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

physics.stackexchange.com

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

#41

Post by Kirby64 »

initplus wrote: post #31
hammock wrote: post #18

Earlier quoted context omitted.

Why would the duration or frequency of the burst matter if RF is harmless?

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.

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.

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

#42

Post by Johnythree »

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.

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

#43

Post by xp84 »

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!

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

#44

Post by nxobject »

TheWoodsy wrote: post #23

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

I never expected the sire of HP's test and measurement division to be posting entertaining casual YouTube content, but here I am.

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

#45

Post by Hamuko »

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…

Arm's length of the front bumper, but about 1-2 car lengths of the front door.

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

#47

Post by jpk »

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 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.

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

#48

Post by martin-adams »

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’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.

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

#49

Post by ace2358 »

jvanderbot wrote: post #28
ace2358 wrote: post #25

Earlier quoted context omitted.

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.

vs holding it out at arms length but same height? Ground interference is a thing.

Yes, holding at same height. I did not know about ground interference! Thanks.

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

#50

Post by DeathArrow »

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…

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.
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