Author Topic: getting radio waves out of the human body  (Read 1197 times)

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Offline Alex EisenhutTopic starter

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getting radio waves out of the human body
« on: January 29, 2020, 02:28:42 am »
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?
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Offline Ed.Kloonk

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Re: getting radio waves out of the human body
« Reply #1 on: January 29, 2020, 02:55:15 am »
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)

It was a ripoff of an TOS episode. https://en.wikipedia.org/wiki/Home_Soil
Quote
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?

Cartman satellite dish, anyone?
Quote
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?
iratus parum formica
 

Offline T3sl4co1l

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Re: getting radio waves out of the human body
« Reply #2 on: January 29, 2020, 03:13:52 am »
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.

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Offline Wimberleytech

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Re: getting radio waves out of the human body
« Reply #3 on: January 29, 2020, 03:57:35 am »
Medical Implantable Communications Service (MICS)
https://en.wikipedia.org/wiki/Medical_Device_Radiocommunications_Service

Pacemakers 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.

 

Offline Someone

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Re: getting radio waves out of the human body
« Reply #4 on: January 29, 2020, 04:48:07 am »
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.
SR44 250mWh
or if you take the smallest common battery:
SR416 13mWh
Its not a µW power budget for the camera or RF.
 

Offline Alex EisenhutTopic starter

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Re: getting radio waves out of the human body
« Reply #5 on: January 29, 2020, 05:36:44 am »
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.
SR44 250mWh
or if you take the smallest common battery:
SR416 13mWh
Its not a µW power budget for the camera or RF.

Perhaps but there are 4 white LEDs on the thing and they fire twice a second.
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Offline Someone

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Re: getting radio waves out of the human body
« Reply #6 on: January 29, 2020, 06:05:33 am »
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.
SR44 250mWh
or if you take the smallest common battery:
SR416 13mWh
Its not a µW power budget for the camera or RF.

Perhaps but there are 4 white LEDs on the thing and they fire twice a second.
This information is easily found:
Quote
the total power dissipation is 33.5 mW, in which, the sensor consumes 8 mW, the RF transmitter consumes 24 mW and LEDS consumes 1.5 mW
https://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.pdf
The inefficient antennas require TX power of mW
 

Offline Wimberleytech

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Re: getting radio waves out of the human body
« Reply #7 on: January 29, 2020, 02:29:24 pm »

More general history of the devices including detail on the RF attenuation and frequencies:
https://ecse.monash.edu/staff/mehmety/J2014I.pdf
The inefficient antennas require TX power of mW


That 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.  Dunno
https://www.bostonscientific.com/content/dam/bostonscientific/quality/education-resources/english/ACL_PillCam_101408.pdf
 

Offline TimFox

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Re: getting radio waves out of the human body
« Reply #8 on: January 29, 2020, 03:47:16 pm »
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.
 


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