Realizing Efficient Wireless Power Transfer in the Near-Field Region Using Electrically Small Antennas

Abstract

In the early 1900’s, Tesla carried out his experiment on power transmission over long distances by radio waves (Tesla, 1914). He built a giant coil (200-ft mast and 3-ft-diameter copper ball positioned at the top) resonating at 150 kHz and fed it with 300 kW of low frequency power. However, there is no clear record of how much of this power was radiated into space and whether any significant amount of it was collected at a distant point. Over the years, wireless power delivery systems have been conceived, tried and tested by many (Brown, 1984; Glaser, 1968; McSpadden et al., 1996; Shinohara & Matsumoto, 1998; Strassner & Chang, 2002; Mickle, et al., 2006; Conner, 2007). For very short ranges, the inductive coupling mechanism is commonly exploited. This is best exemplified by non-contact chargers and radio frequency identification (RFID) devices operating at 13 MHz (Finkenzeller, 2003). Such systems are limited to ranges that are less than the device size itself. For long distance wireless power delivery, directed radiation is required, which dictates the use of large aperture antennas. This line of thinking is best exemplified by NASA’s effort to collect solar power on a single satellite station in space and relay the collected power via microwaves to power other satellites in orbit (Glaser, 1968). In addition to needing large antennas, this scheme requires uninterrupted line-of-sight propagation and a potentially complicated tracking system for mobile receivers. In 2007, MIT physicist Soljacic and his group demonstrated the feasibility of efficient nonradiative wireless power transfer using two resonant loop antennas (Kurs et al., 2007). Since then, there has been much interest from the electromagnetics community to more closely study this phenomenon (Kim & Ling, 2007; Jing & Wang, 2008; Kim & Ling, 2008; Pan et al., 2009; Thomas et al., 2010; Jung and Lee, 2010; Cannon et al., 2009; Kurs et al., 2010; Casanova et al., 2009). It was found that when two antennas are very closely spaced, they are locked in a coupled mode resonance phenomenon. In this coupled mode region, the two antennas see each other’s presence strongly, and very high power transfer efficiency (PTE) can be attained (see Fig. 1). The term magnetic resonance coupling is often used to describe this phenomenon, although such coupled mode resonance can exist in antenna systems dominated by either magnetic or electric coupling, as shown in (Kim & Ling, 2007). It was also found that to maximize the power transfer the antennas need to have low radiation loss