getting radio waves out of the human body - Page 1
Forum section: electronics / projects

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 29, 2020, 02:28:42 am
content: Seems there are swallowable cordless cameras for medical imaging. I thought they recorded onto flash or something but who would want the job of plugging THAT into your USB port?Turns out the thing broadcasts its little journey.The camera uses 3x1.5V button cells. So we're talking microwatts of RF power since the thing might take 8 hours to get out.The antenna can't be big, and the receiver looks like about 8 pads glued to the person that go to a magical box.Star Trek taught me that humans are ugly bags of mostly water. (In one of the dumber episodes of TNG)One day I put my phone into a Ziploc bag and submerged it. I saw the bars go to zero. Water has a high dielectric constant. The human body is a pretty good conductor overall. At a first guess I'd use a sub 1GHz frequency to get data out my belly.Are they maybe using the colon itself as an antenna somehow?This medical stuff is secretive. Maybe a patent search could help me?So... what frequency band do you use to punch through a person like that? Is it spread-spectrum and the receiver does the hard work?

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 29, 2020, 02:55:15 am
content: Quote from: Alex Eisenhut on January 29, 2020, 02:28:42 amIt was a ripoff of an TOS episode.https://en.wikipedia.org/wiki/Home_SoilQuoteCartman satellite dish, anyone?Quote

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2020, 03:13:52 am
content: Right, it'll most likely be low frequency, near field inductive. The body is plenty conductive enough to short out electric fields (hence your finger looks very much like a lossy capacitor say at 100MHz), but is still pretty transparent to magnetic fields. Typical skin depth at 100MHz is 6cm -- even in the very center of the body, you can probably expect a modest say 20dB loss, and obviously, less at lower frequencies.High frequencies aren't even that bad, as the high dielectric constant makes antennas a lot more feasible! The waves don't radiate far, but if your link gain works out alright, eh, there you go.Indeed, because of the large κ, internal fields don't radiate much at all into free space -- total internal reflection hits hard, and most radiation is quickly absorbed. The preferred approach is patch antennas stuck to the body, also scaled appropriately for the medium.I wonder if they use antenna diversity to improve reception in all orientations; if the camera is a simple dipole (solenoid), it has a null plane, which will be crossed sooner or later as it waggles its way through the intestine.Tim

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 29, 2020, 03:57:35 am
content: Medical Implantable Communications Service (MICS)https://en.wikipedia.org/wiki/Medical_Device_Radiocommunications_ServicePacemakers are implanted just under the skin, so not a lot of water to deal with.Knee implants (a new thing) is another matter but still doable.

author: Someone
author_rank: Super Contributor
timestamp: January 29, 2020, 04:48:07 am
content: Quote from: Alex Eisenhut on January 29, 2020, 02:28:42 amSR44 250mWhor if you take the smallest common battery:SR416 13mWhIts not a µW power budget for the camera or RF.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: January 29, 2020, 05:36:44 am
content: Quote from: Someone on January 29, 2020, 04:48:07 amPerhaps but there are 4 white LEDs on the thing and they fire twice a second.

author: Someone
author_rank: Super Contributor
timestamp: January 29, 2020, 06:05:33 am
content: Quote from: Alex Eisenhut on January 29, 2020, 05:36:44 amThis information is easily found:Quotehttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC1402296/More general history of the devices including detail on the RF attenuation and frequencies:https://ecse.monash.edu/staff/mehmety/J2014I.pdfThe inefficient antennas require TX power of mW

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 29, 2020, 02:29:24 pm
content: Quote from: Someone on January 29, 2020, 06:05:33 amThat is a great article you posted. I scanned it and did not find information about how much power is transmitted, but maybe I missed it.Here is a brief article that indicates the transmit power is 1uW radiated power. That seems pretty low to me, however. Maybe that is the power measured at the surface of the skin. Dunnohttps://www.bostonscientific.com/content/dam/bostonscientific/quality/education-resources/english/ACL_PillCam_101408.pdf

author: TimFox
author_rank: Super Contributor
timestamp: January 29, 2020, 03:47:16 pm
content: MRI senses radio-frequency magnetic fields (not radiation, but near-field). Above 64 MHz, corresponding to 15 kG static field, the attenuation due to body fluid becomes noticeable (referred to as “penetration”). Early calculations, treating the body as a bag filled with “normal saline”, over-estimated this problem. The cellular structure confining the saline reduces large-scale induced current flow.