Physics question about remote controls from car talk

[Borodog]

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They make radio towers tall for a reason.

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But that reason is NOT to increase the gain.

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I think he probably just meant the signal going over the other cars in the lot.

[CORed]

I was replying to the question about why sometimes touching the TV antenna makes the picture fuzzier instead of better.

[slickpoppa]

After doing a little bit of research, I've found maybe one piece of the puzzle. Here's the site of a company that did some research on this question and determined that holding the fob up to one's mouth does indeed significantly increase the power of the signal (and to an even much greater extent than holding the fob to another part of one's body), but they don't offer any theoretical explanation for why that happens:

http://www.remcom.com/varipose/varip...ess-entry.html

Their website isn't clear, but it appears that in their experiment they did not actually touch the fob to the person's mouth or chin.

[gumpzilla]

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After doing a little bit of research, I've found maybe one piece of the puzzle. Here's the site of a company that did some research on this question and determined that holding the fob up to one's mouth does indeed significantly increase the power of the signal (and to an even much greater extent than holding the fob to another part of one's body), but they don't offer any theoretical explanation for why that happens:

http://www.remcom.com/varipose/varip...ess-entry.html

Their website isn't clear, but it appears that in their experiment they did not actually touch the fob to the person's mouth or chin.

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What frequency do they use as the carrier? I suppose it's possible that the mouth/throat might be a resonant cavity at those frequencies. Thinking about the dimensions, I'd guess the mouth/throat would probably have a cavity mode at a couple GHz, with how low it goes depending how much of your alimentary canal gets involved.

[diddyeinstein]

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This definitely works. AFAIK, you're basically using the roof of your mouth as a satellite dish to focus and bounce the signal in a specific direction, giving it greater range.

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I think this is the right answer. I also believe this helps to explain why it works better with your mouth open then closed (I think I remember playing around with an ex's and this being true). The waves can pass through your body easily, but the less obstruction the better.

[Ringo]

I read this before, and apparently it does work. Theories abound as to why, and the mouth/body acting as a resonance cavity is a big one. Personally I think that holding the keyfob up to your chin raises the height you would normally click it at. At mouth height, it will be higher than most of the parked cars around you, hence have a longer range.

[Kaj]

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They make radio towers tall for a reason.

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But that reason is NOT to increase the gain.

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I think he probably just meant the signal going over the other cars in the lot.

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Oh. I was assuming that whoever did this study was smart enough to rule out that variable. If not, that's pretty piss poor.

[billygrippo]

i tried this a few times. turns out i didnt even need the remote, my chin alone can unlock my car. i think its my fillings

[uDevil]

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Increasing the size of the antenna can also make static stronger.


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If anything, the signal to noise ratio should increase.

The frequency these things work at is 300-400 MHz (see http://auto.howstuffworks.com/remote-entry.htm ). So the wavelength is ~1m, and the human body is rough1y 2x this length. The antenna idea is plausible for the key transmitter, but a body's effect on TV reception seems like something different.

I think it can be tricky to couple energy to a waveguide when dimensions and connections are well controlled. The idea that having your mouth open matters or that the body's internal structures act as resonant cavities seem pretty goofy to me.

The other proposed effects seem plausible and may occur simultaneously. I don't see how you can tell with the information given which are first and which are second or higher order effects.