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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, free space is not a high impedance, it's 377 ohms. Tesla coils have impedances of hundreds of Kohms and are extremely poor matches to it. That's why Tesla died in poverty and all successful long-distance wireless power has used microwave beams.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
No, no. 377 ohms is for far field radiation.
This isn't far field stuff, it's only near field.
And the impedance here is capacitive, not resistive.
For far field you generate varying magnetic field and electric field at the same time and that launches radio waves. It turns out that a 377 ohm antenna gives you the right mix of fields.
If you only vary one then you get only near field. (It's not possible to totally only vary one, but if you mostly generate only one, then you won't launch much far field/radio at all).
In this application you wouldn't want to generate far field radiation.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Exactly, and launching far field is the most efficient way of transmitting power over long distances. Near field is lousy unless the distance is very short indeed.
I'd like to go one step further and argue that far field is the best way of transmitting wireless power, period.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
wrote ... I'd like to go one step further and argue that far field is the best way of transmitting wireless power, period.
Far field yes.
Short distances, maybe, dunno.
Deployed systems like the oyster cards on the underground use near field (inductive) resonant, and it seems to work well. Whether you could use radio for that instead, dunno, possibly.
Anyway, that wasn't what the OP asked for.
Tesla seems to have been trying to use the ionosphere and the earth plane for doing "nearfield" capacitive power transfer to devices situated between the two; it's unclear to what extent that failed technically, or whether his backers just pulled out. I don't know of any particular reason it couldn't have worked, it's certainly a cute idea!
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Capacitive power transfer: how would you approach this?
