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.
as written by the authors arXiv:2601.12386
A microwave rectifier at 5.8 GHz without any capacitors is presented, which owns a measured MW-to-DC conversion efficiency of 68.1%. A harmonic rejection filter and a DC pass filter, which replace lumped capacitors in conventional microwave rectifiers, are applied to suppressing the harmonics produced by an HSMS-286 Schottky diode during rectifying. At the fundamental frequency, a microstrip impedance transformer which contains a shunt {\lambda}g/8 short-ended microstrip transmission line and two short series microstrip transmission lines are applied to compensating the imaginary impedance of the diode and matching the input impedance of the rectifier. The measured MW-to-DC conversion efficiency agrees well to the simulated results. The novel rectifier without any lumped passive elements may be applied for power transmission system at higher microwave frequencies.
The 5.8 GHz band is used by many Wi-Fi routers. Such rectifiers could power sensors from router signals.