Diffraction and Wireless Charging

26
Mar. 2018

Consider this little girl drinking from a garden hose. Water is “transmitted” from the garden hose and “received” by the girl.

The water stream coming out of the garden hose has a fairly small diameter. Move farther from the garden hose, and the diameter of the water stream widens. With a given receiver size, a smaller percentage of the available water will be captured as you move farther away from the hose. Conversely, more water will be ‘wasted’ or get people or objects things wet.

What is true with water is also true with power beams. When the beam ‘diverges’ (becomes wider) the distance of the transmitter from the receiver impacts the power capture ability. This has two implications:

  • Efficiency. If only a small portion of transmitted energy is captured by the receiver, more energy needs to be sent from the transmitter to power the device. One option to improve efficiency is to increase the receiver size, but a much larger receiver would not fit on a phone or other portable devices.
  • Safety. If only a small portion of transmitted energy is captured, the rest of the power is radiated into the environment. Power not captured by the receiver would be radiated towards people, animals and objects that are not interested in this power radiation. the intended receiver. Depending on the amount and type of energy, this might be unsafe.

For these reasons, it is better to fit all the transmitted energy within the size of the receiver. But is that possible? The laws of physics teach us about diffraction, a phenomenon that prevents beams from being focused into arbitrarily small spots. The distance in which you can contain all the energy within the receiver depends on various factors: the size the transmitter, the size of the receiver, the distance between the two and the frequency (or wavelength) in which you are transmitting.

For example, let’s assume a transmitter diameter of 30 cm (~12 inches) and a receiver diameter of 5 cm (~2 inches). It turns out that the maximum distance in which you can get a contained beam is:

 

As you can see, the working distance of RF is quite small. In contrast, IR light beams can travel very large distances while remaining contained. For practical “room sized” distances, IR provides the opportunity to deliver a beam in which all the transmitted energy falls on a reasonable-size receiver.

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