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Re: About wireless energy transfer



Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Original poster: Jim Lux <mailto:jimlux@xxxxxxxxxxxxx><jimlux@xxxxxxxxxxxxx>

At 07:49 PM 2/9/2007, Tesla list wrote:
Original poster: Ed Phillips <mailto:evp@xxxxxxxxxxx><evp@xxxxxxxxxxx>


I'm not disputing the case for low efficiency, for a single device, but what if there were, say, 20 or 30 of those devices within the field?"
   If there were the transfer to each one would 
have an efficiency determined by its unique 
coupling to the "transmitter".  If they were 
separated from each other by enough that there 
was no mutual coupling between them [the 
receivers] then the total power would be the sum of the individual powers. "
It is indeed true that the total power would be the sum of the individual powers. However, if the spacings are sufficent (more than a wavelength in any case and even less than that if the beam is to be pointed away from the broadside to the distribution of elements) then the energy will add coherently in more than one direction and not all of the power will be in the main beam. For the simplest case, a line array of equally-spaced elements, there can be many discrete beams as large as the "main beam" and energy will be lost to all of them. If the elements are more generally distributed, perhaps not in a plane, the sidelobes are not as well defined but the energy lost in them doesn't change. Do a Google advanced search on "grating lobes" and "array antennas" and you'll find lots of references. One of the problems the designer of an array antenna has is to decide the element spacing. Less elements result in a simpler and cheaper antenna but the limit (for a fixed total aperture width) is set where the grating lobes "rob power" from the main lobe. If the spacing is a half wave the beam can be steered close to 90° from broadside of the array before grating lobes appear but it is more common practice to increase the element spacing to save components and limit the scan to 60°.
"Having spent the better part of the last 5 years 
working on research into novel ways to do 
non-microwave phased arrays, the statement: 
"separated by enough so there's no mutual coupling" says it all...
That's the rub.  At distances <1 wavelength there 
will always be mutual coupling to deal 
with..except for some "special cases" like two coils that are orthogonal.
Yes, one can come up with schemes to adjust 
everything to compensate for the mutual coupling, 
but that requires knowing what the couplings are 
(non trivial), a way to do the adjusting 
(non-trivial), and a way to do the calculations 
(non-trivial).  All of these also require some non-zero amount of power to do."
    Mutual coupling is always a problem in 
designing arrays but the unfortunately the 
designer has to live with it.  In designing 
microwave antennas one common practice is to 
build a small array of elements and to measure 
the coupling of elements near the center in order 
to determine the required impedance matching.  In 
general that impedance matching can't work over 
all possible angles so there is some power lost 
due to it.  Sounds as if you may be working with 
some interesting array concepts - can you share any of them?
Ed