Grumpy The Mule wrote:
Cecil Moore wrote in
:

Mike Y wrote:
And while I realizeit's quite possible to get 'shocks' off an
induction
coupling that essentially goes to a high impedance, it's a far cry
from being
able to 'power a house' with a broadcast transmitter that wasn't
designed for the purpose.
The original story that I heard about 40 years ago is
that it involved 60 Hz high-voltage power lines. A
farmer is supposed to have built an induction coil
in a shed directly beneath the power lines and picked
up free energy.

Hmm.. a bit of physics..

Induced voltage is Nturns*dPhi/dt
dPhi/dt - change in flux (Webers) per second

Flux is area of coil * B field.

AC field under a HV power line is typically around 1-10 microTesla.

So, the peak flux through a 1 square meter coil is about 10 microWeber.
At 60Hz, the maximum slope is 2*pi*frequency, or 377, so now, we're up
to 377*10E-6, or 377E-5 or 3.77E-3.. about 4 millivolts..

Now, lets assume a coil that's 10x10 meters.. that's 100 square meters,
so we're up to 0.4 volts, open circuit voltage, for one turn.

Now, let's say our ambitious farmer winds 100 turns.. now we're up to 40
Volts. But, that coil is 4000 meters of wire (100 turns * 40
meters/turn), and will have non-zero resistance. Let's say our farmer
used AWG 10 wire. about 1 ohm/1000 ft, so call it 13 ohms all told
(4000 m is approximately 13000 ft).

So we have a source with 40V open circuit output voltage and 13 ohm
output Z. If we hook up a matched load (another 13 ohms), it will have
20 Volts across it, and dissipate about 30 watts in the load and 30
watts in the loop.

Now, at $0.10/kWh, and 0.03kW, that's about $26/year

Of course, there's the labor in building that coil.
And, the cost of the wire. (400lb of copper at $3/lb is $1200)

Sure, you'd use aluminum wire, so you could cut that down by a factor of
3 or so.. So, invest a lot of hours stringing up that coil, and a few
hundred bucks in aluminum, and save $30/year...

Excellent economics there..

Hi all,

This again falls under the presumption folks are actually interested
in antennas.

Perhaps it is only one step removed from Art's lamentations of being
shown the door, but hopefully it isn't such a weepy heartbreaker.

Anywho.

It has been reported for several years now that researchers have been
designing antennas to harvest power from transmission antennas (power
lines in the quote above) for the purposes of powering home gadgets. A
report in the New York Times offered an article how Apple was looking
into a way to recharge their iProducts through the air. I've even
mentioned the coupling mechanism inferentially in the past.

Art's tear swollen eyes are too fatigued for such reading, Bart
probably prefaces all his google searches with the keyword xfiles, and
no one else has noticed this either. (Bart should have prefaced with
the keyword Tesla, but to be fair, it would only dump 1,000,000 nutzo
links.)

The term-du-jour is evanescent waves (we call them near fields) and
lab reports offer up to 60% power transfer. I've modeled some that
are more efficient (low ohmic computer bits were used to optimize the
transfer); but to truly model what has been put on the bench
(literally and figuratively) would consume too many segments, and
violate too many NEC prohibitions. Still and all, I've replicated
their results conceptually where I could jimmy the scales.

While Art whines about the lack of science (surprising complaint, or
perhaps I should say an ironic complaint), these constitute SMALL
antennas that would fit into a matchbox if they were modeled on his
claims (again, that hint of irony).

Those so interested may, of course, wire me for files.

73's
Richard Clark, KB7QHC