An isolation technique, which does not require conventional circulators, is proposed for the realization of a simple and low-cost aperture-coupled circularly polarized antenna for application to full-duplex devices. The approach is based on the use of slotlines loops to provide surface current cancellation in specific regions of the antenna structure, leading to improved axial ratio and isolation between the ports in excess of 50 dB. Circular polarization is achieved by introducing a double-box hybrid coupler, which is optimized to obtain good matching and isolation of the quadrature signals. On this basis, both right-and left-hand circularly polarized beams are achieved by interchanging the transmitting and receiving antenna ports, enabling full-duplex operation and reconfigurability. While the antenna structure is designed for 2.45 GHz operation, one can take advantage of the proposed approach to tune the frequency of maximum isolation. Both single-element prototypes as well as a 2x2 array are fabricated and measured, showing good agreement with the simulations and validating the proposed isolation approach. The beam steering capabilities as well as the application to a Van Atta retrodirective antenna array and the possibilities of achieving delta and sum patterns for monopulse operation are also reported. The proposed full-duplex antenna can also represent an excellent solution for narrowband wireless power transmission systems.

Inductive and capacitive impedance matching are two different techniques optimizing power transfer in magnetic resonance inductive wireless power transfer. Under ideal conditions, i.e., unrestricted parameter ranges and no loss, both approaches can provide the perfect match. Comparing these two techniques under non-ideal conditions, to explore fundamental differences in their performance, is a challenging task as the two techniques are fundamentally different in operation. In this paper, we accomplish such a comparison by determining matchable impedances achievable by these networks and visualizing them as regions of a Smith chart. The analysis is performed over realistic constraints on parameters of three different application cases both with and without loss accounted for. While the analysis confirms that it is possible to achieve unit power transfer efficiency with both approaches in the lossless case, we find that the impedance regions where this is possible, as visualized in the Smith chart, differ between the two approaches and between the applications. Furthermore, an analysis of the lossy case shows that the degradation of the power transfer efficiencies upon introduction of parasitic losses is similar for the two methods.

This paper presents a single-sided impedance matching technique, i.e., matching from only the transmitter or receiver side, using a switchable interspiraled coil. This technique is suited for active impedance matching in magnetic resonance wireless power transfer. The switchable coil consists of 9 selectable, single-turn coils placed in-between the 9.5 turns of a spiraled resonator coil with an inductance of approximately 8 $\mu H$ . The self-inductances of the single-turn coils vary between 33-493 $nH$ . The interspiraled coil was realized on a single-sided PCB, making the system compact in size. Circuit analysis and simulation show that the inductance selectability enables impedance matching over a range of distances in a Wireless Power Transfer (WPT) System at 6.78 MHz operation frequency. Measurement and simulation of the efficiency indicate that the proposed interspiraled coil may significantly improve the Power Transfer Efficiency (PTE) when the distance and the load change.

One of the most challenging issues in a magnetic resonance wireless power transfer (WPT) system is to maintain high efficiency at various distances between the transmitter and the receiver. One way to achieve the desired efficiency is to use matching network between the source and the resonating coils. In this paper, we present a comparison between capacitive and inductive impedance matching techniques, which are widely exploited to overcome the efficiency drop. This study includes circuit analysis and verification of these via simulation. During this research, key concepts within WPT such critical coupling and trade-off between efficiency and distance are discussed as well. We show through Simulation that it is possible to achieve a maximum power transfer efficiency (PTE) with both methods at 60 cm distance between the coils (coupling coefficient 0.01) in an ideal case.

This paper presents design and analysis of a switchable inductor suited for active impedance matching in magnetic resonance wireless power transfer systems. The switchable inductor is compact and can be used as both driving coil at the transmitter side and load coil at the receiver side. We first present the concept through design, simulation and measurement of a three loops coil. This model of the switchable coil is used to study the internal couplings between the loops. The coil was fabricated on PCB with inductances between 130–324 nH. In addition, we demonstrate a ten-loop switchable coil for driving a high Q-factor resonance with an almost linear increase in their mutual inductances, which is verified by measurement results.