[OT] Power over radio is it a true thing or just a myth ?

[Oleg Goldshmidt]

On Mon, Aug 24, 2009 at 4:33 PM, Micha<michf at post.tau.ac.il> wrote:

> I don't have a link now but I saw a thread somewhere about the myth of
> stealing power from powerlines with a large coil, and the calculation came
> to the point if I remember the numebers correctly that you may be able to
> run a 1.5v light bulb using several tons of copper if your are several feet
> from the power line.
>
> I doubt wifi is any better in this respect.

The *basic* calculation is not very difficult. I believe the most
powerful long- or medium-wave broadcasting transmitters have a nominal
power of 2.5MW. Assume you have a 2MW transmitter 1km from your house.
Assume it actually radiates 2MW of power - I wouldn't be surprised if
efficiency was around 20% or so, but let's assume 2MW is actually
emitted isotropically. Neglect losses (that are, in reality, very
considerable), and put a round antenna with radius of 1m on your roof.
Assume that your antenna collects 100% of the radiation. The result is

                               2MW*pi*1m^2
                               -------------------- = 0.5W,
                                 4*pi*1km^2

which might be enough to light a 1.5V light bulb with a resistance of
4Ohm (assuming no losses again).

Given that your losses are likely to reduce usable power by a couple
of orders of magnitude compared to the above, you will *not* be able
to light a small light bulb by any stretch of imagination.

Looking up the effective radiated power of your WiFi router (divide
the nominal power by 5 if not quoted directly) and plugging the
parameters in the formula above (what's your receiving antenna size?)
is left as an exercise to anyone who keeps the router's documentation
in the bottom drawer.

-- 
Oleg Goldshmidt | oleg at goldshmidt.org