A dual-band circularly polarized aperture coupled microstrip RFID reader antenna using a metamaterial (MTM) branch-line coupler has been designed, fabricated, and measured. The proposed antenna is fabricated on a FR-4 substrate with relative permittivity of 4.6 and thickness of 1.6 mm. The MTM coupler is designed employing the provided explicit closed-form formulas. The dual-band (UHF and ISM) circularly-polarized RFID reader antenna with separate Tx and Rx ports is connected to the designed metamaterial (MTM) branch-line coupler. The maximum measured LHCP antenna gain is 6.6 dBic at 920 MHz (UHF) and RHCP gain is 7.9 dBic at 2.45 GHz (ISM). The cross-polar CP gains near broadside of the RFID reader antenna are approximately less than compared with the mentioned co-polar CP gains in both bands. The isolations between the two ports are about 25 dB and 38 dB, at 920 MHz and 2.45 GHz, respectively. The measured axial ratios are less than 0.7 dB in the UHF band (917-923 MHz) and 1.5 dB in the ISM band (2.4-2.48 GHz).

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In this article, a novel wideband wavelength conversion is demonstrated using a highly nonlinear fiber consisting of a fluorinedoped fiber that has a high delta core and surrounded by a deeply depressed ring as opposed to the use of photonic crystal fibers. The wavelength conversion range is from 1460 to 1640 nm, which covers the S-, C-and L-bands of the optical network. The interacting waves consist of an arrayed waveguide grating tuned dual-wavelength fiber laser output together with a tunable laser source signal. A four-wave-mixing conversion of efficiency of À20 dB is achieved within a 70 nm tuning range within a 3.9 dB fluctuation. An optical signal to noise ratio of 30 dB is also realized within the same tuning range.

A resistive and capacitive (RC) microwave absorber with a layer thickness less than a quarter of a wavelength is investigated based on closed-form design equations, which are derived from the equivalent circuit of the RC absorber. The RC absorber is shown to have a theoretical 90% absorption bandwidth of 93% when the electrical layer thickness is 57 o (about λ 0/6). The trade-offs between the layer thickness and the absorption bandwidth are also elucidated. The presented formulation is validated by a design example at 3 GHz. The RC absorber is realized using a silver nanowire resistive rectangular structure with surrounding gaps. The measured 90% absorption bandwidth with a layer thickness of λ 0/8 is 76% from 2.3 GHz to 5.1 GHz in accordance with the theory and EM simulations. The presented design methodology is scalable to other frequencies. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. ⓒ

We have proposed and analyzed an equivalent circuit for a magnetically coupled wireless power transmission (WPT) system between two loop resonators by considering its coupling coefficient and radiation-related parameters. A complete formulation is provided for all the necessary circuit parameters. The mechanism of radiation loss is sufficiently explained. The circuit and electromagnetic (EM) simulation results have been shown to be in good agreement. Based on the proposed circuit formulation, a specific load impedance for maximum WPT efficiency was found to exist. The proposed modeling of the WPT in terms of circuit characterizations provides sufficient insight into the problems associated with WPT. Ⅰ. IntroductionWe live in a world of wireless communication. A huge amount of information is wirelessly communicated between mobile terminals. Now, the increasing requirement for the wireless transfer of electric power is making wireless power transmission (WPT) technology increasingly important. Based on the analysis, they successfully lit a 60 W bulb at a distance of 7 feet (more than 2 meters) by using helical coils of high Q. The efficiency was reported to be 60 % at 9.9 MHz. Most of the papers related to WPT focus on the power transmission efficiency. The mechanism of radiation loss and the coupling coefficient have rarely been explained in detail. The radiation loss is surely the most crucial limiting factor in WPT and it warrants an in-depth analysis.In this paper, in order to investigate the mechanism of WPT in a more detail, we focus on the analysis of a WPT system using an equivalent circuit. Based on the equivalent circuit, key parameters-such as WPT efficiency and the radiation loss rate are properly defined and derived.Furthemore, a method for extracting both from electromagnetic (EM) simulations or measurements is proposed. Finally, the proposed modeling for WPT is validated by comparing the circuit and the EM simulations. Ⅱ. Analysis of an Equivalent Circuit for WPT using Two LoopsCommonly, WPT using magnetic coupling is realized using two resonant loops facing each other. One loop is connected to an AC power source and the other loop is connected to the load. Power is wirelessly transmitted from one loop to the other as a result of magnetic coupling between the two resonant structures with the same resonant frequency. Fig. 1 shows the equivalent circuit for a magnetically coupled WPT that considers radiation effects. V1 is the voltage source for the WPT system. R1 and R2 are the conductor loss resistances, R r1 and R r2 are the resistances accounting for radiation loss, and RL is the load resistance.L 1 and L 2 are the inductances, and C 1 and C 2 are the capacitances, for the first and second loops, respectively. M is the mutual inductance between the two loops. I1 and I 2 are the currents flowing on each loop.Using KVL's, we obtain

A wideband single-layer absorber at X-band adopting trumpet structures is presented. One unit of the proposed absorber is composed of a trumpet-shaped resonator loading four chip resistors, a metallic backplane and an FR-4 (ε r = 4.4-j0.02) substrate between them. The absorption is 99% at 10.9 GHz. The full width at half maximum is 95% at 12.7 GHz (6.7-18.7 GHz). The 90% absorption bandwidth is 65% at 12.75 GHz (8.6-16.9 GHz). The size of the unit cell is 5.6 × 5.6 × 2.4 mm 3 . The proposed absorber is almost insensitive to polarisations of incident waves due to the symmetry of the structure. The results of EM simulation and measurements are in good agreement.Introduction: Microwave absorbers are required for anechoic chambers and many other military and commercial applications. One of the conventional absorbers is the Salisbury screen [1]. This type of absorber employs a resistive sheet against a metallic ground plane, separated by a quarter-wavelength. This may be bulky at low frequencies.Recent advancements in metamaterial research have opened the possibility for metamaterial absorbers. The most common designs have been based on split-ring type resonators, which have been widely studied as an element for negative permeability and double negative metamaterial [2]. However, they usually suffer from narrow bandwidth due to the highly dispersive nature of the structure. There have been many other designs suggested to improve the bandwidth of the absorbers. Some absorbers using the Minkowski fractal frequency selective surface or double layer were studied [3,4]. However, these attempts further complicate or enlarge the structure. Meanwhile, a resistive pattern with a predefined surface resistance has been studied for large-scale production and cost reduction [5]. In [6], an absorber with a high-impedance surface was designed for wideband absorption. The reported 90% absorption bandwidth is roughly 85% about the centre frequency of 10 GHz, but with a mixture of different unit structures having different surface resistance per square. The separation between the absorber surface and the conducting plane is 7.5 cm (a quarter wavelength).

Alternative analytic expressions for the mutual inductance (Lm) and coupling coefficient (k) between circular loops are presented using more familiar and convenient expressions that represent the property of reciprocity clearly. In particular, the coupling coefficients are expressed in terms of structural dimensions normalized to a geometric mean of radii of two loops. Based on the presented expressions, various aspects of the mutual inductances and coupling coefficients, including the regions of positive, zero, and negative value, are examined with respect to their impacts on the efficiency of wireless power transmission. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. ⓒ

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