## 1 Introduction  
RF-MEMS, i.e., Micro-Electro-Mechanical Systems (MEMS) for Radio Frequency (RF) applications have been around for a long time. First discussed in scientific papers after the mid-1990s [1; 2], MEMS-based RF possives quickly ignited huge expectations in terms of market breakthroughs, primarily because of their remarkable and unprecedented electromagnetic characteristics, like wideband operability, large reconfigurability/tunability, very-low loss, high-isolation, high Q-factor [3].  
Despite the initial market expectations of RF-MEMS were disappointed, the context started to change quite radically after 2010 when smartphones started to become increasingly more popular. The increasing integration of devices in the handsets led to a progressive worsening of the quality of communications [4; 5]. Analog impedance matching tuners based on RF-MEMS technology were the first in line solution available [6], which made possible solving the problem, and, on the other hand, to score the first successful exploitation of RF-MEMS in mass-market applications. Moreover, the spread of RF-MEMS-based reconfigurable passive networks in the consumer segment is pulling applied research and engineering of other components based on Microsystem technology, with robust switches and switching units first in line [7,8].  
Besides the abovementioned case, another intriguing application field for high-performance RF-MEMS is gaining shape, which is the upcoming fifth generation of mobile communications, i.e., 5G, with all its implications and spillovers in the Internet of Things (IoT) scenario.  
Active development of 5G technologies and satellite communications demand for high-speed stable data transfer services from any location. Satellite systems providing high-speed Internet connectivity utilize K and Ka frequency bands offering a large bandwidth compared to lower frequencies. Conventionally, signal reception and transmission are carried out through two separate frequency channels in the K and Ka frequency bands, which, in turn, increase the demand for the high-performance components intended for signal control of the transceiver modules.  
In light of this scenario, this work presents and discusses a couple of design concepts of RF-MEMS-based reconfigurable phase shifters. From the architectural point of view, we relied on the True-Time Delay (TTD) topology, which is well-known in the literature. Given the field of application mentioned above, we tailored the design of the network in such a way to have the center frequency of operation at 19.5 GHz and 29.3 GHz, i.e., in the K and Ka band, respectively. Concerning reconfigurability, we opted for 4 stages (i.e., 16 configurations), controlled by electrostatically driven ohmic series RF-MEMS micro-relays. In particular, the K band phase shifter employs a classical transversal clamped-clamped switch topology. On the other hand, the Ka band phase shifters, given its higher operating frequency, imposes more stringent requirements in terms of reduced network footprint. Having in mind such a driver, we developed and implemented an innovative in-line longitudinal RF-MEMS switch design, conjugating the compactness typical of cantilevered (single-hinged) micro-relays, with the mechanical robustness and resilience of clamped-clamped solutions.  
The paper is arranged as follows. After the current section framing the evolution of RF-MEMS and the opportunities of the 5G and IoT application fields, the following Section 2 will introduce the reconfigurable phase shifters design concepts, along with modeling and simulations. Section 3 will develop a comprehensive discussion of the RF-MEMS phase shifters, leveraging the acquired experimental data. Finally, Section 4 will collect some conclusive considerations.