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Comment by crimandnakatoya | Hacker News Reader

Isn't that sort of like stealing power? Wouldn't the station need to broadcast with more energy to reach any listeners "behind" your antenna-siphon? And isn't the energy transfer extremely inefficient?

It kind of sounds like the apocryphal tales of farmers who would lay big loops of cabling underneath HV lines to power their electric fences.

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1. No, because the power has already been transmitted. There would be problems if there were billions of these devices around a transmitter, because that would affect the loading. But one isn't going to make a difference.

2. Potentially yes, but the actual shadow is tiny. And transmitter powers are fixed and limited anyway.

3. Yes, of course. So is any form of undirected radio transmission.

4. This actually works. Kind of. You need to run the line parallel or build a resonant mix of L and C at 60Hz (or 50Hz.) You can also do things like power fluorescent tubes by induction.

https://www.trendhunter.com/trends/magnetic-field-fluorescen...

The problem is the voltage/current is very hard to control, and the whole point of electric fences is that they're not lethal. You'll get something out of a resonant circuit, but if you're not a qualified electrical engineer it won't be the clean 110/60 or 220/50 needed for an electric fence.

Points 2 and 3 seem like they could contradict each other. Wouldn't a tiny amount of power for you equate to a much larger amount of power at the radiation's source?

No. The transfer is inefficient because the vast majority of the transmitted power is dissipated in the environment as heat.

The transmitter cannot tell if switch the receiver on and off.

It only works because the LC circuit has a high "Q" and it peaks up at resonant frequency, at the expense of current. It is sort of like a torque convertor for electricity.

Now in most applications a minimum voltage is required and you will counter the small current by using a capacitor to accumulate power, at a usable voltage, until you have enough to briefly do something.

I would make a YouTube video if I could be arsed, but way to busy right now.

'TheOtherHobbes offered a detailed reply, but I'd like to add a useful mental model for such questions. Like all other EM radiation, radio waves are just light. They diffuse a bit differently than visible light due to much longer wavelengths, and different materials are transparent/opaque to them, but to a first approximation, you can view the transmitting antenna as a lightbulb, and the receivers as opaque and weakly-reflective objects.

Your questions are thus similar to asking, "If you put a tiny solar cell on a glass table, wouldn't that steal light from the lightbulb? Wouldn't we need a more powerful lightbulb to illuminate the room? Isn't this energy transfer extremely inefficient?". Yes - it consumes some power; no - it casts a tiny shadow, and enough light is scattered around that you won't see the difference; yes, if the only reason you turned the lightbulb on is to power that solar cell, it's very inefficient.

In Denmark the government have allowed people who live along some of the largest underground powerlines to install geothermal equipment to heat their houses.

You better be really, really sure the person who comes out to mark the utilities before you dig has the correct coordinates...

That might actually save the utility companies some power usage - the resistance of a material increases with temperature and the homeowners would be reducing the temperature of the ground.

seems like that's a win for both parties? the transmission line heating is a waste product, which increases resistance, which increases the waste. so if folks are willing to pump the heat out and productively use it, that's great.

It's very interesting. Do you have a link about this?

Anyway, this is very different.

In an electric wire part of the energy is always transformed into heat and wasted, so it's nice is someone can use it.

In this device, they add something to absorb part of the energy that otherwise would have traveled to your neighbor, so the transition tower must increase (slightly) the energy used.

It's like digging a hole and replacing a part of the high voltage cable with a device that makes more heat on purpose to heat your home.

That would mean listening to the radio was stealing power - they don't care, they want you to listen to their advertisements...

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