sbstn said:
Short term goal is to transmit as close to 50W as I can across 15cm.
After that I want to get the receiving circuit as small as possible to integrate into a quadrotor. I'll do as you advise and everything will be well protected.
I've devoted much thought to the topic, :) but only ever experimented using Tesla Coils lighting up fluorescent bulbs, or running low-volt solar-cell motors remotely using capacitive coupling and a diode bridge to create DC.
The straightforward but hazardous experiment would be to use a microwave oven. See two such projects on youtube:
Microwave powered RC helicopters:
<< links removed by berkeman >>
A safer version would be to use a big desktop Tesla coil, high voltage, and capacitive coupling. To avoid scragging any electronics, start out with just a DC motor and rotor as your "helicopter." Perhaps use a half-volt solar cell motor at first, then go to standard motors as you optimize your setup.
I'd place a Tesla Coil main terminal below my helicopter. To keep destructive arcs from leaping, either seal the TC toroid terminal within a layer of heavy insulation such as a close-fitting oil-filled plastic tank, or put it inside a well-sealed thick plexiglas box, or just keep the helicopter high enough above the toroid that big arcs aren't triggered. (But it all should work better if the helicopter assembly could be very close to the main terminal.)
For your capacitive pickup antenna, make two thin metal foil parallel plates many inches diameter with several inches between them. (Perhaps the edges need a bead of silicone RTV caulk to prevent arcs.) Connect thin wires to the foils and run them up to the motor. Pass the circuit through a diode bridge to convert the high-freq AC into DC for the motor. Normal diode bridges won't handle the 200KHz TC frequency, so you'll have to build your own bridge using four fast-recover Shottky diodes such as 1N5819, or perhaps use a CDBHD2100-G bridge from digikey.com The bigger the spacing between the two plates, the higher the received voltage. The closer the plates come to the TC terminal, the higher the voltage. A TC with higher frequency should work better, or perhaps even find on eBay a 1940s home diathermy quack-medical machine (which is basically a CW continuous-wave vacuum-tube Tesla Coil at industrial heating frequency band near 27MHz.)
A perhaps higher-powered version would use invisible ion-cloud conductive coupling instead of plates. Do the same as above, but instead of the foil plates, provide a grid of downward-pointing needles below the helicopter (all needles connected electrically.) A second independent grid of needles pointed upwards goes above the helicopter. Run a wire to each needle grid, pass it through a high-freq diode bridge, and run your motor. The needle grid arrays could be quite large, yet shouldn't interfere with aerodynamics. And they might create a glowing violet nimbus when viewed in reduced light! (Also... instead of a complicated needle-grid, perhaps a simple tuft of carbon-fiber cloth from Tap Plastics or a fiberglas supply company would work just as well.)
In both of these systems the diode bridge removes about a volt from the received signal. If the output volts from the two wires is too small to run your motor, it should be possible to step it up by many times by using a low-weight small ferrite transformer from a switching power supply, or even build your own hand-made air-core pancake transformer.
The following one won't help you, but... Nikola Tesla in the Colombian Exposition provided spinning copper disks which were placed many yards from the 2-phase "Egg of Columbus" toroidial transformer he was using to create his rotating b-field. He was forming an "induction motor," but with a central stator coil, and many rotors yards distant. He cheated: the disks were on jewel bearings and suspended within sealed glass globes pumped down to vacuum! Hold such a globe adjacent the coil to crank up the RPMs, then remove it to any place in the room, and it keeps spinning forever (since it probably required far less than milliwatts to maintain the rotation once started.) I only found one reference to this old Tesla physics demo, and I've never heard of anyone reproducing it since the original at the 1893 Chicago World's Fair. Most physics departments no longer maintain professional glass blowers on staff!
