[LINK] Re: incand lamps debate

[Stewart Fist]

Daniel wrote
> 
> It's alleged that you can 'harvest' the energy from a radio
> transmission, perhaps those of you with greater experience can confirm
> or debunk this.


There's no doubt you can harvest energy from radio waves, but the question
is "how much."  Detection circuits are extraordinarily sensitive.

The energy is measured in terms of 'power-density'  - usually how many
micro-watts per square metre of surface facing the transmitter.


You can't get more powe outr than was put in. So that, together with the
operations of the inverse-square law over distance, gives you a way of
calculating theoretical possibilities.



My car key-lock battery has never been replace, yet after three years it
still operates the car lock-mechanism over 30 meters.

 The surface area of an imaginary sphere 30 m in radius is 5600 sq meters.
So to harvest this pulse of power (representing the energy that an ant
probably puts into each step), requires an antenna array with a surface area
of approximately the size of a house-block.

When you do the same sort of calculations for, say, a mobile phone radiating
over a few kilometers back to a base-station -- or even a very powerful TV
transmitter covering a region with radius of 100kms (even allowing for some
ground reflections and directionality or beam-narrowing) you always find
that you are dealing with such incredibly small amounts of available power
at the receiving end.

This is what most cell-phone scarem-ongerers forget about base-stations, and
it's also why the handset represents the greatest potential problem, not the
base station.

A satellite transmitter with an Australia-wide beam will operate at a
power-output of about 100 Watts (the power of an incandescent household
lamp) at a distance of 32,000 kms. Eeven with a directional beam, this power
is spread out across the whole 3 million square kilometers of territory.
That's why you need a dish rather than a stick antenna, and also why the
dishes need a special low-noise amplifier.

 You figure the power density of this one.




-- 
Stewart Fist, writer, journalist, film-maker
70 Middle Harbour Road, LINDFIELD, 2070, NSW, Australia
Ph +61 (2) 9416 7458