Recent Advances in Microwave and Terahertz Engineering

In view of the peak-to-average power ratio (PAPR) of wireless communication base stations, a Doherty power amplifier with high efficiency maintained at output power back-off (OBO) can effectively solve the problem of low efficiency of the traditional power amplifier at the point of power back-off. In this paper, we propose a method to implement a dual-frequency Doherty power amplifier (DPA) using a step-impedance low-pass filter to improve the bandwidth and efficiency of the DPA at output power back-off (OBO). Step impedance low-pass filters are used to solve the bandwidth limitations in traditional impedance converters and improve the efficiency of Doherty power amplifiers to a certain extent. In order to verify the proposed scheme, an efficient concurrent dual-band Doherty power amplifier operating at 2.0/3.5 GHz is designed and fabricated for the first range 1 (FR1) of 5G applications. In the measured results, the concurrent dual-band DPA achieves a saturated output power of 44 dBm and drain efficiency of 62% with a 6 dB back-off efficiency of 53% at 2.0 GHz and a saturated output power of 43.5 dBm and drain efficiency of 68% with a 6 dB back-off efficiency of 58% at 3.5 GHz. Full article

Terahertz technology and vortex beams have demonstrated powerful capabilities in enhancing the channel capacity of communication systems. This work proposes a design strategy of dual-band and dual-function 3-bit coding metasurface based on beam polarization characteristics. The unit cell of the metasurface is composed of two pattern structures, which has the ability to flexibly and independently control the reflection phases of incident plane wave at two frequency bands. The metasurface designed in this work is a combination of two patterns according to the addition operation and the convolution operation. The 3-bit coding metasurface generates two orbital angular momentum (OAM) beams with a deflection of 12.1° with modes $l_1=+1$ and $l_2=-1$ under the y-polarized incidence at 0.6 THz. Similarly, the designed metasurface produces two OAM beams with a deflection of 16.5° under the incidence of x-polarized wave at 0.9 THz, and the modes are $l_3=+1$ and $l_4=-2$. The full-wave simulation results agree well with the theoretical predictions, which could prove the correctness and effectiveness of the proposed method. The metasurface designed according to this method has potential applications in multiple-input multiple-output (MIMO) communication systems. Full article

This article presents the methodology to design a single-fed circularly-polarized antenna with low front-to-back ratio (FBR). A circular-patch (CPatch) antenna has been incorporated within the rectangular-cavity, made of, substrate integrated waveguide (SIW). The size of the CPatch and the SIW cavity has been chosen appropriately, in a manner, that the both resonators dominant mode coincide. This arrangement has been adopted to realize the basic radiating unit with no surface-wave and the significantly lower FBR. The circularly polarization has been excited through shorting the periphery of CPatch radiator to the “one of the two metallic grounds” of this SIW cavity. The patch periphery has been shorted from two distinct points, separated by the quarter wavelength—over center frequency of working band. The antenna has been designed and manufactured over Rogers RT/Duroid 5880 substrate with dielectric constant (ε_r) of 2.2, loss-tangent (tan δ, at 10 GHz) of 0.0009, and substrate height of 0.508 mm. Southwest^® end launcher (SEL) along with SIW-to-GCPW (Grounded Co-Planar Waveguide) transition has been used here to facilitate the measurement of antenna’s electrical and the radiation performance. The designed antenna’s impedance bandwidth and the 3 dB axial-ratio (AR) bandwidth is 9.5% and the 2.3%, respectively. It’s simulated and the measured peak gain, within working frequency band, is higher than 8.5dBic. The proposed antenna’s FBR is antenna is significantly lower than the conventional circularly-polarized antennas. Through comparative study, with work in open literature, it has been demonstrated that the designed antenna, based on proposed method, can a potential candidate for applicable in satellite and in the other spaceborne communication system’s module—at ground and in the space station. Full article

The outdoor terrestrial terahertz (THz) communication links have recently attracted great research and commercial interest in response to the emerging bandwidth-hungry demands for extremely high-speed wireless data transmissions. However, their development is hindered by the random behavior of the atmospheric channel due to the molecular attenuation, adverse weather effects, and atmospheric turbulence (along with free space path loss (FSPL) and pointing errors) due to the stochastic misalignments between the transmitter and the receiver. Thus, in this work, we investigate the joint influence of these detrimental effects on both capacities, i.e., average (ergodic) and outage, of such a typical line of sight (LOS) THz communication link. Specifically, atmospheric turbulence-induced intensity fluctuations can be modeled by using either the suitable gamma or the well-known gamma–gamma distribution for weak and moderate to strong turbulence conditions, respectively. Additionally, weak to strong stochastic misalignment-induced intensity fluctuations, due to generalized pointing errors with non-zero boresight (NZB), are emulated by the appropriate Beckman distribution. Taking into additional consideration the unavoidable presence of FSPL and the different but realistic water vapor concentration values along with the influence of weather conditions, an outage performance analysis has been conducted. Considering the abovementioned significant effects, novel analytical mathematical expressions have been extracted for both average (ergodic) and outage capacity, which are critical metrics that first incorporate the total influence of all of the above significant effects on the THz links’ performance. Through the derived expressions, proper analytical results verified by simulations are presented and demonstrate the validity of our analysis. It is notable that the derived expressions can accommodate realistic parameter values involved in all the above-mentioned major effects and link characteristics. In this context, they provide encouraging quantitative results and outcomes for both capacity metrics under investigation. The latter enables the design and the establishment of modern and future high-speed THz links, which are expected to cover longer propagation distances and thus become even more vulnerable to atmospheric turbulence effect. This is modeled and incorporated in our analysis and expressions contrary to most of the previous works in the open technical literature. Full article

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS₂ on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy. Full article

Millimeter-wave (MMW) imaging has a tangible prospect in concealed weapon detection for security checks. Typically, a one-dimensional (1D) linear antenna array with mechanical scanning along a perpendicular direction is employed for MMW imaging. To achieve high-resolution imaging, the target under test needs to keep steady enough during the mechanical scanning process since slight movement can induce large phase variation for MMW systems, which will result in a blurred image. However, in the scenario of imaging of a human body, sometimes it is difficult to meet this requirement, especially for the elderly. Such blurred MMW images would reduce the detection accuracy of the concealed weapons. In this paper, we propose a deblurring method based on cycle-consistent adversarial network (Cycle GAN). Specifically, the Cycle GAN can learn the mapping between the blurred MMW images and the focused ones. To minimize the effect of the shaking blur, we introduce an identity loss. Moreover, a mean squared error loss (MSE loss) is utilized to stabilize the training, so as to obtain more refined deblurred results. The experimental results demonstrate that the proposed method can efficiently suppress the blurring effect in the MMW image. Full article

In this paper, a triple-band terahertz chiral metasurface is proposed, which could realize spin-selective absorption (SSA) effect and efficient independent phase manipulation in three distinct frequency bands. Through the simulation of the surface current distribution, we explain the mechanism of the triple-band SSA effect. Furthermore, the introduction of Pancharatnam–Berry phase endows the metasurface with the ability to manipulate the reflection phase at the chiral resonance frequencies, which enabled simultaneous amplitude and phase manipulation of CP waves through different phase coding strategies. To test this concept, two terahertz SSA-coding metasurfaces were designed and simulated, which have the function of four-beam splitting and vortex wave anomalous reflection, respectively. These simple-structured multifunctional devices demonstrate the application prospects of the metasurface in terahertz chiral sensing, imaging, secure communications, etc. Full article

To meet the existing requirements of multiband communication and improve the efficiency and performance of communication RF modules, a concurrent tri-band high-efficiency power amplifier operating in three frequency bands is proposed. The input and output impedance values of concurrent power amplifier is analyzed, and the input and output-matching circuit and bias circuit are designed. Through the impedance compensation principle, the impedance matching of three frequency bands is realized, and the amplifier can maintain high power and high efficiency at three arbitrary wide interval frequencies. To this end, a simultaneous tri-band power amplifier is designed and tested by using transistor CGH40010F. The experimental results show that the peak power of the designed simultaneous tri-band high-efficiency power amplifier is more than 10 W, the power-added efficiency reaches 55~69%, and the amplification gain is about 10 dB at three frequency bands of 2.2, 2.6, and 3.5 GHz. The design of concurrent tri-band high-efficiency power amplifier is flexible, the calculation of microstrip line parameters is simple, and it can work in three frequency bands simultaneously. It provides an effective structure scheme for designing concurrent power amplifiers in transmitting systems. Full article

We demonstrate analytically the technique and arrangement of nanoparticle antenna arrays with the enhancement of optical characteristics at an optical frequency regime. The optical characteristics of the array are enhanced by introducing an inverse active spherical coated nanoparticle (I-CNP). This inverse active spherical coated nanoparticle is designed and combined with already demonstrated active CNPs. Consequently, three types of active CNPs and their inverse-based plasmonic nano-antenna array configurations have been designed and studied: two CNP configurations, two inverse CNP (I-CNP) configurations and a CNP with an I-CNP configuration in the presence of passive elements. Detailed near-field analysis contains an E-field, radiated power, scattering and absorption examination, whereas far-field analysis includes gain and pattern investigation. The finite-difference time-domain (FDTD) simulation results in CST depict the benefits of a CNP with an I-CNP array configuration in the presence of passive elements over the other two in terms of both near-field and far-field characteristics, at closer inter-element distances because of coupling avoidance with possession of a dipolar pattern. Full article