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Why does holding a key fob to your head increase its range?

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Re: Why does holding a key fob to your head increase its range?

#101

Post by teeray »

Angostura wrote: post #81
mmaunder wrote: post #65

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.

If that were the case, wouldn't simply holding it in your ungloved hand be sufficient?

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.

Re: Why does holding a key fob to your head increase its range?

#102

Post by mcv »

xp84 wrote: post #43

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 b…

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.

Re: Why does holding a key fob to your head increase its range?

#103

Post by gorlilla »

Angostura wrote: post #81
mmaunder wrote: post #65

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.

If that were the case, wouldn't simply holding it in your ungloved hand be sufficient?

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.

Re: Why does holding a key fob to your head increase its range?

#104

Post by abrugsch »

seeknotfind wrote: post #46

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

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.

Re: Why does holding a key fob to your head increase its range?

#105

Post by Aloha »

jpk wrote: post #47
Johnythree wrote: post #42

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 mis…

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 lengt…

Yes - and not surprising, it's functionally acting like a ground plane.

Re: Why does holding a key fob to your head increase its range?

#106

Post by wallaBBB »

greggsy wrote: post #96
wallaBBB wrote: post #94

Earlier quoted context omitted.

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…

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.

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...

Re: Why does holding a key fob to your head increase its range?

#107

Post by sgerenser »

KeplerBoy wrote: post #88
plugin-baby wrote: post #34

Earlier quoted context omitted.

IME Bluetooth headphone volume increases if you put them near your head

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.

Empirical evidence says otherwise. Bluetooth headphones on desk, can hardly hear anything. Touching my head (particularly the ears), loud and clear.

Re: Why does holding a key fob to your head increase its range?

#108

Post by helpfulContrib »

eps wrote: post #85
ratg13 wrote: post #40

Earlier quoted context omitted.

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.

> 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.

[dead]

Re: Why does holding a key fob to your head increase its range?

#109

Post by jimnotgym »

ht85 wrote: post #92

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"

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!
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