A microwave oven heats food, but this circuit harvests energy from radio waves. Engineers have built a rectifier that converts 5.8 GHz microwaves into direct current. The big news: inside, there isn't a single capacitor: those little charge reservoirs that usually sit in every rectifier.

Instead of capacitors, striplines do the job — traces on the board that behave like springs for current. One line, an eighth of a wavelength long, suppresses spurious oscillations, much like a muffler silences an engine's roar. Two short lines tune the impedance so the diode (a tiny valve that lets current flow one way) operates at peak efficiency.

Measurements showed that 68.1% of the wave's energy is converted into useful current. That's like catching 68 out of 100 raindrops in a bucket while the rest fly past. The result matched calculations, meaning the design can be scaled to higher frequencies — where capacitors start to struggle.