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Comment by exabrial | Hacker News Reader

I can't seem to get any info on how much power is actually transmitted/received... it's hard to beat the Inverse Square Law.

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It looks like the FCC approval pertains to a device delivering ~10W at a distance of 0.5m in the 900MHz band. https://apps.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm...

So what makes the radio waves magically drop off the face of the earth after .5m? And who allows 40 dBm (much more if we're talking at .5m distance) in an ISM band?

I mean, that test report is making a lot of assumptions and setup constraints that are now magically not present in the video..

Yes, I have been trying (unsuccessfully) to get my head around how the test setup would translate into a practical application. And that the certification appears to violate the FCC's own 30dBm power limit in the 900MHz ISM band. Very strange.

The ISM field strength levels here are defined in 47 Part 18.305. they are measured in an anechoic chamber and inverse square law scaled to the field strength at the stated distance.

Source: I used to work with RF lighting devices, which need to meet the levels of 18.305(c), and were usually tested the same way in pre-compliance before final OATS testing.

>>>... device delivering ~10W at a distance.

I'm reading it very differently. ~10W is the power of the transmitter, the bit that the FCC actually cares about, not the energy delivered to the target device. With all the inefficiencies and losses, I think they are far below 1w in actual delivered power.

no it's not, you use a directional antenna or phased arrays and beamforming (which is what they're doing).

Previous articles claim 4 watts at 5 feet in the 5GHz ISM band via phased array antennas.

I'm as skeptical as you.

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