These reconfigurable/tunable components such as RF switches, varactors, adaptive matching networks and filters, frequency-agile antennas, frequency-selective surfaces, polarization-agile antennas and polarizer, discrete, and continuous phase shifters, and based on it, beam-steering antennas can be realized with different materials and technologies, which are symbolized in Figure 4 at the lower left:  
* in semiconductor technologies [5; 6; 7; 8; 9; 10], in particular silicon technologies (Complementary Metal-Oxide-Semiconductor (CMOS) and Bipolar CMOS) that offer much lower cost as compared to Indium Phosphide (InP) or Gallium Arsenide (GaAs) technologies, and can address consumer applications,
* with RF MicroElectroMechanical Systems (RF-MEMS) [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25], and
* by using functional materials such as ferrites [26; 27; 28], ferroelectrics, mainly Barium Strontium Titanate (BST) capacitors, filters, and phase shifters in thin or thick-film technology [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44] and the Microwave Liquid Crystal (MLC) technology beyond optics. With the emergence and progress of newly developed LC mixtures specifically synthesized for microwaves since 2002 [45; 46; 47; 48; 49; 50], innovative concepts and designs with appropriate biasing schemes enabled numerous high-performance MLC components and devices in the RF domain, which fully exploit the microwave LC's unique properties: various planar delay line, metallic  
Figure 4: Scenario of various applications and platforms with some important characteristics, including beam-steering antennas and agile components, following the software-controlled radio approach. Reprinted from [2], Cambridge University Press, 2020.  
and dielectric waveguide phase shifters at different frequencies [45; 46; 47; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84], tunable resonators and filters [85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 90; 910; 911; 912; 913; 914; 915; 916; 917; 918; 919; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104], tunable metamaterial structures, frequency-selective and high-impedance surfaces [105; 106; 107; 108; 109; 100; 101; 102; 103; 105; 106; 107; 108; 109; 110; 111; 111; 112], and electronically steerable or reconfigurable antennas, which might be further subdivided into frequency-agile antennas [122; 123; 124; 125; 126; 127; 128], polarizer and polarization-agile antennas [129; 130; 131; 132; 133], deflecting gratings and lens antennas [134; 135; 136; 137; 138; 139; 140], mixed beam-switching and beam-steering antennas [141; 142; 143], beam-steering reflectarrays [144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157], leaky-wave antennas [158; 159; 160; 161; 162; 163; 164], and beam-steering phased array antennas [165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179]. These MLC components are discussed in some detail in [2]. In contrast to traditional microwave reconfigurable components with PIN diodes, Field-Effect Transistor (FET), RF-MEMS, ferrite switches or Monolithic Microwave Integrated Circuits (MMICs), MLC, and ferroelectric components provide continuous tuning with extremely low power consumption (quasi powerless), but with different response times. Compared to MLC components with response times from milliseconds to several tens of seconds (depends on the line topology and biasing scheme), ferroelectric ones can be tuned extremely fast in the nanosecond range; however, these are accompanied with much higher dielectric losses above 10 GHz than MLC components. A first review of the potential of the MLC technology was given in [180] and more specifically in [181], including material properties and characterization as well as some exemplary examples of MLC components.