## 3 Emerging and State-of-the-Art Technologies
### Techniques for Reconfigurable RF Front-End Systems and Antennas
Moreover, a new type of MEMS-based linear RF switches was fabricated and tested in [158] (see Figure 10b), demonstrating operation over a wide frequency range from 20 GHz to 110 GHz, thereby enabling high scalability for future communication systems. Despite these advantages, MEMS presents low switching speeds and challenges in integration. The previously mentioned approaches involve external devices integrated into the structure. An alternative solution is the use of tunable materials, which exhibit properties that can be varied when biased with an appropriate DC field. Two of the most widely researched tunable materials are liquid crystals/ferroelectrics [159] and ferrites [160]. Liquid crystals and ferroelectrics enable permittivity tuning with applied voltage (see Figure 11a), while ferrites allow permeability tuning through the application of a DC magnetic field. These materials are generally considered cost-effective and technologically promising for mmWave frequencies, with applications spanning RIS [161; 162], filters [163; 164], tun able phase shifters [165; 166] and reconfigurable phased arrays [167; 168], among others. An example of a reconfigurable antenna is depicted in Figure 10f. However, at higher and particular at mmWaves frequencies, these materials face several challenges, such as increased insertion losses, reduced efficiency [168], higher power consumption, and the requirement for complex biasing networks [169]. To mitigate the increased losses, thin film ferroelectrics such as \(BaTiO_{3}\) and its composites [170] were utilized for the development of RF surface mount capacitors [171], RIS [172], and tunable phase shifters [173]. For ferrites, the switching speeds are limited by the capabilities and the bulkiness of the electromagnets employed. An example of a 360\({}^{\circ}\) phase shifter with ferrite slab developed utilizing SIW technology is presented in Figure 11b. To address these issues, composite multiferroics have recently been proposed as an alternative solution. These materials combine the advantages of both ferroelectrics and ferrites, offering potential improvements in tunability [174]. The working principle of composite magnetoelectric/multiferroic materials can be described as follows: When an electric field is applied to a piezomagnetic material, the microscopic electric dipoles shift, causing mechanical deformation. This deformation is transmitted across the interface to the magnetic material, where a similar effect, such as magnetostriction or piezomagnetism, takes place, leading to an overall macroscopic magnetization. This process, known as converse magnetoelectric coupling, results in the modification of the material's magnetization by an external electric field, as seen in Figure 11c. Conversely, the direct magnetoelectric effect occurs when a magnetic field is applied to a magnetostrictive material, generating voltage or polarization due to strain-induced effects in the piezoelectric component. Some popular piezoelectric materials are PZT, barium strontium titanate (BST), and lithium niobate, while for magnetostrictive, there are YIG, Terfenol, Metglas, NFO, and Galfenol. Despite these advancements, the tuning range of currently available tunable microwave devices remains limited due to the low electromechanical coupling [175; 176; 177]. Another challenge is that the strong magnetostrictive materials such as Metglas and Terfenol are highly lossy at microwave frequencies, making them unfit for such applications. For sub-THz and THz applications, graphene has been extensively exploited due to its intrinsic property of tunable conductivity via applied chemical potential [180; 181; 182]. [PERSON] et al. [180] proposed a tunable phase shifter where graphene alters both the amplitude and phase response of the device, as shown in Figure 11d. Another key application area is nano-antennas, where conventional metals such as copper and gold approach their plasma frequency and become extremely lossy, rendering them inefficient as radiating elements [183].