# Harnessing energy from radio station.

Discussion in 'Wireless & RF Design' started by Nathan Hale, Apr 17, 2014.

1. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
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Hello All! I read that all forms of EM waves have some energy stored in them, be it microwaves or radio waves. I was wondering if i would be able to take the radio energy from my local radio station which is coming out of my speakers as sound and in turn using that energy to trickle charge a small rechargeable battery. Will this idea work. if yes why? if no why not?

Thank you all

2. ### MrChips Moderator

Oct 2, 2009
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3,451
It is a losing proposition to try to use the energy from the speakers. The energy is not coming from the radio station (except if your radio is a crystal radio). The energy from the speaker is coming from the battery in the radio or from the wall outlet that powers the radio.

If you live right next door to the transmitter antenna you could harvest some energy. Otherwise the power level is way too small to be able to charge the battery. You would be better to use a small wind powered generator or a solar cell.

3. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Ok, i get it. You are basically saying the amplifying power comes from the battery in my radio, not from the signal itself...How about energy from GPS satellites and pulsars?

4. ### WBahn Moderator

Mar 31, 2012
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The energy from the GPS satellites is far less than the from a radio station. Rather Herculean efforts are required to pull the signal up out of the noise. The total transmit power on the L1 signal is only about 25W. By the time that gets to the ground you have an energy density of about -135 dBW/m^2. That equates to 0.03pW per square meter. In other words, even assuming you could harvest all of the energy with 100% efficiency, you would have to collect all of the energy that hits an area the size of 8000 football fields to collect one microwatt.

The energy making to it Earth from a pulsar is even more miniscule. Plus you would have to harvest energy that is in the X-ray and gamma ray regime. Are you up for that?

Last edited: Apr 18, 2014
5. ### alfacliff Well-Known Member

Dec 13, 2013
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look up " square law" the power of a radio wave decrases as the square of the distance increases. at that much loss, it dosnt make much power at the recieving end, unless your under the shadow of a high powered station antenna. that idea of harvesting power from radio waves has been around for quite a while, and still hasnt been usefull, even during the days of the megawatt am stations.

6. ### vk6zgo Active Member

Jul 21, 2012
677
85
If you are very close to a Transmitting aerial,that aerial no longer looks like a "point source",so the simple "square law" no longer applies.

That said,the energy available,even directly across the road from a large Broadcasting Station is minimal.

Back in the days of outside toilets, a guy in Perth across the road from 6WF had a tuned circuit in his "Loo"driving a single "dial light".
It made a nightime visit to the "facilities" a bit less hazardous,but power?

About as much as a nearly flat flashlight battery!

7. ### fernan82 New Member

Apr 19, 2014
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I don't know how well they work but I've seem WiFi phone chargers on BestBuy. I also saw a video (YouTube?) that shows how if you're really close to an AM station you can power a set of headphones from the radio waves with just a wire coil so it may be possible.

A read another article about a guy that went to jail for stealing from the power from high voltage lines that ran by his property with a giant inductor. And I've read about experents building sensor circuits with low power mcus that draw power from nearby power lines in the same way.

So it's possible but it is also illegal.

Apr 19, 2014
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9. ### samuel.whiskers Member

Mar 17, 2014
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A quote from here -

"Radio frequency signals of extraterrestrial origin are extremely weak. As an example, if all the signal energy ever received from all the radio telescopes ever built (viewing objects other than the sun) were combined, there would not be enough total energy to melt a single snowflake."

10. ### vk6zgo Active Member

Jul 21, 2012
677
85
A crystal radio gets all its energy from the radio signal,& can be a reasonable distance away--the secret is in high sensitivity of the headphones.

My guess is that the WIFI chargers are a scam!---A better idea would be a small solar cell.
To draw a reasonable amount of power,it would also be quite large!

11. ### fernan82 New Member

Apr 19, 2014
26
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Agreed. And the phone on the video is probably drawing more power from the battery to display the bars than what it's harnessing of the airwaves. But for example and application that needs to wake every our (maybe even every few minutes) take a measurement and go back to sleep can probably be powered indefinitely this way.

I can't find the article (it was about 10 yrs ago) but it was. The guy had a barn just to house it and it drew enough power for the power company to notice. His lawyer was arguing that since the lines go through his property he's allowed to draw power from it, just like you're allowed to take fruit from your neightbor's tree if it falls on your property. He lost.

12. ### WBahn Moderator

Mar 31, 2012
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You don't have to be close. I built a crystal radio that had no problem receiving KOA from my house which is a good 50 miles from their antenna. Plus I'm way down in a valley.

But this doesn't really indicate that you can get a lot of energy from a radio signal nearly as much as it indicates how amazingly sensitive human hearing is.

13. ### atferrari AAC Fanatic!

Jan 6, 2004
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Doesn't the credit go to the radio? Your ears seem to be there just to benefit from that. The sensitive thing is your crystal set I think.

Even if you are deaf (half of it myself) the radio would still be sensitive.

14. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
The crystal set isn't all that sensitive. The earphones are cheap (as in \$1.50) and are just the kind that used to come with cheap transistor radios 30 years ago. The "crystal" is just a germanium diode. Other than that there's an inductor (a bunch of wire wrapped around a paper towel core) and a capacitor (some aluminum foil wrapped around a 2L soda bottle partly filled with water to tune the frequency) and a resistor (to provide a DC path to ground since the earphone is essentially a capacitor). The antenna is just a long piece of speaker cable (not cut to any particular length) dangling out the window.

But even if the radio was super sensitive, that doesn't change the fact that the total energy you are starting with is miniscule and you aren't adding any to it since you have no power source. So even if the radio were 100% efficient in turning the radio energy into sound energy (and it isn't, not by a LONG shot), it's pretty amazing that your ear can detect it (and do so without any difficulty whatsoever).

15. ### alfacliff Well-Known Member

Dec 13, 2013
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one microvolt of rf into a cystal set isnt much output. a sensitive radio will hear a 1 microvolt signal easily, it has amplification.
if you want to see how much power is avialable from rf in your neighborhood, connect a spectrum analyzer to your antenna.

16. ### Art Distinguished Member

Sep 10, 2007
797
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If you were close enough to an AM station that you could convert useable energy
with no more components than antenna, Earth, coil and diode,
I think it would be more of a worry for your health to be that close to the station.

17. ### vk6zgo Active Member

Jul 21, 2012
677
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Art,the hazards are very overstated!

Back in the day,we used to stroll alongside the feeders from 6WF(at 55kW) & 6WN (at 10kW),down to the Mast Hut under the Dual mast.

This mast was driven from 6WF & 6WN via a combining unit which stopped each Transmitter feeding signal to the other one's output.

This,along with the associated Aerial Coupling Unit,had a bundle of Thermocouple Ammeters which we read each day at around 3pm.

Dozens of guys ,for tens of years did this,with no adverse effects.