Recharging via radio waves?

1. Jun 16, 2009

OAQfirst

I'm not knowledgeable about these things, with all the free energy nonsense I frequently see. So what do you think?

http://tech.yahoo.com/blogs/null/143945 [Broken]
Does this sound right? If so, it looks like exciting news. Even for stand-by charging, that's still a good thing.

Last edited by a moderator: May 4, 2017
2. Jun 16, 2009

mgb_phys

You have to assume Nokia have done their sums.
But assuming the signal is a couple of milliwatts and you leave it on continually.
My phone battery is 700mAh x 3.6V so stores around 2.5Wh, charging with 1mW would take 3months! It's difficult to believe it's even enough to overcome the leakage in the charging circuit.

3. Jun 16, 2009

OAQfirst

It reports their goal is 50 milliwatts. Would that be substantial enough?

4. Jun 16, 2009

mgb_phys

My phone lasts a week on standby with a 2500mWh battery so around 15-20mW - if they can pull 50mW out of the air that would certainly help.
I suppose if they didn't just use the phone antenna they could also use the power form lots of other transmitter sources, TV/radio/wifi etc.

5. Jun 16, 2009

OAQfirst

Hmm. Well, here's to hoping!

Thanks.

6. Jun 16, 2009

Greg Bernhardt

Would this phone technology be useful only to those in major cities?

7. Jun 16, 2009

Staff: Mentor

Presumably. I wonder if anyone has actually calculated how dense the radio energy is in various places.

 calculation deleted. I guess you'd want to do a calculation based on surface area....

Last edited: Jun 16, 2009
8. Jun 16, 2009

Ivan Seeking

Staff Emeritus
...and over what range of frequencies?

9. Jun 16, 2009

flatmaster

I immagine you would want to make your windings as large as possible to contain the maximum amount of magnetic flux. Probabilly the entire surface area of the phone. Also, if the phone wasn't sitting in the correct orientation with the source, you wouldn't get anything.

10. Jun 16, 2009

Staff: Mentor

The surface area of a 1"x2"x4" phone is 8 square inches. At 50mw, that's 900 mw/sq ft. The side surface area of my 19x14x8' living room is 528 square feet, which would mean if I wrapped it with antennas, could generate 475 w. My house is 20'x40'x20', or 2400 square feet side surface area. That's 2.1 kW, which (with storage), would be more than enough to supply all my energy needs.

I find that hard to swallow.

Last edited: Jun 16, 2009
11. Jun 16, 2009

Moonbear

Staff Emeritus
Over what time frame?

When would this be most useful? I'd think if you were stranded out in the wilderness without a charger is when you'd need something like this...one would hope the charge would happen fast enough to call for help before you die of dehydration or exposure to the elements. Otherwise, it's nothing more than a gimmick.

12. Jun 16, 2009

Staff: Mentor

miliwatts is power, Moonbear - it's got a timeframe in it (seconds).

13. Jun 16, 2009

Blenton

Isnt it illegal to 'harvest' energy from radio waves? I could imagine those who live near the transmitters putting up huge coils on their houses to sap up the free kilowatts.

14. Jun 16, 2009

Moonbear

Staff Emeritus
D'oh. Sorry, I made the classic mistake of mixing up power and energy. Thanks for the correction.

15. Jun 16, 2009

Pengwuino

What transmitter?

16. Jun 17, 2009

Blenton

17. Jun 17, 2009

Pengwuino

I would think that's somtehing you could do right now. I was thinking he meant that Nokia was planning on building transmitters specifically for recharging cell phones.

18. Jun 17, 2009

Blenton

No I believe they meant radio waves currently in the air.

19. Jun 21, 2009

xykotik

Before I read the Yahoo! article, I assumed induction, like the contact-free charger for my Braun toothbrush, or the ones used to recharge cardiac pacemakers and ICDs (implanted defibrillators). It doesn't look like that's what they meant.

My immediate image was of holding a four-foot fluorescent tube under an HT power transmission line and seeing it glow dimly. Kinda neat, but not a lot of oomph for being so close to the EM source (albeit 60Hz, not RF). I aint' doin' the math for that, so I'd like to see a white paper or something before I throw away my chargers.

Neat topic