[HN Gopher] Why does holding a key fob to your head increase its...
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Why does holding a key fob to your head increase its range?
Author : jasonhansel
Score : 510 points
Date : 2024-01-09 02:23 UTC (20 hours ago)
(HTM) web link (physics.stackexchange.com)
(TXT) w3m dump (physics.stackexchange.com)
| cranberryturkey wrote:
| your skull acts as an antenna
| pestatije wrote:
| anybody's head range increases by holding anything to it...101
| physics
| icepat wrote:
| I've started using a HAT running the TINFOIL distribution, then
| I've configured ingress with request token validation. I bill
| everyone around me on a per-request basis for UNLOCK calls to
| their car. I've found the system works pretty well at keeping
| unwanted GET requests to my MIND database by various bad
| actors.
| dijksterhuis wrote:
| top gear (UK) demonstrating this in practice (potato video
| quality) https://www.youtube.com/watch?v=_jACSPipPSE
| gnicholas wrote:
| I've found it helps to hold my key with both hands.
| tempestn wrote:
| That would support the head-arm loop theory.
| riffraff wrote:
| I can imagine you may also try to raise your hands to get less
| ground interference, at which point it would not be
| inappropriate to invoke the power of Greyskull.
| Buttons840 wrote:
| It also works holding it near your torso, don't have to use your
| head.
|
| Although, the joke you tell your friends is that it only works if
| they open their mouth and stick out their tongue like a satellite
| dish.
|
| The fact that it works next to your torso made me think it was
| just a matter of bouncing half the radio emissions towards the
| vehicle, thus doubling the signal. Just like someone holding an
| omni-directional campaign lamp close to their body increases the
| light headed in a certain directions.
| tempestn wrote:
| That was my first thought too, but the OPs tests had the range
| only improving when held to the head.
| chii wrote:
| i suspect the torso's cavity isn't as amplifying as the
| liquid in your brain.
|
| an interesting test would be to try a full bladder vs empty
| bladder...
| TeMPOraL wrote:
| We could then measure how the signal range changes as you
| pee, particularly with the length of the urine stream.
|
| I'm serious. This would be an analogue to those navy
| experiments with shooting streams of sea water to fashion
| temporary antennas of arbitrary length, instead of having
| permanent ones that have fixed length and stick out.
| rft wrote:
| Thanks for the liquid antenna pointer, I had no idea this
| was actually tried! Apparently not only the navy thought
| about this, but there are idea floating around about
| using liquid metal for variable length antennas.
|
| One of the links I found: https://www.mwrf.com/technologi
| es/embedded/systems/article/2...
| Buttons840 wrote:
| I've had my fob fail to work multiple times while walking
| away from my car, so I take it out of my pocket and put it
| near my lower back and it works (even though I'm now even
| farther away because I'm walking).
| cstejerean wrote:
| That's slightly different though, sounds like fob went from
| being blocked by your body to no longer being blocked by
| your body.
| ace2358 wrote:
| I've always put the key behind my head when doing this, and can
| confirm, it extends the range through experimentation.
|
| I thought for sure I'd be absorbing it with my head but I
| definitely get extra range.
| jvanderbot wrote:
| vs holding it out at arms length but same height? Ground
| interference is a thing.
| ace2358 wrote:
| Yes, holding at same height. I did not know about ground
| interference! Thanks.
| eps wrote:
| I can confirm that holding the remote _next_ to the head
| doesn 't work, but touching chin, temple or the back of the
| head does extend the range. This works not just with car
| remotes, but also with garage door openers.
| HenryBemis wrote:
| My favorite place (and yes I do this since I saw the Top
| Gear) is under the chin, pointing the fob/key upwards and
| opening my mouth. So it doesn't use the skull's cavity
| but the mouth's (or so I think/believe)
| jimnotgym wrote:
| Have you tried standing on one leg, and touching your
| nose with the free hand. Vast improvement over the
| amatuer technique
| ratg13 wrote:
| It's just using the water in your body as an antenna.
|
| You can see the same effect by holding the car remote to a
| water bottle.
|
| You'll notice the water bottle gives you about the same
| extended range as your head.
| eps wrote:
| > _You can see the same effect by holding the car remote to a
| water bottle._
|
| And _this_ ^ doesn 't actually work.
|
| I know it because I specifically tested it. Went from 2L
| bottles and all the way to 8L jug. Nada. But the head works.
| ratg13 wrote:
| You've piqued my curiosity.
|
| I will have to run more tests.
| eichin wrote:
| _clean_ water isn 't a great conductor - the body is a
| bag of _dirty_ water...
| sushisource wrote:
| > Although, the joke you tell your friends is that it only
| works if they open their mouth and stick out their tongue like
| a satellite dish.
|
| I cannot wait to trick someone into doing this
| userbinator wrote:
| I am disappointed to see no mention of "passive radiator" nor
| "parasitic element" on that page anywhere:
| https://en.wikipedia.org/wiki/Driven_and_parasitic_elements#...
| kevin_thibedeau wrote:
| Must have been the log lady's secret all along.
| rabuse wrote:
| My ex-girlfriend used to do this back in our teens, and I used to
| think she was crazy... until I started to do it. Blew my mind.
| behnamoh wrote:
| In the movie LalaLand, they mention that this is dangerous
| (causing diseases). Is that true?
| acomjean wrote:
| I suspect not. Its a short burst of rf and not done very
| frequently. (I use this technque with an key fob near an open
| mouth probably 4 times a year.. Thanks Car talk.
|
| https://community.cartalk.com/t/does-the-remote-entry-work-b...
|
| Your phone is probably worse.
| buntz wrote:
| During summer my bald head burns in the 5 minutes it takes to
| roll garbage to the curb.
|
| Really doubt even phones are dosing us the way nature does.
| hammock wrote:
| Why would the duration or frequency of the burst matter if RF
| is harmless?
| seba_dos1 wrote:
| Why would the amount or the way it gets into your body
| matter if water is harmless?
| initplus wrote:
| RF is harmless below the point where it directly heats
| tissue.
|
| Having a full strength cell tower transmitter inside your
| home is not safe due to the intensity of the transmission,
| it will directly heat tissue inside your body.
| Kirby64 wrote:
| I think the distinction/point is that it's not harmful in
| the 'this might give you cancer' way that ionizing
| radiation sources will. Worst case, you get an RF burn...
| which is not great, but it's effects are immediate and
| obvious, unlike ionizing radiation which is much more
| insidious.
| coldtea wrote:
| > _RF is harmless below the point where it directly heats
| tissue._
|
| Would it heat tissue anymore than the Sun (in warm
| places) or your heater or air-conditioner set to heat
| already does 10x stronger?
| retrac wrote:
| Microwave RF that is somewhat directional behaves a lot
| like laser light.
|
| If you were to step into the beam carried by a waveguide
| for something like a powerful TV transmitter, it will
| cook you through like a death ray out of science fiction.
| Fortunately it's not actually collimated, so just a few
| feet away the energy is barely enough to cook your
| retinas faster than your blood supply can cool them
| (which can cause blindness). Otherwise, yes, it's much
| like infrared heating effects. If infrared penetrated a
| few centimetres deep, anyway.
| coldtea wrote:
| > _Microwave RF that is somewhat directional behaves a
| lot like laser light._
|
| While fob frequency is microwave, isn't it also much
| lower energy compared to, say a microwave oven, powered
| by mains power to cook food?
|
| And also the fob operating in the low 100s of Mhz, not
| GHz, wouldn't that make it less dangerous?
| hammock wrote:
| https://www.youtube.com/watch?v=yqljaKjKjd4
|
| ^ this guy literally does nothing more than lay a long
| wire out on the grass, and touching it instantly smokes
| and burns his skin
| userbinator wrote:
| As true as the notion that cellphones and microwaves cause
| cancer.
| Buttons840 wrote:
| Should we worry about a radio transmission powered by a 3 volt
| battery while taking on our phone?
| zontorol wrote:
| A phone battery is 3.8 volt. No worry, right?
| Buttons840 wrote:
| Good point. I don't know much about these things. Are the
| radio transmissions from a phone stronger? I would expect
| so, but don't know.
| farkanoid wrote:
| But my remote contains a 12V battery /s
| hutzlibu wrote:
| Depends. There is lots of irrational fear with this and as
| far as I know, no solid proof for harm, but there is extra
| electricity flowing in your head, because of the transmission
| of your phone. I don't like talking on the phone that much
| anyway, so I do not care so much, but if I would talk for
| hours, I would use a headset (also for ergonomic reasons).
| drivers99 wrote:
| https://www.youtube.com/watch?v=w5SRH6Ac1LI&t=25s Love that
| movie. I didn't remember that so I found the scene. The
| character's joking / trying to mess with the other person. "You
| gotta put that thing to your chin. It makes your head into an
| antenna, so... I think it gives you cancer but you find your
| car faster. I mean, you don't live as long but you get where
| you're going quicker so it all evens out. Just a suggestion."
| At 5:15 in that video she tries it and it works.
| havblue wrote:
| I'm pretty sure that was a "neg".
| cableshaft wrote:
| Used to have a radio transmitter that plugged into my car
| lighter, and hook up to an mp3 player (or whatever) and transmit
| that on a radio frequency that the car could pick up. Often it
| would be staticky unless I set the device on my leg, and then it
| got a lot clearer.
|
| Always assumed it just kind of turned my body into an extra large
| antenna. Probably works for a key fob in a similar way.
| entropicgravity wrote:
| It's your conductive nervous system (including your brain) that
| is acting as a directional antenna.
| nsonha wrote:
| My brain could not connect to wifi the other day and the key fob
| trick worked!
| TheWoodsy wrote:
| Keysight Labs did some testing on this a few years ago.
| https://www.youtube.com/watch?v=AjYyjQKW-pU
| nxobject wrote:
| I never expected the sire of HP's test and measurement division
| to be posting entertaining casual YouTube content, but here I
| am.
| piskov wrote:
| Makes one wonder about bluetooth headphones
| plugin-baby wrote:
| IME Bluetooth headphone volume increases if you put them near
| your head
| aqfamnzc wrote:
| I find this especially true when moving them close to the
| ears. Could it be the dished shape?
| KeplerBoy wrote:
| bluetooth is a digital transmission. The volume is encoded in
| the signal, it can't get louder just because it receives a
| clearer signal.
|
| Unless the reduced power consumption by the transceiver
| causes the battery voltage to rise noticeably.
| sgerenser wrote:
| Empirical evidence says otherwise. Bluetooth headphones on
| desk, can hardly hear anything. Touching my head
| (particularly the ears), loud and clear.
| KeplerBoy wrote:
| Ohh, I need to work on my sarcasm detector.
| Johnythree wrote:
| A more likely theory is that the fob transmitter (and antenna) is
| tuned to put out max signal when near the body. This is it's
| normal use.
|
| If however you tested it with the fob somewhat isolated it would
| then be out of tune.
|
| eg in the factory it would be tuned at a set distance from a
| "dummy body".
|
| It's the same with a walkie-talkie radio. The antenna must be
| tuned when held in the hand, as your body provides the missing
| earth or ground-plane.
|
| You can easily demonstrate this. Mount an antenna on a ground-
| plane with a length of coax, a SWR meter and a transmitter. Tune
| the antenna so the SWR is 1:1 and then move you hand close to the
| antenna. Once your hand gets withing a wavelength or so, the
| effect of the "detuning" can be readily seen on the SWR meter.
| jpk wrote:
| This reminds me of a class in college where one project was to
| make a waveguide WiFi antenna. You know the ones; often built
| out of a Pringles can.
|
| On the day the project was due, we each demo'd our antennas and
| the instructor recorded their range, SNR, etc. Most in the
| class followed the Pringles can tutorials, but I was always
| kinda extra. So I used some kind of roof flashing coiled into a
| cylinder to get the length and radius tuned just right, and I
| fashioned a comfy handle to aim it using some scrap aluminum
| flat, and round wood stock.
|
| Mine did really well, but it also had the widest variance in
| performance. We figured out that my wood handle could be held
| in a way in which you didn't actually touch any metal, but if
| you put your thumb up on the aluminum part of the handle, it
| instantly got way better SNR!
|
| The instructor told us that touching the housing improved the
| quality of the ground because the capacitance of our meat acts
| like a short at those high frequencies.
| Aloha wrote:
| Yes - and not surprising, it's functionally acting like a
| ground plane.
| DeathArrow wrote:
| Even simpler, tune in an FM radio and move your hand near the
| antenna. It will sound better or worse depending on your hand
| position and how close it is to the antenna.
| matsemann wrote:
| So a homemade theremin?
| Thlom wrote:
| Back in the early 00's when we still had analogue VHF TV
| signals, we wanted to watch the football world cup at school.
| Of course there was no cable or anything at the school and no
| VHF antenna and streaming was not a possibility yet so we had
| to be creative. We found an antenna cable and tried to
| manipulate the cable and use whatever as an antenna. We tried
| the sheet metal roofing, but that didn't work very well, we
| tried some metal rods and that didn't work great either. By
| chance we found out that if someone held the cable in one
| hand and leaned out of the window with their other hand
| stretched out we got a signal that was good enough to watch
| the game. Well, for everybody except the poor bastard that
| had to stand behind the TV halfway out the window.
| yrro wrote:
| Fond memories here of watching Babylon 5 on a Performa 5300
| with an Apple TV Tuner Card. There was no antenna to the
| room with the computer, but I found that I was able to get
| an acceptable picture by holding the coax cable attached to
| the antenna input in the right way while keeping one hand
| attached to the central heating radiator...
| playingalong wrote:
| A metal fork is the best antenna (among non-antennas)
| ipsum2 wrote:
| Reminded me of discovering this when I was a kid and thought
| I was magic. I had forgotten about that.
| beAbU wrote:
| You might be right about this, it makes sense. So standing on
| your tippytoes with the remote-arm stretched out to try and get
| better range is actually counter productive. Take 2 steps
| closer, and hold the remote close to your body.
| puzzlingcaptcha wrote:
| More likely than what? SE comments provide at least three
| distinct theories. IMO this one (also provided in the comments)
| seems fairly conclusive
| https://www.remcom.com/examples/keyless-entry.html
| andyjohnson0 wrote:
| How does it detect the presence of a nearby human body?
| hmottestad wrote:
| OP means that while developing the radio and antenna part of
| the fob the engineers have adjusted the radio and antenna to
| work the best when in close proximity to a body. The human
| body has a certain effect on the radio signals, so adjusting
| the radio and antenna to work best while close to the human
| body will mean that it won't work as optimally when further
| away from a human body.
|
| It doesn't detect the human body, it is just designed to work
| the best when it is physically close to the human body.
| yunohn wrote:
| The scenarios being compared are:
|
| - Fob in hand
|
| - Fob in hand, held against head
|
| Both of these already fulfill your "near body" requirement.
| Your point around tuning would not explain the specific
| difference being questioned.
| wallaBBB wrote:
| Not really - antenna is tuned to the maximum allowed emission
| power per local regulations (e.g. big differences between EU,
| US and JPN regulations). Certification tests do not include any
| human handling, since it's really unpredictable and varies.
| Therefore, tests are just the fob with a (per specifications)
| positioned Rx antenna (highly controlled environment).
|
| BTW similar effect is observed with the fob being next to the
| bottle of water. There are peer reviewed studies on these
| topics, that my RF engineer shared with me when I asked him way
| back about it, but are heavy on the RF/antenna theory, beyond
| my understanding.
| greggsy wrote:
| I read it in the sense that they would retune or reengineer
| it if it was discovered that holding it in a hand resulted in
| poorer performance - ie. they need to factor in the
| environment it operates in (a hand beside a body) after they
| design it on paper.
| wallaBBB wrote:
| There are 2 limiting factors taken into account during
| design - regulations and peak current that the battery can
| supply.
|
| The battery thing (usually a coin cell CR2032 that can
| supply ~13mA per spec) can be a bit mitigated with a beefy
| capacitor (though RF packet length and space in-between
| have to be appropriate for the capacitor recovery, not to
| brown out the system).
|
| Usually the bigger bottleneck is the regulation, so you
| tune for best performance in a clean environment. Hand and
| body during operation are really unpredictable to model
| in/retune, but luckily usually adds for the better. Anyways
| the range of the classical RF fobs (@3xx-4xx MHz and
| 8xx-9xx MHZ bands) is well beyond the real world needs that
| the 10-20% drop of performance due to certain fob positions
| will make a difference. If it does, you probably have a low
| battery.
|
| Next gen fobs (if they hit the mass market) are BLE and
| there the range performance is more critical. Dunno what
| will be the approach there.
|
| What can cause problems are ornament and the mechanical key
| insert. It has to be extensively tested that there is no
| coupling with those metal parts that are close to the
| antenna. Problem comes sometimes when they decide to
| redesign a bit the keyfob without going through the tests
| internally (e.g. oh it's just a minor tweak of the radius
| of the metal ring around the fob housing...) and then the
| certification fails and has to be repeated (not just money,
| but getting the slot for it...). Some OEM might even be
| inclined to think they don't need a certification for such
| a minor change and push it to the market, making the
| problem worse if there is an issue.
|
| Regarding the ornament things on fobs, there were even
| instances of them magnetically coupling with the wireless
| chargers or even immobilizers and that lead to some
| melting...
| bbarnett wrote:
| Or pointing it at your head, is the same as holding it up high
| in the air.
| hulitu wrote:
| > A more likely theory is that the fob transmitter (and
| antenna) is tuned to put out max signal when near the body.
| This is it's normal use.
|
| This is true.
| xp84 wrote:
| The answer posted with all the experiments is conducted with a
| 2009 GTI, which reminds me of my first car, a Jetta from a
| generation before that. Its manual included this helpful
| illustration of the full expected range of the remote:
|
| https://imgur.com/a/1Pdfyg4
|
| Yes, apparently they were only willing to promise it would work
| within about arm's length of the car! It wasn't that bad, but
| also it wasn't a heck of a lot better, either!
| Hamuko wrote:
| Arm's length of the front bumper, but about 1-2 car lengths of
| the front door.
| maccard wrote:
| My first thought was "I wonder if the car is to scale or the
| fob"
| chaorace wrote:
| Well, it's in a 3D perspective so we have the third option of
| saying that they're _both_ to scale and that the perceived
| size difference is a forced-perspective illusion. In that
| case you could actually measure the distance depicted in the
| diagram, though I have my doubts about how accurate they
| bothered to make it!
| mcv wrote:
| Short range isn't necessarily bad. Modern cars unlock
| automatically if you try to open the door while the keyfob is
| in range, so long range could mean someone could unlock your
| car while you're out of sight.
|
| I don't know if that system uses the same range as when you
| press the button on the keyfob, but I have read about a hack
| (probably here) where thieves used a transmitter to extend the
| range of both the car and the keyfob to unlock the car while
| the owner is out of sight. That's easier to do the longer the
| range is.
| HnUser12 wrote:
| I don't know the inner workings of the fob, but my car has a
| pretty long range when unlocking with the key fob click, but
| the automatic unlock without click(keyless entry) works only
| when I'm next to the handle. It doesn't work even if I'm at
| the back of the car and someone tries to unlock driver door.
| eitally wrote:
| I had a 2001 GTI and it similarly only reliably worked within
| probably 25' of the vehicle. In contrast, my 2017 F150 fob
| works as much as 150-200' from the vehicle. I tremendously
| appreciate this, especially with remote start to warm it in the
| winter.
| fragmede wrote:
| If range is desirable, Tesla's, and I imagine other cars have
| this as well, have a cell modem and an app, so you can
| control it via the Internet, which is effectively infinite
| range. It's occasionally handy to be able to unlock the doors
| to let someone into the car from elsewhere in the world. Eg
| friend left their bag in the car but I'm not around to let
| them in.
| seeknotfind wrote:
| Because I saw this headline, I tried to hold my key fob to my
| head today to lock my car, but it didn't work and I had to walk
| closer. So, there's one data point for you. Maybe I'm doing it
| wrong
| martin-adams wrote:
| I've been using this trick for many years. Things like when in
| a car park and trying to confirm I've locked my car at some
| distance, this has worked. It's been pretty reliable for me.
| LargeTomato wrote:
| One more anecdote, this has not consistently worked for me.
| Raising the height of the receiver may have an effect but I'm
| not sure.
| srcreigh wrote:
| We need to cross analysis people across the axes of
| tall/short and large/small head for posterity
| zro wrote:
| Tall with large head here, works pretty consistently for
| me
| pbhjpbhj wrote:
| Maybe there's a difference in resonance of the people
| reporting their experiences here; also differences in the
| frequency tuning of the fobs?
| ta8903 wrote:
| Maybe the one case where "you're holding it wrong" is valid
| advice.
| redhale wrote:
| "Works on my skull"
|
| I typically press it firmly against my chin, and I have
| observed increased range many times (including testing this
| trick explicitly with a few different fobs a few years ago out
| of curiosity). Try pressing more firmly, or rotating the fob.
| abrugsch wrote:
| I too have been using this trick for many years on many cars
| since first seeing it done by Clarkson on Top Gear (circa 2006
| IIRC and linked in the PSE question). I still baffle people
| with the considerable amount of extra range it can achieve.
| Half a football pitch was probably the best example I got on a
| 2008 Nissan Qashqai.
| avalys wrote:
| This effect 100% works, I've tried it many times over 20 years
| with many different cars.
|
| It is best if you hold the flat side of the remote up against
| your chin.
| jurassicfoxy wrote:
| It's really funny when people don't believe it though. At first
| glance it looks like such an obvious "you're a gullible idiot"
| hoax, haha.
| ipython wrote:
| Funny I came here to share the "chin" trick as well.
| MaXtreeM wrote:
| Sixty symbols (same author as Numberphile) video on this from 9
| yeas ago: https://www.youtube.com/watch?v=0Uqf71muwWc
| pyinstallwoes wrote:
| I always thought it was because my skull acted as an antenna :P
| martijnvds wrote:
| Sixty Symbols did a video on this almost a decade ago:
| https://www.youtube.com/watch?v=0Uqf71muwWc
| risfriend wrote:
| Fascinated by this, going to dig deep.
| snapetom wrote:
| Wow, I first heard about this in the early 90's in high school! I
| was told to hold it under your chin. It _seemed_ to work, but I
| never formally did any experimentation nor did I have the
| statistics base back then. I still did it up until I got a
| proximity fob a few years ago. However, I always felt silly
| because I thought it was an old wives tail from my childhood.
| Very cool to learn there 's actual science behind it.
| mmaunder wrote:
| Not sure I agree with the cavity theory. I suspect it's you, you
| big ole bag of salt water, acting as a capacitive ground plane
| for the antenna. It's like mounting a ham or CB antenna on a car
| and having the metal of the car not be in direct contact with the
| ground, but it acts as a capacitive ground plane. You can do
| something similar with a vertical antenna using ground radials in
| a star pattern on the ground.
| phendrenad2 wrote:
| I think this is the real answer.
| Angostura wrote:
| If that were the case, wouldn't simply holding it in your
| ungloved hand be sufficient?
| teeray wrote:
| Probably not. Position of the antenna relative to the ground
| plane alters the radiation pattern. That's why you'll see
| police cruisers mount their antennas dead center on the roof
| --it creates an omnidirectional pattern. Mounting it to one
| side or another causes the field to radiate more in the
| direction with more roof in front of it (kind of the opposite
| of intuition). So the remote in your hand still likely
| interacts with your body, but not in a controlled or
| desirable way.
| gorlilla wrote:
| The ground plane is typically orthogonal to the antenna. Both
| TX and Rx antennae will have their own 'effevtive' ground
| plane relative to each other at any give point in time that
| will effect their Tx/Rx capabilities. Add into that, parking
| lots are full of metal objects and typically nearby large
| buildings. Those, also, will change the propagation
| characteristics of the general environment.
|
| There's a lot at play in open-air environments. This does
| seem quite easy for DIY experiments using RTL-SDRs in varying
| environments.
| throwaway892238 wrote:
| I mean, it's electromagnetic waves. The right wavelength will
| bounce off things and increase likelihood of receiving a coherent
| signal. Works for a lot of things; RF, sound, light. RF has low
| energy and huge wavelengths. IIRC keyfobs are in the 200-600MHz
| range.
|
| Ha, found some (kind of) evidence:
|
| Radiowave Effects on Humans - March 28, 1980 / T. Neil Davis
| (https://www.gi.alaska.edu/alaska-science-forum/radiowave-eff...)
| One reason the question is unanswered is that the energy absorbed
| by a human from radio waves depends upon the relationship between
| the size of the human and the frequency of the radio waves. Just
| as a TV antenna of the right length and orientation picks up the
| best signal (the most energy) from a transmitted wave, so it is
| with a human being. It appears that the cranial cavity of a
| mammal will resonate at specific radio frequencies determined by
| the size of the brain cavity. At these resonant frequencies the
| human head will absorb vastly more radiowave energy than it will
| at other nearby frequencies. An adult's head will
| resonate at a frequency between 350 and 400 MHz (megahertz).
| Being smaller, a child's head will resonate at a higher
| frequency, somewhere between 600 and 850 MHz. Since each
| individual may have his or her own resonant frequency, a
| particular frequency radiowave might affect one person more than
| another. Consequently, testing on humans--even if people are
| willing to let this happen--can be rather complicated.
|
| Basically the human head is a resonance chamber that probably
| amplifies the signal. But also your body is made of water, and RF
| bounces off metal and water. The capacitive coupling of skin
| probably adds an enhancement to the effect.
| HPsquared wrote:
| The same reason your phone speaker gets louder when you put it on
| the table.
| WithinReason wrote:
| An explanation I read somewhere is that the radio waves reflect
| from the inside of your skull, the curvature acts as a parabolic
| antenna.
| eps wrote:
| That's a pretty dumb explanation if you pardon the bluntness.
| The effects shows itself regardless of how the head is turned
| reltive to the target.
| willis936 wrote:
| Given the conductance of flesh and the frequencies at question
| the skin depth would only be a few cm at most. The RF energy
| never makes it through the brain.
| K0balt wrote:
| It's because the antennas in fobs lack an effective ground and
| are typically unbalanced. The body acts as an RC choke, creating
| a short on the ground side of the antenna (a typically
| intentional part of the design). The closer the fob is to your
| body mass, the better it will perform. Bone may also act as a
| reflector to some degree, and height can play a role.
| ht85 wrote:
| From the top answer:
|
| > 1/2 m [?] 1.5 ft
|
| Thanks US units for the quote of the day: "Meters are for waves,
| feet are for antennas"
| jimnotgym wrote:
| Excuse me, feet are US units??? Surely you mean feet are units
| that were adopted in the US... and then dropped by the rest of
| the world!
| elzbardico wrote:
| Funny how everybody readily accepts the metric system for
| measuring radio wavelengths without complaints. Even the most
| hardcore traditionalist ham radio operator in America never
| complains about naming bands with meters.
| karol wrote:
| Can we now discuss it in the context of 5g? That would be so much
| more interesting.
| hprotagonist wrote:
| _There were no large structures around accept for the concrete
| encased stainless steel vacuum tube of the LIGO Livingston
| Interferometer which runs parallel to the measurement axis and
| extends for kilometers in both directions._
|
| amazing.
| johnny_canuck wrote:
| I used to use this trick when I worked as a photographer of used
| cars. You'd get some 10-20 fobs and be expected to walk a lot of
| ~300+ cars and take images of them. Holding the fob up to your
| chin was the trick to finding the right vehicle... assuming the
| battery wasn't dead.
|
| I still use this trick today to find my car when I park somewhere
| I am unfamiliar with.
| nativeit wrote:
| I just had to tip my cap. I worked as an imaging clerk for
| Copart Auto Auctions about 18-years ago. My job was more about
| putting the images on the website (and ordering new photos if
| they weren't to-spec, or if there was visible gore--it being an
| insurance auction), but I have an intimate awareness of your
| experience. It was an interesting, if poorly paid, job
| experience.
| aadityaubhat wrote:
| Good explanation I found on YouTube few years back -
| https://youtu.be/0Uqf71muwWc?si=eLDi_zvMtXJeh_WZ
| mypgovroom wrote:
| It doesn't
| peter_d_sherman wrote:
| Interesting!
|
| >"This is a really interesting question. It turns out that your
| body is reasonably conductive (think salt water, more on that in
| the answer to this question), and that it can _couple to RF
| sources capacitively_. "
|
| Related: Capacative Coupling:
| https://en.wikipedia.org/wiki/Capacitive_coupling
| marcus0x62 wrote:
| When goofing off while tracing wires with a tone generator[0], we
| used to press the detector wand into our head, lick our finger
| tips, then run our fingers along the contacts on the 66 block[1]
| until we heard the warble from the tone generator.
|
| 0 - A troubleshooting and wire identification tool consisting of
| a battery-powered generator that produces a distinctive warbling
| sound and an inductive detector with a speaker that allows you to
| hear the warble when placed near the correct wire pair. Most look
| like this: https://www.grainger.com/product/EXTECH-Tone-and-
| Probe-Kit-4...
|
| 1 - a wiring terminal commonly used in the telephone system.
| https://en.wikipedia.org/wiki/66_block
| chaorace wrote:
| Those tone probes are so fun to play with! I highly recommend
| anyone with a spare $15 to grab a cheapo model and just... run
| around the home with it. It's an enlightening experience to
| actually perceive with your own ears the normally invisible
| world of EM radiation
| dicroce wrote:
| When I was a kid I had a tv where the quality of the picture
| noticeably improved when you touched the antenna... and I had
| recently heard that humans were mostly water... So I grabbed a
| glass of water and set it on top of the tv near the antenna and
| the picture cleared right up. Unfortunately this event made me
| the IT support guy for the family from then on...
| 99catmaster wrote:
| Am I the only one for whom this doesn't work? If I try to unlock
| the car with the fob against my head, my car doesn't seem to
| receive any signal. But if I try and unlock the car from the same
| distance, key fob not on head, it works like normal.
| arbitrage wrote:
| Hold the fob under and in contact with your chin and aim your
| face at your car. The beam gets focused in a specific direction
| when you're using your head to amplify the signal. This is how
| you get improved distance with your head. Your skull is not an
| isotropic resonant emitter. The signal ends up beam-shaped.
| BoThrowAway wrote:
| I'm not touching this one- I want to, but no.
| svnt wrote:
| You might try flipping the fob over
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