By the Power of Wi-Fi
Two Wi-Fi (what does that even stand for, anyway?) stories from recently.
- A device that claims to get power from Wi-Fi.
- A man who claims his neighbor’s Wi-Fi signal makes him sick.
I doubt the plausibility of both of these claims. A run-of-the-mill household wireless router is rated by the manufacturer as having a total power output of 15 dBmW. This is a logarithmic scale describing the power output of radiative devices in such a way that the same units can be used for very powerful and not so powerful antennas. In good old SI units, this is about 32 mW.
Electromagnetic waves typically propagate isotropically. That is, wavefronts take the form of spheres expanding in every direction from the point of origin at the speed of light. When we talk about the power of a transmitter we’re usually talking about the power obtained by integrating over the size of the wavefront. So the power is the same from any point that is the same distance from the antenna, but constantly decreasing with distance. Because the wavefront is spherical, the rate at which the power density falls off is one over the square of the distance.
An order of magnitude estimate for the power of an FM radio station is 100 kW. How far away would we have to be from the radio station in order to see the same power density (watts per meter squared) as we would 1m away from the Wi-Fi router? About 1.8 km (1.1 miles). So I might ask, does being near a radio station make the electromagnetically sensitive gentleman ill? One mile isn’t an unreasonable distance to be from a transmitting tower. And if it’s possible to pick up power from a 32 mW signal, why hasn’t it been developed for FM radio earlier? And what’s the efficiency? Is 32 mW (assuming you’re right next to the transmitter) enough to charge a battery, anyway?
