Abstract-In this paper, a loading technique for improving pulse radiation from bow-tie antennas is introduced. This technique allows transmission of short transient pulses with very small latetime ringing and relatively high radiation efficiency. It makes use of a combination of a constant resistive loading along the antenna and a capacitive loading with linearly increasing reactance toward the antenna ends. The constant resistive loading is applied using volumetric microwave absorbers to cover one side of the antenna and the linear capacitive loading is realized by constructing narrow slots on the antenna surface. Relatively high radiation efficiency is achieved by choosing the location of the slot nearest to the feed point in such a way that radiation from it combines constructively with radiation from the feed point. Using a 0.8-ns monocycle for excitation, the technique results in a level of late-time ringing of lower than 40 dB and at the same time the peak value of the transmitted pulse is 54% higher than that of the same antenna without loading.Index Terms-Bow-tie antennas, capacitive loading, resistive loading, ultrawide-band antennas.
In this article, a MIMO antenna was designed to operate for 4G mobile terminals. By using the resonance of ground planes and the inductive coil, the proposed antenna achieves simplified design, improved bandwidth and efficiency. The proposed MIMO antenna has an excellent ECC value below 0.1 due to the nearly orthogonal radiation patterns from different antenna elements over LTE band 13. Moreover, an additional antenna element is adopted to simultaneously cover WiMAX, WLAN, and Bluetooth services in the frequency range from 2 to 2.7 GHz. These properties make the proposed MIMO antenna a promising choice for 4G mobile applications. In addition, we proposed a general guideline for designing small MIMO antennas simultaneously with improved ECC and isolation. These approaches could also be helpful for research in small MIMO mobile antennas. ABSTRACT: In this work, a step-by-step development of a compact microstrip rat-race coupler (RRC) has been presented and discussed.A high degree of miniaturization has been obtained by substituting six quarter-wavelength uniform atomic building blocks of a RRC by their nonuniform counterparts. The miniaturization procedure has been realized in three progressive steps: (i) the first layout solution of a miniaturized RRC has been acquired using an accurate mathematical model of a nonuniform transmission line (NUTL); (ii) the second layout solution has been obtained by a manual refinement and electro-magnetic (EM) fine-tuning of the NUTL geometry aimed at the utilization of the interior of the RRC in a highly efficient manner; (iii) the final layout, occupying only 8.5% estate area in comparison to a conventional RRC, is the result of the EM optimization. The experimental results confirm the theoretical predictions, proving the vital usefulness of the approach.
Abstract. In this paper, electromagnetic wave scattering from a slightly rough interface inside a stratified medium is considered. A three-layer model for the medium is chosen, which allows us to investigate both possible cases of the medium's stratification: above and below the rough interface. It also is a simple but realistic model for the ground, in which the upper layer is thought to be vegetation or snow, while the middle layer and the homogeneous half-space below it represent the ground itself. The vegetation-ground interface is considered to be rough with statistically homogeneous corrugation, while the two other interfaces are flat. The backscatter coefficients for vertically and horizontally polarized waves are found in the framework of the small perturbation method combined with the Green function formalism. Such an approach allows one to consider arbitrary layers and, in this respect, go far beyond the radiative transfer theory. The influence of the stratification of the medium above and below the rough interface on the scattered field is analyzed numerically. Qualitatively new angular and frequency dependencies of backscattering are found. It is shown that in the case of small ohmic losses stratification of a medium can cause the deviation of backscattering up to 20 dB.
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