Current Issue : July - September Volume : 2017 Issue Number : 3 Articles : 6 Articles
Performance evaluation is an important aspect in the study of microwave-absorbing\nmaterial coatings. The reflectivity of the incident wave is usually taken as the performance indicator.\nThere have been various methods to directly or indirectly measure the reflectivity, but existing\nmethods are mostly cumbersome and require a strict testing environment. What is more, they cannot\nbe applied to field measurement. In this paper, we propose a scheme to achieve field performance\nevaluation of microwave-absorbing materials, which adopts a small H-plane sectoral horn antenna\nas the testing probe and a small microwave reflectometer as the indicator. When the size of the\nH-plane sectoral horn antenna is specially designed, the field distribution at the antenna aperture can\nbe approximated as a plane wave similar to the far field of the microwave emitted by a radar unit.\nTherefore, the reflectivity can be obtained by a near-field measurement. We conducted experiments\non a kind of ferrite-based microwave-absorbing material at X band (8.2ââ?¬â??12.4 GHz) to validate the\nscheme. The experimental results show that the reflectivity is in agreement with the reference data\nmeasured by the conventional method as a whole....
A method for balancing thermal and electrical packaging requirements for\ngallium nitride (GaN) high power amplifier (HPA) modules is presented. The\ngoal is to find a design approach that minimizes the junction temperature of\nthe GaN so that it is reliable and has interconnects that meet electrical performance\nrequirements. One benefit of GaN is that it can simultaneously\nachieve high power density and operate at microwave and millimeter-wave\nfrequencies. However, the power density can be so high that the necessary\nthermal solutions can have negative impact on electrical performance. This is\nespecially a concern for the electrical interconnects required for the input/\noutput ports on high power amplifier devices. This is because the signal interconnects\nmust operate at GHz frequencies, which means that special care\nmust be taken to avoid problems such as undesired signal coupling and\nground path inductance. Therefore, this work focuses on GaN packaging and\nits integration into a module. The results show that an optimum thickness for\nthe GaN heat spreader exits for thermal performance but the electrical design\nis impacted negatively if the optimum thermal design is chosen. Therefore, a\nbalanced design is chosen which meets overall system level requirements....
A wave interference filtering section that consists of three stubs of different lengths, each with an individual stopband of its own\ncentral frequency, is reported here for the design of band-stop filters (BSFs) with ultra-wide and sharp stopbands as well as large\nattenuation characteristics. The superposition of the individual stopbands provides the coverage over an ultra-wide frequency\nrange. Equations and guidelines are presented for the application of a new wave interference technique to adjust the rejection\nlevel and width of its stopband. Based on that, an electrically tunable ultra-wide stopband BSF using a liquid crystal (LC) material\nfor ultra-wideband (UWB) applications is designed. Careful treatment of the bent stubs, including impedance matching of the\nmain microstrip line and bent stubs together with that of the SMA connectors and impedance adaptors, was carried out for the\ncompactness and minimum insertion and reflection losses. The experimental results of the fabricated device agree very well with\nthat of the simulation.Thecentre rejection frequency asmeasured can be tuned between 4.434 and 4.814GHz when a biased voltage\nof 0ââ?¬â??20Vrms is used. The 3 dB and 25 dB stopband bandwidths were 4.86GHz and 2.51GHz, respectively, which are larger than\nthat of other recently reported LC based tunable BSFs....
A novel metamaterial, which exhibits a wideband double negative behavior in X-band, is proposed, designed, and investigated in\nthis paper. The metamaterial is composed of modified S-shaped split-ring resonators (S-SRR). The periodic structure is designed\nand simulated using CST MWs. Next, the experiments are carried out, and it is shown that the simulation and the experimental\nresults agree well and the designed structure has a wide bandwidth in X-band. An absorber application of this metamaterial is also\nprovided, and the structure can be used as an absorber with absorption rate of over 80% for the polarization angles between 0âË?Ë? and\n40âË?Ë?....
Microwave technology has a potential application in the extraction of zinc from sulphide\nores, knowledge of the dielectric properties of these ores plays a major role in the microwave\ndesign and simulation for any process. The dielectric properties of zinc sulfide concentrate for two\ndifferent apparent densitiesââ?¬â?1.54 and 1.63 g/cm3ââ?¬â?have been measured by using the resonance cavity\nperturbation technique at 915 and 2450 MHz during the roasting process for the temperature ranging\nfrom room temperature to 850 ââ??¦C. The variations of dielectric constant, the dielectric loss factor,\nthe dielectric loss tangent and the penetration depth with the temperature, frequency and apparent\ndensity have been investigated numerically. The results indicate that the dielectric constant increases\nas the temperature increases and temperature has a pivotal effect on the dielectric constant, while the\ndielectric loss factor has a complicated change and all of the temperature, frequency and apparent\ndensity have a significant impact to dielectric loss factor. Zinc sulfide concentrate is high loss material\nfrom 450 to 800 ââ??¦C on the basis of theoretical analyses of dielectric loss tangent and penetration depth,\nits ability of absorbing microwave energy would be enhanced by increasing the apparent density as\nwell. The experimental results also have proved that zinc sulfide concentrate is easy to be heated by\nmicrowave energy from 450 to 800 ââ??¦C. In addition, the experimental date of dielectric constant and\nloss factor can be fitted perfectly by Boltzmann model and Gauss model, respectively....
Microwaves have been widely used in the treatment of materials, such as heating, drying,\nand sterilization. However, the heating in the commonly used microwave applicators is usually\nuneven. In this paper, a novel multi-material turntable structure is creatively proposed to improve the\ntemperature uniformity in microwave ovens. Three customized turntables consisting of polyethylene\n(PE) and alumina, PE and aluminum, and alumina and aluminum are, respectively, utilized in\na domestic microwave oven in simulation. During the heating process, the processed material\nis placed on a fixed Teflon bracket which covers the constantly rotating turntable. Experiments\nare conducted to measure the surface and point temperatures using an infrared thermal imaging\ncamera and optical fibers. Simulated results are compared qualitatively with the measured ones,\nwhich verifies the simulated models. Compared with the turntables consisting of a single material,\na 26%ââ?¬â??47% increase in temperature uniformity from adapting the multi-material turntable can be\nobserved for the microwave-processed materials....
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