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Monday, January 1, 2018

Further Safety Limits for Energous? MPE

It's just as well Energous got their FCC Part 18 approval in a holiday period - any other week and I simply wouldn't have had the time to read the reports, analyze them, and write these blog posts. (You can find these earlier posts here) Over the last four days it's been around 7,500 words and 15 pages worth of writing, and there was a lot of reading and work for each of those pages. This will likely be the last in-depth analysis for a few days, as I actually have to get back to real work tomorrow - the exception possibly being a short piece on Powercast I'm working on and may finish tonight (Edit: nope, that didn't happen).

Here's the main point of this post - Maximum Permissible Exposure is a further safety limitation for RF radiation that has not been applied to the approved Energous system, as far as I can see. If it were, using limiting power density for the general public, my calculations say they're over the limit by a factor of 3. If they use the "Controlled Exposure" higher limits, then they are within the limit (just) - however in that case this is a system that would not be allowed to be used around the general public. I can't find exemptions that would apply so I'm wondering why it's not implemented for Energous, or if they are simply not admitting this is a system not fit for consumers.

Update: Energous may be classifying their transmitter as "mobile" to allow them to select the more lenient SAR tests than the MPE. 

Maximum Power Exposure - MPE
Re-reading FCC regulations (fun times!) I found another safety limitation for RF that I had forgotten about. While the FCC Part 18 approval documents cover the Specific Absorption Rate (SAR), where Energous sit right at the limit of at 50 centimeters (hence the keep-out/danger zone), there is also Maximum Permissible Exposure or MPE. The FCC document detailing this can be found here, it's Title 47, Section 1, 1310. Table 1 lists the exposure limits for the general public.

The power density limit is set as f/1500, where f is the frequency in MHz. Given the frequency for Energous is 913 MHz that sets the maximum power per cm2 at 0.61mW/cm2. An iPhoneX has a surface area of 14.4 by 7.1 = 102 cm2, so if the MPE limit is to be observed then maximum power to an iPhone X at any distance, in a perfect orientation, is 62 mW. Assuming a 60% conversion efficiency from RF to DC that means no more than about 37 mW at the battery.

Energous are claiming 100 mW received or more at the 50 cm mark, so I'm now confused as to how this is possible without exceeding the MPE. There are apparently exemptions for mobile transmitters, listed here under 2.1093, but they define this as:

For purposes of this section, a portable device is defined as a transmitting device designed to be used so that the radiating structure(s) of the device is/are within 20 centimeters of the body of the user. 

Given there is a "keep-out" zone at 50 centimeters and it is supposed to be "at-a-distance" wireless charging, it's really hard to imagine how they can get this exemption. Even then the only benefit seems to be that the system can use duty cycle to calculate the power - so if Energous were to, at the 100 mW received range, run their system for 3.7 seconds on then 6.3 seconds off, constantly repeating (or equivalent) then they could meet the 62 mW RF power density limit. 

So if MPE were to be applied Energous would, in addition to their (IMO) half-assed and useless safety zone, need to switch the system off about 2/3 of the time in any 30 minute period of charging. If they manage to up the power from where they are now, not only will the keep-out zone increase in size, they'd have to decrease the duty-cycle further as well.

Not Usable by the Public?
The only other way I can see this regulation not applying is if they use the less stringent safety limits for Occupational/Controlled Exposure. These are 5 times higher than for the general public (f/300 rather than f/1500). This would place the maximum RF power received by an iPhone X under ideal circumstances at 310 mW, which at 60% efficiency translates to 186 mW at the battery - barely more than the 100 mW currently received (apparently), and essentially within a 2 to 3 dB safety margin usually required. Once again, by my calculations, the Energous system as-is sits at the safety limit and cannot increase power from here. Worse, if the general public exposure limits are applied then the system power, these numbers say it would have to be reduced by a factor of 3 from the already low values.

Wearables and MPE
Things get much worse now for consumer wearables as well - for a 2 by 2 cm patch (big for a wearable), that 4 cm2 will get you a stunning 2.44 mW of RF power, or 1.5 mW after conversion. That would be enough to charge your AirPods in about 4 days. Patch antenna would also be highly directional.

Does MPE Apply to Energous?
So how do Energous get away without meeting the Maximum Power Exposure limit? I don't see it referenced anywhere in the FCC Part 18 approval documents. It's not as if they don't know about it, an IEEE Spectrum interview with CTO Leabman in January 2016 has him admitting the MPE applies:

IEEE Spectrum: When you go through this process with the FCC, is it like they have this checklist of criteria you have to fulfill in order to be safe, or is it much more nuanced than that?

Leabman: Yes and no. Certainly there are regulations for RF, what's safe and what's not safe. There's SAR [specific absorption rate] for your phone. There's MPE [maximum permissible exposure]. The nice thing about RF is that they know what's safe, and they have regulations, so in that respect, that's known. 

I've tried to find other exemptions to MPE in the regulations, but so far have not. If there are any readers with a knowledge of FCC RF regulations and MPE who care to let me know, I'd appreciate that. I'll update this post should I find that.

Edit: In discussion with some other engineers, the KDB 447498 D01 General RF Exposure Guidance document may offer some insight here. Energous may be classifying their device as "mobile" under 47 CFR 2.1091 by meeting the requirement that it is not "physically secured at one location", is "able to be easily moved to another location", and that the user is at least 20 cm from the transmitter. In this case, they may be using this section to choose between SAR or MPE as the limit. Given MPE is the more stringent of the two, I can see why they chose SAR instead. Further, this document shows why they picked 50 cm as the "keep-out zone" as there appears to be a different way SAR is calculated when you cross that 50 cm boundary, and the safety limit is even lower. It also seems to definitively limit the RF power at 50 cm and 913 MHz to 158 mW. (I misread units here, it's 50mm not 50cm, hence the scorethrough)

So the questions here are: "Would this exclusion still apply if the transmitter were "portable" and not potentially mobile?" but mostly "Have Energous chosen SAR instead of MPE knowing they would fail MPE?"

Looks like lots of gaming the system is going on.

Unsafe Demo, Faked, or Something Else?
Last point: The IEEE Spectrum article has a Jan 2016 demonstration of their system charging via an antenna at 5 feet at 1.6 Watts, while in a Las Vegas hotel room with a journalist present. They state that 0.673 Watts is about 10% of power transmitted, so 7 Watts total or so. At 5.8 GHz the MPE limit for an iPhone X receiver would be about 100 mW and 500 mW RF received, "general public" and "controlled" respectively. This was a demonstration then that exceeds regulatory limits, and if we scale linearly from the approved system I'd guess the entire room exceeded the SAR safety limit. So did Energous give an unregulated demonstration that exceeded safety limits, was it faked, or were they otherwise somehow exempted?


  1. Another issue with RF charging that I haven't seen discussed much is the potential for permanently damaging radios (as opposed to temporarily blocking them). Most wireless chipsets have a stated maximum survivable signal strength in the region of -10 to 0 dBm. Even the SAW filters that are often used between the antenna and chipset can be damaged by signals as low as 10dBm.
    I strongly suspect that many devices that don't nominally receive at 913MHz band nonetheless don't have sufficient selectivity in front of the sensitive bits to handle 20+dBm input at 913MHz, and would therefore risk permanent damage from being put near one of these chargers.

    1. Interesting point, and not something I'd considered. I'll see if I can get time to look into this. Thanks.