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Spherical Helices for Resonant Wireless Power Transfer

The capabilities of electrically small spherical helical antennas for wireless power transmission at small and moderate distances are analyzed. Influence of design on antenna radiation resistance, efficiency, and mode ratio is examined. These are the factors that, according to the theoretical considerations depicted herein, govern the maximum transfer performances. Various designs and configurations are considered for the purpose, with accent on small-size receivers suitable for implementation in powering common-sized gadgets. It is shown that spherical helix design is easily manipulated to achieve a reduced antenna size. Good radiation characteristics and impedance match are maintained by multiple-arm folded antenna design and by adjusting the separation between the arms.

Gen2 RFID as IoT Enabler: Characterization and Performance Improvement

RFID has become an enabling technology for IoT implementation. In dynamic RFID scenarios, such as smart shops or industrial surroundings, it is crucial to identify every good, with an applied RFID tag, before it leaves the interrogation area. Currently, commercial reader solutions adopt DFSA protocol as a simple MAC that manages the communication between a reader and multiple tags. To increase DFSA throughput (the number of read tags in the unit of time) and thus speed up tag identification, simple calculations show that the number of tags should equal the frame size. However, the literature exhibiting RFID performance shows that tag responsiveness is stochastic, while this has been often neglected when considering the throughput. To investigate the influence and to define related research challenges in the RFID domain, this work provides the idea of the required measurements by using SDR technology, while arguing that PHY and MAC layers should be looked at integrally. If not, tag identification will be delayed, while at the same time unnecessary energy waste will occur. In the measurement campaigns, the metric of TRP is employed, given as tag response probability distribution, which can be used for modeling the MAC layer.

Impact of Tag Responsiveness on Gen2 RFID Throughput

It is well known in the research community that the throughput of dynamic frame slotted ALOHA (DFSA) in a radio frequency identification (RFID) system can be maximized when DFSA frame size is set to the number of tags. However, related works often neglect the fact that tag responsiveness is of a stochastic nature, depending on the tag hardware performance in the wireless channel. Therefore, as number of tag responses may not be equal to the number of tags located in the interrogation area, corrections to the standard simulation models should be applied. In this letter, we provide the corrections that are based on tag read probability (TRP) metrics. At first, numerically, TRP was retrieved by the usage of fully configurable open-source software defined radio RFID reader in an indoor nonfading radio channel at the single frequency and constant output power, while considering reliable communication parameters in the environment of single and a pair of commercial Gen2 RFID tags. Then, TRP is statistically modeled, and the implications to the throughput are shown.

Helical Antennas in Satellite Radio Channel

Monofilar and multifilar helical antennas are the most widely proposed antennas in satellite communications systems. The main reason why these antennas constitute an asset in applications concerning satellite and space communications generally is circular polarization. Good axial ratio provides precise measurement of the polarization of the received signal due to immunity of the circularly polarized wave to Faraday rotation of the signal propagating through the ionosphere. In addition to circular polarization, monofilar helical antennas offer the advantage of high gain in axial direction over a wide range of frequencies which makes them suitable for applications in broadband satellite communications.

Interaction Between Human and Near-Field of Wireless Power Transfer System

In this paper we provide new recommendations for a type of antenna design in applications where a human is present in the vicinity of a wireless power transfer (WPT) system by means of power transfer efficiency (PTE) and specific absorption rate (SAR). The interaction between a homogenous human model and different WPT systems is investigated at 13.56 MHz using spherical mode theory antenna model (SMT-AM) and full-wave numerical analysis. The human model exposure and the performance of the proposed WPT system are analyzed further for some typical scenarios. It is shown that the position in which the human model is closer to the receiver is favorable over the position closer to the transmitter, concerning both PTE and SAR. Also, the consideration of variable receiver load indicates that different levels of SAR coupled by degraded PTE can be expected. The proposed antennas are designed, and proof of concept WPT measurements are carried out.

2.4 GHz micro-strip patch antenna array with suppressed sidelobes

In this paper, we present the design and simulated results for a patch antenna array at 2.4 GHz. The antenna consists of one linear array with six series-fed patch antennas. In radar and smart-antenna applications it is very important to use antennas with high gain and low sidelobes in order to avoid the interference and to increase the gain of antenna array. The aim of this research is to design an antenna array that has suppressed sidelobes as much as possible in azimuth by using Kaiser-Bessel amplitude coefficients. Our designed antenna has a gain of 15.1 dB with an angular width of 14.6° at a frequency of 2.4 GHz. The measured 10 dB return loss bandwidth equals 18.7 MHz. Patch antenna array is analyzed using CST MICROWAVE STUDIO 2014.

Interaction between humans and wireless power transfer systems

The performance of wireless power transfer (WPT) systems depends on antenna radiation efficiency and operating resonant frequency. In HF WPT systems, nearby objects have a certain impact on antenna characteristics and consequently on the WPT system performance. In this paper, human-WPT system interaction is investigated in order to provide basic recommendations regarding WPT system design and the human exposure to WPT systems. Preliminary results show that an efficient WPT system with four-arm SHAs is less susceptible to the degradation of system performance due to the influence of the nearby fantom, in comparison with the inefficient WPT between spirals. Also, it is possible to achieve a smaller risk of the electromagnetic human exposure to WPT systems when using an efficient WPT system rather than an inefficient one or a standalone transmitter.

Increasing the radiation efficiency and resistance of electrically small spherical helical antenna for wireless power transfer

The performance of wireless power transfer system depends highly on antenna radiation efficiency, according to the theory. Practical problem of power transfer between electrically small antennas is their impedance match to standard transmission lines. This paper shows how the maximum power transfer efficiency can be reached with new design approach at 10 MHz. Various helical antenna designs are investigated and compared. A combination of two methods is proposed for increasing the radiation efficiency and the input resistance of spherical helical antenna. The results show that it is possible to achieve significantly higher power transfer efficiency at 10 MHz than in the reported literature. The wireless power transfer system consisted of two four-arm spherical helical antennas with kresa = 0.064, can achieve maximum power transfer efficiency of 95 percent at 2 m.

Electromagnetic characterization of SNR variation in passive Gen2 RFID system

Passive Gen2 RFID systems based on the wireless communication between readers and tags have become the enabling technology for Internet of Things (IoT) application. Such passive RFID tags are powered up by incident RF power and they respond back to the reader by modulating the backscattering power in terms of changing the tag IC impedance. In various environments this incident power can oscillate and have a certain influence on the power of tag signal. This may have a significant influence on the data available at the reader, especially when analyzing the multiple tag responses, which may happen at the same time (usually referred to as a collision of multiple tags). This paper associates the electromagnetic problem of chips impedance matching with the variation of tag SNR at the reader. The simulation results depict the difference of tags backscattering for some chip impedance states. By using fully configurable Software Defined Radio (SDR), the performances of the actual system and characterize the variation of SNR at the reader was retrieved.

Multiple-folded antenna design for maximizing power transfer efficiency at 6.78 MHz

In this paper, we present the transmitter and receiver multiple-folded design for maximizing power transfer efficiency (PTE) for wireless power transfer (WPT) systems at 6.78 MHz. Using this design, it is possible to achieve higher PTE than the one obtained with square loops provided in the standard. The multiple-folded antennas are designed and tested with numerical electromagnetic software in free space, their quality factors are calculated and compared, and the problem of transmitter and receiver misalignment is investigated. The proposed transmitter and receiver are fabricated and proof-of-concept WPT measurements are performed using vector network analyzer.