[ffk]

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. Impressive!

These kinds of applications have also been a long time coming. With IoT devices and NFC readers looming around the corner, I think it is likely that we will see some pretty innovative medical inventions. Likewise, as mentioned by NKosmatos, we need to take the security aspect very seriously. Some of these NFC devices are programmable, and should defend against attacks that could lead to events such as withholding life-saving medicine or misreporting biometrics.

For reference, check out this paper I co-authored:

Suitability of NFC for Medical Device Communication and Power Delivery (2007)

https://ieeexplore.ieee.org/document/4454171/

And if you're interested in an early paper about RFID-delivered viruses, check out this by Tanenbaum et al.:

http://www.rfidvirus.org/papers/percom.06.pdf

[darkmighty]

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 theory mistake, and I wonder what the actual device construction is like. (since it cannot work the way he describes at steady state)

Maybe he's interpreting phase modulation as frequency modulation? Of course at steady state all frequencies would be the same, making this a weird/incorrect interpretation I think.

[ffk]

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. :)

[darkmighty]

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.