This paper presents a novel low-complexity method to characterize the permittivities of low-loss dielectric slabs at millimeter-wave and sub-terahertz bands. The method assumes plane-wave illumination of the material under test and utilizes reflected fields on two air-material interfaces, resolved through a sufficiently wide-band measurement. Unlike conventional methods, it does not need a reference measurement using a metal plate and a back metal plate. The applicability of the method is discussed. The misalignment of a reference plane and the effects of the gap between the back metal and the material under test are analyzed for published reflected-field based methods to show the advantages of the method. Finally, the proposed method is applied to estimate the permittivities of both plexiglass with 30 mm thickness and nylon with 21 mm thickness across 140-210 GHz and compared against the free-space transmission method, showing good agreement. The proposed method is suitable for permittivity characterization of low-loss materials when a wide-band plane-wave measurement is possible.

This letter introduces a permittivity characterization method for thin materials. It can operate without reference-plane calibrations in free space, reducing system complexity, especially at THz frequencies. The relevant solving formulas and resolution analysis are derived. Full simulations validate the proposed method and analyze its performance with different thicknesses and loss tangents, showing excellent performance for common materials. When applying the method to characterize the permittivities of different samples in the G-band, it provides accurate estimates, particularly for high-loss materials. Therefore, it is a proper candidate for permittivity characterization of thin materials.

Penetration of cellphones into markets requires their robust operation in time-varying radio environments, especially for millimeter-wave communications. Hands and fingers of a human cause significant changes in the physical environments of cellphones, which influence the communication qualities to a large extent. In this paper, Referential cellphone antenna arrays at 28 and 39 GHz are designed, and then simplified electromagnetic models of them for simulations are developed. Their radiation properties are evaluated through near-field scanning of the two prototypes in free space, first for de-embedding the feedline loss of cellphone antennas. The similarity of radiation patterns between simplified models and cellphone mock-ups ensures the reliability for comparing simulated and measured results involving real-hand effects. Next, hand-antenna-interaction radiation measurements are set up so that we validate radiation performance simulations that use computational models of real hands that are generated by the approach in [1] and cellphones, by carrying out measurements of the same real hands and cellphone mock-ups and comparing measured to simulated results, which we show to agree well. This enables future research and development of mm-wave handset antennas relying on realistic simulations during the design phase.

This article introduces a novel approach to experimentally characterize the effective human skin permittivity at sub-terahertz (sub-THz) frequencies, specifically from $140$ to $210$ GHz, utilizing a quasi-optical measurement system. To ensure accurate measurement of the reflection coefficients of human skin, a planar, rigid, and thick low-loss dielectric reference plate is utilized to flatten the human skin surface. Then, a permittivity characterization method is proposed that considers phase correction for displacements and inclination of the reference plate due to the nonrepeatable behavior of human subjects in placing their body parts on the reference plate to flatten skin surfaces. The phase correction ensures good measurement repeatability. We show that the measured complex relative permittivity of the finger, palm, or arm of seven volunteers shows small standard deviations for repeated measurements of the same skin area of the same person. Meanwhile, we see significant permittivity variations across different skin regions of a human body and for different persons most likely due to varying conditions of layered structures of the skin and their moist levels. The proposed measurement system holds significant potential for skin permittivity characterization in sub-THz bands, facilitating in-depth studies on human-electromagnetic wave interactions based on the measured permittivity values.

A novel synthesis method of designing polarization rotation pattern manipulation surface (PRPMS) with enhanced performance under oblique incidences and scanning to arbitrary directions is presented in this article. Firstly, the wave conversion process of the structure with incident angle, scan angle, and polarization is theoretically investigated by the proposed equivalent circuit model (ECM), which reveals that the polarization conversion ratio (PCR) is determined by the impedance transforming via the patch. Then, two methods are proposed to achieve a relatively constant relationship of PCR with the incident angle or scan angle, including broadening the beamwidth of the active element patterns and adjusting the feeding positions according to our proposed theory. To reach these goals, four open-ended stubs are loaded on the patch, so as to broaden the 3-dB beamwidth of the patch by reducing the electrical length between two equivalent slots. Taking advantage of the transmission line theory, the desired PCR with different incident angles and scan angles can be easily achieved by only tuning the equivalent feeding positions of the patch. Finally, based on the phased array theory, three PRPMS prototypes are calculated, designed, fabricated, and measured to verify our design concept. Good agreements between these results show that the PRPMS can successfully alter the incident wave to the wanted angle and rotate the polarization while maintaining the PCR over 80%. The results verify that it is possible to use this method in the design of functional surfaces.

Under-sampled arrays exhibit low mutual coupling, alleviating heat dissipation challenges associated with active radio frequency (RF) components in integrated front-end designs. This paper presents an effective dielectric dome lens (DDL) for a uniform array with one-wavelength inter-element spacing, enabling beam scanning with reduced grating lobes, making it suitable for millimeter-wave communication systems. The shape of the DDL is optimized using the geometric optics (GO) algorithm, allowing for the reflection of grating lobes while maintaining effective main beam scanning. The DDL is then miniaturized, integrated with absorber sheets to dissipate reflected grating lobes, and equipped with matching layers to improve radiation performance. The subsequent full-wave simulation further optimizes the design, accounting for factors overlooked by the GO algorithm and finalizing the DDL structure for operating with a under-sampled array. Finally, a 3 × 8 under-sampled rectangular patch array working at 28 GHz, integrated with a cylinder DDL, is fabricated and measured to validate the design approach. The experimental results are in agreement with the simulations. With the help of the DDL, the array with one-wavelength inter-element spacing achieves a continuous, symmetrical-to-boresight beam-scanning range of 110°, with a scan loss (SL) within 3 dB. The grating lobe is consistently reduced, maintaining peak side-lobe levels more than 9 dB below the main beam. This scanning performance remains effective over a bandwidth of 2 GHz.