>
> In the case where they are inductively coupled there is very little
> radiation, but if they even charge more than a couple inches away it's
> going to take a substantial signal and it certainly wouldn't be green.
Mostly I think they use a pair of coupled mag-loops.
They can be a couple loop diameters apart and still achieve 50-70%
efficiency.
You can model the effect in EZNEC. Build a couple identical resonant
magloops, put a resistive load in one and a source in the other.
Compare the radiated power (average gain test) to the "load loss," which of
course, in the case of resonant wireless power transfer is the whole useful
bit ...
http://n3ox.net/files/eznec/wireless_power.ez
It's kind of a neat effect. Just near field coupling that you could build
at home at the basic level. I assume that the actual devices will be fairly
sophisticated... measuring impedance of the primary circuit with very low
power levels and then cranking it up when a load is detected (look at the
source impedance in EZNEC with and without a load.... almost purely
inductive without, very resistive with... that's part of the key to the
method).
Of course, if you tightly couple them (fraction of a loop diameter) you can
get essentially 100% efficiency. This isn't actually true for a pair of
single wire loops stacked right on top of each other without resonating
them. If you try to build a non-resonant transformer that way (that is,
without a core) it doesn't work very well. Too much leakage.
If you very tightly couple them, you get great power transfer and they
really can be considered non-radiative. The back EMF on the load loop
induces a current 180 degrees out of phase with the current in the source
loop, and from a radiation standpoint you have a parasitic phased array very
close spaced of antennas with 1% radiating efficiencies anyway, so it ends
up really low gain.
I built a pair of resonant coupling rings to transfer ultrasound signals on
and off my rotating Ph.D. experiment:
www.n3ox.net/files/us_ring.jpg
The EZNEC file of that one shows 99+% of the power going to the load and an
"average gain" too low to list (<-99.99dBi)
You move 'em apart by a couple loop diameters and they end up more like
-40dBi and 70% efficient if I recall correctly.
That could still be a RFI generator, but I'm hoping that the fact that *very
sharp resonance* is the entire key to the operation will limit the RFI
potential to hams.
I get kind of a chuckle out of the hype.
I've been using this sort of thing to couple ultrasound signals to rotating
equipment for a few years now, ever since I got the idea from the Tenth
Edition ARRL Antenna book (circa 1964), Figure 3-74, which shows how to use
large inductive loops to both match to and couple open wire line to your
rotary beam antenna ;-)
I still haven't tried to light up a lightbulb on the other side of the shack
but I could, and you could too.
73
Dan
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