With the rapid development of millimeter wave technology, it is a fundamental requirement to understand the permittivity of materials in this frequency range. This paper describes the dielectric measurement of undoped silicon in the E-band (60–90 GHz) using a free-space quasi-optical system. This system is capable of creating local plane wave, which is desirable for dielectric measurement in the millimeter wave range. Details of the design and performance of the quasi-optical system are presented. The principle of dielectric measurement and retrieval process are described incorporating the theories of wave propagation and scattering parameters. Measured results of a sheet of undoped silicon are in agreement with the published results in the literature, within a discrepancy of 1%. It is also observed that silicon has a small temperature coefficient for permittivity. This work is helpful for understanding the dielectric property of silicon in the millimeter wave range. The method is applicable to other electronic materials as well as liquid samples.

A compact broadband four-port multiple-input-multiple-output (MIMO) antenna for autonomous aerial vehicles to assist automotive communication is proposed. By utilizing a 3-D ground dielectric layer, the microstrip patch elements are vertically placed on each side of the cube to achieve omnidirectional coverage. Grounding branches are introduced on both sides of the radiating patch to realize short circuit, significantly improves impedance matching, resulting in a broadband of 4.16–8.84 GHz and a compact size of 70 mm $\times $ 70 mm $\times $ 10 mm. Additionally, parasitic ground structures are incorporated inside the 3-D ground dielectric layer, effectively enhancing port isolation. Measurements show that the port coupling coefficient is better than -18 dB in the entire frequency band, and it is even superior to -25 dB in the V2X frequency band. The envelope correlation coefficient remains below 0.0048 throughout the operating band. Furthermore, the diversity gain achieves 9.992 dB, while the channel capacity loss is maintained below 0.4 bits/s/Hz. This design provides an efficient and stable signal transmission solution for drone-assisted automotive communication systems.

A broadband polarization rotator built on single substrate is presented in this work. The device is designed for operation in the K and Ka bands. A slant array is used to achieve polarization rotation by 90° in a reflective manner. Broadband has been obtained, with the operation frequency range covering 15–45 GHz for 3 dB criteria, which is almost 100% fractional bandwidth. In addition, the insertion loss is less than 0.3 dB over a moderate broad incident angle from 0°–20°. Furthermore, the polarization conversion ratio can be as high as 0.95. By using a bi-static method, the fabricated prototype is measured, and the measured results demonstrate satisfactory agreement with the simulation ones. In comparison with other reflective designs in the literature, this design provides good bandwidth as well as polarization conversion ratio. Miniaturization can be investigated to increase the angular stability.

A compact antenna array is developed for W-band vehicle mounted millimeter-wave radar, also suitable for internet of things (IoT) and intelligent vehicular transportation systems (IVTS). The proposed array provides a flat shoulder shaped radiation pattern, obtained using a beamforming algorithm, enabling medium-and long-range radar detection and information transmission between wireless devices. The beamforming algorithm determines the excitation amplitudes and phases of each sub-array. Power dividers are designed using a scheme of equal/unequal power division to meet the requirements of amplitude. And phase shifters are used to create required phases. To obtain a compact design, a substrate integrated waveguide (SIW) feeding structure are employed to integrate power dividers, phase shifters, and waveguide transitions into a single part. Quadratic recursive exhaustive search in combination with full-wave simulation is used to optimize the design. Further improvements are made to achieve a more compact feeding structure. The realized array provides a stable flat shoulder shaped radiation pattern in the H-plane. In addition, its maximal gain appears in the boresight direction, which is usually a challenge for patch element array. The realized gain is 22 dBi, and the ripple in the shoulder range is less than 2.3 dB, which is better than other designs. The feeding structure is only 14mm× 16mm large, showing a minimized design. The demonstrated antenna presents itself to be an excellent hardware candidate for IoT and IVTS.

A low-profile flexible tri-band antenna deployed on autonomous aerial vehicles (AAV) for both communication and navigation uses is presented. The patch antenna is designed on a 0.1mm thick flexible polyimide substrate, conforming to the surface of an AAV. In addition, polyimide substrates bear the advantage of low weight. Slot structures are used to create three operating bands. By adding two asymmetrical spiral units, the impedance matching is realized in the ranges of 1.53 GHz to 1.61 GHz, 2.37 GHz to 2.50 GHz, and 5.13 GHz to 6.50 GHz. These bands are allocated by the Ministry of Industry and Information of China for AAV communication and navigation band. Moreover, the spiral units enable circular polarization from 1.53 GHz to 1.61 GHz. Its 3 dB axial ratio spans 1.564 GHz to 1.584 GHz. The antenna measures 50mm × 60mm × 0.1mm, showing a satisfactorily low profile. The antenna has been mounted on different curved surfaces. It is observed that though the working frequencies undergo slight shift, they remain within the allocated bands, demonstrating good flexibility. Its nature of low profile, flexibility, lightweight, and multi-band design envisages potential application for AAVs.