Lighting a LED with ambient RF (was candle)

Hello, There are apparently a few working RF energy harvesters, one by a Joe 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.

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 wrote in :

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 saw that at the VOA station at their 'Bethany' facility in Mason Ohio. The original W.W. II Crosley transmitters were the worst offenders, with large glass doors in front of the output tubes. Of course, having up to 1 MW of HF RF in a half acre made it simple to light the lamp. They pulled a 4' tube out of a locker to show off the effect to a bunch of teenagers who were there on a tour from our highschool's ham radio club.

People apparently don't build crystal sets any more, what a pity. 10s of Km from a MW broadcast transmitter, 10-20 metres long wire antenna into an LC circuit provides enough voltage to push through a low foward voltage diode, and generate a low level but audible signal in hi-Z earphones. That provides a rough indication for what can be achieved. Accessible energy density is proportional to distance squared, unless you're very close, and the receive antenna efficiency is likely to be a dominant factor.

I have no idea how much power can be extracted from a ferrite rod antenna. Googling produces vague results.

A "longwire" antenna would probably pick up more power. Any reasonable antenna will be a fraction of a wavelength for AM, so it looks sort of like a capacitor driven by a current source. That could drive a resonant circuit, of which the LED capacitance itself is part of the LC capacitance. That wouldn't need a separate rectifier diode, which would just waste power. That could be Spiced easily. Antenna, L, variable C, LED. One of those trimpot-looking ceramic variable capacitors would work. It's an impedance matching problem.

When I was a kid, I had a crystal radio with proper high-impedance headphones. With roughly a 40 foot longwire antenna on the roof, it was *loud*, maybe milliwatts of audio power. A good LED is visible in room light at a microwatt.

Larkin took another crap, and his hat covered his eyes ?:-) ...Jim Thompson

If you are so good at web searching, tell us how much power a ferrite rod can extract from AM stations.

Or maybe "engineering." You wouldn't understand.

It's not self-defeating if it lights the LED. I think I mentioned broadband power harvesting in a recent post.

You don't know or care about this, you just want to whine.

Are you so helpless that you can't connect four parts in Spice?

No. Milliwatts and microwatts are the same dimensional units.

You keep repeating the same childish attempts at insults, and you can't post circuits that work. Sounds like senility to me.

I don't suppose you have anything intelligent to say about lighting LEDs from ambient RF. JF, also getting senile, doesn't either.

from ambient RF" I tried several different types of diodes and Ferrite rods and coils and could never get a measurable voltage across a .01 uf cap.

The key here, is that RF thermal noise isn't harvest-able (unless your receiver is cryogenic). You need to identify a frequency at which the RF is strong (depends on your area, of course) and then build the equivalent of a crystal radio - a tuned antenna with a rectifier that isn't too lossy. It should be possible to use a voltage-multiplier and neon lamp in most urban environments, if you tune to a local AM station. The neon lamp will blink after enough charge gets deposited.

If the neon lamp lights when you're holding it in your fingers, there's a real transmitter nearby. If an incandescent lights when you put a rabbit-ear TV antenna onto it, the transmitter is perhaps right there in the room... I've seen that happen.

scope

You have no idea of "effective area" ??:-)

Senility is due to shit tightening your hat band so no blood reaches your brain... has nothing to do with age... I can cite all kinds of octogenarians with far more smarts than Larkin. ...Jim Thompson

scope

More word salad.

from ambient RF" I tried several different types of diodes and Ferrite rods and coils and could never get a measurable voltage across a .01 uf cap.

I had in mind an AM station and a longwire antenna. Just hang the LED across an LC tank, and let it rectify the RF. LED capacitances are often in the 10 pF range. A good crystal set is LOUD! in a city environment.

scope

When it's your ignorance, it's "word salad" ?:-) Bwahahahahaha! You just keep getting deeper and deeper into it. Keep trying >:-) ...Jim Thompson

scope

The effective area of a half wave dipole is about 0.12 square wavelengths, thus at 1 MHz with 300 m wavelength, the capture area is about 10000 m² or 1 ha.

The typical ferrite rod antenna has a gain of -30 to -60 dB below the full size dipole, thus the effective area is somewhere between 10 m² and 1 dm². Multiply this by the local power density [W/m²] and you get a ballpark figure of the available power.

scope