Why does holding a key fob to your head increase its range?(physics.stackexchange.com) |
Why does holding a key fob to your head increase its range?(physics.stackexchange.com) |
amazing.
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.
https://community.cartalk.com/t/does-the-remote-entry-work-b...
Your phone is probably worse.
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.
Would it heat tissue anymore than the Sun (in warm places) or your heater or air-conditioner set to heat already does 10x stronger?
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.
^ this guy literally does nothing more than lay a long wire out on the grass, and touching it instantly smokes and burns his skin
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?
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.
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.
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.
Actually it's not. The antenna is tuned to give a peak in output. It's either correctly tuned, or it's off tune.
It's the design of the whole transmitter/battery/antenna which chosen to give a compromise between battery life and max allowed output.
Few transmitters will come close to the max legal power, and if they did, de-tuning the antenna would result in a very inefficient compromise.
If you include a (dummy) human in the test environment you will (with luck) be close to optimum. By not including a human, you can be sure that the results will be worse in actual use.
- 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.
And of course different surroundings will also effect the directivity (eg the radiaton pattern).
So some will give better results than others. Why is hard to understand?
It doesn't detect the human body, it is just designed to work the best when it is physically close to the human body.
This is true.
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!
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
It is best if you hold the flat side of the remote up against your chin.
There's a lot at play in open-air environments. This does seem quite easy for DIY experiments using RTL-SDRs in varying environments.
I still use this trick today to find my car when I park somewhere I am unfamiliar with.
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.
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.
an interesting test would be to try a full bladder vs empty bladder...
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.
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.
I thought for sure I’d be absorbing it with my head but I definitely get extra range.
I cannot wait to trick someone into doing this
> 1/2 m ≈ 1.5 ft
Thanks US units for the quote of the day: "Meters are for waves, feet are for antennas"
>"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
Always assumed it just kind of turned my body into an extra large antenna. Probably works for a key fob in a similar way.
Unless the reduced power consumption by the transceiver causes the battery voltage to rise noticeably.
Warm car in winter, yeah
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.
Oh, no. Yet another reason not to buy a new car. (Hope it can be disabled in the preferences somewhere.)
Technology trying to guess what I want is almost never a good thing. It'll get it right 80% of the time, but that last 20% is a killer, and means I'll have to monitor it carefully all the time.
My aunt had an older Prius, and sometimes when she locked it and walked away it would still see the fob and unlock itself. She had to carefully sneak away from the car to make sure it stayed locked.
MCUs are highly specialized (also not publicly available), and you can play with the tuning registers up to several digits without much performance loss. 3xx/4xx MHz ones you can easily go over the regulation limits (IIRC in JPN occupancy is taken together with the power into the calculations, it's a very crowded spectrum...).
If a spread spectrum is used at 8xx/9xx range, there there is no worry of hitting the limit, here usually battery life and max peak current are limiting, and fine tuning is often done for individual samples at EOL in the plant. It doesn't necessarily mean that all at 8xx/9xx are spread spectrum, but the "premium" (usually half-duplex) are. They provide a lot better performances at low SNR (and thus great range) compared to the standard FSK/Manchester 1 or 2 CH fobs.
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...
However he is stuck with a compromise design and knows that there are various effects that a nearby conductive body can have on the antenna's performance.
- Anything nearby will effect the tuning of the antenna. And if the antenna is no longer at resonance, then it's efficiency is greatly reduced.
- Anything nearby will disturb the antenna due to its missing ground plane, which will change both the tuning and the loading (eg impedance).
- And anything nearby will act as Directive or Reflective elements thus greatly effecting the radiation pattern (eg directivity).
In the early days, it was common for phones to have an external whip antenna, and also a socket so an outdoor antenna could be plugged in if required.
But forcing the designed to include the antenna within the body of the phone, (and without an effective ground plane) results in a mess of compromises.
Unfortunately the general public thinks that phone works by "magic", and is somehow exempt from the fundamental need for a radio to have an efficient external antenna.
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.
One of the links I found: https://www.mwrf.com/technologies/embedded/systems/article/2...