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Wireless system can power devices inside the body

news.mit.edu

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Re: Wireless system can power devices inside the body

#41

Post by ffk »

darkmighty wrote: post #33
ffk wrote: post #27

The main thing I got from the MIT article is this: "This system relies on an array of antennas that emit radio waves of slightly different frequencies. As the radio waves travel, they overlap and combine in different ways." This is known as constructive interference, pretty nifty to see it used in this context. The range has been increased from 10 cm to 1 meter since the paper I co-authored on this subject. Impressiv…

Isn't this description flawed though? Waves of different frequencies are independent in power. Thus they don't interfere (when time-averaged) -- at all. Zero. (this is because signals of different frequencies are orthogonal in the time-average integral of product sense) What creates interference is difference is phase among waves of the exact same frequency. It's odd that the paper author is making such basic EM theo…

My understanding is all frequencies interfere with all other frequencies. When averaged, the total power is probably not affected, but the power at any given point at a small enough time interval should be noticeable.

But then, I'm not an antenna designer, I may have something fundamentally wrong in my understanding. :)

Re: Wireless system can power devices inside the body

#42

Post by darkmighty »

ffk wrote: post #41
darkmighty wrote: post #33

Earlier quoted context omitted.

Isn't this description flawed though? Waves of different frequencies are independent in power. Thus they don't interfere (when time-averaged) -- at all. Zero. (this is because signals of different frequencies are orthogonal in the time-average integral of product sense) What creates interference is difference is phase among waves of the exact same frequency. It's odd that the paper author is making such basic EM theo…

My understanding is all frequencies interfere with all other frequencies. When averaged, the total power is probably not affected, but the power at any given point at a small enough time interval should be noticeable. But then, I'm not an antenna designer, I may have something fundamentally wrong in my understanding. :)

The problem is as soon as you're speaking of "frequency" you're already on a time-scale where the power doesn't interfere with other frequencies (as long as the medium is linear, etc -- which is almost certainly true here). In the mathematical/technical sense (in linear system theory)it's quite clear that different frequencies would interfere, it's a basic entire principle of linear circuit design that's been used for centuries -- systems have a Frequency Response where each frequency is independent as a result of the eigenfunction property. Personally I just think it's a big oversight, maybe they were trying to give a laymen explanation and confused themselves. What they're doing must be some form of phase modulation, maybe they're interpreting as short bursts of frequency modulation (with a stable steady-state frequency), but as I said this is not an illuminating view imo.

Edit: Actually I think they might have frequency control indeed. That would be a good idea to find a peak in frequency response (i.e. "resonant power transfer"), not for any directivity gains through constructive interference. That might be the source of confusion.

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