Lighting a LED with ambient RF (was candle)

There are apparently a few working RF energy harvesters,  
one by a Joe <Something> who in fact has a patent on it.
This basically uses a voltage doubler, and if I remember
correctly a 30 feet antenna. Another design was discussed
on this newsgroup, and this person was having some  
grounding issues. Be warned though that the output is  
very low, in the milliWatts.
    

On Mon, 1 Oct 2012 20:54:20 -0700, "Billyb97113"


Did you try to activate your cell phone close to your system ?

The capture area of a dipole is proportional to the square of the
wavelength. Thus, much more power is available at lower frequencies
with the same field strength (V/m).

Repeat your measurement close to a medium wave (AM) broadcast station
using a tall vertical antenna. Since your antenna most likely will be
much shorter than 1/4 wavelength, it will be very low impedance and
highly capacitively reactive, thus a series inductance is needed, in
order to get any power delivered to the load.  

A LED at 2 V and 1 mA would be a 2 kohm load, while an electrically
short antenna would have a few ohms or  even milliohms radiation
resistance, thus some impedance step up is needed.

A full size dipole would have 50-75 ohm impedance and might drive
nicely a back to back LEDs at 20-30 mA. At 100 MHz, this would require
about 2 V/m field strength, thus the system must be quite close to the
transmitter.

Any systems using ferrite rods on the MW band would be practically
useless, since the typical ferrite rod antenna gain is -30 to -50 dB
below the dipole.

I vaguely remember something about fluorescent tubes glowing near TV  
transmitters. Maybe Sutro tower.

On a sunny day (Tue, 02 Oct 2012 02:29:39 -0700) it happened miso


Fluorescent tubes also glow when stuck in the ground under a HV power line.
I used to walk around with a glowing neon next ot my 7 MHz xmtr antenna.

I need to try something like that up the road, where there are no houses,
but feed from power station goes through. Three phase might screw it up.

Greg

On a sunny day (Tue, 9 Oct 2012 02:33:00 +0000 (UTC)) it happened gregz


 http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/

Have fun!

http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/

That's how I pictured it !!

Greg

http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/

I've heard stories (urban legends?) about people who were prosecuted for  
running a wire down
their fence and stealing power from the power company.
If the fence is there, how does hooking a light bulb across it cost the
power company additional money?  Aren't you just diverting the losses
through a light bulb as they make their way into the ground?

   How many wire fences are grounded?  At one time, they were used for
telephone service in rural areas.  The other side of the phone was
grounded.

Normally, the fence wires are either open circuit (e.g., barbed wire on  
wood posts) or short circuit (e.g., chain link on steel posts).  Either  
way, the "radiated" power is reflected, not absorbed.  A small phase shift  
occurs, but that's all.  A resistive load, however, really does drain  
power from the lines.

In principle, every radio station can know exactly how many radio  
receivers are tuned to their station, though getting quite that much  
accuracy (nW out of kW, while modulating with program information) is  
impractical.

It's slightly easier for a power company to notice watts, or hundreds of  
watts, out of gigawatts, at a surprisingly constant, unmodulated  
frequency.

Tim

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Really?

How do they know if I am listening vs. having a receiver turned off vs.  
a tree absorbing the signal?

All this time I thought the signal was radiated away from the antenna  
and if no one received it the signal just kept going...

If a transmission is sent and no one is there to receive it, was it ever  
really sent at all?

What!!??

Rick

Yes!

In principle.

You might've shot off a reply before noticing the "impractical" part  
though.


They don't.  I said "tuned", implying the antenna is a net absorber, not  
just a phase-shifting reflector (i.e., reactive, but unmatched loading).


It does, but the antenna either absorbs or re-radiates it.  Actually, if  
the antenna is resonant, it does that anyway regardless -- this has  
applications in passive RADAR, of course.

This is a trivial consequence of the boundary conditions imposed by a hunk  
of metal.  Fields don't just pass willy-nilly, they interact in an  
analytical and in-principle-detectable manner.  Whether you can detect ppm  
or ppt in practice is for the engineers to figure out.


Absolutely!

If a heat source at 500K is radiating into a room at 500K, is it really  
radiating?

Physics tells us, yes, and the room is likewise radiating.  The net  
balance is zero, so no heat transfer occurs (they are in equilibrium).  
But that doesn't mean there's nothing there.

What really amazes me is, John doesn't know this.

Tim

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Ok, but unless that re-radiation is re-absorbed by the source, it can't tell
the difference.  The coupling coefficient is very small.

No, it's not.
That's a closed system.
The only way for the room to be in equilibrium at 500K and the radiator  
at 500K is if the
net power radiated is zero.

Exactly!  I *did* use words like "in principle", "it's up to engineering",  
and "10^-12" in there. :)


But I didn't say net power radiated.  I said radiated, i.e. emitted alone.

All things radiate all the time!

Tim

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Ok, but so what?
All things absorb all the time.  In equilibrium, the absorption and  
radiation
cancel.
If  you have a covered bucket of water, you have evaporation and  
condensation
happening in equilibrium.  But there's no LOSS of water.

Exactly!  And just as a sealed container, sitting in the refrigerator,  
collects condensation on one side (which is actually due to net work, due  
to a thermal gradient, but it needn't be much), so too, an antenna can  
detect the nature of the EM waves around it, and anything affecting them  
(the net power transfer, back and forth, likewise being potentially very  
small).

Tim

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On Tue, 9 Oct 2012 18:56:17 -0500, "Tim Williams"


How does the transmitter know that a receiver is there, as opposed to
the signal passing on into space?


Not really. A mile of wet mud under the line would extract a lot of
power, and not be accountable. Line losses will vary with temperature,
and they can't know the temperature of every point on the transmission
line. And they can only meter to a reasonable fraction of a per cent
accuracy, to detect line losses. A kilowatt out of a gigawatt would
never be detected... 1 PPM.

Hey, this is cool...

http://www05.abb.com/global/scot/scot245.nsf/veritydisplay/fa4150f852382867c12577f8004c0d5d /$file/br_hv-tg(en)c_2gja708402-1012.pdf

There are huge potential transformers, too.




--  

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

On a sunny day (Tue, 09 Oct 2012 14:16:47 -0700) it happened mike


http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/

If the fence is an open circuit not much power will be delivered.
If you take the ends and connect a light bulb maybe some power is delivered.
That *could* be the reasoning.

line.

http://www.doobybrain.com/2008/02/03/electromagnetic-fields-cause-fluorescent-bulbs-to-glow/

Ok, but where'd the differential power come from?
Does it increase the line loss?
Or does it merely reroute loss power through the light bulb
as it moves into the ground and radiate it as light?