# Passive Planar Microwave Devices
## 2 Contributions  
Planar technologies allow for a wide number of possibilities when facing new designs and challenges. In this Special Issue, a compilation of twelve relevant papers on the topic of passive planar microwave devices has been published. In [1], [PERSON] et al. presented the design of a two-port electronically tunable phase shifter with progressive impedance transformation in the K band. The phase shifter consists of a 3 dB hybrid coupler loaded with reflective phase-controllable circuits. They measured the prototype at a frequency of 18 GHz, showing a dynamic range of 600 degrees. [PERSON] et al. [2] proposed a new method to monitor patients with [PERSON]'s disease (HD) using wireless sensing technology. Experimental data were firstly collected by the self-developed microwave sensing platform (MSP) and subsequently preprocessed. Finally, support vector machine (SVM) and random forest (RF) algorithms were used to train the model. The MSP system continuously monitors patients in a noncontact manner, which offers more convenience and privacy. The experimental results show that the prediction accuracy of SVM can be as high as 98.0%, and that of RF can reach 96.7%. [PERSON] et al. [3] proposed a design for a multiband textile antenna for LTE and 5G communication services, composed of a rectangular microstrip patch, two concentric annular slots, and a U-shaped slot. They showed that the coral reef optimization, with the substrate layer (CRO-SL) algorithm, can produce a robust multiband textile antenna, including the LTE and 5G frequency bands. For the optimization process, the CRO-SL is guided by means of a fitness function obtained after antenna simulation by a specific simulation software for electromagnetic analysis in the high-frequency range. In [4], [PERSON] et al. presented a planar, microstrip line-fed, quad-port, multiple-input multiple-output (MIMO) antenna with dual-band rejection features for ultra-wideband (UWB) applications. The proposed MIMO antenna design consists of four identical octagonal-shaped radiating elements that are placed orthogonally to each other. The dual-bandrejection property (3.5 GHz and 5.5 GHz corresponding to Wi-MAX and WLAN bands) was obtained by introducing a hexagonal-shaped complementary split-ring resonator (HCSRR) in the radiators of the designed antenna. Isolation was observed to be higher than 18 dB, and the envelope correlation coefficient (ECC) was less than 0.07 for the MIMO/diversity antenna in the operating range of 3-16 GHz. [PERSON] et al. [5] proposed an ultra-wideband and miniaturized spoof plasmonic antipodal Vivaldi antenna (AVA) for ultra-wideband antenna applications, such as in the 2G/3G/4G/5G base stations. They designed and measured an antenna operating from 1.8 GHz to 6 GHz, with an average gain of 7.24 dBi. [PERSON] et al's [6] review paper focuses on microwave sensors, particularly planar sensors based on resonant elements for material characterization, including solids and liquids. In addition, they reported the main advantages and limitations of microwave sensors based on electrically small planar resonant elements. Moreover, they presented several sensing strategies, with a special emphasis on differential-mode sensors. In [7], [PERSON] et al. proposed a new dual-band balanced bandpass filter based on magnetically coupled open-loop resonators in multilayer technology. The lower differential passband, centered at the global positioning system (GPS) L1 frequency, 1.575 GHz, was created by means of two coupled resonators etched in the middle layer of the structure, while the upper differential passband, centered at a Wi-Fi frequency of 2.4 GHz, was generated by coupling two resonators on the top layer. Magnetic coupling was used to design both passbands, leading to an intrinsic common-mode rejection of 39 dB within the lower passband and 33 dB within the upper passband. [PERSON] et al. [8] demonstrated the beneficial effects of introducing glide symmetry into a two-dimensional periodic structure.