1. Introduction

The 5^th generation new radio (5G NR) communication network systems demand high data rate, wide bandwidth, and reliable connection throughout the band spectrum. These features can be achieved in the antenna systems with multiple frequency transformations and also with the beam shifting phenomenon. Therefore, reconfigurability will be the significant tool to operate the antenna on multiple frequencies and in multiple directions. An antenna that can dynamically change its frequency and radiation characteristics in a controlled and reversible way is called reconfigurable antenna (RA). Therefore, in general, three parameters, namely, frequency, radiation pattern, and polarization, can be reconfigured or the combination of these three can be induced to achieve the high degree of reconfigurability. These alterations in the parameters can be achieved by disturbing the current and variations in the electromagnetic (EM) fields on the radiating surface. In addition to this, some of the new features to be added in the RAs include software and cognitive radio to match with the reconfigurable multistranded and multifunctional operation as well as to maintain the performance characteristics with excellent utilization of spectrum. The RAs are more significant during the past decade and the structure of these antennas can be introduced with various techniques as shown in Figure 1 which describes the four methods to reconfigure the antennas based on the different applications and size of the proposed design. For instance, in case of physical transformation, the antenna size will be significantly large and complex in structure which cannot be feasible for integration into compact devices. Therefore, other methods have been adopted like optical switches, material used, and electrical switching. Among these methods, electrical switching is widely used and popular among the researchers due to the miniature size of the switches and comfortable integration in the compact and low-profile antenna structures.

However, there are certain following challenges in the design of RA. (a) To achieve application-specific reconfigurable parameter (frequency, pattern, and polarization). (b) To optimize dimensional parameters in order to achieve specified reconfigurability. (c) The performance of the antenna should not vary significantly while reconfiguring. There are several advantages of using the RAs which can accommodate multiple wireless spectrums with minimum cost and resources. It also minimizes the requirement of front-end filtering and possesses multifunctional capabilities. Along with the advantages there are some limitations and these include the nonlinearity effects of the switches as well as the losses, interference, and adverse effect of the biasing lines that are utilized to regulate the switching components’ states on the radiation pattern of the antenna. Despite the many problems with such reconfiguration techniques, antenna researchers have been drawn to this sort of RAs due to the ease of integration of such switching elements into the antenna structure.

2. RAs Using Radio Frequency Micro-Electromechanical System (RF-MEMS) Switches

RF-MEMS switches are one of the most widely used components to configure the frequency, pattern, and polarization. A number of researchers have already used this method to achieve the reconfigurability of the antennas. For instance, in [1], the authors designed a reconfigurable microstrip patch antenna in the form of a hexagon. The radiating length of the antenna may be altered with the use of RF-MEMS switches. As a result, the antenna’s frequency was changed from 8.56 to 8.90 GHz. Thus, the antenna may be modified to operate at various frequencies by turning on or off RF-MEMS switches. Body area networks (BANs) are one use case of the antenna. Moreover, in order to cover the whole long-term evolution (LTE) spectrum, with an emphasis on the 600 MHz band, the authors in [2] suggested the designing of an adjustable antenna array. Four multiband antennas are mounted on the frame of the suggested design, and MEMS adjustable capacitors are included to reduce the inherent resonance frequency up to the targeted value. The virtualized component performs well in terms of bandwidth and reciprocal coupling, which keeps the distance between the radiating parts at a manageable level. The power network analyzer (PNA) was used to evaluate the tunability once the construction was produced. The findings showed that three tuning conditions were required for the antenna system to report a very wide bandwidth of 6 dB and resonance at the beginning of the 71 and 32 MHz bands. The same actions were seen in the top and intermediate bands. According to the tests made in the MVG SG24, the overall efficiency is −5.3 dB at 615 MHz, which corresponds to market-available mobile phone frequencies. Furthermore, when comparing performance in free space to the user effects study, which was done using simulations in both conversation mode and data, it was found that the user hand and user hand-head, respectively, contributed to an average reduction in overall efficiency of 4 and 8 dB. The authors in [3] presented the design and manufacturing process of a Ku-band reconfigurable patch antenna. For the patch to have higher RF capabilities, both bulk and surface micromachining methods were used. Shunt capacitive switches and microstrip patch antennas were integrated to provide reconfigurability. This article also highlights the impact of feed mechanisms and the function of micromachining on antenna characteristics. The simulation was performed using a commercially available finite element method (FEM)–based EM solver on a 675 μm high resistive Si substrate. For the designed antenna, a frequency drift of 200 MHz was attained at Ku-band. Moreover, RF-MEMS switch–integrated reconfigurable patch antenna has been described in [4] and is shown in Figure 2. At the chosen dual frequencies of 4.8 and 7.6 GHz, the suggested antenna emitted a right-hand circularly polarized wave with a high frequency ratio of 1.6:1. To regulate the operating frequencies, the switches were integrated into the square patch that was diagonally fed. The suggested antenna offers gain between 5 and 6 dB at both operational frequencies and meets the 3 dB axial ratio (AR) requirement. An approach to adjusting an antenna array without changing the individual components has been represented by the authors in [5].

The array is made up of planar radiating elements that may be joined together to create bigger elements that radiate at frequencies lower than the resonance frequency of their individual components using computer-controlled MEMS switches by means of capacitive RF shorts. The concept’s feasibility was shown by simulations with a 2 × 2 array of patch antennas linked by either RF shorts or hard shorts. The simulated arrays were created, and the measured array attributes matched the findings rather well. The individual element’s radiation pattern was preserved despite the resonance frequency being reduced by half using both shorting techniques. This paper [6] demonstrates the construction and modeling of a tunable antenna that has MEMS switches mounted on it and is loaded with a coplanar waveguide (CPW) stub. To achieve tunable performance usage of monolithic integration of MEMS switches, the antenna has been constructed on a silicon substrate with an air cavity between the patch and ground. To provide a suitable signal transition between the patch antenna’s ground plane and the CPW ground, the CPW ground’s dimensions are increased. A frequency shift of 0.28, 1.6, 0.4, and 1.97 GHz is provided when the antenna shifts from an up to a down state when loaded with one, two, three, and four switches, respectively, as shown in Figure 3. This study showed that by adjusting the capacitive gap and amount of reconfiguration boosted by adding more switches, it is controllable in terms of resonant frequency, bandwidth, and radiation pattern. A unique wideband E-shaped patch antenna with frequency reconfigurability is shown in this study. The suggested layout may be used to advance the performance of bigger terminals or access points that employ patch antennas, including base station antennas or laptop antennas. Particle swarm optimization (PSO) was used to verify the suggested notion of frequency reconfigurability for E-shaped patch antennas. The notion was proven with an original prototype that used perfect switches. A number of MEMS switch model modifications were shown, and the circuit model was selected based on its quick optimization time and high simulation accuracy [7]. An E-shaped patch antenna element is used in a unique broadband right-hand circular polarized/left-hand circular polarized (RHCP/LHCP) reconfigurable patch antenna array that is being studied. A difficult combined S11-AR bandwidth of 17% was attained by employing PSO and confirmed by measurement for the isolated element using MEMS switches at an overall substrate thickness of 0.092λ₀. Compared to the state of the art in single-layer, single-feed CP patch element designs with comparable substrate thickness, the realized bandwidth is much greater. Similar to other thick substrate CP patch antennas, a tiny portion of the higher frequency band exhibits a noticeable beam squint. A unique rotated-element design is used to induce pattern symmetry and overcome the beam squint [8]. Based on the microstrip patch, a novel complementary bifractal Sierpinski gasket antenna is created and coupled with MEMS switches for use in multiband wireless applications such as GPS. The investigation focused on a new kind of self-similar fractal antenna that uses a complementary Sierpinski gasket in relation to a square patch. DCS-1800, PCS-1900, IMT 2000/UMTS, and U-N11 were integrated. The antenna has advantageous features like efficiency and small size [9]. This paper included a demonstration of the dynamic reconfigurability of MEMS Vee antennas, which included beam steering and beam shaping capabilities. This demonstration is the first step toward integrated smart antenna systems, where the functionality and performance of the antenna can be dynamically modified and MEMS impedance tuners, MEMS passive components, active devices, and control circuits can be monolithically combined on the same chip [10]. Using MEMS switches on high resistive substrate (HRS), a reconfigurable dual band dipole antenna operating at 4.86 and 8.98 GHz is described. The substrate area is 1.6 cm²; however, if the dipole arms are folded, this may be further decreased. There was a frequency shift at both frequencies when MEMS switches are used. The bandwidth of the antenna at the lower and higher frequencies, including the MEMS switches, was 1.9% and 13.6%, respectively. At the lower band, the antenna radiation efficiency was 86%, while at the upper band, it was 94%. At the lower and higher frequencies, the dipole antenna’s directivity is 2 and 3 dB, respectively. The 3-dB beamwidth of the E-plane was broad, measuring 84° at low frequency and 60° at high frequency [11]. Research was done on the first self-similar antenna that used RF-MEMS switches. The remarkably multiband performance of the series ohmic contact cantilever RF-MEMS switches is made possible by the way the antenna’s patches are connected to one another. Additionally, more intricate self-similar antennas may be studied, such as high-iteration Sierpinski gaskets or other structures. The findings should demonstrate resonances in a variety of frequency bands with both directed and dipole-like emission patterns. Future research would examine how well this antenna performs for military or communication purposes throughout the whole 10–14 GHz spectrum [12].

This study presents two RAs using the ON/OFF states of MEMS switch. A fully printed inverted F antenna (IFA)-CPW feed antenna is the second one, while a typical metallic planar inverted F antenna (PIFA) is the first. By modeling the ON state of the MEMS switch with a tiny piece of copper, probant experimental verifications have been accomplished. These outcomes were contrasted with theoretical ones by using an RLC comparable electrical model in place of the MEMS in the CAD tool HPADS and moment unit. A good agreement between the simulated and experimental findings is obtained for both structures. Thus, it seems that the second fully printed structure is a better fit for more intricate Sniart incorporated Systems [13]. It is suggested to use a brand-new reconfigurable Hilbert curve patch antenna (RHCPA). The third-order Hilbert curve is used to figure out the patch. At least two antenna designs with distinct radiation patterns are created by adding a few slots. The radiation patterns are rearranged by the use of MEMS switches to flip the configurations. The outcomes of the simulation are provided. This antenna may be used in radar communications system [14]. A square bowtie microstrip antenna with square patches that can be adjusted for frequency and pattern has been introduced. With few switches, the antenna offers beneficial radiation and frequency characteristics. MEMS switches are now being included into scaled-down versions of this antenna. This device represented a new paradigm in the design of individual antenna elements, and by offering more design freedom for element radiation pattern and frequency response, it can significantly increase the performance of large phased arrays as well as portable wireless devices. In the future, more switching components would be added to provide more frequency-stable radiation patterns and to widen the operational frequency ranges [15].

Reconfiguring the size of an antenna using another patch that is linked to it by RF MEMES switches resulted in a reconfigurable dual frequency microstrip multi-input-multi-output (MIMO) patch antenna. Two antenna configurations have been used to examine the design: the open switch case resonates at 5.8 GHz, while the closed switch case resonates at 2.4 GHz. The developed antennas are inexpensive, simple to fabricate, and offer superior isolation [16]. In this research, the authors suggested a patch antenna construction that can be reconfigured by employing MEMS switches. With this idea, a single antenna construction may have numerous functions while taking up less area. The 2.4 GHz version of the microstrip patch antenna may be adjusted to operate at 5.5 or 7.5 GHz. Using MEMS switches, the antenna has been reconfigured. The results of the simulation demonstrate that the MEMS switch minimized the insertion and return losses. As a result, lossless transmission and reception have been achieved and antenna reconfiguration is possible without requiring physical modifications thanks to MEMS. Through simulations, the impact of the slots in microstrip antennas was confirmed [17]. The design, construction, and testing of a tunable PIFA are presented in this study and shown in Figure 4. The electrical length and antenna behavior of the proposed PIFA antenna may be adjusted using RF-MEMS switches. According to measurements, the tuning range is between 1.4 and 1.1 GHz, depending on the MEMS switches’ condition. Over the whole tuning range, a match of better than −9.5 dB down to −19 dB was achieved. Using an innovative adjacent channel power ratio (ACPR) measuring system, the antenna has also been tested against power handling, and the results demonstrate good linearity up to 40 dBm [18].

This work examines the effects of a reconfigurable frequency selective surface (FSS) with capacitive MEMS switches on the scanning and scattering properties of an antenna array made up of X-band rectangular open ends. It is shown that when the MEMS is turned on, the FSS transmits the AA radiation field inside the 45° scan angle sector. When the MEMS is in the “off” state, the FSS blocks a radiation field with a radius of A; as the scanning angle increases, the upper limit of the frequency range shifts toward lower frequencies. When MEMS is in the “on” state, the FSS reduces the phased array’s radar cross section (RCS) by 10–15 dB up to 45° of incidence angles, at least in the vicinity of the specular lobe [19]. This study presents the development of a tunable power factor analysis (PFA) to address impedance mismatch caused by the human hand effect. To adjust the resonance frequency of the capacitor-loaded PIFA, which is piezoelectrically actuated with a low actuation voltage, a metal-to-metal direct contact type RF-MEMS switch has been used. To demonstrate the viability of using human hand effects to rectify resonant frequency shift, the properties of the adjustable PIFA have been evaluated in the presence of phantom hands. The phantom hand effect is responsible for the PIFA’s resonance frequency shifting from 762 to 690 MHz while the switch is in the on position. The initial resonance frequency of the PIFA may be restored with the phantom hand by switching the switch status to the off position. With the phantom hand, the PIFA’s observed resonant frequency in the switch-off state is 760 MHz, and the return loss is −22 dB at that frequency [20]. In order to show a low-cost monolithic passive electronically scanned array (PESA), a frequency reconfigurable triangular microstrip patch antenna enabled by RF-MEMS has been built for monolithic integration with RF-MEMS phase shifters. This article presents our first reconfigurable triangular patch antenna prototype, which is now undergoing construction. Surface micromachining is used to create the aperture linked patch antenna on a dual-layer quartz/alumina substrate. In the oral presentation, the outcomes of the full-wave MoM simulation will be contrasted with laboratory data [21]. In this study, Ansoft HFSS was used in the design of a reconfigurable microstrip patch antenna with a resonance frequency of 6 GHz. RF-MEMS switches were used to get the various operating frequencies of 5.38, 5.68, and 5.75 GHz. The system capacity may be increased using the reconfigurable microstrip patch antenna. Through modifications to its radiation pattern, polarization, and resonance frequency, the RA may adapt to a range of wireless communication needs. Because of its flat design and low weight, microstrip antennas are commonly utilized in satellite and mobile communications systems. There are some of the limitations which include surface wave excitation, limited power handling capability, poor endfire radiator with the exception of tapered slot antennas, and extraneous radiation from feeds and junctions [22]. A defected ground structure–based circular ring patch antenna with frequency reconfigurable utilizing RF-MEMS switches has been presented in this article. The proposed design shows dual band frequency operation while maintaining the promising results with measurements [23]. In this article, two monolithically integrated MEMS switches are used in the construction of a microstrip patch antenna with radiation pattern adjustable characteristics. It is possible to alter the antenna’s radiation pattern by adjusting its physical dimensions. Additionally, we provide the intricate architectures of these RF-MEMS switches, which at operational frequency have isolation and insertion losses of −23.12 and −0.09 dB, respectively. Additionally, 35.4 GHz is the antenna’s resonance frequency, and 6.69% is its bandwidth. Every outcome is simulated [24]. The state of the art in RF-MEMS device development is reviewed in this work, with a focus on switches and Si-micromachined circuit components for use in high-density, high-performance on-wafer packaged circuits. An overall circuit integration approach that may greatly lower the size, weight, and cost of microwave and millimeter-wave components may be made possible by micromachined Si ICs and RF-MEMS devices. The potential for true multifunction chips that combine analog, digital, RF, and optoelectronic capabilities is made possible by the ability to integrate several substrate technologies. In addition to allowing for larger circuit densities, three-dimensional vertical integration with high-performance MEMS devices releases RF circuit design from the constraints of two-dimensional architecture, enabling it to achieve performance and functionality levels that are not achievable in a planar geometry [25].

This paper presents a reconfigurable monopole ultra-wideband (UWB) antenna that incorporates single notched characteristics. The UWB antenna under consideration functions within the frequency range spanning from 3.2 to 12 GHz. The achievement of the notched frequency band is facilitated with the integration of a switched meandered slot that is carved into the radiating patch. The reconfiguration of switching is accomplished through the utilization of RF-MEMS series switches, which effectively mitigate interference to the principal users who operate inside the WiMAX bands spanning from 3.3 to 3.9 GHz. The RF-MEMS switch regulates the two modes of the construction. The UWB antenna proposes a voltage standing wave ratio (VSWR) value of 2 across the frequency range of 3.1–12 GHz. Additionally, this study presents the simulated radiation pattern and current distribution at various frequencies [26]. For the first time, two unique frequency reconfigurable patch antenna components with built-in MEMS actuators are shown. The operating frequencies of these patches may be dynamically changed to be separated by about 0.8–15 percent of the nominal operating frequency. Moreover, the actuators are compatible with array antennas and easy to build. Currently, these actuators are used in an array configuration to adjust the phase as well as the resonant frequency of the signal emitted by each individual element [27]. This study presents the design of a unique reconfigurable microstrip patch antenna for Ku-band application that uses a MEMS switch to adjust the frequency from 15 to 15.65 GHz. Using HFSS version 13, a 3D EM modeling tool, the proposed antenna exhibits reconfigurability behavior in the Ku-band with high gain (average gain greater than 6 dB) and broad bandwidth. A design frequency of 15 GHZ was selected. Microstrip typically provides modest gain and a limited band. Here, it is seen that the bandwidth is more than 900 MHz, the gain is around 6 dB, and the return loss is approximately 42 dB [28]. This work presents a low actuation voltage capacitive shunt switch that may be utilized in conjunction with micromachined antennas. Ansoft HFSS is used to develop and simulate a process flow for the fabrication. To determine the low actuation voltage features of the suggested design, the electromechanical analysis findings are provided and contrasted with those of a fixed flexure-based switch membrane and discussed. It offers a low actuation voltage RF-MEMS capacitive switch that may be used to create micromachined antennas that can be reconfigured. The flexures supporting the switch membrane have a serpentine design, which lowers the spring constant to the point where a pull-in voltage of 1.0 V is achieved. Fabrication files with the necessary masks are created together with the design of a process flow [29].

The design and EM analysis of a quad-band frequency RA for Ku-band applications are presented in this paper. Four distinct resonant frequencies are produced by the monolithic integration of rectangular microstrip patch antenna and MEMS capacitive switches, enabling reconfigurability of the antenna. Using FEM analysis, the microwave performance of the antenna is examined for a high resistive silicon substrate that is 675 ± 20 μm thick. For appropriate impedance matching, the fluctuation of input impedance throughout the microstrip patch antenna’s longitudinal length has also been studied. With a return loss of less than −10 dB and strong directivity at the resonant frequency, a frequency drift of 1.3 GHz, spanning from 14.5 to 13.2 GHz, was attained for the RA [30]. In this study, self-similar planar antennas were combined with ohmic contact cantilever RF-MEMS switches to create a RA system that radiates comparable patterns throughout a broad frequency range. This article described the many problems that arose while integrating the MEMS switches with the overall system design process. The final device has very comparable radiation patterns and radiates at three frequencies that are rather far apart. With significant gains in antenna performance, the suggested approach may be expanded to more complicated antenna structures or programmable linear antenna arrays [31]. RF-MEMS capacitors for resonant frequency tuning were monolithically incorporated into a new reconfigurable microstrip patch antenna. By equipping the microstrip patch antenna with a CPW stub and periodically adding variable MEMS capacitors to it, the operating frequency of the antenna was reconfigured as visualized in Figure 5. Using surface micromachining technology, MEMS capacitors are constructed by placing a 1-m thick aluminum structural layer with a 1.5-m capacitive gap on a glass substrate. Electrostatically driven MEMS capacitors have a low tuning voltage between 0 and 11.9 V. When increasing the actuation voltage from 0 to 11.9 V, the antenna resonant frequency may be continuously adjusted from 16.05 to 15.75 GHz [32].

To accomplish main beam switching, a reflectarray antenna monolithically integrated with 90 RF-MEMS switches has been developed and constructed. Ten reconfigurable reflectarray antenna components operating at 26.5 GHz were formed using aperture coupled microstrip patch antenna (ACMPA) elements. By varying the length of the open-ended transmission lines in the elements using the RF-MEMS switches, the progressive phase shift between the elements may be changed. According to the measurement findings, at 26.5 GHz, the primary beam in the H-plane may be altered between broadside and endfire. Using 90 RF-MEMS switches in the ACMPA components, beam switching of a 26.5 GHz 10 × 10 reconfigurable reflectarray antenna is accomplished as depicted in Figure 6 [33]. A new packaging platform was shown together with a suggested linear polarization (LP)/circular polarization (CP) switchable RA that uses a RF-MEMS switch. An aperture-coupling feed arrangement was used since the radiation aperture is located on the top side of the package substrate. A stub that serves as the other feed is added to a slot ring and linked to the RF-MEMS switch with an on/off state in order to create the LP-CP-switchable antenna based on the package platform as illustrated in Figure 7. The LP state’s measured 10 dB impedance bandwidth is 22.90%, whereas that of the CP state is 28.43%. In the CP condition, the observed 3 dB AR BW is 13.07%. At 21 GHz, the observed gains were 3.90 dBi (CP state) and 2.63 dBi (LP state) [34]. The detailed analysis of the RAs is presented in Figure 8 and also tabulated in Table 1.

Table 1 presents a comprehensive overview of various RAs using RF-MEMS switches. It details different antenna designs, each with unique specifications and applications. The table includes information on the antenna design type (such as hexagonal shaped, E-shaped patch, fractal, and PIFA), the type of reconfigurability (primarily frequency, but also polarization and radiation pattern in some cases), operating frequency range, gain (where available), and intended applications. The frequency ranges of these antennas vary widely, from as low as 0.5 GHz to as high as 39 GHz, covering a broad spectrum of potential uses in wireless communication systems.

These RAs are designed for a diverse range of applications across multiple fields. Some are tailored for specific uses such as BANs, LTE spectrum, Ku-band applications, cognitive radio systems, and GPS. Others are more broadly applicable to wireless communication standards like WLAN, WiMAX, and various satellite communication systems. The table also includes antennas designed for specialized applications such as RADAR, remote sensing, and military uses. The gain values, where provided, range from about 2 to over 22 dB, indicating a wide variety of performance characteristics. This extensive collection of antenna designs showcases the versatility and adaptability of RAs in meeting the diverse needs of modern wireless communication systems.

3. RAs Using PIN Diode

Another method to achieve the reconfigurability is the utilization of PIN diode as these switches are easily available and inexpensive. Therefore, various authors have used this technique in the antennas. For instance, a RA with minimal complexity that can use fewer switches to guide its beam into nine distinct directions, equivalent to θ = {−42°, 0°, 42°} in the φ = {25°, 0°, 155°} planes, and θ = ±25° in the φ = 90° plane, was shown. The 3.4–3.8 GHz RA was made up of four switchable parasitic dipoles positioned at various height levels above a ground plane. These dipoles acted as a director for elements at a lower layer and a reflector for those at an upper layer, enhancing the antenna’s radiation characteristics. The central dipole element was excited by a coaxial probe. A p-i-n diode was used as a switch in each parasitic dipole, allowing these components to be activated and resulting in the required modes of operation. The produced and measured RA showed findings that were in excellent accord with the numerical and theoretical simulation [35]. In this study, a reconfigurable microstrip patch antenna with polarization switching is initially built and then enhanced to provide polarization switching in three switchable frequency bands via the use of extra patch connection/disconnection and corner connection/disconnection methods. Five p-i-n diodes are used to provide three switchable operating bands with center frequencies of 5.1 GHz (first band), 5.45 GHz (second band), and 6.3 GHz (third band). In all three switchable frequency bands, the proposed structure is able to be configured to operate in LHCP, RHCP, or LP due to the corner connection/disconnection of additional patches and use of p-i-n diodes. Wide AR and impedance bandwidths are obtained by feeding the structure with a proximity coupled triangular tapered feed. In every working band, the prototyped antenna exhibits a respectable AR ≤ 3 dB and reflection coefficient. The suggested design’s polarization and frequency diversities might improve the dependability of contemporary wireless networks [36].

In this work, a second-order orbital angular momentum (OAM) mode with programmable vortex direction is achieved by introducing two symmetrical arc segments on opposite sides of the microstrip line that are turned ON and OFF using p-i-n diodes. The synthesis of degenerate mode currents is the basis for OAM mode creation; therefore, the building of this vortex wave antenna may be effectively guided by the characteristic mode theory (CMT). The modal weighting coefficients can be utilized to analyze the performance of the antenna [37]. This paper proposes a low profile, broad impedance bandwidth, dual-LP reconfigurable omnidirectional antenna. The suggested antenna’s radiating structure seems to have two primary distinctive modes, according to CMT. Four PIN dipoles were positioned in a dipole feeding network that has been designed to provide excellent omnidirectional radiation and adjustable polarization as shown in Figure 9. With a strength variation between −1 and 0.4 dBi, the suggested antenna achieved a broad overlapping bandwidth of 49.6% (from 48–0.2 to 797.5 MHz) with a low profile of 0.006λ₀. Both unmanned aerial vehicle (UAV) pattern measurement system and digital terrestrial television service may use the suggested antenna [38]. In this work, a second-order OAM mode with programmable vortex direction is achieved by introducing two symmetrical arc segments on opposite sides of the microstrip line that are turned ON and OFF using p-i-n diodes. The synthesis of degenerate mode currents is the basis for OAM mode creation; therefore, the building of this vortex wave antenna may be effectively guided by the CMT. The modal weighting coefficients can be utilized to analyze the performance of the antenna. The antenna operates in a number of significant commercial bands, including S-band (2–4 GHz), Wi-Max (3.5 and 5.8 GHz), Wi-Fi (3.6, 5, and 5.9 GHz), 5G sub-6-GHz (3.5 and 4.4–5 GHz), and ITU-band (7.725–8.5 GHz), with the added benefit of structural conformance, according to simulation and experimental results [39].

A programmable radiation pattern dual-polarized composite patch-monopole antenna has been described. It is made up of four vertical monopoles loaded into a double differential-feed patch that is linked to the GND by means of p-i-n switches. For both x- and y-polarization, multidirectional beam reconfigurability with three narrow-beam and one wide-beam mode may be accomplished by varying the ON/OFF states of switches. The completed prototype, measuring 0.98λ₀ × 0.98λ₀ × 0.2λ₀ at 2.45 GHz, can operate in eight modes, each having a bandwidth of 2.40–2.51 GHz. The radiation pattern can be reconfigured for each polarization as follows: (i) broadside narrow beam with peak gain of 7.0 dB and HPBW of 68°; (ii) titled narrow-beam at θ = 25° and peak gain of 5.3 dB; (ii) titled narrow-beam at θ = −25° peak gain of 5.3 dB; and (iv) widebeam with HPBW of ≥ 160° and peak gain of 3.7 dB. Because of these characteristics, the suggested antenna is a good fit for contemporary wireless communication systems that need wide coverage and polarization diversity [40]. A 1.57λ_g × 1.25λ_g monopole antenna in the form of an inverted triangular staircase fractal (ITSF) was conceived and built, including a bent arm cross-shaped (BACS) on the radiating patch enabling polarization reconfigurability. The BACS’s two opposing arms were equipped with two PIN diodes arranged asymmetrically to provide polarization reconfigurability as depicted in Figure 10. The antenna displays dual (linear/circular) polarization reconfigurability characteristics. In state 4, the antenna displayed LP with an AR of more than > 5 dB throughout the frequency range of 7.95–12.64 GHz (BW = 4.69 GHz), while in state 3, the antenna displays CP with measured 3 dB AR frequency ranging from 9.88 to 11.06 GHz with a bandwidth of 1.18 GHz. The measured findings verify the performance of the antenna. Fixed satellites, space research, fixed mobile radiolocation, and fractal-based polarization reconfigurable monopole antennas were among the long-distance microwave X-band applications that were well suited for the suggested antenna [41].

This article describes the development and testing of a reconfigurable patch antenna based on a double-ring-slot feeding structure, including both CP and LP. The antenna achieves either LP bandpass-type filtering radiation response or CP operation by employing a p-i-n diode as the switch, eliminating the need for complex filter resonators and reconfigurable circuits. Filtering radiation response for the LP antenna function has been achieved. High-frequency selectivity and an adequate amount of out-of-band rejection were guaranteed by the generation of three programmable radiation nulls. By inserting an additional L-shaped microstrip line and a p-i-n diode for the CP antenna function, a 12.6% 3 dB AR bandwidth was achieved. Other parameter values of the LP and CP antenna types are as follows, with the exception of the additional L-shaped microstrip line and the biasing network [42]. This study described the design of a new dual-band CP RA with a 2 × 2 array. The suggested antenna could resonate at dual-band with dual-CP radiation since it was built on a double inverted asymmetrical U-slot patch. CP might be switched in a RA by asymmetrically regulating the diodes positioned at the arms of two U-slots. A 2 × 2 reconfigurable array was developed using the aforesaid RA and the sequential rotation approach to further increase the AR bandwidth. Furthermore, the reconfigurable dual-band feeding networks were made to provide the array’s necessary changeable phases. The manufactured 2 × 2 array exhibited favorable radiation characteristics, including broad AR bandwidths (28.4% and 16%), broad 3 dB gained bandwidths (23.8% and 18.2%), and low polarization level [43].

In this study, a planar pattern diversity antenna for 2.4 GHz Wi-Fi applications was presented and proven by investigating reconfigurable endfire EM complementary pairs made of dipoles. The printed electric dipoles only have four embedded PIN diodes, allowing for four types of flexible beam steering. The suggested planar, compact construction, cheap cost, and adaptable design were realized by the pattern-RA. Wide application potential for 2.4 GHz WLAN intelligent communication systems was provided by the achievement of a high radiation efficiency of 72%, a high front to back (F/B) ratio of 15 dB, a flexible beam scanning capability, and a steady gain of higher than 3.20 dBi in varied conditions [44]. A CP slot antenna design has been shown. This design might be used to create RAs. The experimental findings showed that both operating modes may provide excellent CP radiation. Two example designs were given: the frequency RA and the polarization RA. Although doubling the system’s available bandwidth and enhancing isolation between two adjacent channels were possible benefits of the designs examined in this work, certain wireless communications applications might not be able to leverage the antenna’s CP capacity. By widening the shortened square-ring slot, the issue would be resolved [45]. With an operational frequency of 5.8 GHz, this work offers an electrically reconfigurable beam steering antenna using a unique integrated RF PIN-based parasitic array (ERPPA) as illustrated in Figure 11. The key characteristic of the suggested antenna is that the integrated RF pin approach reduces the complexity of the DC biasing circuitry design while maintaining the compact size and fulfilling the beam-steering angle criteria found in the proof-of-concept design. A reconfigurable beam steering capability may be achieved by controlling the current flow via the use of only two sets of embedded RF PIN switches. This behavior has resemblance to the Yagi–Uda idea and allows the parasitic components to function as either directors or reflectors [46].

This research investigates polarization reconfigurable patch antennas with four kinds of polarization agility. By adjusting the perturbation elements on a square patch antenna, the dual-fed antenna may operate on two orthogonal LPs concurrently or on two orthogonal CPs alternately. Proposed are two antenna layouts with perturbation components on the ground and on the patch. It is seen that the loaded diodes have a little impact on the radiation pattern and input impedance characteristics but a clear effect on the CP AR. With only two PIN diodes, the suggested two polarization RAs offer a compact design and simple controlling circuitry. The input impedance match, gain, cross-polarization, isolation between the two orthogonal LPs, and the AR of the two orthogonal CP operations are among the positive attributes of both antennas [47]. In this article, PIN diodes were used to create a frequency reconfigurable pixel antenna. The patch antenna was made on a FR4 substrate and was 26.9 mm × 24.5 mm overall. S11 parameter and radiation pattern measurements and simulations were used to examine the design. Using various PIN diode biasing condition combinations, the antenna might be adjusted to operate at 2.5, 3.9, or 10 GHz. All of the rearranged frequency bands exhibit a constant radiation pattern from the antenna. Low levels of cross polarization and an average gain of 6 dB were also reported in the accessible frequency range. A strong correlation between the simulated and measured outcomes substantiates the proposed notion of frequency reconfiguration [48].

Infrastructures for intelligent communication would be incomplete without wireless systems that use electronically reconfigurable antennas. Two PIN diodes are used as switching components in this arrangement to create a multiband fractal reconfigurable antenna (FRA) that can have its performance parameters changed. Lower resonant frequencies and more operational bands are the result of decreasing the antenna patch size and lengthening the electrical length on the radiating patch via the use of fractal slots. With only two RF PIN diodes, the proposed FRA has reconfigurable features in terms of gain and bandwidth, enabling operational bandwidths in the S, C, and X microwave bands. In addition to displaying multiband properties, the suggested antenna offers electronic parameter tweaking for improved performance [49]. In this paper, a wideband 2 × 1 reconfigurable beam steering array architecture for wireless communication systems is shown. The proposed antenna comprises of a rectangular planar ground, two rectangular radiating components, and a microstrip wire for power. These are its measurements: 0.67λ₀ × 0.53λ₀ × 0.03λ₀. By varying the direction of the main beam and the beam tilt between ±30 and ± 38, it may operate in three reconfigurable modes. This is achieved by turning on and off two PIN diodes. A step-by-step study is provided to improve the antenna characteristic performances. The theoretical bandwidth of the RA is 18.84%, the observed bandwidth is 19.42%, and the simulated bandwidth is 18.18%. The antenna that is being shown has a greater efficiency ratio of 80%–86% over the operational range (5–6 GHz) with a maximum gain of 8.62 dB (simulated) and 8.45 dB (measured). A 2.2 relative permittivity low loss Rogers RT5880 substrate is used to create the intended antenna. The findings, including the S11 parameter and radiation patterns, show excellent agreement between the simulated, theoretical, and observed values. The findings gained are helpful not only for pattern reconfiguration but also for enhancing overall gain, antenna bandwidth, and efficiency [50].

The novel frequency-reconfigurable antenna presented in this work may be the first to employ a bias circuit and a dip switch integrated on the same substrate as the antenna. Since this kind of antenna is tiny and adaptable enough to function in several frequency bands with various modes, it may be employed for a wide range of wireless applications. The proposed structure is 28 × 26.35 × 1.6 mm³ in size, with a relative permittivity of 2.2 and a tangent loss of 0.0009. It is printed on a Rogers RT5880 substrate. In between radiating patches, three-PIN diode switches are put. The suggested antenna has four operating modes that encompass nine bands: three dual bands in Mode 1, Mode 2, and Mode 3 (i.e., 4.36 and 7.78 GHz, 3.56 and 6.89 GHz, and 3 and 6.2 GHz), and a triple band in Mode 4 (i.e., 2.88, 5.87, and 8.17 GHz). The proposed antenna has a 97.66% efficiency with a gain that ranges from 1.38 to 4.89 dBi. The bandwidths obtained at the respective frequencies fall between 5.5% and 31.17%. The simulated results are validated experimentally, and the proposed structure is modeled in the CST MWS. The proposed antenna might be used in the Internet of Things (IoT) and modern portable (5G) devices [51]. In terms of design and construction, this study provides a small multifrequency reconfigurable patch antenna intended for use in the S and C bands of the RF spectrum, which are heavily occupied by wireless applications. On the ground plane, reconfiguration is accomplished using a single PIN diode. The diode may display three modes with primary resonant frequencies at 2.07, 4.63, and 6.22 GHz by adjusting the applied voltage. Less than 0.9 V is the minimum voltage needed for resonance switching. The antenna has a radiating patch element with a rectangular ring shape, and it is constructed on a FR-4 substrate with a volume of 70 × 60 × 1.5 mm³. The suggested inexpensive antenna may be simply installed in a standard university lab setting. The three modes’ combined bandwidth is almost one gigahertz, the constructed version of the antenna has a VSWR of no more than 1.02, and the return loss for the three principal resonant frequencies is much less than −40 dB [52].

This work presented a unique reconfigurable monopole slot antenna with a variable radiation pattern and frequency. Additionally, the antenna parameters’ theoretical behavior was shown. Results of the antenna S parameters that were measured and simulated agree rather well. With few switches, the antenna offered beneficial radiation patterns and frequency characteristics. In order to illustrate the performance improvements made achievable by employing frequency and pattern RA [53], a bow-tie antenna with frequency reconfigurability was suggested for WLAN, Bluetooth, and WiMAX applications. An electrically adjustable working band may be achieved by varying the effective electrical length of the antenna by using p-i-n diodes over the bow-tie arms. The results of the simulation and measurement were found to be in excellent agreement, indicating that the antenna has consistent radiation patterns and functions well at the targeted frequency ranges. In addition to these benefits, the reconfigurable bow-tie antenna was a viable option for cognitive radio and multiradio wireless applications due to its small size, consistent radiation properties throughout the whole adjustable frequency range, and low levels of cross-polarized radiation [54]. This article described the design and research for a planar multiband antenna that used PIN diodes to reconfigure its frequency and pattern. By adjusting the controlled activation of the slots positioned on a circular disc that was supplied by a CPW feed, the reconfiguration process was accomplished. With seven PIN diodes on either side of the circle that cut the longitudinal axis, the antenna was symmetrical along its longitudinal axis. The antenna could run in seven distinct modes thanks to a total of 15 PIN diodes. By using all of the antenna’s modes, reconfigurable beam form pattern reconfiguration was accomplished. Operating at 2.4 GHz, the basic antenna was a circular disc with no holes. By turning on the switches at the proper locations during various operating modes, one might alter the electrical length of the slot and obtain frequency reconfigurability. Switch locations were oriented in different directions in each half of the circular, but the total electrical length remains the same in each working mode to provide pattern reconfigurability. Using Ansys HFSS, the investigation simulations were run. The suggested antenna was suitable for WLAN, WiMAX, and Wi-Fi wireless services [55].

This research reported on the integrated frequency and polarization RA made of PIN diode RF switch. In order to alter the surface current distributions and adjust the current route length, three pin diodes were used. This produced variations in both polarization and frequency. The antenna functioned in the 2.42 GHz band with LP when all switches were in the ON state (Mode 1), and it resonated at 2.13 GHz with CP when all switches were in the OFF state (Mode 2). Through optimization in Ansys EM Suite, the patch’s surface was modified to provide the intended outcomes. The antenna exhibited LP in Mode 1 when its AR was more than 10.68 dB and CP in Mode 2 when its AR was less than 0.78 dB. The antenna prototype was constructed, and the measured and simulated results were compared.

For Internet of Vehicles (IoV) applications, a unique, low-cost, high-performance, and easily reconfigurable segmented patch antenna was suggested. The radiating patch’s form was controlled by frequency reconfiguration via the integration of three p-i-n diodes into two slots, and the size of two parasitic components placed in close proximity to the patch was controlled by pattern reconfiguration through the application of the Yagi–Uda principle. This study, to the best of the authors’ knowledge, was the first in the literature to demonstrate 12 different operational modes utilizing just five p-i-n diodes. These modes included broadside and endfire radiation across four frequencies (4.1–5.7 GHz) in the H-plane and four operating modes in the E-plane. With regard to gain, efficiency, and reconfiguration capabilities, the suggested low-cost, readily-fabricated antenna was a promising option for incorporation into vehicular communication networks, including both linked and self-driving cars [57]. This communication presented a multifunctional phased array for advanced satellite applications based on a 2 bit phase-shifting network and planar monopulse comparator. It included beam scanning, polarization reconfiguration, and monopulse functionality. The proposed phased array’s polarization modes, which include LP, LHCP, and RHCP, are individually controlled by the diodes implanted in the top patch. In both azimuth and elevation planes, the intermediate layer’s planar monopulse comparator was able to dynamically flip between the sum and difference patterns. With 8 × 2 2-bit phase shifters making up the bottom phase-shifting network, the proposed phased array exhibited an azimuth plane beam-scanning feature. Consequently, at all polarizations, the sum and difference beams might be scanned in the xoz plane from −50° to +50°. A working prototype was made for display. The observed peak gain of the total beam in the LP (LHCP) radiation mode varied from 11.8 to 9.1 dBi (12–9.5 dBic) when the scanning angle was varied from 0° to −50° in the xoz plane. Additionally, all of the recorded maximum null-depths of the difference beam at boresight in the yoz plane were below −19.8 dB (−17.6 dB) with good amplitude balance in the LP (LHCP) radiation mode. The findings of simulations and measurements corresponded well [58].

This study showcased a small, CPW-fed frequency-reconfigurable antenna with remarkable performance characteristics that was constructed with the use of the Ansys HFSS tool. The antenna, measuring 23 × 23.5 × 1.6 mm³, was designed to span many wireless application frequency bands while keeping a small form factor. Outstanding impedance matching was shown, and strong signal transmission capabilities are ensured by S11 values that stay below −10 dB throughout a number of frequency ranges. In addition, the antenna achieved a high radiation efficiency of almost 96% at 5.48 GHz and an amazing peak gain of 8.61 dB at 10.02 GHz, all while maintaining a VSWR below 2 dB, which indicated little signal loss. These astounding accomplishments highlighted how it might improve signal quality and connectivity for 5G sub-6 GHz and UWB applications [59]. A 23 mm × 18 m small frequency reconfigurable fractal-based maple leaf (MPL)–shaped antenna was conceived and produced. Two PIN diodes were attached onto the inverted U-shaped slots to electrically alter the MPL-shaped antenna’s working frequency range. This was allowed for frequency reconfigurability, which was accomplished by constructing two inverted U-shaped slots etched at the top of the radiating patch of the antenna. The MPL-shaped antenna had an average peak gain of 8.21, 3.91, 4.82, and 4.37 dBi for four different biasing states of the PIN diodes. It might be switched between the operating bands of 2.20–3.08 GHz, 3.20–4.90 GHz, 3.48–4.50 GHz, and 4.97–7.37 GHz. The suggested MPL-shaped antenna allowed for frequency reconfigurability over ISM band [60].

Table 2 presents detailed specifications for various RAs using PIN diodes. The table explains the antenna design type (such as multilayer structure, conventional patch with stub, and fractal monopole), the type of reconfigurability (including frequency, polarization, radiation pattern, and in some cases multiple types), operating frequency range, gain (if available), and intended applications. The frequency ranges of these antennas vary widely, from as low as 0.48 GHz to as high as 12.64 GHz, demonstrating the versatility of PIN diode–based RAs across different parts of the RF spectrum.

These RAs are designed for a diverse range of modern wireless communication applications. Some are tailored for specific uses such as 5G NR FR1, digital television service, Wi-Fi, satellite communication, and cognitive radio. Others are more broadly applicable to wireless communication standards like WLAN, WiMAX, and 4G LTE. The table also includes antennas designed for specialized applications such as polarization diversity, intelligent Wi-Fi systems, and vehicular communication systems. The gain values, where provided, range from about 0.4 to over 23 dB, indicating a wide variety of performance characteristics. This collection of antenna designs highlights the adaptability and efficiency of PIN diode–based RAs in meeting the diverse needs of current and emerging wireless communication technologies.

4. RAs Using Varactor Diode and Liquid Metal Alloy and Optical Switches

Another type of switches is varactor diode which can be used for the reconfigurability in the antennas. The authors have presented some of the research work in this area. For instance, in [61], a dual-frequency reconfigurable MIMO PIFA has been developed in this research for use in LTE bands in handheld devices. The antenna elements adjust their operating frequency between the 1.65–2.2 GHz and 0.8–0.98 GHz bands using the varactor diodes. The suggested MIMO antenna offers isolation of more than 20 dB for all frequencies that are feasible. It is composed of two symmetrical PIFAs with a center-to-center spacing of 43 mm (0.1906λ₀). Multiplexing efficiency, envelope correlation coefficient (ECC), total active reflection coefficient (TARC), mean effective gain (MEG), directive gain (DG), and channel capacity loss (CCL) were used to assess the diversity performance. Additionally, it was discovered that the SAR distribution in human head and hand models was under the 1.6 W/kg allowable SAR limit. Consequently, the suggested antenna may effectively include many antennas into a constrained area platform intended for handheld mobile devices. Similarly, for cognitive radio applications, a unique RA with an 11.5:1 bandwidth was conceived and built. The new antenna that was being suggested comprises two separate lines that span the frequency range of 430 MHz to 5 GHz. The first route had a direct connection to an UWB antenna that operates in the frequency range of 1–5 GHz. The second approach used a DC-controlled varactor-based matching network to process frequencies in the 430 MHz–1 GHz range. Two discrete switches provided the switching capability between reconfigurable area (430 MHz–1 GHz) and wideband (1–5 GHz). The 60 mm by 100 mm designed antenna was small and has a straightforward construction. The use of this innovative antenna in cognitive radio systems was very promising [62]. Moreover, this study presented a unique active compound-five-ring nesting-artificial magnetic conductor (CFRN-AMC) with varactor diodes and developed and analyzed its comparable circuits. It was discovered that the varactor capacitance not only makes it easier to manage the AMC’s operating frequency range, but it also reduced the AMC’s resonant frequency while reducing its overall size. Over the active CFRN-AMC plane was positioned a triangle-shaped dipole antenna in order to realize the frequency-reconfigurable antenna. The results of the modeling and measurements demonstrate that the proposed CFRN-AMC-based antenna had a very low profile, high gain enhancement, and frequency reconfiguration. At 1.8 GHz, the dipole antenna and AMC plane were separated by a mere 2 mm, or 0.012λ [63].

A polarization-reconfigurable wideband high-gain antenna based on liquid metal tuning is presented in this paper. The driving element of the antenna was a C-shaped slot in a center-feed circular patch antenna. Thirteen circular directors and a parasitic circular disc were used to increase the realized gain and expand the bandwidth. The placement of the liquid metal alloy within the tube may be adjusted to adjust the direction of polarization. An analysis has been conducted about the link between the dimensions and the maximum attainable gain. From 4.3 to 5.3 GHz, the antenna demonstrated an impedance bandwidth of −10 dB (a fractional bandwidth of 21.2%). At 5.3 GHz, a 14.4 dBi gain was obtained. There was excellent agreement between the measurement and simulation findings [64].

The frequency and polarization reconfigurability of a microstrip circular patch antenna employing liquid metal is shown. The antenna has a C-shaped slot carved in the middle of the patch, and it can be reconfigured to operate in four different states by utilizing liquid metal and two putty containers. At 5.83 GHz, LP is detected in the absence of liquid metal inside the containers. Two liquid metal droplets deposited in the containers result in CP at 6 GHz. When the rightmost container was filled, the RHCP was acquired; similarly, when the leftmost container was filled with the liquid metal, the LHCP was obtained. LP was detected at 6.15 GHz once every container was full. In the LP scenario, the antenna’s observed gains were 2.68 dB for all full containers and 3 dB for empty ones. The corresponding ARs were 19.65 dB for filled containers and 23.74 dB for empty containers. At 6 GHz, the gain was 2.44 dB and the AR was 0.54 dB when the LHCP was turned on. At 6 GHz, the gain for RHCP was 2.37 dB and the AR was 1.5 dB [65]. In this work, the authors integrate the phase change material (PCM) GeTe (germanium telluride) into the structure of an antenna operating in the millimeter wave (approximately 30 GHz) domain. This allowed the antenna to be reconfigurable in three different polarizations: LP, LHCP, and RHCP. The GeTe material was incorporated into the four corners of a standard patch antenna, which was the basis of the device. The patch was stimulated by a microstrip line. This material may be directly irradiated with ultraviolet (UV) short laser pulses to regulate the phase transition between its insulating (OFF) and metallic (ON) states. This enabled the antenna to be reconfigured between an LP, LHCP, and RHCP. The manufactured device’s measured performances reveal ARs of less than 3 dB across a 400 MHz bandwidth at around 29.5 GHz. For the CP configurations, overall efficiencies might reach up to 75%, while the LP states could achieve a maximum gain of up to 8.3 dBi [66].

This work presented a unique frequency reconfigurable patch antenna with a rectangular patch surrounded by two metallic rings. The antenna’s radiating surface might be changed to provide frequency reconfigurability. Switches were used to link the central patch to the surrounding metallic rings. When the switches were switched ON and OFF, respectively, the antenna operated in the 1.8 and 2.4 GHz frequency ranges. When the switches were switched on, they increased the radiating surface’s size by adding exterior rings, which altered the antenna’s working frequency. Depending on the application, a variety of switching components, such as varactor diodes, PIN diodes, MEMS, or optical switches, could be used [67]. A CP antenna for a foldable smartphone is suggested in this communication. At the top edge of the phone’s two halves are two dipoles that are part of the antenna. Variations in the folding angle may alter the state of polarization. The antenna is LP whether the phone is completely folded or unfolded. The antenna is CP when the folded angle is 90°. Additionally, it is simpler to position the antenna toward the satellite when portable since the highest radiation direction is always in the +z-direction. In addition, it is anticipated that the suggested antenna would be included into the foldable smartphone’s metal frame to enhance satellite communication performance [68]. The authors looked at an antenna in this communication that could reconfigure its polarization and emission pattern. Two parasitic dielectric resonator antennas (DRAs) made of ethyl acetate and a central DRA make up the antenna. The liquid flow may be used to adjust the antenna. The antenna generates omnidirectional radiation in its TM01δ mode when there is just the central DRA present. The antenna generates unidirectional radiation when there is a parasitic LDRA in addition to the central DRA; the polarization is dependent on the parasitic LDRA’s position. Liquid flow may be used to modify the radiation properties, resulting in the realization of three states: omnidirectional radiation, unidirectional radiation, and unidirectional radiation with x- and y-polarization. A 2.4 GHz prototype antenna was manufactured, measured, and simulated for verification [69].

One way to get autonomous control over RHCP/LHCP 2D scanning for RA is via this communication. Through its reconfigurability, the suggested RA unit with its simple design may accomplish the 1 × 1-bit dual-CP phase correction, which in turn allows the realization of a dual-CP RA with polarization-decoupled 2D beam steering. Additionally, we created a unique control structure that significantly reduces the complexity of the mechanic control for the suggested design. It has a straightforward construction, is simple to apply, has excellent stability, and offers flexible control over the unit states. Owing to a lack of resources, a manual control prototype is created and evaluated under two predetermined working situations using appropriate and safe simplifications. With scan loss of no more than 3 dB, the prototype can accomplish decoupled dual CP 2D scanning of ± 50 [70].

The material selection process for RF-MEMS switches—which are used in RAs—is reported in this work. To achieve the required performance, three main performance indicators are used: thermal residual stress, RF loss, and pull-in voltage. According to the selection chart, aluminum is the optimum material to employ as a bridge material in RF-MEMS switches in order to maximize RA performance [71]. A brand-new integrated RF-MEMS switch was unveiled for uses with RAs. The main benefits of the suggested switch over the previously published findings were its multistate operation, simplicity of the DC biasing mechanism, and integration with the antenna structure. An investigation was conducted on how the suggested switch operated in various states. Three switches were used on the U-slotted rectangular antenna to illustrate the use. The switches were placed in such a way that the user may alter the antenna’s resonance frequencies to suit their needs. The antenna may cover the whole range of the conventional wireless spectrum by generating several resonance frequencies. Other slotted antenna layouts may also make advantage of the newly introduced switch. An introduced antenna prototype was constructed [72].

This letter presents a unique polarization reconfigurable slot antenna that is built on two layers of metasurface (MS), has a tunable 3 dB AR bandwidth, and is small, light, and inexpensive. The MSs positioned atop the source antenna may be rotated to provide a variety of polarization modes with customizable frequencies. The simulated results of the antenna presented in this letter not only show that the antenna has excellent characteristics of frequency and polarization reconfigurability but also show that the proposed approach was feasible for implementing a mechanically operated design that can reconfigure the antenna’s multiple characteristics. Thus, this correspondence may provide an innovative approach to the design of multiparameter RAs [73]. A polarization reconfigurable ultra-high-frequency radio-frequency identification (UHF RFID) antenna system working in the EU frequency range has been designed and reported in this research. When compared to normal CP techniques, the polarization agility that has been presented achieves a more efficient polarization matching in the usual RFID situation where the RFID reader antenna must interrogate several tags with various orientations. Furthermore, loading capacitors have been employed to reduce the reader’s overall size, enhancing the antenna’s integration. For narrowband RFID reader applications, the switching feeding network offers a versatile and affordable way to increase overall efficiency. A more condensed solution for the reconfigurable feeding network will be created as an additional viewpoint. Additionally, a feeding arrangement that is more symmetric will be looked at [74].

A single-polarized, continuously-tunable slot-ring antenna working in the S band was presented in this study. To accomplish frequency tunability, varactors were loaded into the slot-ring antenna. The slot-ring antenna may span 2–4 GHz if the feeding network and biasing circuits were designed appropriately. An antenna prototype was constructed and measured. Between 2.66 and 4.09 GHz, the observed return loss was better than 9 dB. The radiation pattern and antenna gain closely resemble the outcomes of the simulation. Through careful placement of switches and varactors inside the antenna’s slots, this tunable slot-ring antenna might be expanded to create a RA array that covers both the L and S bands. The emission pattern, polarization, and frequency of this innovative antenna array might all be altered [75]. A liquid metal-based Yagi–Uda antenna had been demonstrated. Two spinning rods, one passive parasitic dipole, and one balun-fed active dipole make up the antenna. In contrast to previous efforts, the semiclosed channel and microfluidic channels were made without a furnace, significantly streamlining the preparation process. There was not a single disadvantage as compared to those produced in a furnace. In the meanwhile, the suggested antenna’s practical stability might be ensured by using both chemical (gluing) and physical (welding) connections at each part’s joints. The antenna might achieve a pattern-reconfigurable feature that allowed it to transition from bidirectional radiation to directional radiation by spinning the rotating rods at the correct angle. The suggested antenna’s maximum gain increased from a bidirectional to a directional condition by around 2.4 dBi. It was possible to achieve a maximum F/B ratio > 16 dB in a directed radiation condition [76].

Through the use of codesign techniques that combine EM simulation and mechanical guidance, a new 3D frequency RA for the V-band was presented. By simply extending and releasing the elastomer substrate below the antenna, the center frequency of the device could be dynamically shifted from 47.0 GHz (with a 7.0 GHz bandwidth) to 56.6 GHz (with a 13.9 GHz bandwidth). Furthermore, the antenna’s emission patterns at various states were comparable. This antenna was a good option for use in V-band multifunctional, high-resolution RF microsystems, especially in sectors like body sensing and portable health monitoring, due to its considerable bandwidth and reconfigurability [77]. A small MS antenna with adjustable polarization has been successfully constructed and shown. To see the specific mode features and their behavior in relation to surface current and field pattern, the MTS was analyzed using CMA. According to the research, CP radiation was caused by the two basic modes of MTS, which were orthogonal to one another in length and breadth. These modes generate CP with regard to MTS orientation when stimulated with a slot utilizing the CPW approach. When the polarization is directed in the u and v directions, it changes from LP to either RHCP or LHCP. Antenna polarization reconfigurability is achieved via mechanical rotation. For sub-6 GHz 5G applications, where polarization reconfigurability is a crucial need, the suggested architecture works well [78]. This work investigated a low cost 1 bit reconfigurable beam steering 1 × 4 antenna array for mobile terminals. It was possible to reach the overlapping −10 dB impedance bandwidth of 26.6–28.4 GHz. By steering the beam to five distinct angles of 0°, ±20°, ±40°, and so on, this array might attain a beam-steering range of 56.5° to 49.5° with an element spacing of 0.5λ₀. The feeding network’s switch ON/OFF states were combined to provide the 1 bit phase shifting. A curved microstrip feedline was used to reduce the 1 bit phase’s quantization error. An array that was more compact was obtained by building a compact feeding network. The small size and cheap cost of the proposed array make it a suitable choice for mobile terminal applications [79]. The authors have presented in this study a unique 2.4 GHz ISM band split-ring resonator-based frequency RA for tissue-independent medical applications. Between the split-ring resonators, a conductive ON/OFF switch was included to sustain S11 performance in various bodily tissues. The numerical findings showed that the 2.4 GHz ISM band was fully covered by the antenna found in the skin, muscle, and fat tissues. Additionally, it displayed appropriate gain values and consistent radiation behavior across the ISM band [80].

Again Table 3 presents the analysis for various RAs using varactor diodes, liquid metal alloy, and optical switches. It catalogs antenna designs, each with its unique characteristics and applications. The table provides detailed information on the antenna design type (such as MIMO PIFA, monopole patch, artificial magnetic conductor, and MS-based antennas), the type of reconfigurability (including frequency, polarization, and radiation pattern), operating frequency range, gain (where available), and intended applications. The frequency ranges of these antennas vary widely, from as low as 0.43 GHz to as high as 56.6 GHz, demonstrating the versatility of these RAs across different parts of the RF spectrum, including millimeter-wave frequencies.

These RAs are designed for a diverse range of modern wireless communication applications. Some are tailored for specific uses such as mobile hand-held devices, cognitive radio, WLAN, radar communication systems, and 5G sub-6 GHz networks. Others are more broadly applicable to wireless communication standards like GSM and Wi-Fi. The table also includes antennas designed for specialized applications such as polarization diversity, foldable smartphones, UHF RFID, and body sensing. The gain values, where provided, range from about 2 to over 22 dB, indicating a wide variety of performance characteristics.

5. Other Different Ways to Achieve the Reconfigurability in the Antennas

Various other methods apart from the abovementioned methods have been discussed in this section. For instance, in [81], the authors describe a compact frequency-reconfigurable patch antenna designed for the Beidou (COMPASS) Navigation System. The antenna can switch between three operating bands (E2, E6, and E5b) via manual circular rotation of the top layer. It uses a shorting load structure to achieve frequency agility—by rotating the top layer, the shorting probe connects to different sets of shorting pins in the bottom layer, changing the resonant frequency. The antenna has a dual-feed configuration with a broadband 90° phase shifter to generate CP. Measured results show good performance across all three bands, with VSWR below 1.3, AR below 2.5 dB, and gains of 2–3.3 dB. The compact design (50 × 50 × 14.8 mm) provides a solution for multiband patch antennas that are difficult to design using traditional approaches. In another research [82], the authors proposed a novel multi-LP reconfigurable unidirectional circular patch antenna. The antenna can switch among four LPs at 45° rotations by controlling the connections between four shorting posts and the ground plane using PIN diodes. The antenna operates in the 2.4 GHz WLAN band with a size of about 0.57λ × 0.57λ × 0.07λ at 2.45 GHz. Measured results show an overlapping impedance bandwidth of 2.33–2.50 GHz for different polarization states, with realized gains ranging from 5.3 to 5.9 × dBi. Similar research [83] describes a novel reconfigurable microstrip patch antenna that can switch between LP, LHCP, and RHCP. The design uses two square loop slots on the ground plane with PIN diodes to control the polarization state, simplifying the biasing circuit and keeping the patch unperturbed. Experimental results show good performance, with 10-dB impedance bandwidths of about 60 MHz for LHCP and RHCP, 3-dB AR bandwidths of 20 MHz, and gains above 6.4 dB for CP states. The antenna offers a simple structure and low cost, making it suitable for wireless communication systems operating around 2.4 GHz. In another research [84], a novel reconfigurable microstrip patch antenna that can switch between LP, LHCP, and RHCP has been discussed. The design uses two square loop slots on the ground plane with PIN diodes to control the polarization state, simplifying the biasing circuit and keeping the patch unperturbed. Experimental results show good performance, with 10-dB impedance bandwidths of about 60 MHz for LHCP and RHCP, 3-dB AR bandwidths of 20 MHz, and gains above 6.4 dB for CP states. Other research group [85] describes the design of a 2 bit reconfigurable UWB planar antenna array for beam scanning applications. The antenna array operates in the C-band frequency range of 4.0–5.5 GHz and consists of 16 Vivaldi antenna elements, 16 wideband phase shifters, and a feeding network. It achieves beam scanning capability from −45 to 45° with sidelobe levels below −10 dB. The design features a broad bandwidth, simple structure, and low cost compared to traditional phased array antennas. Another document [86] describes the design of a frequency and polarization reconfigurable patch antenna with a stable gain. The antenna can operate at eight discrete frequency bands and three polarization states (LHCP, RHCP, and LP) by using switchable shorting pins and controllable perturbation segments. A key feature is that the antenna maintains a relatively stable gain (variation less than 1.8 dB) across all operating states by utilizing the loss characteristics of PIN diodes. The design achieves good performance in terms of impedance matching, radiation patterns, and AR over a wide frequency range of 1.84–2.63 GHz. Similarly, in [87], a novel frequency and pattern reconfigurable slot antenna is designed. The proposed antenna can reconfigure to three different frequency bands around 1.8–2.1 GHz using switches in the slot. It also achieves pattern reconfigurability, with the ability to steer the main beam to three different angles (0°, ±15°, and ±30°) by manipulating switches in slits at the edges of the ground plane. The antenna uses a reflector to produce a directional radiation pattern and achieve high gain. Further in [88], the authors describe a frequency-reconfigurable antenna design using a MS. The antenna consists of a circular patch antenna with a circular MS placed directly on top of it. By physically rotating the MS relative to the patch antenna, the operating frequency of the antenna can be tuned over a range of 4.76–5.51 GHz. The MS acts like a dielectric substrate with variable permittivity, allowing the antenna’s resonant frequency to be adjusted by rotation while maintaining good radiation characteristics across the tuning range. A frequency-reconfigurable loop antenna is designed using a graphene nanosheet–loaded branch in [89]. A composite impedance network is proposed to improve the antenna’s impedance and balance radiation efficiency. The fabricated antenna demonstrates continuous frequency tuning from 2 to 2.4 GHz with improved low-frequency radiation efficiency. At the last but not the least, in [90], a slot-based electronically steerable parasitic array radiator (ESPAR) antenna is proposed for IoT applications. In another article, pattern reconfigurability has been achieved from CPW-to-slotline transitions in the frequency range of 3.5–6.5 GHz [91].

Table 4 presents detailed specifications for various RAs using other methods beyond RF-MEMS switches, PIN diodes, varactor diodes, liquid metal alloy, and optical switches. The table provides information on the antenna design type (such as stacked cylindrical, center-fed circular, slotted ground patch, and graphene-loaded antennas), the type of reconfigurability (including frequency, polarization, and radiation pattern), operating frequency range, gain (where available), and intended applications. The frequency ranges of these antennas vary from 1.19 to 5.51 GHz, covering a significant portion of the sub-6 GHz spectrum. These antennas are designed for a diverse range of applications, including navigation systems, wireless communications, tablet computer applications, RADAR detection, cognitive radio, and large-scale IoT networks. The gain values, where provided, range from 2.5 to 6.4 dB.

6. Challenges in Designing the RA

7. Suggestions to Improve the Performance of the RAs

Based on the rigorous analysis on the different types of RAs, various challenges have been discussed in above section. Now, the authors would like to present some of the suggestions and alternative solutions which can improve the performance parameters of the RAs. One of the prominent suggestions is to incorporate advanced and smart materials like graphene and utilize alloys for adaptive structures and exploration and exploitation of metamaterials for overall improvement of EM features [101]. One of the key elements is the efficiency of switching mechanism in these types of antennas. Therefore, researchers and antenna designers should consider optimized switching mechanism, which includes fast and high-quality RF switches. These switches should possess the lowest insertion loss and ability to integrate with advanced control algorithms [102]. In addition to this, another method like use of various optimization, machine learning techniques, and real-time optimization can be utilized to optimize the antenna at its best level for dynamic environments [103].

Further, the antenna designers should focus on the designing and analyzing of wider bandwidth, multiband antennas which include the cognitive radio features [104]. Further, the antenna designs can be made more efficient by incorporating phased array features using beamforming ICs and developing analog, digital, or mixed beamforming solutions for mm and high-frequency applications. This design leads the antenna designers toward miniaturization and integration with the help of fractal geometries, slotted ground structures, and usage of active component with on-chip solutions. Further, a discrepancy has been observed in the simulated and measured results. It is due to some of the fabrication tolerances in the design which can be avoided by utilizing 3D printing, implementing additive manufacturing, and exploring the opportunities to use nanotechnology [105]. Other suggestions for power control design include improving power handling capability, implementing adaptive impedance matching for optimal power transfer, and improving robustness and reliability including incorporating redundancy and fault tolerance in critical applications. Moreover, the antenna designers could use advanced modeling and simulation by developing digital twins for real-time performance prediction.

In addition to the suggestions, the researchers can opt for various alternative solutions for designing the high-performance RAs. These solutions can be in the form of using advanced material selection and using artificial intelligence and machine learning for the optimized version of the final antenna design [106]. Moreover, high-precision MEMS switching mechanism, metamaterial/MS structures, and optical control mechanisms can be used to develop the hybrid optoelectronic reconfiguration systems. In addition to this, newest technologies like plasma-based and quantum annealing algorithm can be explored for the global optimization of the antenna parameters.

8. Advantages of Switching-Based RAs

9. Conclusions

This paper discusses the detailed analysis of the RAs along with the challenges faced by the researchers while designing and manufacturing these antennas. The study was based on the different types of the RAs. The article emphasizes on the switching mechanism to achieve the various types of the reconfigurability. These switching mechanisms include the utilization of RF-MEMS switches, PIN diodes, varactor diode, use of liquid metal alloy, and mechanical methods. RF-MEMS switches and PIN diodes are the most widely used methods for the smooth operation of the systems due to their low pull-in voltage. Moreover, the frequency is the most favorite parameter to reconfigure among researchers as compared to radiation pattern and polarization due to ease in the manufacturing and excellent impedance matching. The RAs find applications in various sectors including terrestrial and satellite communications, vehicular communication due its movement in rural and urban areas, and cognitive radio.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

This work was supported by the Deanship of Scientific Research, Northern Border University, Arar, Kingdom of Saudi Arabia, under grant number “NBU-FFR-2024-2467-04.”