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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?

#151

Post by yunohn »

Johnythree wrote: post #146
yunohn wrote: post #90

Earlier quoted context omitted.

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.

Different body positions are going to affect the antenna tuning differently. 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?

So your direct point is manufacturers have tuned fobs for best connectivity when held /against the user’s forehead/? I must’ve never seen the user manuals…

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

#152

Post by egorfine »

This saved me from blistering cold just yesterday night. Car was too far for the webasto heater to receive signal from the fob. Tried multiple times, then remembered this trick. And yes, car picked up the signal when sent from the forehead.

Warm car in winter, yeah

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

#153

Post by stef25 »

fragmede wrote: post #135
eitally wrote: post #131

Earlier quoted context omitted.

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.

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.

Even cheap BMW had this over 5 years ago

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

#156

Post by green-salt »

avalys wrote: post #53

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.

Thanks to this trick I have saved countless hours searching parking lots

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

#157

Post by differentView »

dicroce wrote: post #129

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...

The lesser known origin of Aquaman.

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

#158

Post by dtgriscom »

mcv wrote: post #102
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 th…

> Modern cars unlock automatically if you try to open the door while the keyfob is in range

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.

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

#159

Post by buescher »

TheWoodsy wrote: post #23

Keysight Labs did some testing on this a few years ago. https://www.youtube.com/watch?v=AjYyjQKW-pU

That's actually pretty good. Their data (https://youtu.be/AjYyjQKW-pU?t=198) is consistent with my experience: +/- 6 dB or more depending on who is holding the remote and how they hold it. Range checks are not very reproducible and if you start worrying about reproducibility your time is better spent setting up a basic outdoor 3m test if you don't have a chamber. Most of the "hold to your chin" effect is just "try holding it a different way".

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

#160

Post by wallaBBB »

Johnythree wrote: post #147
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…

> antenna is tuned to the maximum allowed emission power per local regulations 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 ante…

Not sure what products your experience comes from, but I am talking explicitly about keyfobs. This is generally produced by Tier 1 suppliers, not the OEMs themselves, and not too many players in the field. With the MCU suppliers it shrinks even more (talking about handsfree models, most common in the market, utilizing LF for localization and standard RF bands for Tx).

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.

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