Author Topic: Human body as a 50mhz antenna application to indoor presence monitoring systems.  (Read 1428 times)

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Offline MusclorTopic starter

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Hi, it's me, the noob who knows enough to be dangerous ;)

I've read a lot of literature on the human body acting as a 50mhz monopole antenna (in fact depending on posture between 40 and 60 i believe).
There's even an old (10 years +) US patent by the DOD on using this as a means to not only track individuals behind walls, but even identify if they carry a metal object (possibly, in this case, a weapon).

My question is as follow: since nowadays pseudo radars such as the RCWL-0516 are available for around  a dollar each, why haven't we seen the implementation, or at least the development of full scale indoor tracking, especially as part of open source projects?

In other words, imagine placing such device at 5 yards/meters interval in a grid pattern on a floor (you probably need to add some sort of directional component to them, such as placing them on a rotating servo and track the azimuth) connect them all to a server,  run the  system for a few minutes with NO humans on the floor, then let humans in, and spot the difference :) - Why isn't it trivial to then derive a heat map from the interference patterns aggregated on the server, a bit like we do on a daily basis when running netspot-like wifi surveys?

I feel like i'm missing the obvious - because there are hundreds if not thousands of papers on the matter of presence monitoring and human tracking (including leveraging mimo multipathing/time of flight analysis on off-the-shelf wireless routers), but I have yet to see a single commercial application or even a proof of concept video on youtube. 

Thank you for illuminating me on the matter if possible at all!

 

Offline radiolistener

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I've read a lot of literature on the human body acting as a 50mhz monopole antenna (in fact depending on posture between 40 and 60 i believe).

that's false information
 

Offline MusclorTopic starter

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I've read a lot of literature on the human body acting as a 50mhz monopole antenna (in fact depending on posture between 40 and 60 i believe).

that's false information

This may explain a lot, including why the two companies selling MIMO-based human detection went bust (i think they were based on an MIT project at https://news.mit.edu/2018/artificial-intelligence-senses-people-through-walls-0612 which is however real).

A summary is at https://bkibret.com/human-body-antenna. I've seen the USPTO document with my own two eyes and it was the DOD quoting the body was a 50mhz monopole antenna.

Could you please provide more information? I'm building an assistive technology project with my own limited experience for my mother who suffers from Alzheimer, so it's not just a passing interest on my part but a genuine need for knowledge in order to build the best system possible. Thank you.
 

Online tggzzz

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Motion sensors fitted to alarms are used for monitoring the elderly. Such a system might be more reliable than a homebrew hack, and greater peace of mind.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline radiolistener

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Could you please provide more information?

Any piece of conductor works as antenna for any desired frequency. The difference is just input impedance and efficiency.
Human body may have resonance at near to 50 MHz frequency, but it has high loss, so it's not a good antenna.

Just for fun, I just measured body impedance :)
It really has self resonance near 50 MHz (48 MHz), it's input impedance at that frequency is about 1500 Ohm.

 

Online Bud

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What was your test setup?
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Offline radiolistener

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What was your test setup?

I inserted small piece of wire into VNA connector and tight touch it with wet hand.
I dipped my hand into salt water for better connection  ;D
« Last Edit: May 13, 2019, 11:38:30 pm by radiolistener »
 

Offline MusclorTopic starter

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Any piece of conductor works as antenna for any desired frequency. The difference is just input impedance and efficiency.
Human body may have resonance at near to 50 MHz frequency, but it has high loss, so it's not a good antenna.


Thank  you for the information and for the test!
I understand it's a terrible antenna - but, for the sake of discussion and curiosity, why would the setup i described not work (at all)?

Feel free to assume a high budget :)
 

Offline Whales

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What was your test setup?

I inserted small piece of wire into VNA connector and tight touch it with wet hand.
I dipped my hand into salt water for better connection  ;D


Connecting to the hand (~middle) of a human would make you a shorter (two-prong) monopole antenna, wouldn't it?  Cmon, feet or head feed point please :)

Follow proper ESD precautions to avoid damaging your human. Excess moisture can be removed via the oven profile shown in figure 3.  Compatibility with alcohol-based solvents varies depending on sample.  Do not attempt to solder. 
« Last Edit: May 14, 2019, 04:06:57 am by Whales »
 


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