This paper presents high-sensitivity sensors based on an open complementary square split-ring resonator and a modified open complementary split-ring resonator operating at 4.5 GHz and 3.4 GHz, respectively. The sensors are designed for the detection of multiple liquid materials, including distilled water, methanol, and ethanol. The liquid under test is filled in a glass container loaded using a pipette. Compared to the conventional OCSSRR, the modified OCSSRR with multiple rings exhibits a higher frequency shift of 1200 MHz, 1270 MHz, and 1520 MHz for ethanol, methanol, and distilled water, respectively. The modified sensor also demonstrates a high sensitivity of 308 MHz/RIU for ethanol concentration which is the highest among the existing microwave sensors. The sensors in this manuscript are suitable for multiple liquid-material-sensing applications.

An ultrathin and wideband reflective cross-polarization converter is presented in this paper. The design consists of a pair of modified dumbbell shaped resonators which are connected together with a conductive strip. The proposed single-layer THz metasurface achieved a high polarization conversion ratio (PCR) of $\ge$ 95% in the operating frequency ranging from 1.76-4.26 THz. This wide-bandwidth cross-polarization conversion is obtained by exciting multiple closely spaced resonant frequencies at 1.98 THz, 2.61 THz, 2.95 THz, and 3.93 THz. The working mechanism of the proposed structure is analyzed and the polarizer exhibits an excellent angular stability upto 40 $^{0}$ of incidence. Further, the proposed cross-polarizer exhibits continuous tunability of PCR over the operating frequencies when photoconductive silicon (Si) is filled in the gap between pair of dumbbell-resonators. The wideband characteristics and the switchable property show that the proposed reflective cross polarizer is an excellent candidate for manipulation and control of polarization in THz communication systems.

A self-multiplexing SIW antenna based on hexagonal SIW cavity is proposed. The structure consists of resonating elements of different sizes, which provide the hexaband operations. The antenna resonates at 5 GHz, 5.17 GHz, 5.32 GHz, 5.53 GHz, 5.62 GHz, and 5.72 GHz by employing different slot lengths between the resonating elements. The antenna geometry is arranged so that independent tuning of individual operating frequencies is possible without affecting the remaining center frequencies. The self-hexaplexing antenna exhibits a port isolation better than 29 dB between the resonating elements with a low frequency ratio of 1.14. The simulated gain of the antenna is 5.32 dBi, 5.68 dBi, 5.41 dBi, 5.91 dBi, 5.43 dBi and 5.14 dBi at the respective operating frequencies. The proposed self-hexaplexer operates in the NR band (n46) therefore being suitable for communication system applications.