A low-profile and broadband metasurface antenna is proposed for circular polarization radiation. The metasurface is an array of subwavelength square patches. The modal behaviors of the proposed metasurface are investigated by using the characteristic mode theory. Two characteristic modes with the same resonant frequencies and orthogonal current distributions are chosen as operation modes. Furthermore, a hybrid feed system consisting of a cross-slot and a microstrip line is employed to excite the two orthogonal modes having 90° phase difference to obtain circular polarization radiation. Based on these concepts, an antenna with low profile of 0 0.07 $λ_\text{0}$ (0 $λ_\text{0}$ is the free-space wavelength at operation frequency of 5.5GHz) is designed. The measurement results show that the proposed antenna has-10-dB impedance bandwidth of 4.8-6.35 GHz and 3-dB axial-ratio bandwidth of 4.85-6 GHz. Moreover, the antenna gain is 6.8-9.7 dBic in the whole axial-ratio bandwidth.

A methodology to design a multi-functional device (MFD) with switchable absorption and polarization conversion (PC) modes is proposed in this research. The methodology combines graphene absorbing metasurface (GAM) and polarization conversion metasurface (PCM) in one component. The key point is that the absorbing metasurface (AM) should be made completely by conductivity tunable material, such as graphene. The PCM can be constructed by fine metal. A prototype is designed to verify the designing methodology by stacking a layer of gold pattern PCM and two layers of GAM with fishnet shape patterns. In the PC mode, the chemical potentials of the two GAMs are µ_c1 = µ_c2 = 0 eV, then, the graphene layers act as thin dielectric layers, which can be treated as transparency. Thus, the PCM plays a leading role in the performances of the whole structure. Therefore, a linear polarized incidence is rotated by 90° in the 2.10-3.13 THz (39.5% at 2.61 THz) with polarization conversion ratio (PCR) larger than 90%. In the absorption mode with µ_c1 = 0.5 eV and µ_c2 = 0.8 eV, the GAMs have strong surface plasmon-polaritons (SPPs), and they concentrate most of the incident power, which makes the GAMs dominate roles of the whole structure. Therefore, the incident power is dissipated in the GAMs, and the absorption rate is larger than 90% in the 1.30-3.01 THz (79.5% at 2.15 THz). The absorption band for absorption rate larger than 70% is 1.19-3.90 THz (106.7% at 2.54 THz). The design methodology is useful for the designing of components with switchable absorption and PC modes.

Intelligent radios collect information by sensing signals within the radio spectrum, and the automatic modulation recognition (AMR) of signals is one of their most challenging tasks. Although the result of a modulation classification based on a deep neural network is better, the training of the neural network requires complicated calculations and expensive hardware. Therefore, in this paper, we propose a master–slave AMR architecture using the reconfigurability of field-programmable gate arrays (FPGAs). First, we discuss the method of building AMR, by using a stack convolution autoencoder (CAE), and analyze the principles of training and classification. Then, on the basis of the radiofrequency network-on-chip architecture, the constraint conditions of AMR in FPGA are proposed from the aspects of computing optimization and memory access optimization. The experimental results not only demonstrated that AMR-based CAEs worked correctly, but also showed that AMR based on neural networks could be implemented on FPGAs, with the potential for dynamic spectrum allocation and cognitive radio systems.

Terahertz electromagnetic (EM) wave components usually have a single function, such as they can only convert the polarization state of an incident wave or absorb the incident energy, which would be a limitation for their applications. To make a breakthrough, a multi-functional device (MFD) is proposed in this paper, and it is capable of switching between absorption mode and polarization conversion mode. The device has a low-profile and simple structure, and it is constructed by graphene-based absorbing metasurface (AM) and gold-based polarization conversion metasurface (PCM). By controlling the chemical potential (μc) of the graphene, the leading role is transferred between the AM and the PCM, which leads to steerable absorption and polarization conversion (PC) modes. For the PC mode, the simulated polarization conversion ratio (PCR) is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz). For the absorption mode, the simulated absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). The physical mechanisms and operating characteristics of the MFD are discussed. This research has potential applications in terahertz imaging, sensors, photodetectors, and modulators.

We propose a high refractive index metamaterial （HRIM）that is formed by symmetrically etching square metal patches on both sides of a dielectric plate. By reducing the gap of adjacent patches and the thickness of dielectric plate, the proposed metamaterial can suppress the diamagnetic response and enhance the capacitance coupling, so that the refractive index of the metamaterial is higher than 10 in wideband from 4 GHz to 8 GHz. Attributing to the property of high refractive index, the metamaterial can effectively improve the performance of a microstrip antenna when it is loaded as a superstrate. The measured results reveal that the impedance bandwidth of S11< -10 dB is 690 MHz (12.4%) and the maximum gain reaches to 13 dB at 5.6 GHz. The 1 dB and 3 dB gain bandwidth are 380 MHz (6.7%) and 590 MHz (10.3%), respectively. The antenna has a subwavelength profile of 0.19"λ" _"0" " (" "λ" _"0 " is the free space wavelength of the antenna central frequency). Furthermore, the profile can be further reduced to 0.15"λ" _"0" when the radiation patch of the antenna is surrounded by metasurface.

A high-gain Fabry-Perot cavity antenna (FPCA) with wide 1-dB gain bandwidth in the S-band is proposed. A partially reflective surface (PRS) with a positive phase gradient in a wide band is adopted to effectively improve the radiation characteristics of the designed broadband source antenna. The proposed FPCA has a measured -10 dB impedance bandwidth of 2.9-3.95 GHz (30.65%), and a 1-dB gain bandwidth is 2.9-3.6 GHz (21%). In such a wide band, the realized gain is enhanced by up to 6 dB, and the measured maximum aperture efficiency is 39.3%.