Table of Contents

    Topic review

    Reconfigurable Microstrip Filter–Antenna Integration

    View times: 178
    Submitted by: Yasir Al-Yasir

    Definition

           Reconfigurable and tunable radio frequency (RF) and microwave (MW) components have become exciting topics for many researchers and design engineers in recent years. Reconfigurable microstrip filter–antenna combinations have been studied in the literature to handle multifunctional tasks for wireless communication systems. Using such devices can reduce the need for many RF components and minimize the cost of the whole wireless system, since the changes in the performance of these applications are achieved using electronic tuning techniques. However, with the rapid development of current fourth-generation (4G) and fifth-generation (5G) applications, compact and reconfigurable structures with a wide tuning range are in high demand. Also, meeting these requirements comes with some challenges, namely the increased design complexity and system size. Accordingly, this paper aims to discuss these challenges and review the recent developments in the design techniques used for reconfigurable filters and antennas, as well as their integration. Various designs for different applications are studied and investigated in terms of their geometrical structures and operational performance. This paper begins with an introduction to microstrip filters, antennas, and filtering antennas (filtennas). Then, performance comparisons between the key and essential structures for these aspects are presented and discussed. Furthermore, a comparison between several RF reconfiguration techniques, current challenges, and future developments is presented and discussed in this review. Among several reconfigurable structures, the most efficient designs with the best attractive features are addressed and highlighted in this paper to improve the performance of RF and MW front end systems.

    1. Introduction

           The increasing demand for compact, simple, and efficient transceivers continues to impact the development of microwave (MW) and radio frequency (RF) applications [1][2][3][4][5]. Some of the essential elements in such devices are the planar antennas and filters [6][7][8][9], which significantly affect the whole performance of the wireless communication systems. Generally, RF interference is a big issue in the current and future wireless systems, such as the green RF front ends and wideband applications [10][11]. Microstrip bandpass filters (BPFs) are commonly used in several applications, mainly in RF and MW wireless communications, due to their effective role in suppressing interference and noise signals [12][13][14]. Recently, the office of communications (Ofcom) has identified a low bandwidth at 700 MHz, mid bandwidth (3.4–3.8 GHz), and an upper millimeter-wave bandwidth (24.25–27.5 GHz) for possible use with fifth-generation (5G) systems [15]. However, microstrip BPFs are utilized to attenuate the harmonic signals in fourth-generation (4G) and 5G applications [16][17][18][19][20]. For microstrip BPFs, the number of poles and zeros, input and output external quality factors, coupling coefficients, and the configuration of the resonators are important parameters that define the filter performance [21]. Most microstrip filter miniaturization approaches aim to analyze, control, or optimize these parameters [22]. Additionally, several design techniques have been introduced in the literature, such as stepped-impedance resonator (SIR), combline, open-ring, coupled-line, and stub impedance filters [22][23][24][25][26][27].

           On the other hand, reconfigurability can also be utilized using RF electronic components, such as varactors and PIN diodes, which allow for the current distribution on the patches to be modified and then for the reflection coefficient and radiation patter characteristics to be adapted. Micro-electro-mechanical switches (MEMS) can also be considered, however these involve additional costs and extra circuits. In recent years, several reconfigurable microstrip BPFs have been introduced [28][29][30][31][32][33][34][35][36]. However, with the rapid development of current 4G and 5G applications, compact, efficient, and reconfigurable planar filters with a wide tuning range will be urgently needed [37].

           In addition to reconfigurable microstrip filters, frequency-reconfigurable microstrip antennas have been investigated and developed for many years to provide important features to enhance the innovation and development of RF systems [38][39][40][41]. Another important factor to be considered by antenna designers and researchers these days, especially when designing antennas for mobile devices, is the geometrical size and design complexity of the RF elements. Therefore, antenna miniaturization techniques are continuously under review and study by many researchers and engineers. However, there are always new developments and updates in the literature related to these aspects. Due to the high demand for very small structures, the construction of more compact components is required, while the gain and radiation pattern properties should be maintained at the same time and for the same configuration [38]. Compact frequency-reconfigurable microstrip antennas have been introduced for several applications, such as mobile communication devices. Furthermore, these antennas are also needed for other applications, such as global systems for mobile communication (GSM), digital communication systems (DCS), personal communication systems (PCS), universal mobile telecommunication systems (UMTS), Bluetooth, wireless local area networks (LAN), and long-term evolution (LTE) [42][43][44][45][46][47][48][49][50][51][52].

           In recent years, the microstrip filter–antenna integration designs have become some of the most desired structures because of their low profile, compact size, light weight, and ease of fabrication [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71]. Microstrip filtering antennas are also beneficial because they can be printed directly onto the dielectric substrate materials [53]. Filtering antenna designs have many applications, mostly in modern wireless communication systems, where filtering and efficient radiation pattern responses can be obtained simultaneously [55]. Furthermore, reconfigurable microstrip filtering antennas have attracted increasing interest nowadays as they can deliver more efficient and multiple functionalities [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86]. These designs do not implement microstrip antennas and filters separately, rather the filter is loaded onto the radiating patch instead, resulting in more compact structures and improving the entire performance of the RF and MW systems. 

    2. Microstrip Filter–Antenna (Filtenna) Integration

           Recently many microstrip filter–antenna designs using different types of substrate materials have been proposed [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71]. In [56], a co-design of a filter–antenna using a multilayered substrate is introduced for future wireless applications. The design consists of three-pole open-loop ring transmission lines and a T-shaped microstrip antenna. The multilayer technology is utilized to achieve a compact size structure. A Rogers RT5880 substrate with a relative dielectric constant of 2.1 and a thickness of 0.5 mm is used in this structure. The filter–antenna design operates at 2.6 GHz, with a fractional bandwidth of around 2.8% and a measured gain of 2.1 dB. While the main advantage of this structure is the compact size, it has a complex structure due to the use of a multilayer substrate configuration. The design presented in [57] also used the same design procedures and achieved similar performance, having a circular polarization characteristic. However, the filter–antenna design can involve different design techniques based on substrate-integrated waveguide (SIW) technology.

           In [58], a dipole microstrip filter–antenna with quasi-elliptic gain performance using parasitic resonators is presented. The parasitic elements were designed based on the stepped-impedance resonators and utilized to generate two transmission zeros in the in-band transmission, as well as two radiation nulls in the out-of-band bandwidth. The design was fabricated using an F4B-2 substrate with a dielectric constant of 2.4 and a thickness of 1.1 mm. The design also has an air layer located between the radiator and the ground layers, with a height of 9 mm. The deigned filter–antenna works at 1.85 GHz and has a fractional bandwidth of 4.2%. The design offers not only good radiation in the passband region but it also efficiently attenuates the noise signals in the stopband spectrum. Moreover, a wideband balun filter–antenna design with a high roll-off skirt factor is presented in [61]. The design is composed of a fourth-order quasi-Yagi radiator cascaded with a multilayer balun microstrip filter. The balun filter is formed by five stepped impedance resonators, which improves the rejection ratio of the passband. The designed filter–antenna operates at 2.5 GHz with a fractional bandwidth of 22.9% and generates two transmission zeros at both edges of the passband. The design has achieved 5.4 dBi realized gain, with a high roll-off rejection level. Although the design has shown some advantages, such as the wide bandwidth and high suppression level, it also requires the use of multilayer substrate technology.

           Recently, a very compact wideband microstrip filter antenna design with high gain and high selectivity was proposed in [71]. The design consists of a rectangular microstrip, four parasitic lines, two strip lines, and three shorting vias. The design is printed on an 80 × 80 mm2 F-4B substrate with a dielectric constant of 2.6, loss tangent of 0.003, and a height of 4 mm. The center frequency of the design is 2.4 GHz, with an impedance bandwidth range of 2.19 GHz to 2.68 GHz (fractional bandwidth of 20.1%). The filter antenna has a realized gain of 9.5 dBi and flat radiation efficiency of more than 90%. Figure 1 shows the simulated and measured results with a prototype of the fabricated filtering antenna.

    Figure 1. The filtering antenna design reproduced from [71]. 2020, IEEE: (a) S-parameter and gain; (b) efficiency and a photograph of the fabricated prototype.

           However, design complexity and system size are other challenges facing designers of filtering antenna structures. As explained in the literature, many design approaches have been carried out to offer a simple structure and compact size, which can be easily integrated with other RF front end systems. The multilayer structures presented in [56,58,59,62] have not managed these requirements. Moreover, substrate integrated waveguide (SIW) technology and the balun configuration were other notable attempts, as presented in [57] and [61], respectively. To summarize these approaches, Table 1 shows the performance comparison between the surveyed microstrip filter–antenna designs from the literature, which have similar performance. It should be noted that the filter–antenna design proposed in [71] has a compact size with a simple structure and offers higher gain, higher selectivity, a wider fractional bandwidth, and good reflection coefficient characteristics. In summary, without a need for extra filtering circuits, the design presented in [71] offers a new solution for current and future filtering antenna designs.

    Table 1. Comparison between the presented filter–antenna designs.

           Additionally, many reconfigurable microstrip filter–antenna structures have been presented and discussed [72][73][74][75][76][77][78][79][80][81][82][83][84][85][86]. In [79], a multiband tunable filter cascaded with a monopole antenna for cognitive radio communications is presented. The reconfigurable design covers four useful applications, including 1.9 GHz (GSM), 2.5 GHz (Bluetooth), 3.6 GHz (WiMAX), and 5.3 GHz (WLAN). Additionally, the deigned multiband filter–antenna provides a gain range from 1.2 dBi to 3.5 dBi in the four operating bands, with small variations of about 0.5 dBi between the adjacent bands, delivering a radiation efficiency above 60%. Table 2 compares some of the similar reconfigurable filtering antenna designs in the literature with the design presented in [79]. However, it is shown that the reconfigurable filtering antenna presented in [79] has a smaller size and wide tuning range, covering four discrete configurations for four important wireless applications.

    Table 2. Comparison between some reconfigurable filter–antenna designs.

           A filter–antenna design with a reconfigurable frequency and bandwidth using an F-shaped feeding network is presented in [77]. The new feeding technique generates a multipath coupling scheme and provides the cross-coupling required to improve the out-of-band characteristics. Additionally, two varactor diodes are used and designed within the feeding network. The achieved performance shows that the proposed reconfigurable filter–antenna design has tunable frequency ranges from 2 GHz to 2.52 GHz, a fractional bandwidth that is tunable from 2.2% to 21.3%, a measured maximum gain of about 7.6 dBi, and a measured peak total efficiency of 85%. Figure 2 shows a photograph of the implemented reconfigurable filtering antenna design with simulated and measured reflection coefficients and boresight gain. Table 3 presents the performance comparisons between some recently published reconfigurable filtering antenna designs.

    Figure 2. The reconfigurable filtering antenna reproduced from [77]. 2020, IEEE: (a) prototype structure; (b) S-parameter performance; (c) boresight gain performance.

    Table 3. Performance comparisons between reconfigurable filter–antenna designs.

           It should be noted that considering both filter–antenna integration and reconfigurability properties at the same time will lead to some more advantages. However, this will also pose some challenges for both the biasing circuit and the structure configuration. In [48], two PIN diodes and four varactors are utilized in the basing circuit. Despite this configuration adding more complexity to the structure, it also results in a compact size and good performance in terms of the tuning range and the realized gain. It is also shown that wideband and tunable bandpass performance can be achieved by using the filter antenna integration design presented in [86]. This configuration has a high degree of freedom in terms of controlling the S-parameter characteristics and the radiation pattern behavior using a compact size structure. Thus, this makes the designed reconfigurable filter antenna a good candidate for current and future wireless applications

    3. Comparison between Switching Techniques

           The common types of reconfiguration techniques that can be utilized to implement reconfigurable structures are illustrated in Table 4 [87][88][89][90][91][92][93][94][95][96]. Structures based on RF-MEMS [88], PIN diodes [89], and varactors [90] that redirect their surface currents are called “electrically reconfigurable.” RF structures that use photoconductive configuration switch components are called “optically reconfigurable” [92]. Electronically reconfigurable or tunable elements are the best option when size and efficiency are required. However, the power handling capability and the lifetimes of these reconfiguration techniques cause some essential issues. PIN diodes operate in two configurations. The “on” state is where the diode is forward biased and the “off” state is where the diode is not biased or reverse-biased, while RF-MEMS uses mechanical movement to obtain a short circuit or an open circuit in the surface current path of RF elements. Unlike PIN diodes and RF MEMS, varactors can provide a continuous tuning range, with typical capacitance values range from tens to hundreds of picofarads. Moreover, unlike electrical reconfiguration, the photoconductive technique does not require the use of bias circuits and can be loaded in the RF PCB board without adding a complex design to modify the radiating elements. Additionally, the activation–deactivation mechanism for the switch does not create harmonic issues or intermodulation distortion. Conversely, in contrast with active switches, the optical switches are less common because of lossy characteristics and the need for complex activation approaches [96]. A description of the operation of the switches and comparisons between them are summarized in Table 4.

    Table 4. Comparison between switching techniques [90,91,92,93,94,95,96,97,98,99].

    4. Current Challenges and Future Developments

           Over the last few years, RF designers, researchers, and engineers have made a huge effort to explore reconfigurable filters and antennas and their integration as alternatives to the existing approaches and topologies, along with developing high-RF front end performance. Compared to the classical and passive filters and antennas, some essential challenges accompany the integrated and reconfigurable filters and antennas, which are efficient, compact, and multifunctional. Although recent researches show that microstrip planar configurations are capable of reducing the structure size, having the ability to produce a wider and flexible tuning range with low power and low loss is currently an important issue. As can be observed from the previous sections of this review, filter–antenna integration with reconfigurable characteristics requires a complex configuration, which can be considered as a common challenge for all reconfigurable transceivers. To overcome this challenge, some reconfigurable or tunable planar filters employing dual-mode ring resonators were introduced in [29][32][35]. Furthermore, the reconfigurable filter introduced in [36] has excellent performance in terms of the S-parameter group delay and S21 phase. Other features were also observed for this design, such as having a compact size, limited number of tuning diodes, and a simple structure.

           Additionally, the realization of reconfiguration approaches in RF and MW components improves the multifunctional performance of the entire system. In the literature, several studies have stated the importance of reconfiguration techniques. For instance, an E-shaped microstrip wideband antenna with polarization diversity was presented in [97] to work in the frequency range of 2.3 GHz to 2.6 GHz. In a similar way, radiation pattern reconfigurable wideband microstrip antennas are also introduced in [98][99] to operate in the spectrum ranges of 2.3 GHz to 2.55 GHz and 1.6 GHz to 4 GHz, respectively. As shown in these papers, the integration of slots, lumped elements, and surface mount components in the radiating patch penetrates the radiation pattern performance. To overcome these problems, several papers in the literature utilize the feed line of the antenna to achieve filtering performance with reconfigurable characteristics. Some of the recent research studies in the literature that apply this technique to obtain filtering performance include [61][62][63][64]. Additionally, a filter–antenna design with a reconfigurable frequency and bandwidth using an F-shaped feeding network was presented in [100]. This technique generates a multipath coupling scheme and provides the cross-coupling required to improve the out-of-band characteristics.

           Additionally, wideband filtering antenna designs are essential components of future wireless applications used to tackle high-speed and high data rate transmissions. For these designs, it is noticed that the size, insertion loss, and differential-mode bandwidth should also be taken into consideration and carefully investigated by the designers. Most of the introduced wideband and ultra-wideband filtering antenna configurations are designed based on a single-layer substrate. Therefore, it should be pointed out that using liquid crystal resonators and low-temperature co-fired ceramics can enhance the out-of-band rejection, thus improving and enhancing the performance of the wideband communication systems [101][102][103].

           Reconfigurable filtering antennas based on substrate-integrated waveguide (SIW) technology can also be used for mmWave and 5G wireless communications to provide lower losses, higher quality factors, and more power handling capability when compared with the other surveyed approaches [104]

    . Additionally, using these techniques offers some advantages, such as enhancing the bandwidth and reducing the losses and sizes of the configurations. According to what is shown in this review, the design technique proposed in [86] can also overcome the challenges facing these technologies by using only one single-layer, half-mode, substrate-integrated waveguide resonator loaded with four slot lines. Furthermore, and with as any RF or microwave element, reconfigurable filters and antennas and systems combining both of these can also be designed, analyzed, and optimized using artificial intelligence, neural networks, and bio-inspired optimization algorithms [105][106][107][108]. These approaches can be utilized for future reconfigurable structures, since these designs require more analysis and parameter studies than classical and passive configuration. Therefore, using these approaches in the future could lead to overcoming several issues and challenges by processing many variables at one time. It is anticipated that new design techniques with high efficiency and fully reconfigurable characteristics will be seen shortly.

    The entry is from 10.3390/electronics9081249

    References

    1. Hussaini, A.; Abdulraheem, Y.I.; Voudouris, K.N.; Mohammed, B.A.; Abd‐Alhameed, R.A.; Mohammed, H.J.; Elfergani, I.; Abdullah, A.S.; Makris, D.; Rodriguez, J.; et al. Green Flexible RF for 5G. In Fundamentals of 5G Mobile Networks, 1st ed.; Rodriguez, J., Ed.; John Wiley and Sons: Hoboken, NJ, USA, 2015.
    2. Al-Yasir, Y.I.A.; Tu, Y.; Bakr, M.S.; Parchin, N.O.; Asharaa, A.S.; Mshwat, W.A.; Abd-Alhameed, R.A.; Noras, J.M. Design of multi-standard single/tri/quint-wideband asymmetric stepped-impedance resonator filters with adjustable TZs. IET Microw. Antennas Propag. 2019, 13, 1637–1645.
    3. Liu, H.; Ren, B.P.; Li, S.; Guan, X.H.; Wen, P.; Peng, X.X.Y. High-Temperature Superconducting Bandpass Filter Using Asymmetric Stepped-Impedance Resonators with Wide-Stopband Performance. IEEE Trans. Appl. Supercond. 2015, 25, 1–6.
    4. Al-Yasir, Y.I.A.; Tu, Y.; Parchin, N.O.; Abdulkhaleq, A.; Kosha, J.; Ullah, A.; Abd-Alhameed, R.; Noras, J. New Multi-Standard Dual-Wideband and Quad-Wideband Asymmetric Step Impedance Resonator Filters with Wide Stop Band Restriction. Int. J. RF Microw. Comput. Aided Eng. 2019, 29, 1–17.
    5. Tu, Y.; Guo, X.R.; Wang, C.H.; Jin, J. An improved 860–960 MHz fully integrated CMOS power amplifier designation for UHF RFID transmitter. Int. J. Electron. Commun. 2013, 67, 574–577.
    6. Ghouz, H.H.M.; Sree, M.F.A.; Ibrahim, M.A. Novel Wideband Microstrip Monopole Antenna Designs for WiFi/LTE/WiMax Devices. IEEE Access 2020, 8, 9532–9539.
    7. Lu, J.; Zhang, H.C.; He, P.H.; Zhang, L.P.; Cui, T.J. Design of Miniaturized Antenna Using Corrugated Microstrip. IEEE Trans. Antennas Propag. 2020, 68, 1918–1924.
    8. Ogurtsov, S.; Koziel, S. A Conformal Circularly Polarized Series-Fed Microstrip Antenna Array Design. IEEE Trans. Antennas Propag. 2020, 68, 873–881.
    9. Al-Yasir, Y.I.A.; Alkhafaji, M.K.; A. Alhamadani, H.; Ojaroudi Parchin, N.; Elfergani, I.; Saleh, A.L.; Rodriguez, J.; Abd-Alhameed, R.A. A New and Compact Wide-Band Microstrip Filter-Antenna Design for 2.4 GHz ISM Band and 4G Applications. Electronics 2020, 9, 1084.
    10. Hilt, A. Availability and Fade Margin Calculations for 5G Microwave and Millimeter-Wave Anyhaul Links. Appl. Sci. 2019, 9, 5240.
    11. Moghaddasi, J.; Wu, K. Multifunctional Transceiver for Future Radar Sensing and Radio Communicating Data-Fusion Platform. IEEE Access 2016, 4, 818–838.
    12. Hong, J.-S.; Lancaster, M.J. Microstrip Filters for RF/Microwave Applications; John Wiley and Sons: Hoboken, NJ, USA, 2004; Volume 167.
    13. Richard, J.C.; Chandra, M.K.; Raafat, R.M. Microwave Filters for Communication Systems Fundamentals, Design, and Applications; John Wiley and Sons: Hoboken, NJ, USA, 2017.
    14. Ian, H. Theory and Design of Microwave Filters; IET Electromagnetic Waves Series 48; IET: London, UK, 2006.
    15. Statement: Improving Consumer Access to Mobile Services at 3.6 GHz to 3.8 GHz. Available online: https://www.ofcom.org.uk/consultations-and-statements/category-1/future-use-at-3.6-3.8-ghz (accessed on 21 October 2018).
    16. Al-Yasir, Y.I.A.; Ojaroudi Parchin, N.; Abdulkhaleq, A.M.; Bakr, M.S.; Abd-Alhameed, R.A. A Survey of Differential-Fed Microstrip Bandpass Filters: Recent Techniques and Challenges. Sensors 2020, 20, 2356.
    17. Hou, Z.; Liu, C.; Zhang, B.; Song, R.; Wu, Z.; Zhang, J.; He, D. Dual-/Tri-Wideband Bandpass Filter with High Selectivity and Adjustable Passband for 5G Mid-Band Mobile Communications. Electronics 2020, 9, 205.
    18. Guan, Y.; Wu, Y.; Tentzeris, M.M. A Bidirectional Absorptive Common-Mode Filter Based on Interdigitated Microstrip Coupled Lines for 5G “Green” Communications. IEEE Access 2020, 8, 20759–20769.
    19. Al-Yasir, Y.I.A.; Parchin, N.O.; Abdulkhaleq, A.; Hameed, K.; Al-Sadoon, M.; Abd-Alhameed, R. Design, Simulation and Implementation of Very Compact Dual-band Microstrip Bandpass Filter for 4G and 5G Applications. In Proceedings of the 16th International Conference on Synthesis, Modeling, Analysis and Simulation Methods and Applications to Circuit Design (SMACD), Lausanne, Switzerland, 15–18 July 2019; pp. 41–44.
    20. Al-Yasir, Y.I.A.; Parchin, N.O.; Alabdallah, A.; Abdulkhaleq, A.M.; Sajedin, M.; Elfergani, I.T.E.; Abd-Alhameed, R.A. Design, Simulation and Implementation of Very Compact Open-loop Trisection BPF for 5G Communications. In Proceedings of the 2019 IEEE 2nd 5G World Forum (5GWF), Dresden, Germany, 30 September–2 October 2019; pp. 189–193.
    21. David, M.P. Microwave Engineering; John Wiley and Sons: Hoboken, NJ, USA, 2012.
    22. Hotopan, R.; Cos, M.; Las-Heras, F. Reduced size C-band bandpass filter with 2nd harmonic suppression. In Proceedings of the 8th European Conference on Antennas and Propagation (EuCAP), The Hague, The Netherlands, 6–11 April 2014; pp. 971–974.
    23. Ghatak, R.; Sarkar, P.; Mishra, R.K.; Poddar, D.R. A Compact UWB Bandpass Filter With Embedded SIR as Band Notch Structure. IEEE Microw. Wirel. Compon. Lett. 2011, 21, 261–263.
    24. Liu, H.; Liu, T.; Zhang, Q.; Ren, B.; Wen, P. Compact Balanced Bandpass Filter Design Using Asymmetric SIR Pairs and Spoof Surface Plasmon Polariton Feeding Structure. IEEE Microw. Wirel. Compon. Lett. 2018, 28, 987–989.
    25. Yuceer, M. A Reconfigurable Microwave Combline Filter. IEEE Trans. Circuits Syst. II Express Briefs 2016, 63, 84–88.
    26. Cho, Y.; Baek, H.; Lee, H.; Yun, S. A Dual-Band Combline Bandpass Filter Loaded by Lumped Series Resonators. IEEE Microw. Wirel. Compon. Lett. 2009, 19, 626–628.
    27. Velez, P.; Naqui, J.; Fernandez-Prieto, A.; Duran-Sindreu, M.; Bonache, J.; Martel, J.; Medina, F.; Martin, F. Differential Bandpass Filter With Common-Mode Suppression Based on Open Split Ring Resonators and Open Complementary Split Ring Resonators. IEEE Microw. Wirel. Compon. Lett. 2013, 23, 22–24.
    28. Al-Yasir, Y.I.A.; Parchin, N.O.; Abdulkhaleq, A.; Abd-Alhameed, R.; Noras, J. Recent Progress in the Design of 4G/5G Reconfigurable Filters. Electronics 2019, 8, 1–17.
    29. Tang, W.; Hong, J. Varactor-Tuned Dual-Mode Bandpass Filters. IEEE Trans. Microw. Theory Tech. 2010, 58, 2213–2219.
    30. Long, J.; Li Cui, C.; Huangfu, J.; Ran, L. A Tunable Microstrip Bandpass Filter With Two Independently Adjustable Transmission Zeros. IEEE Microw. Wirel. Compon. Lett. 2011, 21, 74–76.
    31. Al-Yasir, Y.I.A.; Parchin, N.O.; Alabdallah, A.; Abdulkhaleq, A.M.; Abd-Alhameed, R.A.; Noras, J.M. Design of Bandpass Tunable Filter for Green Flexible RF for 5G. In Proceedings of the 2019 IEEE 2nd 5G World Forum (5GWF), Dresden, Germany, 30 September–2 October 2019.
    32. Ebrahimi, A.; Baum, T.; Scott, J.; Ghorbani, K. Continuously Tunable Dual-Mode Bandstop Filter. IEEE Microw. Wirel. Compon. Lett. 2018, 28, 419–421.
    33. Chen, C.; Wang, G.; Li, J. Microstrip Switchable and Fully Tunable Bandpass Filter With Continuous Frequency Tuning Range. IEEE Microw. Wirel. Compon. Lett. 2018, 28, 500–502.
    34. Chen, F.; Li, R.; Chen, J. Tunable Dual-Band Bandpass-to-Bandstop Filter Using p-i-n Diodes and Varactors. IEEE Access 2018, 6, 46058–46065.
    35. Lu, D.; Tang, X.; Barker, N.; Feng, Y. Single-Band and Switchable Dual-/Single-Band Tunable BPFs With Predefined Tuning Range, Bandwidth, and Selectivity. IEEE Microw. Wirel. Compon. Lett. 2018, 66, 1215–1227.
    36. Al-Yasir, Y.; Parchin, N.O.; Rachman, Z.-A.S.A.; Ullah, A.; Abd-Alhameed, R. Compact tunable microstrip filter with wide-stopband restriction and wide tuning range for 4G and 5G applications. In Proceedings of the IET’s Antennas and Propagation Conference, Birmingham, UK, 11–12 November 2019; pp. 1–6.
    37. Iqbal, A.; Tiang, J.J.; Lee, C.K.; Mallat, N.K.; Wong, S.W. Dual-Band Half Mode Substrate Integrated Waveguide Filter With Independently Tunable Bands. IEEE Trans. Circuits Syst. II Express Briefs 2020, 67, 285–289.
    38. Bernhard, J. Reconfigurable Antennas; Morgan and Claypool: San Rafael, CA, USA, 2007.
    39. Abdulraheem, Y.I.; Abdullah, A.; Mohammed, H.; Abd-Alhameed, R.; Noras, J. Design of Frequency-reconfigurable Multiband Compact Antenna using two PIN diodes for WLAN/WiMAX Applications. IET Microw. Antennas Propag. 2017, 11, 1098–1105.
    40. Al-Yasir, Y.; Abdullah, A.; Ojaroudi Parchin, N.; Abd-Alhameed, R.; Noras, J. A New Polarization-Reconfigurable Antenna for 5G Applications. Electronics 2018, 7, 293.
    41. Al-Yasir, Y.I.A.; Abdullah, A.; Mohammed, H.; Mohammed, B.; Abd-Alhameed, R. Design of Radiation Pattern-Reconfigurable 60-GHz Antenna for 5G Applications. J. Telecommun. 2014, 27, 1–6.
    42. Peroulis, D.; Sarabandi, K.; Katehi, L.P.B. Design of reconfigurable slot antennas. IEEE Trans. Antennas Propag. 2005, 53, 645–654.
    43. Panagamuwa, C.J.; Chauraya, A.; Vardaxoglou, J.C. Frequency and beam reconfigurable antenna using photoconducting switches. IEEE Trans. Antennas Propag. 2006, 54, 449–454.
    44. Yasir, I.A.A.; Hasanain, A.H.A.; Baha, A.S.; Parchin, N.O.; Ahmed, M.A.; Abdulkareem, S.A.; Raed, A.A. New Radiation Pattern-Reconfigurable 60-GHz Antenna for 5G Communications. Available online: https://www.intechopen.com/online-first/new-radiation-pattern-reconfigurable-60-ghz-antenna-for-5g-communications (accessed on 26 September 2019).
    45. Yang, S.-L.S.; Kishkand, A.A.; Lee, K.-F. Frequency Reconfigurable U-Slot Microstrip Patch Antenna. IEEE Antennas Wirel. Propag. Lett. 2008, 7, 127–129.
    46. Valkonen, R.; Luxey, C.; Holopainen, J.; Icheln, C. Frequency-reconfigurable mobile terminal antenna with MEMS switches. In Proceedings of the Fourth European Conference on Antennas and Propagation, Barcelona, Spain, 12–16 April 2010; pp. 1–5.
    47. Yu, Y.; Xiong, J.; Li, H.; He, S. An Electrically Small Frequency Reconfigurable Antenna with a Wide Tuning Range. IEEE Antennas Wirel. Propag. Lett. 2011, 10, 103–106.
    48. Majid, H.A.; Rahim, M.K.A.; Hamid, M.R.; Ismail, M.F. A Compact Frequency-Reconfigurable Narrowband Microstrip Slot Antenna. IEEE Antennas Wirel. Propag. Lett. 2012, 11, 616–619.
    49. Majid, H.A.; Rahim, M.K.A.; Hamid, M.R.; Murad, N.A.; Ismail, M.F. Frequency-Reconfigurable Microstrip Patch-Slot Antenna. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 218–220.
    50. Jin, G.; Deng, C.; Xu, Y.; Yang, J.; Liao, S. Differential Frequency-Reconfigurable Antenna Based on Dipoles for Sub-6 GHz 5G and WLAN Applications. IEEE Antennas Wirel. Propag. Lett. 2020, 19, 472–476.
    51. Al-Yasir, Y.I.A.; Parchin, N.O.; Elfergani, I.; Abd-Alhameed, R.A.; Noras, J.M.; Rodriguez, J.; Al-jzari, A.; Hammed, W.I. A New Polarization-Reconfigurable Antenna for 5G Wireless Communications. In Broadband Communications, Networks, and Systems. BROADNETS 2018. Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering; Sucasas, V., Mantas, G., Althunibat, S., Eds.; Springer: Cham, Germany, 2019; Volume 263.
    52. Al-Yasir, Y.I.A.A.; JaroudiParchin, N.O.; Alabdullah, A.; Mshwat, W.; Ullah, A.; Abd-Alhameed, R. New Pattern Reconfigurable Circular Disk Antenna Using Two PIN Diodes for WiMax/WiFi (IEEE 802.11 a) Applications. In Proceedings of the 2019 16th International Conference on Synthesis, Modeling, Analysis and Simulation Methods and Applications to Circuit Design (SMACD), Lausanne, Switzerland, 15–18 July 2019; pp. 53–56.
    53. Khan, A.; Nema, R. Analysis of five different dielectric substrates on microstrip patch antenna. Int. J. Comput. Appl. 2012, 55, 44–47.
    54. Chuang, C.-T.; Chung, S.-J. A new compact filtering antenna using defected ground resonator. In 2010 Asia-Pacific Microwave Conference (APMC); IEEE: Piscataway Township, NJ, USA, 2010; pp. 1003–1006.
    55. Coonrod, J. Choosing Circuit Materials for Millimeter-Wave Applications. High Freq. Electron. 2013, 1, 22–30.
    56. Cui, J.; Zhang, A.; Yan, S. Co‐design of a filtering antenna based on multilayer structure. Int. J. RF Microw. Comput. Aided Eng. 2020; 30, e22096, doi:10.1002/mmce.22096.
    57. Hua, C.; Liu, M.; Lu, Y. Planar integrated substrate integrated waveguide circularly polarized filtering antenna. Int. J. RF Microw. Comput. Aided Eng. 2019; 29, e21517, doi:10.1002/mmce.21517.
    58. Niu, B.‐J.; Tan, J.‐H. Dipole filtering antenna with quasi‐elliptic peak gain response using parasitic elements. Microw. Opt. Technol. Lett. 2019, 61, 1612–1616, doi:10.1002/mop.31848.
    59. Park, J.; Jeong, M.; Hussain, N.; Rhee, S.; Park, S.; Kim, N. A low‐profile high‐gain filtering antenna for fifth generation systems based on nonuniform metasurface. Microw. Opt. Technol. Lett. 2019, 61, 2513–2519, doi:10.1002/mop.31931.
    60. Al-Yasir, Y.I.A.; Parchin, N.O.; Abd-Alhameed, R.A. A Differential-Fed Dual-Polarized High-Gain Filtering Antenna Based on SIW Technology for 5G Applications. In Proceedings of the 14th European Conference on Antennas and Propagation (EuCAP), Copenhagen, Denmark, 15–20 March 2020; pp. 1–5.
    61. Song, L.; Wu, B.; Xu, M.; Su, T.; Lin, L. Wideband balun filtering quasi‐Yagi antenna with high selectivity. Microw. Opt. Technol. Lett. 2019, 61, 2336–2341, doi:10.1002/mop.31903.
    62. Wu, W.; Fan, R.; Wang, J.; Zhang, Q. A broadband low profile microstrip filter-antenna with an omni-directional pattern. In Proceedings of the 2014 3rd Asia-Pacific Conference on Antennas and Propagation, Harbin, China, 26–29 July 2014; pp. 580–582.
    63. Lin, C.; Chung, S. A Compact Filtering Microstrip Antenna with Quasi-Elliptic Broadside Antenna Gain Response. IEEE Antennas Wirel. Propag. Lett. 2011, 10, 381–384.
    64. Wu, W.; Ma, B.; Wang, J.; Wang, C. Design of a microstrip antenna with filtering characteristics for wireless communication systems. In Proceedings of the 2017 Sixth Asia-Pacific Conference on Antennas and Propagation (APCAP), Xi'an, China, 16–19 October 2017; pp. 1–3.
    65. Al-Yasir, Y.I.A.; Parchin, N.O.; Abd-Alhameed, R.A. New High-Gain Differential-Fed Dual-Polarized Filtering Microstrip Antenna for 5G Applications. In Proceedings of the 14th European Conference on Antennas and Propagation (EuCAP), Copenhagen, Denmark, 15–20 March 2020; pp. 1–5.
    66. Wu, W.; Yin, Y.; Zuo, S.; Zhang, Z.; Xie, J. A New Compact Filter-Antenna for Modern Wireless Communication Systems. IEEE Antennas Wirel. Propag. Lett. 2011, 10, 1131–1134.
    67. Ohira, M.; Ma, Z. An efficient design method of microstrip filtering antenna suitable for circuit synthesis theory of microwave band-pass filters. In Proceedings of the 2015 International Symposium on Antennas and Propagation (ISAP), Hobart, Australia, 9–12 November 2015; pp. 1–4.
    68. Al-Yasir, Y.I.A.; Alhamadani, H.A.; Kadhim, A.S.; Ojaroudi Parchin, N.; Saleh, A.L.; Elfergani, I.T.E.; Rodriguez, J.; Abd-Alhameed, R.A. Design of a Wide-Band Microstrip Filtering Antenna with Modified Shaped Slots and SIR Structure. Inventions 2020, 5, 11.
    69. Abdel-Jabbar, H.; Kadhim, A.S.; Saleh, A.L.; Al-Yasir, Y.I.A.; Parchin, N.O.; Abd-Alhameed, R.A. Design and optimization of microstrip filtering antenna with modified shaped slots and SIR filter to improve the impedance bandwidth. TELKOMNIKA 2020, 18, 515–545.
    70. Mohammed, K.A.; Alhamadani, A.; Al-Yasir, Y.I.A.; Saleh, A.L.; Parchin, N.O.; Abd-Alhameed, R. Study on the effect of the substrate material type and thickness on the performance of the filtering antenna design. TELKOMNIKA 2020, 18, 72–79.
    71. Yang, D.; Zhai, H.; Guo, C.; Li, H. A Compact Single-Layer Wideband Microstrip Antenna with Filtering Performance. IEEE Antennas Wirel. Propag. Lett. 2020, 19, 801–805.
    72. Tawk, Y.; Costantine, J.; Christodoulou, C.G. A varactor-based reconfigurable filtenna, IEEE Antennas Wirel. Propag. Lett. 2012, 11, 716–719.
    73. Soltanpour, M.; Fakharian, M.M. Compact filtering slot antenna with frequency agility for Wi-Fi/LTE mobile applications. Electron. Lett. 2016, 52, 491–492.
    74. Augustin, G.; Chacko, B.P.; Denidni, T.A. Electronically reconfigurable uni-planar antenna for cognitive radio applications. Microw. Antennas Propag. 2014, 8, 367–376.
    75. Deng, J.; Hou, S.; Zhao, L.; Guo, L. Wideband-to-narrowband tunable monopole antenna with integrated band-pass filters for UWB/WLAN applications. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 2734–2737.
    76. Kingsly, S.; Thangarasu, D.; Alsath, M.G.N.; Thipparaju, R.R.; Palaniswamy, S.K.; Sambandam, P. Multiband Reconfigurable Filtering Monopole Antenna for Cognitive Radio Applications. IEEE Antennas Wirel. Propag. Lett. 2018, 17, 1416–1420.
    77. Hu, P.F.; Pan, Y.M.; Zhang, X.Y.; Hu, B. A Filtering Patch Antenna with Reconfigurable Frequency and Bandwidth Using F-Shaped Probe. IEEE Trans. Antennas Propag. 2019, 67, 121–130.
    78. Rodrigues, L.; Varum, T.; Matos, J.N. The Application of Reconfigurable Filtennas in Mobile Satellite Terminals. IEEE Access 2020, 8, 77179–77187.
    79. Mahmoud, K.R.; Montaser, A.M. Design of Compact mm-wave Tunable Filtenna Using Capacitor Loaded Trapezoid Slots in Ground Plane for 5G Router Applications. IEEE Access 2020, 8, 27715–27723.
    80. Bi, X.; Zhang, X.; Wong, S.; Guo, S.; Yuan, T. Design of Notched-Wideband Bandpass Filters with Reconfigurable Bandwidth Based on Terminated Cross-Shaped Resonators. IEEE Access 2020, 8, 37416–37427.
    81. Qin, P.-Y.; Wei, F.; Guo, Y.J. A wideband-to-narrowband tunable antenna using a reconfigurable filter. IEEE Trans. Antennas Propag. 2015, 63, 2282–2285.
    82. Tang, M.-C.; Wen, Z.; Wang, H.; Li, M.; Ziolkowski, R.W. Compact, frequency-reconfigurable filtenna with sharply defined wideband and continuously tunable narrowband states. IEEE Trans. Antennas Propag. 2017, 65, 5026–5034.
    83. Horestani, A.K.; Shaterian, Z.; Naqui, J.; Martín, F.; Fumeaux, C. Reconfigurable and tunable S-shaped split-ring resonators and application in band-notched UWB antennas. IEEE Trans. Antennas Propag. 2016, 64, 3766–3776.
    84. Malakooti, S.-A.; Mousavi, S.M.H.; Fumeaux, C. Tunable bandpassto-bandstop quasi-Yagi–Uda antenna with sum and difference radiation patterns. IEEE Trans. Antennas Propag. 2019, 67, 2260–2271.
    85. Yang, X.; Lin, H.; Gu, H.; Ge, L.; Zeng, X. Broadband pattern diversity patch antenna with switchable feeding network. IEEE Access 2018, 6, 69612–69619.
    86. Yassin, M.E.; Mohamed, H.A.; Abdallah, E.A.F.; El-Hennawy, H.S. Circularly Polarized Wideband-to-Narrowband Switchable Antenna. IEEE Access 2019, 7, 36010–36018.
    87. Rohde, U.L.; Newkirk, D.P. RF/Microwave Circuit Design for Wireless Applications; Wiley: New York, NY, USA, 2000.
    88. Cetiner, B.A.; Crusats, G.R.; Jofre, L.; Biyikli, N. RF MEMS integrated frequency reconfigurable annular slot antenna. IEEE Trans. Antennas Propag. 2010, 58, 626–632.
    89. Chen, R.-H.; Row, J.-S. Sing-fed microstrip patch antenna with switchable polarization. IEEE Trans. Antennas Propag. 2008, 56, 922–926.
    90. Behdad, N.; Sarabandi, K. A varactor-tuned dual-band slot antenna. IEEE Trans. Antennas Propag. 2006, 54, 401–408.
    91. Al-Yasir, Y.; Abd-Alhameed, R.A.; Noras, J.M.; Abdulkhaleq, A.M.; Ojaroudi, N. Design of Very Compact Combline Band-Pass Filter for 5G Applications. In Proceedings of the Loughborough Antennas & Propagation Conference (LAPC), Loughborough, UK, 12–13 November 2018; pp. 1–4.
    92. Zhao, D.; Lan, L.; Han, Y.; Liang, F.; Zhang, Q.; Wang, B.Z. Optically controlled reconfigurable band-notched UWB antenna for cognitive radio applications. IEEE Photon. Technol. Lett. 2014, 26, 2173–2176.
    93. Parchin, N.O.; Al-Yasir, Y.I.A.; Abd-Alhameed, R.A. Microwave/RF Components for 5G Front-End Systems; AVID SCIENCE: Telangana, India, 2020.
    94. Chen, Z.; Wong, H.; Kelly, J. A polarization-reconfigurable glass dielectric resonator antenna using liquid metal. IEEE Trans. Antennas Propag. 2019, 67, 3427–3432.
    95. Al-Yasir, Y.I.A.; Tu, Y.; Parchin, N.O.; Elfergani, I.; Abd-Alhameed, R.; Rodriguez, J.; Noras, J. Mixed-coupling multi-function quint-wideband asymmetric stepped impedance resonator filter. Microw. Opt. Tech. Lett. 2019, 61, 1181–1184.
    96. Zheng, S.H.; Liu, X.; Tentzeris, M.M. Optically controlled reconfigurable band-notched UWB antenna for cognitive radio systems. Electron. Lett. 2014, 50, 1502–1504.
    97. Ullah, U.; Koziel, S.; Mabrouk, I.B. Rapid Redesign and Bandwidth/Size Tradeoffs for Compact Wideband Circular Polarization Antennas Using Inverse Surrogates and Fast EM-Based Parameter Tuning. IEEE Trans. Antennas Propag. 2020, 68, 81–89.
    98. Row, J.; Kuo, L. Pattern-Reconfigurable Array Based on a Circularly Polarized Antenna with Broadband Operation and High Front-to-Back Ratio. IEEE Trans. Antennas Propag. 2020, 68, 4109–4113.
    99. Wu, Z.; Tang, M.; Li, M.; Ziolkowski, R.W. Ultralow-Profile, Electrically Small, Pattern-Reconfigurable Metamaterial-Inspired Huygens Dipole Antenna. IEEE Trans. Antennas Propag. 2020, 68, 1238–1248.
    100. Li, Y.; Zhao, Z.; Tang, Z.; Yin, Y. Differentially Fed, Dual-Band Dual-Polarized Filtering Antenna with High Selectivity for 5G Sub-6 GHz Base Station Applications. IEEE Trans. Antennas Propag. 2020, 68, 3231–3236.
    101. Hao, Z.; Hong, J. Quasi-Elliptic UWB Bandpass Filter Using Multilayer Liquid Crystal Polymer Technology. IEEE Microw. Wirel. Compon. Lett. 2010, 20, 202–204.
    102. Zhou, P.; Li, B.; Lu, H.; Shi, Y.; Tang, W. Novel Ultrawideband and Multimode LTCC Common-Mode Filter Based on the Dual Vertical Coupling Paths. IEEE Trans. Compon. Packag. Manuf. Technol. 2019, 9, 1345–1353.
    103. Shen, G.; Che, W. Compact Ku-band LTCC bandpass filter using folded dual-composite right- and left-handed resonators. Electron. Lett. 2020, 56, 17–19.
    104. Shen, Z.; Xu, K.; Shi, J. Compact single-layer bandwidth-enhanced balanced bandpass filter using half-mode substrate-integrated waveguide. Electron. Lett. 2019, 55, 697–699.
    105. Sans, M.; Selga, J.; Vélez, P.; Bonache, J.; Martín, F.; Rodríguez, A.; Boria, V.E. Optimized wideband differential-mode bandpass filters with broad stopband and common-mode suppression based on multi-section stepped impedance resonators and interdigital capacitors. In Proceedings of the IEEE MTT-S International Conference on Numerical Electromagnetic and Multiphysics Modeling and Optimization for RF, Microwave, and Terahertz Applications (NEMO), Seville, Spain, 17–19 May 2017; pp. 10–12.
    106. Jamshidi, M.; Lalbakhsh, A.; Lotfi, S.; Siahkamari, H.; Mohamadzade, B.; Jalilian, J. A neuro-based approach to designing a Wilkinson power divider. Int. J. RF Microw. Comput. Aided Eng. 2020, 30, 1–10.
    107. Mohammed, H.J.; Abdulsalam, F.; Abdulla, A.S.; Ali, R.S.; Abd-Alhameed, R.A.; Noras, J.M.; Abdulraheem, Y.I.; Ali, A.; Rodriguez, J.; Abdalla, A.M. Evaluation of genetic algorithms, particle swarm optimisation, and firefly algorithms in antenna design. In Proceedings of the 13th International Conference on Synthesis, Modeling, Analysis and Simulation Methods and Applications to Circuit Design (SMACD), Lisbon, Portugal, 27–30 June 2016; pp. 1–4.
    108. Mohammed, H.J.; Abdullah, A.S.; Ali, R.S.; Abd-Alhameed, R.A.; Abdulraheem, Y.I.; Noras, J.M. Design of a uniplanar printed triple band-rejected ultra-wideband antenna using particle swarm optimisation and the firefly algorithm. IET Microw. Antennas Propag. 2016, 10, 31–37.
    More
    1. Please check and comment entries here.