Diffraction, contained beams and wireless charging

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 three implications:

  1. If only a small portion of transmitted energy is captured by the receiver, more energy needs to be sent from the transmitter to power a particular device. However, the ability to increase the transmitted energy might be limited by safety considerations.
  2. If a smaller portion of transmitted energy is capture, a larger portion is ‘’wasted’ outside the receiver. This could lead to exposing people, animals, plants and objects to unintended energy.
  3. An impact on the receiver size. If the goal is to capture a larger portion of the energy from a transmitted divergent beam, a larger receiver is needed. It might not be practical to integrate large receivers into small devices.

For these reasons, it is better to contain all the energy within the size of the receiver. This is called a “contained beam”. 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, RF significantly diverges over distance. In contrast, IR light beams can travel very large distances while remaining contained. For practical “room sized” distances, only IR provides the opportunity to deliver a ‘contained beam’ in which all the transmitted energy falls on a reasonable-size receiver.

Tags: , , ,