Editorial for the Special Issue on the New Trends in Microwave/Millimeter Antennas/Filters: From Fundamental Research to Applications  
The rapid growth of wireless communication systems has led to high demand for the design of microwave/millimeter components with multiband characteristics, high performance, and ease of integration with other devices [1, 2, 3]. Recently, 5G wireless communication networks have started to stimulate the development of beam-steering techniques [4, 5, 6]. In comparison with previous technologies, including 4G wireless applications, 5G is shifting to higher frequencies to offer wider bandwidths and provide a higher capacity [7, 8, 9]. The use of millimeter wave and sub-6 GHz bands has been proposed to benefit services supporting networks of small/large cells facilitating high-capacity hotspot zones while increasing area efficiency [10, 11, 12]. In this Special Issue, we target the latest technology and developments in microwave/millimeter system components and aim to overcome technical challenges by bringing together academics and industrial researchers to identify and discuss novel results within this continuously evolving field. The goal of this Special Issue is to stimulate the community by addressing the key issues on the topic in the hope that antennas and filters will make a greater impact in our society. The 19 published papers in this volume have presented cutting-edge research findings. In total, 17 papers have discussed antennas, MIMO antennas, and RFID, and two of them discussed filters. The submitted papers are from authors from different countries, including Australia, China, Egypt, India, Iraq, Ireland, Italy, Jordan, Malaysia, Morocco, Pakistan, Palestine, Saudi Arabia, Spain, Turkey, and the United Kingdom. Novel techniques have been reported in this issue for the development of filters and antennas. Multiple-input multiple-output (MIMO) antennas are of great interest due to their attractive features. Five published papers have discussed the design of MIMO antennas for 5G, ultra-wideband (UWB), and other wireless applications. Ultra-wideband (UWB) technology is widely used in many communication scenarios. However, narrowband systems can easily interfere with a UWB system, which generates multipath fading. In order to solve these interferences and meet the design requirements of high isolation of multiple-input multiple-output (MIMO) antennas, two MIMO antennas with double-notch structures are designed (contribution 1). Firstly, two U-shaped slots are etched on the radiating patch and feeder to achieve notch characteristics in WiMAX and ITU bands. Using this antenna element, a two-element antenna is put symmetrically in parallel, and two rectangular branches are loaded to improve the isolation. The size is 0.57\(\lambda\) 0.32\(\lambda\) \(\times\) 0.013\(\lambda\) (at 2.5 GHz). Then, a four-element antenna is designed to meet the requirements for another application; here, each element is placed orthogonally to each other, and the isolation is improved by loading a cross-shaped branch in the middle of these elements. The size is 0.57\(\lambda\)\(\times\) 0.57\(\lambda\)\(\times\) 0.013\(\lambda\). Both antenna samples are tested to verify the design. The measurement results show that the working bandwidths are 2.45-14.88 GHz and 2.14-14.95 GHz, the isolations are greater than 17 and 20 dB, and the peak gains are 5.7 and 5.9 dBi for the two- and four-element MIMO antennas, respectively. Compared to the references, the designed antennas have a wider bandwidth and a higher gain and radiation efficiency. They are well-suited for diverse wireless applications. In another work, a 4 \(\times\) 4 miniaturized UWB-MIMO antenna using FR-4 material with reduced isolation was designed and analyzed using characteristic mode analysis (contribution 2). To minimize the antenna's physical size and to improve the isolation, an arrangement of four symmetrical radiating elements was positioned orthogonally.