charge your car from radio waves?
 

Nikola Tesla Page, Tesla's power receiver
It sure sounds interesting. Biggest hurdle to overcome is the superconductor problem...

I had a little science kit that extracted a tiny bit of power off radio waves....

Given the energy density, you'll be hard pressed to get anything useful for your car :p

And remember, this is "free to me" energy - not "free energy."

A tiny bit being, what, however?

And energy that is being dissipated into the air should be used for something at least. It's a better use of the energy than listening to any of the useless radio stations in my area that are constantly running ads for car lots trying to sell their over-stocked, under-performing gas guzzling SUVs. :p

Enough to make crackling sounds on a piezo speaker :p

If a myriad of devices start drawing power off the waves, that load needs to be made up for. It's a wireless energy transfer - so if you take an additional watt, one additional watt must be provided from the transmission side.

For some reason I'm now reminded of WLW's 500kw transmitter from the 30/40's... :D

How is pulling some power to run your car any different than dissipating the same power in the environment as far as the transmitter is concerned? Does a light bulb magically use more power just because you put up a solar panel to absorb some of the light?

Unlike a photovoltaic panel, your radio frequency receiver is a resonant circuit excited by sympathetic oscillations originating at the transmitter. Though such a circuit seems passive, it actually creates a load on the transmitter (in the same way the transformer secondary winding loads the primary).

This load is infinitesimally small but is measurable with a grid-dip meter. Of course, the amount of current is equally small (on the order of micro amps, or even pico amps). That's why a typical superheterodyne radio must have an RF amplifier followed by an IF amplifier to boost the signal strength enough to pass through a demodulator without being lost altogether. You'd die of old age long before you charged a flashlight, let alone a car battery. There's probably more energy available from parking under power lines, but it still wouldn't get you very much.

It's not technically impossible, it's just that the power is 10 to 12 orders of magnitude too small to be practical.

They'll make electricity from sewage first.

Poop power? Sewage turned into electricity - Green Machines - MSNBC.com

In the link I have posted, it is possible to capture a significant amount of energy if superconductors are used.

If the power receiver isn't there, the waves will simply propagate until they are absorbed by something else. As far as the transmitter is concerned, how is it any different to have the power receiver absorb the waves as opposed to, for instance, a body of water?

The advantage is that radio wave power collection will work in the shade and at night.
I think it would be powering the cars of the future if someone manages to make it work well enough.