Wireless power with RF - how much can you get?
A common question that we get asked is “How much power can I get?”. The answer is not straightforward because there are multiple factors involved, with the main controllable factors being:
- Transmitter power
- Transmitter antenna gain
- Transmission frequency
- Distance between transmitter and receiver
- Receiving antenna gain
There are other considerations too, like multi-path, antenna impedance matching, polarization etc.
Like most things, radio waves are subject to the laws of physics. That being the case, a formula known as Friis equation , named after Harald T. Friis, can be used to estimate the amount of power received at a certain distance.
P = Power, G = Antenna Gain, R = Distance
r = Receive, t = Transmit, λ = Wavelength (inverse of frequency)
Unless you are close to a transmitter (radio, TV, mobile, etc), or use a large antenna, pure ambient RF sources provide very little power. Powercast’s RF harvesting technology can also be coupled with a local transmitter to create a reliable wireless power system.
Since today’s wireless sensors have low power requirements (and are decreasing), harvesting power in the milliwatt and microwatt range can be sufficient to provide enough power for perpetual operation depending on the frequency of operation and duty cycle of the sensor system.
Energy Harvesting and Micro-Power