majid tayarani

In this study, a novel dual-band multiple-feed microstrip antenna is introduced for GNSS applications. The proposed antenna has a compact size of 1548 \(mm^2\) and counts as a low-cost, low-profile antenna. The mathematical equations of multiple-feed configuration are driven to analyze circular polarization (\(CP\)) generation and find the effective parameters on \(CP\) value. It is shown that for an n-feed symmetric antenna, the phase shift of \(\frac{2\pi}{n}\) between ports leads to pure \(CP\), regardless of the electrical field pattern. However, if the first feed electric field has equal value in the x and y direction and also 90$^{\circ}$ phase shift, the \(CP\) value is increased. A  symmetric four-feed configuration is employed on the antenna to generate \(CP\). Also, fractal circular rings and perturbing sectors are applied to miniaturize the antenna and improve the \(CP\) value. Numerical simulation shows that all feeding network requirements are met and the phase shift is satisfied at both frequencies by using the coupler technique. Furthermore, results show desirable band width of 48 \(MHz\) and 60 \(MHz\) and high front to back ratio of 16.1 and 14 at \(f_1\)=1.227 \(GHz\) and \(f_2\)=1.575 \(GHz\), respectively. The axial ratio (\(AR\)) beam-width at both frequencies is more than 66$^{\circ}$.

In this study, a novel blind adaptive algorithm with discrete periodic variables is introduced. The proposed discrete periodic variable algorithm (DPVA) has been applied to the new designend 7-element antenna array. The DPVA is based on minimizing the output power to steer null (or nearly null) gain in the direction of interference. Discrete phase shift leads to the use of hybrid phase shifter in practice and thus reduction of implementation costs. In addition, the proposed algorithm has low computational complexity. Results show that DPVA has fast and reliable convergence. It is converged within less than 400 iterations and less than 1 millisecond time duration. The null depth created by this algorithm is 90 \(dB\) which is an indication of pure cancellation of the interference. Furthermore, the effect of a number of interference sources is investigated. It is shown that the null depth is decreased by the increase of interference sources. In the studied 7-element array, increasing the interference sources up to 6 decreases the null depth to 20 \(dB\).

In this paper, a comprehensive study is performed on the realization of inhomogeneous materials, the extraction of the electromagnetic parameters, and the applications in microwave engineering. First, the concepts and definitions of inhomogeneous materials are presented as one of the main categories of electromagnetic complex media. Then, the applications of these materials in microwave engineering are introduced, namely gradient refractive index lenses, radar absorbing materials and radomes. In the next step, the realization methods of these materials are presented. Designing devices and achieving various applications requires intimate knowledge of the electromagnetic parameters. In the last part, the methods for retrieving and measuring the electromagnetic properties of inhomogeneous materials in the microwave frequency range are explained. Numerous articles which deal with the subject of inhomogeneous materials in microwave engineering, confirm the importance of this topic. According to the applications mentioned for inhomogeneous materials, these can be used in military and naval systems to increase the country's defense and military capabilities. Therefore, it is important to perform a comprehensive review of inhomogeneous materials in microwave engineering and to make researchers more familiar with this field and facilitate the future researches. In this research, in the form of a review article, this issue has been addressed.

We study through a drywall imaging using synthetic aperture radar (SAR) method. Transmission and reflection characteristics of the wall are analyzed by a rigorous coupled-wave analysis (RCWA) method. The Bragg modes are investigated on analytic formulation and reconstructed image. The cutoff frequency for different Bragg modes is calculated and the relative power carried by each of these modes is shown. The Green function of such a periodic structure is derived by representing a line source radiation in the spectral domain. The analytical results are then validated by the numerical FEM results using the COMSOL software and the effect of the number of the Bragg modes is shown. Having numerically calculated backscattered fields of a target behind the wall and the free-space Green function, the SAR image of the target is computed and the image artifacts due to the presence of the wall are addressed. Firstly, we show that the target image is still distorted and not focused even with the background subtraction. Secondly, by properly employing the phase of the wall Green function instead of the free-space Green function, we indicate that the target image is successfully refocused.

This paper presents the design and fabrication of two second harmonic (SH) signal generators based on GaAs (HG1) and GaN (HG2) technologies, which simultaneously perform the same function as a variable amplifier, phase shifter, and attenuator. With the capability to adjust its impedance matching capacitors, this integrated network produces a variable amplitude and phase shift at the second harmonic frequency. This design is a good choice for performance in active feedback linearization. It has been shown that by injecting the output signal of the power amplifier to the input of the SH signal generator and applying its output to the input of the power amplifier, a significant improvement in third-order intermodulation (IMD3) and adjacent channel power ratio (ACPR) can be achieved. To investigate the linearization performance of both categories, 2-tone and modulated signal tests are performed on a 10 W GaN power amplifier at 1.5 GHz with the proposed circuit. The measured data show a 24 dB decrease in IMD3 and a 13 dB decrease in ACPR for a 4-MSPS 64-QAM modulated signal compared to the power amplifier at 3 dB back-off.

In this study, a new simple method for design and implementation of rectangular waveguide leaky wave antenna with the longitudinal long slot is presented. In this method which has been done using HFSS, firstly, the waveguide attenuation is calculated based on the position of the slot in relation to the waveguide broad wall. Then the desired aperture illumination is fitted to the slot position on the broad wall of the waveguide by MATLAB. Finally, a Taylor-distributed rectangular waveguide leaky wave antenna with 30 dB SLL designed and fabricated in ku band. The good agreement between the simulation and the measurement results for radiated patterns verifies the effectiveness of the presented method.

An effective method is introduced to overcome the narrow strip width in unequal Wilkinson power divider with high dividing ratio. In the proposed Wilkinson power divider، the separated microstrip lines are replaced by a uniform asymmetrical microstrip coupled lines. To design the power divider، a suitable error function is defined and then minimized which led to the final dimensions of the proposed unequal Wilkinson power divider. A sample of the proposed power divider with 4:1 dividing ration is designed and simulated، which indicate the effectiveness of the proposed power divider and its design method.

The Application of elliptic shaped ridge waveguide is presented for excitation of a centreline longitudinal shunt slot antenna. The proposed method is realized by adding elliptic shaped to the ridge of the single ridge waveguide. The elliptic shaped ridge is placed exactly under the longitudinal slot and the slot placed along centreline of the waveguide. It is shown that for a typical slot، the slot normalized conductance can be increased by increasing the elliptic shaped depth. The simulation results show that the proposed structure can be considered as a proper candidate for replacing the conventional longitudinal shunt slot.