[scientizscht (OP)]

What is the state of the art for wireless energy transmission?

[alancalverd]

The state of the science is the inverse square law, which translates to the state of engineering as "not worth doing". It's surprising that a genius like Tesla couldn't see the problem.   

[Bored chemist]

The state of the science is the inverse square law, which translates to the state of engineering as "not worth doing". It's surprising that a genius like Tesla couldn't see the problem.   

To be fair, that should be "not usually worth doing".
My wireless 'phone charger works fine and it avoids my having to repeatedly make and break a mechanical connection. That is (I hope) likely to improve long term reliability.

I'm not sure of the maths but broadly speaking I think it's almost certainly not worth doing if you want to transmit power over a distance that's bigger than the size of the thing you are transmitting to.

There's an interesting question about transmitting information, rather than power, where we are prepared to accept very large losses.
It is acceptable to use kilowatts of radio power to transmit TV and radio programmes, because the receiver can get most of its power requirement "locally" from the mains or a battery.

The bit where it is interesting is a so called crystal set- a radio without ay battery or electrical power supply, but which is driven entirely by the radio signal from the transmitter. They are no longer popular because... they just aren't very good.

It is, somewhat "challenging" to describe this- the oldest  type of radio receiver- as " state of the art".

[alancalverd]

You can find "no battery" radio circuits on line. I made a few back in the 1960s. Essentially a tuned "acceptor" circuit feeding a crystal diode detector, with the rejected RF energy from the aerial being rectified and used to power a germanium single-transistor AF amplifier to produce a remarkably loud signal compared with the unamped crystal.

The wireless phone charger and similar devices (wireless electric toothbrushes were an earlier manifestation) use the magnetic field of an airgapped transformer to transmit energy over a short distance, thanks to the ability of a magnetic core to provide tight coupling if the gap is smaller than the diameter of the core. Resonant transfer of EM radiation is effective over slightly longer distances but   

Nikola Tesla believed that wireless power transfer was possible and probable. He built what was called the “Tesla Tower” which was a giant coil connected to a 200 foot high tower with a ball 3 feet in diameter. Tesla pumped 300 kW of power into the device; the coil resonated at 150 kHz. The experiment failed due to the fact that the power diffused in all directions.

Microwaves can be usefully focussed by a dish for line-of-sight transfer but the overall efficiency tends to be poor. It's useful for passive snooping, where the microwave input effectively interrogates a passive microphone (in the most famous case hidden in a "gift" of a wooden plaque (the Presidential seal) from the Russians to the US Embassy in Moscow, but hasn't found any "legitimate" applications of which I am aware.   

I used the airgapped transformer (toothbrush) technique to make a waterproof connection to an underwater x-ray machine but whilst it looks attractive for transcutaneously recharging an artificial heart, AFAIK none of my colleagues  has solved all the practical engineering problems yet - the human body is as aggressive an environment as the North Sea, but won't tolerate as much lost power dissipation!

[Bored chemist]

Come to think of it, is the RFID tag the 21st century version of the crystal set and the present from Moscow?

[alancalverd]

It's possibly part of the solution to noninvasive continuous glucometry.

[scientizscht (OP)]

It's possibly part of the solution to noninvasive continuous glucometry.

Yes and part of the ion measuring in capillary rise from a fixed satellite.

[alancalverd]

Tell us more!

[scientizscht (OP)]

Tell us more!

Nosy

[teragram]

The old idea of beaming energy from orbiting solar pv cells to earth via microwaves seems to be still on the agenda I think?

[Hayseed]

EM emission has a 50% duty cycle.  There is a way to make it 100%.  But it takes two TX ants and two RX ants.

One can also double the information transfer, without increasing bandwidth.

[Bored chemist]

EM emission has a 50% duty cycle. 

In what way?
As far as I can tell, the Sun shines all the time.

[Hayseed]

Do fluorescent light tubes shine all of the time?