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WoolfkeeperQuoteI think that's why they choose >1 Mhz frequency for the witricity system. Metals have a very thin skin depth at those frequencies, and this gives high resistance; so you don't get much eddy current, and not much power is dissipated.I think you have this the wrong way round, I'm afraid. The higher the resistivity of the metal, the greater the loss - which is what one would expect --ain't it? No resistance and the skin depth is zero - no loss at all.
I think that's why they choose >1 Mhz frequency for the witricity system. Metals have a very thin skin depth at those frequencies, and this gives high resistance; so you don't get much eddy current, and not much power is dissipated.
Skin depth is a FUNCTION of resistance. You can't have one without the other.
I just had another thought on the subject of interference and here is a seriously (unspurious) objection. The ratio of received MF (say) broadcast signal and local 'power' signal in an adjacent channel would be tiny, in most cases : say -60dB. That would impose unrealistic limits on the receiver selectivity filter which is designed on the assumption that the adjacent channel will be 30dB LOWER (that's 90dB adrift - using ball park figures)). This requirement would certainly not be met by existing designs. Rather than needing a 9kHz free band I think you would be needing to free-up spectrum space corresponding to three or four channels on either side of the power channel. That's a huge chunk of the MF band wiped out at a stroke. You always have to be backward compatible in these matters and you couldn't expect to retrofit filters into every existing MF receiver.
That statement just has to be wrong.
A transformer will be highly inefficient if you have an air gap but a small one still allows some useful coupling. That's the obvious argument where 50 Hz coupling is concerned- as with toothbrushes.
Then for thr RF situation: The radiation resistance of a small structure (e.g. Oyster pad) is very low and it will only radiate if tuned very heavily.
The near field is very near. A structure which will give near field coverage for a 'room' will have a radiation resistance which is more or less proportional to its size (for long wavelengths) and will need to lay down a proportionally larger total flux if the receiver is to work anywhere within / near it.
That indicates considerably more power is needed. (Remember- you can't have a huge 'receive' structure on your hand held device which could mitigate this, to some degree.)
I don't know what coupling factor you are assuming but it would be several tend of dB, I believe - unless you can show me some sums to the contrary.