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Thursday, July 19, 2018

Energous FCC FOIA Docs

I had a chance to go through the FCC/Energous FOIA documents mentioned in my last blog post. They span a period from around February to December 2017 when Energous were trying to get their mid-range transmitter approved under Part 18 (unlimited power) rules. Most of them have the really useful information redacted, and are Energous bugging the FCC to "please can we visit so we can move this on quickly" and the FCC engineers saying "errr, sorry out of town that day" like an ex-girlfriend trying to avoid an annoying and slightly psycho ex. One of them was super interesting with lots of info in it though, which reveals a lot of the history of the system.

The data shown seems to be for the system as they were trying to push through in early 2017, with 21 W out, but to ensure they hit the SAR safety limit by December had to dial it back to 10 Watts out. This ties with the December device power at 90 cm being 30 mW, while in March it was 60 mW at 1 meter. They pushed the size of the safety cutoff zone up to 50 cm from around 35 cm, possibly changing the focal point along the way (just eyeballing the structure it looks to have changed between March and December). 

The main FCC concerns seemed to be safety via the SAR limit, as well as ensuring that energy was in "pockets of concentration" (Doc 18). There's a significant discussion on corrective factors applied as safety margins, and basically they have to scale all measured results by ~1.5 and still be under the 1.6W/kg limit (Doc 30). This means, as I suspected, that Energous cannot raise the power output of their system from where they are today (0.966 W/kg) - what they have now is as powerful as it gets.

My read of this - Energous just kept resubmitting and resubmitting, each time with the FCC telling them to go away and what to do next, and eventually they dialed everything back to the point where they got it through under the SAR limit. An undercurrent in the notes is sloppy work by Energous, with the FCC constantly having to ask for clarifications, updates, or actually doing calculations for them! (Doc 30) 

I don't see any evidence of pressure on the staff to push the device through, more exasperation on their part with second rate engineers just throwing stuff at the wall hoping it would eventually stick.

The rest of this analysis is a little dry for those not interested in the details, just warning you.

There's a few things though that stand out as important, beyond what I listed earlier. First of all, they had moved to 915 MHz from 5.8 GHz by February 2017. Document 1 indicates that the February submission is a second clarification or change in response to a meeting they had with the FCC in October 2016. I could speculate that a 5 or 6 month response time indicates that some significant changes had been made, as new measurements or clarifications could be made quickly. This may be the timeframe in which the frequency switch was made. This IMO is a significant change with implications for performance of the system, yet was never mentioned in SEC calls or quarterly reports.

The work presented by Energous also appears to be sloppy, with the FCC multiple times noting how poor the data consistency and quality is (Doc 9 "trying to understand field distribution", Doc 41 requesting "proper and consistent information", Doc 48 "We understand that it was prepared quickly, but we suggest paying attention to some details.").  Their two tables don't match, for example the received power listed below when converted from dBm in Table 2 are 512 mW and 47 mW (27.1 and 16.7 dBm) for 30 cm and 1 meter respectively, but in Table 1 (previous post) are 190 mW and 60 mW (22.8 and 17.7 dBm).


Basically, the numbers don't add up. I expect the numbers here are for an earlier system that was very tightly focused to try and maximize power at a single point to get to ~500 mW to charge a phone, but ended up going way over SAR. There may be some other reason, one being estimated and one measured, but I'm tending to the "sloppy" for now.

Document 32 shows the "keep out" zone changed around November 2017, increasing it to 50 cm. This may also be when the physical structure of the power bar was changed, or may simply be that they were forced to update their SAR measurements and this was required.

Document 18 references the need not only to be concerned with safety and states that data "should also show that the energy distribution through field maps demonstrate that there are pockets of concentration". This indicates that the safety restriction was not only a single SAR number, but the physical distribution of the energy. 

Figure 11 above shows a typical on axis beam, with a peak at one point showing the transition from near to far field, and then a gradual but continuous reduction with distance. If we assume a "pocket" of energy implies a region where there is a lower value of energy both before and after the charge location, then that would restrict the use of the system to just beyond near field only. Even with a phased array, where that transition zone is, it's a function of frequency and physical size of the transmitter. For any practical size transmitter, (equations here) it's basically likely to never be viable beyond 1 meter. If this is the case, that's a huge limitation for any at-distance wireless power system unless they make the transmitter the size of a wall.

Also, that peak in Figure 11 is at 42cm, but they say a focus at 65cm. They might want to take a look at their work there...


The size of a region of constructive interference, a focus, is often defined by the half power, or -3dB points. Looking at Figures 11 and 13, it seems that region is about 60 cm in the x- and z-axes, but only 15cm or so in the y-axis. Some "pocket"!

The "safety system" that detects if anyone is in range of the device and shuts it off (supposedly) is ultrasonic, using TI PGA450 chips and my best guess would be Murata ultrasound transmitters (sound familiar?), used in car parking sensors. I'd be very interested to see how this is setup, because as someone who has worked with them before and ultrasound a lot, I think that system might be easy to fool if you don't design it very carefully.

So overall this data is interesting, nothing too amazing, but confirms what was suspected - that this system is at the limit of what it can transmit safely, that it doesn't have enough power to charge a phone in any realistic way, that efficiency is low, and that FCC staff weren't too impressed with Energous' consistency and quality of work. But it does prove that persistence pays off.

(Seemingly obligatory statement - I have no financial position, short or long, in Energous or any related company)

3 comments:

  1. Paul - Good work on this, and your other postings!

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  2. Interesting stuff.
    Minor nitpick - the half power point is -3dB.

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    Replies
    1. You're right - I've been looking at acoustic pressure plots lately where the half amplitude point is -6dB. I'll correct, thanks for pointing that out.

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