Core–shell composites, Fe3O4@C, with 500 nm Fe3O4 microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe3O4 surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe3O4 microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe3O4 micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe3O4 micropsheres can promote their graphitization degree to a certain extent. Coating Fe3O4 microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very inte...

Shell thickness-dependent microwave absorption of core-shell Fe3O4@C composites.

In recent years, the dielectric resonator antenna (DRA) has emerged as a new and viable alternative to conventional low-gain elements such as dipoles, monopoles, and microstrip patches. This practical resource presents complete, up-to-date details on DRAs in a single volume. The book provides you with clear guidance on the mode of operation and radiation behavior of DRAs, the main methods of excitation, and the major advances in DRA technology. This hands-on reference equips you with simple equations and graphs that help you rapidly design DRAs of spherical, cylindrical, and rectangular shapes, without having to resort to complex analytical or numerical calculations. You find guidelines for designing feeds required to excite the DRAs, such as probes, apertures, and microstrip lines. In addition, the book offers you various techniques for enhancing the bandwidth performance of DRAs for wideband applications. You learn how to design low profile DRAs and DRAs with circular polarization. Several approaches for designing linear and planar DRAs arrays are also considered. Moreover, this comprehensive book provides advice on the fabrication of DRAs and measurement methods used to characterize their performance. Numerous design examples are included to give you a sense of the versatility that DRAs afford.

Dielectric resonator antenna handbook

Metal-organic-frameworks derived porous carbon-wrapped Ni composites with optimized impedance matching as excellent lightweight electromagnetic wave absorber

This paper focuses on a tutorial overview of defected ground structure (DGS). The basic conceptions and transmission characteristics of DGS are introduced and the equivalent circuit models of varieties of DGS units are also presented. Finally, the main applications of DGS in microwave technology field are summarized and the evolution trend of DGS is given.

/pdf/an-overview-on-defected-ground-structure-3dt7yj00ew.pdf

An Overview on Defected Ground Structure

It is well accepted that the microwave absorption performance (MAP) of carbon nanotubes (CNTs) can be enhanced via coating magnetic nanoparticles on their surfaces. However, it is still unclear if the magnetic coating structure has a significant influence on the microwave absorption behavior. In this work, nano-Fe3O4 compact-coated CNTs (FCCs) and Fe3O4 loose-coated CNTs (FLCs) are prepared using a simple solvothermal method. The MAP of the Fe3O4-coated CNTs is shown to be adjustable via controlling the Fe3O4 nanocoating structure. The results reveal that the overall MAP of coated CNTs strongly depends on the magnetic coating structure. In addition, the FCCs show a much better MAP than the FLCs. It is shown that the microwave absorption difference between the FLCs and FCCs is due to the disparate complementarities between the dielectric loss and the magnetic loss, which are related to the coverage density of Fe3O4 nanoparticles on the surfaces of CNTs. For FCCs, the mass ratio of CNTs to Fe3+ is then opti...

Enhanced Microwave Absorption Performance of Coated Carbon Nanotubes by Optimizing the Fe3O4 Nanocoating Structure

This paper discusses the analysis of a novel two-segment rectangular dielectric resonator antenna (DRA) for broadening of the impedance bandwidth. In the proposed configuration, two rectangular dielectric sections are used which are separated by a metal plate. With this configuration, it is possible to excite two adjacent resonant frequencies. Utilizing the two-segment thin DRA and skillfully varying its aspect ratio, an appropriate structure is obtained that illustrates more than 76.8% impedance bandwidth (for S11>10 dB) at 3.32- 7.46 GHz frequency band.

/pdf/design-of-wide-band-dielectric-resonator-antenna-with-a-two-1krfsxc6vt.pdf

Design of Wide-Band Dielectric Resonator Antenna with a Two-Segment Structure

In this paper,pattern search (PS) algorithms are introduced as a new tool for array thinning. It is shown that by selection of a fitness function which controls more than one parameter of the array pattern,and also by proper setting of weight factors in fitness function,one can achieve very good results. The method is tested on linear arrays of isotropic and non-isotropic elements and is shown to be useful in both cases. Mainlobe scanning to different angles is also tested and the results are success. In all cases studied in this paper,relative sidelobe level (SLL) is less than −20 dB with only small increase in mainlobe beamwidth compared to the case of a uniform array. Results of PS optimization are compared in two cases,which their starting points of optimization are different from each other. There is also a comparison made between results of array thinning using Genetic Algorithm (GA) and PS,and PS is shown to be a fast and reliable algorithm to be used in array thinning problem.

/pdf/thinned-arrays-using-pattern-search-algorithms-1afiylakp6.pdf

Thinned Arrays Using Pattern Search Algorithms

In recent years, there has been significant research interest in microwave applications of defected ground structures. In this paper, three kinds of Periodic Defected Ground Structures are analyzed to assess their effects on surface and leaky waves. To achieve this purpose, the finite element method accompanied by Periodic boundary conditions is used to show the propagation characteristics of these periodic structures. Also, the existence of surface wave reduction property is investigated and finally the simulation Results are compared to the experimental results with reasonable agreement.

/pdf/guided-and-leaky-wave-characteristics-of-periodic-defected-3deu33qc4v.pdf

Guided and Leaky Wave Characteristics of Periodic Defected Ground Structures

This letter presents a $\pi$ -model dual-band impedance transformer which can match two unequal arbitrary complex loads at two frequencies. The required nonlinear simultaneous equations are derived to obtain the physical parameters of transmission lines and stubs. Return loss for the proposed matching circuit is measured to validate the results of the presented structure simulations. Excellent agreement between analytical and measured results confirms the proposed structure.

$\Pi$ -Model Dual-Band Impedance Transformer for Unequal Complex Impedance Loads

In this paper, two new line-fed loaded planar antennas are proposed for ultra-wideband applications. The first antenna is a circular patch with a circular ring as a Defected Ground Structure (DGS). A 50 Ohm microstrip line passes through the antenna which is symmetrical between the feed and the load. The impedance bandwidth of the first antenna with S11 < −10 dB is more than 10 GHz, from 3 GHz to more than 13 GHz, in both simulation and measurement. It will be shown that the antenna has quite a stable radiation pattern and also high gain over its bandwidth. The second configuration is a rhomboidal patch which a 50 Ohm microstrip line passes through it. A rhomboidal DGS ring is employed to widen the bandwidth of the proposed antenna. The impedance bandwidth with VSWR < 2 is more than 10 GHz, from 3 GHz to more than 13 GHz, in both simulations and measurement. The second antenna has also quite a stable radiation pattern and high gain values in its frequency band. For these antennas, a wideband 50 Ohm load has been used. Finally, it should be mentioned that the antennas have very compact structures as well as very simple configurations.

/pdf/two-new-loaded-compact-palnar-ultra-wideband-antennas-using-1f7znu0pvc.pdf

Keyvan Forooraghi

Papers

Design of Wide-Band Dielectric Resonator Antenna with a Two-Segment Structure

Thinned Arrays Using Pattern Search Algorithms

Guided and Leaky Wave Characteristics of Periodic Defected Ground Structures

$\Pi$ -Model Dual-Band Impedance Transformer for Unequal Complex Impedance Loads

Two New Loaded Compact Palnar Ultra-Wideband Antennas Using Defected Ground Structures