How metamaterial designs boost multiband rectenna efficiency?

[HannahW]

I’ve been prototyping a small device that needs to harvest ambient RF energy, but my current rectenna setup is terribly inefficient at the low power levels I’m working with. I’m wondering if anyone has practical experience with newer metamaterial designs for this, specifically around tuning for multiple frequency bands.

[Eric.S]

I’ve tinkered with a few rectenna layouts for ambient RF and the gains stay stubbornly small. When I tried a small patch with a handful of resonators around 900 MHz and 2.4 GHz, the harvested power barely hit tens of microwatts before the rectifier stopped conducting. Temperature drift and substrate losses were constants, so I looped back to a single band and spent more time on matching than chasing more frequency slots.

[Robert.W]

One time I tried a metamaterial inspired fringe design aiming for a couple bands in a compact patch. The tuning was a nightmare; tiny fabrication or temperature shifts would wreck the impedance match and the rectifier never saw the right conjugate input. I ended up shelving the multi band idea and sticking with a single, better controlled path after I did a few quick bench measurements.

[RobertVT]

Could the real problem be the ambient power density rather than the tuning tricks? In a normal room or on a desk you get a mess of reflections and path loss that makes it hard for a tiny antenna to develop a usable match.

[AndrewJ]

I did a quick bench run with a wideband patch and a broadband rectifier and kept seeing microwatts per square centimeter, sometimes less, sometimes more depending on location. The impedance flip was so sensitive that results varied from day to day, so I stopped chasing multi band schemes for now and just tried to characterize the detector itself.