This study aims to introduce a simple equivalent circuit model of circular loop shaped frequency selective surface (FSS), predicting the plane‐wave transmission characteristics for oblique angles of incidence. The equivalent circuit model is based on a series of basic equations in order to calculate the inductance and capacitance of strip gratings. Through provided relations, the values of circuit elements can be calculated as well. The proposed relations are applied to many circular loop FSSs with different dimensions, and the results are compared with the full‐wave simulations obtained from computer simulation technology microwave studio. Moreover, some comparison graphs between the simulated and analytical resonant frequency for different values of design parameters have been carried out to show the circuit model follows the trend of the simulated responses. Finally, both the FSS structures have been fabricated and measured in an anechoic chamber. A good agreement between the experimental results and the simulated responses demonstrates the validity of the circuit model.

This article presents a new noniterative inverse modeling technique based on machine learning regression and its applications to microwave design optimization. The proposed inverse model accepts the high-dimensional S-parameters computed at many frequency points as the input and estimates the optimal geometrical/physical parameters of the microwave component as its output. The least-squares support vector machine regression is combined with the principal component analysis to simultaneously overcome both the high-dimensional input space and ill-posed challenges of the inverse modeling. We also propose a new empirical method to find the optimum number of principal components (i.e., the compression level) for each example in an automated way. This makes our proposed model general and easy to use compared with the existing data-driven inverse modeling techniques. The inverse model is trained by a set of scattering parameters computed via a 2-D/3-D solver for few configurations of the geometrical parameters. The feasibility and the accuracy of the proposed optimization scheme are investigated by comparing its predictions with the corresponding optimal configuration estimated via a commercial solver.

This study aims to present a thin and broadband meta‐surface for both mono‐static and bi‐static radar cross‐section (RCS) reduction being able to operate in large range of incident angles and different polarisations. Meta‐surfaces are formed through the combination of two artificial magnetic conductor (AMC) cells arranged in periodic and randomised fashions in order to suppress the scattered fields in low levels in all directions as well as minimising the maximum RCS of the metallic surface. By appropriate design of each AMC, and using equivalent circuit method, the reflection coefficients of these surfaces are designed in a way to reach 180 (±37°) phase difference over a wide range of frequencies and incident angles. The bandwidth for 10 dB mono‐static RCS reduction is about 53% for both periodic and randomised structures at the normal incidence. It is illustrated that the randomised arrangement structure has 3 dB lower maximum bi‐static RCS compared with the periodic structure. Finally, the periodic and randomised arrangement prototypes are fabricated and measurements of their mono‐static RCS are conducted. The validity of the study design is fulfilled by achieving an acceptable agreement between the measured and simulated results.

An analytical approach for calculating resonant frequencies of a circular microstrip antenna loaded with a shorting pin in arbitrary position is presented based on the cavity model. The antennas can be used as small antennas since their fundamental resonance frequency is lower than that of a regular circular patch antenna. Two scattered waves are used to modify the electric field of the antenna in the presence of the pin. The scattered waves are considered based on the theory of scattering in conjunction with the image method. The first scattered wave is produced by the pin while the second one is the wave backscattered by the patch edge of the antenna. The backscattered wave is assumed to be generated by the images of the pin with respect to the patch edge. It is shown that the proposed approach can predict the resonant frequency of the antenna reasonably adequate. Furthermore, it can be extended in the case of several shorting pins. The obtained results from the method are compared with the achieved results from the simulations, the measurements, and the relative previous analytical models, to confirm the quality of the approach.

A high-power narrow-linewidth low-threshold single-mode microsphere laser is investigated by coupling an undoped microsphere with different size to a single-microsphere laser. A model is proposed for microsphere lasers and it is verified by comparing its results to the experimental results published in the literature for a thulium doped tellurite glass single 12.5 μm microsphere laser. It is illustrated that by coupling an 11.5 μm undoped microsphere to a multimode 50 μm thulium doped tellurite glass microsphere laser, single-mode operation is obtained and despite the relatively high output power, the quality factor also remains high. For the proposed structure, an output power of 208 μW at 30 mW pump power is achieved. Its threshold power and FWHM linewidth are 0.06 mW and 0.53 fm (40 kHz) respectively which are significantly enhanced compared to the single 12.5 μm microsphere laser.