**3.1 Microwave coupling**

Microwaves are electromagnetic waves of frequencies ranging from 1 to 30 GHz. They are widely used in today's applications, especially in communications. Microwaves, differently from radio waves, can be sent in narrow beams, allowing the transmitter to concentrate its energy on the receiver. Microwaves are emitted or radiated from an antenna that is fed with a high frequency current in low power applications such as mobile phones. Another antenna will then pick up the microwaves and transform them back to an electric current.

The conversion of microwaves back to electricity was the biggest barrier to overcome in order to convert back the highest amounts of power. When an antenna picks up a microwave signal, it generates an alternating current of the same frequency as the microwave signal and equal to the microwave's signal strength. Since all applications and devices run on either an AC voltage of 50 Hz or 60 Hz or a constant DC voltage, the microwave antenna's high frequency current must be converted to a suitable voltage type. A great development of this technology was the invention of the 'rectenna' or 'rectifying antenna' by W. Brown. Using a rectifier, the rectenna converts the microwave antenna's high frequency current into a DC voltage. Further advancements in the semiconductor technology coupled with the availability of Schottky-barrier diodes resulted in higher efficiencies, higher power capabilities and smaller rectenna designs [16].
