|Fig. 1: South Mountains broadcast towers in Phoenix, AZ. (Source: Wikimedia Commons)|
Wireless energy harvesting has recently gained increased attention in the research community as an enabler of always-on, ubiquitous electronic devices. Unlike a device powered by a battery, a wirelessly powered device can continue to operate for an indefinite amount of time without requiring human intervention. This capability will certainly be necessary when the number of connected "smart objects" exceeds the world's population by two orders of magnitude, which is projected to happen by 2020.  It has been established that the ambient radiofrequency (RF) energy from television stations can sufficiently power devices consuming on the order of 10's of microwatts at distances of kilometers.  Unlike solar energy, ambient RF energy has the advantage of being present around the clock. However, the power that an RF energy receiver, such as a smart object, can gather from an RF energy transmitter, such as a television tower (Fig. 1), decreases with the physical distance between them.
For the case of isotropic (undirected) antennas at the transmitter and receiver, the path loss varies inversely with the square of the distance. When both the direct line-of-sight path and the ground reflection are taken into account, the received power varies inversely with the fourth power of the distance.  Hence, it makes sense to consider building dedicated RF sources that can be located closer to a set of smart objects than a remote tower. Although this adds the maintenance overhead of the dedicated RF source, its cost is amortized over the set of smart objects that it powers. Solutions currently exist for providing 100's of microwatts at distances of meters, while complying with FCC regulations.  Nonetheless, the subject of RF power transmission remains a subject of controversy due to the health concerns associated with exposure to RF radiation. Somewhat counter-intuitively, healthcare applications of smart objects are expected to contribute $1.1-$2.5 trillion in annual growth to the global economy by 2025.  In the next decade, the scientific community must gain a more thorough understanding of the health risks associated with RF power transmission.
Several studies have been conducted to assess the effects of RF radiation from mobile phones on the human brain, using both cognitive tests and electroencephalogram (EEG) activity as metrics.  The results suggest a wide range of contradictory possibilities. Among the cognitive studies reviewed in Habash et al., two studies are cited in which physical response times and memory reaction speeds are slowed in the presence of RF radiation.  However, nine studies are cited in which the RF radiation had no effects on the tested cognitive function. Among the EEG studies reviewed in Habash et al., the majority indicate that some change in EEG activity occurs in the presence of RF radiation, but it is unclear whether or not the altered EEG activity has adverse health implications. 
In Chou, consideration is given to the distinction between thermal and non-thermal effects of RF radiation.  An increase in temperature in a body subject to intense electromagnetic radiation is the basis for the microwave oven. Naturally, there is a scientific consensus that exposure to high intensity RF radiation can cause thermal effects with adverse health implications. However, Cho suggests that the measured non-thermal effects of low-intensity RF radiation may simply be misunderstood thermal effects, and that more careful attention must be paid by the scientific community to determine if biological changes occur in response to low-intensity RF radiation for reasons other than temperature increase.  Specific to neurological effects, Chou cites a 2001 study claiming that low-intensity RF exposure affected electrical activity in hippocampal slices of rats. [5,6] However, it was previously known that the tips of metal electrodes absorb RF energy at a much higher rate than the surrounding tissue. Six years later, the author reported that electrode-induced heating artifacts were present in the 2001 measurements. Using a new exposure system without the electrode heating problem, no effects were observed due to low-intensity RF, even with an order of magnitude stronger RF power flow than that used in the 2001 experiment.
Today, the case against RF power transmission due to adverse health effects from low-intensity RF is lacking in evidence. The experiments needed to provide this evidence are difficult to construct and have produced misleading conclusions in the past. Furthermore, findings from different studies in the past decade are contradictory. On the other hand, in order to rule out adverse health effects due to low-intensity RF, precise and repeatable measurements must be taken under conditions reflecting exposure in a realistic situation. We expect that this area of research will see significant activity in the next decade, given the economic incentive to begin powering smart objects with dedicated RF sources.
© Danny Bankman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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