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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?

#81

Post by Angostura »

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?

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

#82

Post by andyjohnson0 »

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…

How does it detect the presence of a nearby human body?

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

#83

Post by hmottestad »

andyjohnson0 wrote: post #82
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…

How does it detect the presence of a nearby human body?

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.

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

#84

Post by eps »

WithinReason wrote: post #80

An explanation I read somewhere is that the radio waves reflect from the inside of your skull, the curvature acts as a parabolic antenna.

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.

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

#85

Post by eps »

ratg13 wrote: post #40
Buttons840 wrote: post #6

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 camp…

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.

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

#86

Post by redhale »

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

"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.

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

#87

Post by eps »

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.

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.

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

#88

Post by KeplerBoy »

plugin-baby wrote: post #34
piskov wrote: post #24

Makes one wonder about bluetooth headphones

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.

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

#89

Post by HenryBemis »

eps wrote: post #87
jvanderbot wrote: post #28

Earlier quoted context omitted.

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

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.

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)

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

#90

Post by yunohn »

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…

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.

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