From my understanding, common industrial standards such as Qi Wireless Charging will use frequencies ranging from 80 to 300 kHz. However, there is research carried out on higher frequencies as well. What are the advantages and drawbacks when it comes to using higher frequencies?
Low frequency wireless charging is near-field, because the wavelength is so long compared to the distance between the charger and the thing being charged. This means the coupling is inductive or capacitive (usually inductive) and power mainly goes into the thing you want to charge and nowhere else. It is working like a transformer.
At 300 kHz the charger is not going to radiate much unless it is VERY big (100 metres, for instance). This mean it doesn't waste power by radiating it.
With inductive coupling, if the thing to be charged is not there then almost no power is used up. If there is radiation, this carries on all the time, whether the thing to be charged is there or not (unless the charger has some way of telling and can be switched off), and also in directions where there is nothing to be charged, so it is usually very inefficient.
At 2.4 GHz it is hard to build structures that will couple well in the near field over ranges of more than a few mm without radiating. However at 2.4 GHz is is possible to build a far-field antenna that will form a beam enabling it to be used to charge a drone, for instance, using a beam steered to point at a receiving antenna on the drone. This is not possible at 300 kHz, and operating at 300 kHz in the near-field at ranges of more than a few metres would require very large antennas or very large fields.
Dear Mr. Chamath Vithanawasam,
the reason can be the little electromagnetic loss
in this frequency range.
Sincerely Yours
F. Gräbner
Microwave power transfer from orbiting satellites has long been a staple of science fiction writers based on early research in the 60s and 70s. The issue is really about power transfer efficiency at both ends and coherence of the beam over distance. Any microwave array will typically have sidelobes, so even a tiny percentage of the desired power transfer going in an undesired direction is a major problem. With advances in lasers and solar cells, optical power transfer may be more practical than microwave transfer for those sorts of applications. You can find more on Wikipedia under wireless power transfer https://en.wikipedia.org/wiki/Wireless_power_transfer and this is a decent short article. http://large.stanford.edu/courses/2011/ph240/shu2/
As far as commercial/residential applications and the idea of charging your cell phone with microwaves, there are plenty of concepts for extracting parasitic power from existing transmissions, but these would generally be in the nano-Watts or less range. I wouldn't want to be anywhere near the fields needed to charge your cell phone in any reasonable time period. That'd be more like standing in your microwave oven!
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