Human body as a 50mhz antenna application to indoor presence monitoring systems. - Page 1
Forum section: electronics / beginners

author: Musclor
author_rank: Contributor
timestamp: May 13, 2019, 10:36:41 am
content: Hi, it's me, the noob who knows enough to be dangerousI'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!

author: radiolistener
author_rank: Super Contributor
timestamp: May 13, 2019, 11:10:10 am
content: Quote from: Musclor on May 13, 2019, 10:36:41 amthat's false information

author: Musclor
author_rank: Contributor
timestamp: May 13, 2019, 11:20:09 am
content: Quote from: radiolistener on May 13, 2019, 11:10:10 amThis 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 athttps://news.mit.edu/2018/artificial-intelligence-senses-people-through-walls-0612which is however real).A summary is athttps://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.

author: tggzzz
author_rank: Super Contributor
timestamp: May 13, 2019, 11:29:17 am
content: 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.

author: radiolistener
author_rank: Super Contributor
timestamp: May 13, 2019, 11:53:03 am
content: Quote from: Musclor on May 13, 2019, 11:20:09 amAny 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 impedanceIt really has self resonance near 50 MHz (48 MHz), it's input impedance at that frequency is about 1500 Ohm.

author: Bud
author_rank: Super Contributor
timestamp: May 13, 2019, 01:22:43 pm
content: What was your test setup?

author: radiolistener
author_rank: Super Contributor
timestamp: May 13, 2019, 07:48:39 pm
content: Quote from: Bud on May 13, 2019, 01:22:43 pmI 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

author: Musclor
author_rank: Contributor
timestamp: May 14, 2019, 02:13:31 am
content: Quote from: radiolistener on May 13, 2019, 11:53:03 amThank 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

author: Whales
author_rank: Super Contributor
timestamp: May 14, 2019, 04:03:40 am
content: Quote from: radiolistener on May 13, 2019, 07:48:39 pmConnecting 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 pleaseFollow 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.