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Comment by jcims | Hacker News Reader

I remember this article from the same site a while back - https://hforsten.com/heartbeat-detection-with-radar.html

I bought some cheap 10ghz and 24ghz dopper radar units off of amazon and started tinkering with them. You can absolutely pick up heartbeats and breathing just visually in the spectrogram.

Here's a few samples from that era:

10ghz pointed at ceiling fan:

https://www.youtube.com/watch?v=tIiFvByf1CQ

10ghz pointed straight up underneath a quarter that I flipped and allowed to land on the surface

https://www.youtube.com/watch?v=8riretP8ylE

10ghz pointed at a quarter spin on the surface

https://www.youtube.com/watch?v=5lnYvJoxRak

The comb filtering of the signal from the spinning surface is really cool.

10ghz module on amazon - https://www.amazon.com/HiLetgo-Microwave-Detector-Wireless-1...

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interesting does/can it work behind structures? is it safe to point this at yourself?

yes, it's safe. It's not that different from regular WiFi or cellular signals.

It's non ionizing (aka it doesn't have enough energy to instantly destroy cells unlike uv radiation) but it can heat up tissue, which is linked to cancer and worse (think microwave ovens).

There’s no risk of cancer from heating up tissue.

There is but it's sun burn type risks, not nuclear reactor type.

A few unlucky people have been literally cooked to death by military radar. It's as awful as it sounds.

> It's as awful as it sounds.

Well to me it sounds like you'd get heat stroke and pass out before anything reaches particularly painful temperatures, because it's heating you pretty evenly and not via contact, but maybe that's not the right way to think about it?

At higher frequencies I would guess it gets closer to normal burning?

> because it's heating you pretty evenly and not via contact

My microwave oven disagrees

Without knowing anything about tissue interactions the reason why microwaves heat so unevenly is the sanding pattern that waves make.

If you want to get heated evenly, don't remove the spinning glass plate before entering your microwave oven.

This isn't really correct I'm afraid. The cause of SAR is predominantly dielectric losses and the loss tangent is a strong function of tissue type -- CSF, fat and bone are really quite different in terms of epsilon r and sigma, and one has to solve Maxwell with a human voxel model to work out SAR effectively which computationally is a pain.

Once tissue heating has occurred what happens next is well described by the bioheat equation, which is basically the thermal diffusion equation with a massive percussion term. The blood supply is very different to different tissue types and the depth at which peak heating occurs is a very strong function of wavelength.

For frequencies, the combination of these effects means that your eyes are most at risk -- water like and terrible blood supply. This gives rise to the first piece of advice I was given when a graduate student playing with electron paramagnetic resonance -- never look down a waveguide and treat them like a loaded gun!

Perfusion not percussion! Damn autocorrect...

Isn't sun burn damage and nuclear reactor damgage the same type but at different magnitudes? If your skin becomes red and hot a few hours after being exposed to the sun it's because your immune system is killing off cells that had their DNA damaged. The damage is not caused by heat but by radiation.

https://www.thenakedscientists.com/articles/questions/why-do...

I did some reading and it seems you are right.

No Idea why phones come with a SAR rating then and why there are safe limits defined, when no one found unsafe limits.

Because you tend to hold phones right to your brain.

Personally, I perceived it as a matter of good engineering: your phone shouldn't spend its energy to heat your ear.

Interesting! There might be other unintended issues from transmitting heat, I suppose?

Even from transmitting heat to the phone's components itself or other items that are nearby, e.g. laptops: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5219578/.

Afaik the transmission energy of a cell phone is limited to a maximum of 1 or 2 Watts (depending on which frequency it operates on). This power is a maximum that will only be used if the reception is bad.

In the wavie scene (people who are paranoid about radio waves controlling them) shielding those devices is a typical approach — this is also where the meme of the tinfoil hat comes from. Ironically that shielding makes your cellphone operate with the maximum transmission power at all times.

Tiny amounts of cell heating/cooling have been found to change the cell differentiation mechanism. That's the reason ultrasound energies are limited during pregnancy and it's recommended not to do too many ultrasounds.

I don't see why microwave heating wouldn't cause the same effect.

But there is a risk of dying from heating up tissue.

I have a "human presense sensor" which is a 5GHz radar. It works behind structures, detection is weird sometimes but much better than a passive infrared sensor for this use case.

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