Lighting a LED with ambient RF (was candle)

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

Have fun!

We had that with a hydraulic servovalve!

"this is the BBC world service...."

To be precise 70 m tall (vertical polarization).

at what impedance level ?

The radiation resistance drops inversely proportionally to the square of wavelength below 1/4 wavelengths, thus the matching network not only needs to tune out the antenna capacitively reactance, but also transform the very low (a few ohms or less) to the standard 50/75 ohm impedance levels.

Definitively _NOT_

While the provincial US organization "IHF" tried to introduce the dBf (femptowatt decibels above 1 W) in order to make some sense into advertisement, those 10 dBf figures apply _only_ to receivers in the

Due to the band noise around 1 MHz, those dBf figures are useless.

There is no "p", idiot.

Oh, yeah. We engineers use powers of 3, mostly. Deci and hecta and all those are less common here.

A longwire, or a big loop, sounds more promising.

Hey, we're engineers. We know how to match impedances.

That would reduce the 'Q' to an unusable level since a parallel L/C circuit is high impedance.

The Q would be reduced in proportion to how much power is extracted by the LED. Delivering power into the LED is the whole point, so naturally that will reduce Q.

Of course, the impedance match should be optimized for maximum power transfer. Crystal sets often used tapped inductors for that reason, so thet might be reasonable here.

And the voltage would be lower at that tap.

That's how I pictured it !!

Greg

Depends on what you connect to which tap.

scope

Yup, we are famous, often just minutes after we post.

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?

Have you ever seen a _working_ crystal radio with the capacitor connected to the tap?

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.

In this whole conversation the only thing you chose to comment on is a typo and you have to call the guy an idiot at that...

What??!!!

Rick

No, but we're making a night light, not a crystal radio. And I didn't say that the cap had to be connected to the tap, but it might make sense.

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