan, C.; Feng, X.; Zhao, Y.-T.; Ning, J.; Wang, D.; Su, T. Dynamically tunable filtering attenuator based on graphene integrated microstrip resonators. IEEE Trans. Microw. Theory Tech. 2020, 68, 5270–5278. [Google Scholar] [CrossRef]
  23. Fan, C.; Wu, B.; Sun, S. Controllable design of filtering attenuators based on graphene integrated dual-mode microstrip resonator. In Proceedings of the IEEE 4th International Conference on Electronic Information and Communication Technology (ICEICT), Xi’an, China, 18–20 August 2021; pp. 833–835. [Google Scholar]
  24. Yi, Y.; Zhang, A.-Q. A tunable graphene filtering attenuator based on effective spoof surface plasmon polariton waveguide. IEEE Trans. Microw. Theory Tech. 2020, 68, 5169–5177. [Google Scholar] [CrossRef]
  25. Hou, R.; Chen, J.; Zhao, Y.-T.; Su, T.; Li, L.; Xu, K.-D. Varactor-graphene-based bandpass filter with independently tunable characteristics of frequency and amplitude. IEEE Trans. Compon. Packag. Manuf. Technol. 2022, 12, 1375–1385. [Google Scholar] [CrossRef]
  26. Yang, Z.-Q.; Wen, Q.-L.; Fan, C.; Wu, B.; Qiu, Y. Dynamically Tunable Multifunction Attenuator Based on Graphene-Integrated Dual-Mode Microstrip Resonators. Electronics 2025, 14, 137. [Google Scholar] [CrossRef]
  27. Chen, J.; Zhang, J.; Zhao, Y.; Li, L.; Su, T.; Fan, C.; Wu, B. High-Selectivity Bandpass Filter with Controllable Attenuation Based on Graphene Nanoplates. Materials 2022, 15, 1694. [Google Scholar] [CrossRef]
  28. Lin, Z.-C.; Liu, Y.; Li, L.; Li, J.; Chen, J.; Xu, J.; Xu, K.-D. Dual-Band Bandpass Filter with Independently Tunable Attenuation Characteristics Using Graphene Nanoplates. IEEE Trans. Plasma Sci. 2022, 50, 5031–5037. [Google Scholar] [CrossRef]
  29. Li, J.; He, W.; Xu, K.-D.; Chen, J.; Zhang, X.Y. Compact Tunable Broadband Filtering Attenuator Based on Variable Resistors. IEEE Microw. Wirel. Technol. Lett. 2023, 33, 983–986. [Google Scholar] [CrossRef]
  30. Wen, P.; Jiang, Y.; Liu, F.; Ma, Z.; Wang, Y. Direct Synthesis of Continuously Tunable Wideband Bandpass Filtering Attenuator with Multiple Transmission Zeros. IEEE Trans. Circuits Syst. II Express Briefs 2024, 71, 4346–4350. [Google Scholar] [CrossRef]
  31. Knowles, J.M.; Sigmarsson, H.H.; McDaniel, J.W. Design of a symmetric lumped-element bandpass filtering attenuator (Filtenuator). In Proceedings of the 2022 IEEE 22nd Annual Wireless and Microwave Technology Conference (WAMICON), Clearwater, FL, USA, 27–28 April 2022; pp. 1–4. [Google Scholar]
  32. Pal, B.; Mandal, M.K.; Kahar, M.; Dwari, S. Filtering attenuator with electronically tunable attenuation. In Proceedings of the IEEE MTT-S International Microwave and RF Conference (IMaRC), Kanpur, India, 17–19 December 2021; pp. 1–4. [Google Scholar]
  33. He, W.; Li, J.; Xu, K.-D.; Yan, S.; Chen, J. Voltage-Controlled Tunable Filtering Attenuator Using PIN Diodes. IEEE Trans. Circuits Syst. II Express Briefs 2024, 71, 562–566. [Google Scholar] [CrossRef]
  34. Nadeem, A.; Nikolaou, S.; Psychogiou, D.; Vryonides, P. Multifunctional and Reconfigurable Bandpass Filters with Continuously Tunable Attenuation or Quasi-Reflectionless Behavior. IEEE Trans. Circuits Syst. II Express Briefs 2024, 71, 4466–4470. [Google Scholar] [CrossRef]
  35. Wei, F.; Liu, H.-Y.; Zhou, X.-C.; Cui, P.-Z.; Jin, G. Tunable Loss Reflectionless Filtering Attenuator with Ultrawide Bandwidth and Extended Tunable Attenuation Range. IEEE Microw. Wirel. Technol. Lett. 2025, 35, 666–669. [Google Scholar] [CrossRef]
  36. Li, J.; Chen, J.; He, W.; Song, Y.; Zheng, Z.; Xu, K.-D. A Wideband Continuously-Tunable Quasi-Reflectionless Filtering Attenuator. IEEE Microw. Wirel. Technol. Lett. 2025, 35, 1304–1307. [Google Scholar] [CrossRef]
  37. Knowles, J.M.; Sigmarsson, H.H.; McDaniel, J.W. Generalized Theory and Realization of Continuously Loss-Programmable Bandpass Filtering Attenuators. IEEE Trans. Microw. Theory Tech. 2023, 71, 5280–5294. [Google Scholar] [CrossRef]
  38. Liu, X.; Xiang, Q.; Feng, Q. Voltage-controlled filtering attenuator with fully tunable passband and attenuation, AEU—International. J. Electron. Commun. 2025, 200, 155926. [Google Scholar]
  39. Knowles, J.M.; Sigmarsson, H.H.; McDaniel, J.W. Higher-Order Filtering Attenuator Design Considerations for Filter Shape Optimization. In Proceedings of the 2023 IEEE Wireless and Microwave Technology Conference (WAMICON), Melbourne, FL, USA, 17–18 April 2023; pp. 85–88. [Google Scholar]
  40. Han, B.-Y.; Xu, J.; Su, J.-H.; Zhou, G.-Q.; Liu, F.; Wan, H. Broadband Filtering Attenuator with Bandwidth Enhancement in High Attenuation Level. IEEE Microw. Wirel. Technol. Lett. 2025, 1–4. [Google Scholar] [CrossRef]
  41. Chao, S.-F.; Lin, W.-C. Filtering power divider with good isolation performance. Electron. Lett. 2014, 50, 815–817. [Google Scholar] [CrossRef]
  42. Yuan, C.-L.; Quan, X.; Xiu, Y.-Z. Single-and dual-band power dividers integrated with bandpass filters. IEEE Trans. Microw. Theory Tech. 2013, 61, 69–76. [Google Scholar]
  43. Chau, W.-M.; Hsu, K.-W.; Tu, W.-H. Filter-based Wilkinson power divider. IEEE Microw. Wirel. Compon. Lett. 2014, 24, 239–241. [Google Scholar] [CrossRef]
  44. Da Xu, K.; Bai, Y.; Ren, X.; Xue, Q. Broadband filtering power dividers using simple three-line coupled structures. IEEE Trans. Compon. Packag. Manuf. Technol. 2018, 9, 1103–1110. [Google Scholar]
  45. Zhang, G.; Wang, J.-P.; Zhu, L. Dual-band filtering power divider with high selectivity and good isolation. IEEE Microw. Wirel. Compon. Lett. 2016, 26, 774–776. [Google Scholar] [CrossRef]
  46. Wu, S.F.; Li, J.X.; Li, Y.J.; Yan, S.; Zhang, X.Y. SLM printed dual-band dual-polarized shared-aperture filtering antenna array for millimeter-wave integrated sensing and communication. IEEE Trans. Antennas Propag. 2024, 72, 2855–2860. [Google Scholar] [CrossRef]
  47. Zhang, G.; Liu, Y.; Wang, E.; Yang, J. Multilayer packaging SIW three-way filtering power divider with adjustable power division. IEEE Trans. Circuits Syst. II Express Briefs 2020, 67, 3003–3007. [Google Scholar] [CrossRef]
  48. Feng, W.; Shi, Y.; Zhou, X.Y.; Shen, X.; Che, W. A bandpass push–pull high power amplifier based on SIW filtering balun power divider. IEEE Trans. Plasma Sci. 2019, 47, 4281–4286. [Google Scholar] [CrossRef]
  49. Moznebi, A.-R.; Afrooz, K.; Danaeian, M.; Mousavi, P. Four-way filtering power divider using SIW and eighth-mode SIW cavities with ultrawide out-of-band rejection. IEEE Microw. Wirel. Compon. Lett. 2019, 29, 586–588. [Google Scholar] [CrossRef]
  50. Wu, S.; Li, J.; Chen, X.; Yan, S.; Zhang, X.Y. A Ka-band SLM printed filtering divider-fed magnetoelectric dipole antenna array using embedded gap waveguide. IEEE Antennas Wirel. Propag. Lett. 2023, 22, 774–778. [Google Scholar] [CrossRef]
  51. Chi, P.-L.; Chen, Y.-M.; Yang, T. Single-layer dual-band balanced substrate-integrated waveguide filtering power divider for 5G millimeter-wave applications. IEEE Microw. Wirel. Compon. Lett. 2020, 30, 585–588. [Google Scholar] [CrossRef]
  52. Liu, B.-G.; Lyu, Y.-P.; Zhu, L.; Cheng, C.-H. Compact square substrate integrated waveguide filtering power divider with wideband isolation. IEEE Microw. Wirel. Compon. Lett. 2021, 31, 109–112. [Google Scholar] [CrossRef]
  53. Guo, Y.-J.; Xu, K.-D.; Tang, X. Spoof plasmonic waveguide developed from coplanar stripline for strongly confined terahertz propagation and its application in microwave filters. Opt. Express 2018, 26, 10589–10596. [Google Scholar] [CrossRef]
  54. Xu, K.-D.; Guo, Y.-J.; Deng, X.-J. Terahertz broadband spoof surface plasmon polaritons using high-order mode developed from ultra-compact split-ring grooves. Opt. Express 2019, 27, 4354–4363. [Google Scholar] [CrossRef] [PubMed]
  55. Zhang, D.; Zhang, K.; Wu, Q.; Dai, R.; Sha, X. Broadband high-order mode of spoof surface plasmon polaritons supported by compact complementary structure with high efficiency. Opt. Lett. 2018, 43, 3176–3179. [Google Scholar] [CrossRef]
  56. Liu, L.; Li, Z.; Xu, B.; Gu, C.; Chen, C.; Ning, P.; Yan, J.; Chen, X. High-efficiency transition between rectangular waveguide and domino plasmonic waveguide. AIP Adv. 2015, 5, 027105. [Google Scholar] [CrossRef]
  57. Gao, X.; Zhou, L.; Yu, X.Y.; Cao, W.P.; Li, H.O.; Ma, H.F.; Cui, T.J. Ultra-wideband surface plasmonic Y-splitter. Opt. Express 2015, 23, 23270–23277. [Google Scholar] [CrossRef] [PubMed]
  58. Li, M.; Wu, Y.; Qu, M.; Li, Q.; Liu, Y. A novel power divider with ultra-wideband harmonics suppression based on double-sided parallel spoof surface plasmon polaritons transmission line. Int. J. RF Microw. Comput.-Aided Eng. 2018, 28, e21231. [Google Scholar] [CrossRef]
  59. Farokhipour, E.; Komjani, N.; Chaychizadeh, M.A. An ultra-wideband three-way power divider based on spoof surface plasmon polaritons. J. Appl. Phys. 2018, 124, 235310. [Google Scholar] [CrossRef]
  60. Pan, B.C.; Yu, P.; Liao, Z.; Zhu, F.; Luo, G.Q. A compact filtering power divider based on spoof surface plasmon polaritons and substrate integrated waveguide. IEEE Microw. Wirel. Compon. Lett. 2022, 32, 101–104. [Google Scholar] [CrossRef]
  61. Yu, P.; Pan, B.-C. Band-pass power divider based on spoof surface plasmon polaritons and substrate integrated waveguide. In Proceedings of the 2020 Cross Strait Radio Science & Wireless Technology Conference (CSRSWTC), Fuzhou, China, 13–16 December 2020; pp. 1–3. [Google Scholar]
  62. Mirhadi, S.; Soleimani, S. A band-pass power divider based on substrate integrated plasmonic waveguide. In Proceedings of the 31st International Conference on Electrical Engineering (ICEE), Tehran, Iran, 9–11 May 2023; pp. 1–4. [Google Scholar]
  63. Lv, S.; Li, J.; He, W.; Wu, S.; Chen, J. Dual-band millimeter-wave filtering power divider using groove gap waveguide and spoof surface plasmon polariton. In Proceedings of the IEEE 6th International Conference on Electronic Information and Communication Technology (ICEICT), Qingdao, China, 21–24 July 2023; pp. 1–4. [Google Scholar]
  64. Pan, B.C.; Yu, P.; Guo, B.J.; Qian, Y.H.; Luo, G.Q. Unequal bandpass filtering power divider based on hybrid HMSIW-SSPP modes. Front. Phys. 2022, 10, 102345. [Google Scholar] [CrossRef]
  65. He, W.; Li, J.; Wu, S.; Yan, S.; Chen, J. A mmWave Waveguide Filtering Power Divider Based on SSPP-RWG Structure. In Proceedings of the 2023 IEEE 11th Asia-Pacific Conference on Antennas and Propagation (APCAP), Guangzhou, China, 22–24 November 2023; pp. 1–2. [Google Scholar]
  66. Li, J.; Lv, S.; He, W.; Li, Q.; Yan, S.; Xu, K.-D. Millimeter-Wave Filtering Power Divider with Sharp Roll-Off Skirt Using Rectangular Waveguide-Spoof Surface Plasmon Polariton Structure. IEEE Trans. Compon. Packag. Manuf. Technol. 2025, 15, 1454–1461. [Google Scholar] [CrossRef]
  67. Lin, C.-K.; Chung, S.-J. A compact filtering 180° hybrid. IEEE Trans. Microw. Theory Tech. 2011, 59, 3030–3036. [Google Scholar] [CrossRef]
  68. Chen, C.-F.; Li, J.-J.; Wang, G.-Y.; Zhou, K.-W.; Chen, R.-Y. Design of compact filtering 180-degree hybrids with arbitrary power division and filtering response. IEEE Access 2019, 7, 18521–18530. [Google Scholar] [CrossRef]
  69. Lin, F.; Chu, Q.-X.; Wong, S.-W. Design of dual-band filtering quadrature coupler using λ/2 and λ/4 resonators. IEEE Microw. Wirel. Compon. Lett. 2012, 22, 565–567. [Google Scholar] [CrossRef]
  70. Chou, P.-J.; Yang, C.-C.; Chang, C.-Y. Exact synthesis of unequal power division filtering rat-race ring couplers. IEEE Trans. Microw. Theory Tech. 2018, 66, 3277–3287. [Google Scholar] [CrossRef]
  71. Rosenberg, U.; Salehi, M.; Bornemann, J.; Mehrshahi, E. A novel frequency-selective power combiner/divider in single-layer substrate integrated waveguide technology. IEEE Microw. Wirel. Compon. Lett. 2013, 23, 406–408. [Google Scholar] [CrossRef]
  72. Rosenberg, U.; Salehi, M.; Amari, S.; Bornemann, J. Compact multi-port power combination/distribution with inherent bandpass filter characteristics. IEEE Trans. Microw. Theory Tech. 2014, 62, 2659–2672. [Google Scholar] [CrossRef]
  73. Chen, X.; Wang, Y.; Zhang, Q.-F. Ring-shaped D-band E-plane filtering coupler. IEEE Microw. Wireless Compon. Lett. 2021, 31, 953–956. [Google Scholar] [CrossRef]
  74. Uchida, H.; Yoneda, N.; Konishi, Y.; Makino, S. Bandpass directional couplers with electromagnetically-coupled resonators. In Proceedings of the IEEE MTT-S International Microwave Symposium Digest, San Francisco, CA, USA, 11–16 June 2006; pp. 1–4. [Google Scholar]
  75. Zheng, S.-Y.; Zheng, Y.-X. Bandpass filtering coupler based on dual-mode dielectric resonators. In Proceedings of the Sixth Asia-Pacific Conference on Antennas and Propagation (APCAP), Xi’an, China, 16–19 October 2017; pp. 1–3. [Google Scholar]
  76. Hagag, M.F.; Abu Khater, M.; Sinanis, M.D.; Peroulis, D. Ultra-compact tunable filtering rat-race coupler based on half-mode SIW evanescent-mode cavity resonators. IEEE Trans. Microw. Theory Tech. 2018, 66, 5563–5572. [Google Scholar] [CrossRef]
  77. Crestvolant, V.T.; Iglesias, P.M.; Lancaster, M.J. Advanced Butler matrices with integrated bandpass filter functions. IEEE Trans. Microw. Theory Tech. 2015, 63, 3433–3444. [Google Scholar] [CrossRef]
  78. Shao, Q.; Chen, F.C.; Chu, Q.X.; Lancaster, M.J. Novel filtering 180° hybrid coupler and its application to 2 × 4 filtering Butler matrix. IEEE Trans. Microw. Theory Tech. 2018, 66, 3288–3296. [Google Scholar] [CrossRef]
  79. Shao, Q.; Chen, F.-C.; Wang, Y.; Chu, Q.-X.; Lancaster, M.J. Design of modified 4 × 6 filtering Butler matrix based on all-resonator structures. IEEE Trans. Microw. Theory Tech. 2019, 67, 3617–3627. [Google Scholar] [CrossRef]
  80. Shao, Q.; Chen, F.-C. Design of 2 × 8 filtering Butler matrix with arbitrary power distribution. IEEE Trans. Circuits Syst. II Exp. Briefs 2021, 68, 3527–3531. [Google Scholar] [CrossRef]
  81. Shao, Q.; Chen, F.-C.; Wang, Y.; Chu, Q.X. Design of 4 × 4 and 8 × 8 filtering Butler matrices utilizing combined 90° and 180° couplers. IEEE Trans. Microw. Theory Tech. 2021, 69, 3842–3852. [Google Scholar] [CrossRef]
  82. He, W.; Wu, S.; Li, J. Design of a millimeter-wave 4 × 4 filtering Butler matrix. In Proceedings of the TENCON 2022—2022 IEEE Region 10 Conference (TENCON), Hong Kong, China, 1–4 November 2022; pp. 1–4. [Google Scholar]
  83. Qi, S.-S.; Guo, Y.; Wang, J.; Wang, X.; Wu, W. A dual-band filtering 4 × 4 Butler matrix based on SLR. IEEE Antennas Wirel. Propag. Lett. 2023, 22, 3167–3171. [Google Scholar] [CrossRef]
  84. Zhang, Z.; Psychogiou, D. Multifunctional Bandpass Filter with Codesigned Tunable Attenuator and Reflectionless Phase Shifter Functionalities. IEEE Microw. Wirel. Technol. Lett. 2024, 34, 737–740. [Google Scholar] [CrossRef]
  85. EWilkinson, J. An n-way hybrid power divider. IEEE Trans. Microw. Theory Tech. 1960, 8, 116–118. [Google Scholar] [CrossRef]
  86. Pendry, J.B.; Martín-Moreno, L.; Garcia-Vidal, F.J. Mimicking surface plasmons with structured surfaces. Science 2004, 305, 847–848. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Development history of wireless communication systems.
Figure 1. Development history of wireless communication systems.
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Figure 2. Compact microstrip broadband filtering attenuator based on variable resistors [29]. Copyright 2023, IEEE. (a) Ideal circuit model. (b) Layout. (c) Attenuation versus variable resistor at the center frequency.
Figure 2. Compact microstrip broadband filtering attenuator based on variable resistors [29]. Copyright 2023, IEEE. (a) Ideal circuit model. (b) Layout. (c) Attenuation versus variable resistor at the center frequency.
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Figure 3. Voltage-controlled tunable filtering attenuator using PIN diodes [33]. Copyright 2024, IEEE. (a) Ideal circuit model. (b) Layout. (c) Measured S-parameters of the filtering attenuator.
Figure 3. Voltage-controlled tunable filtering attenuator using PIN diodes [33]. Copyright 2024, IEEE. (a) Ideal circuit model. (b) Layout. (c) Measured S-parameters of the filtering attenuator.
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Figure 4. Millimeter-wave waveguide filtering power divider based on SSPP-RWG structure [66]. Copyright 2023, IEEE. (a) Structure of the filtering power divider. (b) Simulated S-parameters.
Figure 4. Millimeter-wave waveguide filtering power divider based on SSPP-RWG structure [66]. Copyright 2023, IEEE. (a) Structure of the filtering power divider. (b) Simulated S-parameters.
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Figure 5. The 4 × 4 mm-wave filtering Butler matrix [82]. Copyright 2023, IEEE. (a) Air model of the filtering Butler matrix. (b) Simulated magnitude S-parameters. (c) Phase difference between output ports.
Figure 5. The 4 × 4 mm-wave filtering Butler matrix [82]. Copyright 2023, IEEE. (a) Air model of the filtering Butler matrix. (b) Simulated magnitude S-parameters. (c) Phase difference between output ports.
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Table 1. Comparison of tunable attenuators and filtering attenuators.
Table 1. Comparison of tunable attenuators and filtering attenuators.
Ref.TechnologyFiltering ResponseFrequency RangeAttenuation Range (dB)Key Features
[4]PIN diodesNo—0.5–15Voltage-controlled design
[6]Graphene flakesNo—0–60Voltage-controlled design
[8]Graphene sandwich structures (GSSs)No—0.3–15Voltage-controlled design
[18]RF-MEMS 8-bit No—10–45Compact chip-level implementation
[22]Graphene sandwich structures (GSSs)YesAbout 1.6–1.8 GHz1.3–7.6Single and dual-band designs
[29]Variable resistorsYes1.3–2.8 GHz 3.26–15.61Broadband, low-cost implementation
[32]PIN diodesYes2.19–2.6 GHz3.8–30.4Voltage-controlled design
Table 2. Comparison of filtering power dividers.
Table 2. Comparison of filtering power dividers.
Ref.Structure/TechnologyFrequency RangeIsolation (dB)Key Features
[41]Wilkinson-based0.81–1.23 GHz>23Filtered impedance lines
[44]Three-line coupled1.42–3.42 GHz>23Tunable bandwidth
[46]Waveguide (coupling matrix)26.9–30.7 GHz—Rectangular resonators
[65]SSPP–RWG hybrid22.65–27.32 GHz—Low-loss, compact
Table 3. Comparison of filtering couplers.
Table 3. Comparison of filtering couplers.
Ref.StructureCenter FrequencyFiltering Order/ModesKey Features
[67]Microstrip resonators2.4 GHz2nd-order ChebyshevBasic 180° filtering coupler
[71]SIW resonators11 GHz2nd-orderCompact single-layer design
[73]E-plane ring waveguide150 GHz2nd-orderMiniaturized structure
[77]Waveguide12.5 GHz2nd-orderUsed as basis of filtering matrix
Table 4. Comparison of filtering Butler matrices.
Table 4. Comparison of filtering Butler matrices.
Ref.Matrix SizeStructureCenter Frequency Phase DifferencesKey Features
[77]4 × 4Waveguide12.5 GHz—First waveguide filtering Butler matrix
[78]2 × 4Microstrip resonator2.4 GHz0°/180°Equal amplitude, dual input modes
[82]4 × 4Waveguide30 GHz0°, 180°, ±90°Stable phase balance, mmWave operation
[83]4 × 4 (dual-band)SLR-based12.25/17.25 GHz—Dual-band operation, compact design
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He, W.; Xu, K. RF Multifunctional Components with Integrated Filtering Characteristics: A Review. Microwave 2025, 1, 11. https://doi.org/10.3390/microwave1030011

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He W, Xu K. RF Multifunctional Components with Integrated Filtering Characteristics: A Review. Microwave. 2025; 1(3):11. https://doi.org/10.3390/microwave1030011

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He, Weiyu, and Kaida Xu. 2025. "RF Multifunctional Components with Integrated Filtering Characteristics: A Review" Microwave 1, no. 3: 11. https://doi.org/10.3390/microwave1030011

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He, W., & Xu, K. (2025). RF Multifunctional Components with Integrated Filtering Characteristics: A Review. Microwave, 1(3), 11. https://doi.org/10.3390/microwave1030011

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