My head amplifies the signal from a keyless entry tag: how?

Try this. Stand a distance away from your car. Try the keyless entry. If it works, move further away. Get to a distance that exceeds the transmission maximum of the keyless entry. Now, place the keyless entry tag directly under your chin and press the button. If you are like me and your head amplifies the keyless entry RF signal, then your car doors will unlock.

How does my head do this?

Taking your word for what happens -

WAG. Your head isn’t “amplifying” the signal exactly. It is redirecting toward your car some signal power that originally went in other directions. Skin conductivity, reflections from it, etc. Your skin is acting like the reflectors of a directional antenna.

Wasn’t there a thread on this a while ago? Ah… here it is!

The physics of car alarm remotes

Damn, I searched under alarm before posting. It’s odd, if “alarm” is the search term for GQ, limited to thread titles, from yesterday and older, the thread does not come up. Using “alarms” doesn’t get it either. Don’t know why. Oh well, carry on.

I think I have experienced something similar but with a radio. If my radio is acting fuzzy, when I touch my antenna it will clear up. The farther I move away from it, the fuzzier it would get.

The same thing happens with my cell phone. If I leave my cell phone on my desk and read the signal strength, it’s usually at 2 or 3 bars. When I place my hand on it (not moving at all) the signal strength will go up, and lower when I remove my hand.

I really want to know why this happens. Sounds like something similar to your occurence.

They did a thing about this on a programme called “Top Gear” - a motoring show here in the UK.
People came up with hundreds of different reasons why this could happen, but I don’t think it was ever actually explained.

I get the same thing with my remote… it’s funny how some directions from the car don’t work as well, but if I’ve moved out of range, I tend to try the “under the chin” approach before I walk back towards the car.

Crazy but it does work.

Well, in physics jargon, your skull is acting as a spherical lens, collimating the radiation from your keyless entry emitter, thereby improving the signal coupling between the two.

Have a look at this Edmunds Optics page on ball lenses.

For a fair comparison you need to hold the remote at the same level as your chin. I’ve found that holding the remote high above my head increases the range.

bughunter, wouldn’t his skull absorb the radiation?

The head acts as a ball lens because the radio waves travel more slowly though the tissues than the air. You probably saw some demonstration of how this causes refraction in school.

While ca 9-10" of head will absorb some RF energy, you should keep in mind that even a modest refraction can dramatically amplify the signal at the reciever. A good lens would focus essentially all the incident EM on a single point; let’s say the head only vaguely focuses the RF, so that the radio waves that would otherwise have spread over 2º of angle only cover 1º. This would cause a 400% increase in signal strength. Unless 75% of the RF was absorbed, you’d still -er- come out ahead. Since, in the case given, the transmitter signal is just on the verge of being strong enough, even a 10-20% effective amplification would be enough.

RF is notstrongly absorbed by human tissues (note that you aren’t noticeable affected by all the radio stations and random EM interference that pass through you every second of your life, and standing up in front of a TV/radio antenna won’t block the signal, unless it’s already borderline). If it were an IR (infrared) keyless device, however, the signal would be almost completely absorbed.

Note that in keyless entry, you’re operating at the transmitter end. The question of how the human body affects borderline TV/RF broadcast reception is more complicated. The tuners are better and the signal more diffuse and often reflected. Your body may actually cause a signal to fade, if it amplifies noise more than the desired signal, or if your body (or the effect of its capacitance touching the antenna) filters the desired band. In other cases, you may effectively become a low-Q additional antenna.

Some people (like myself) seem to be able to make TV/RF signals improve or degrade strongly by entering the room. Being a lifelong electronics buff, I experimented with many possible mechanisms for this effect as a youth, with no definitive result. Anyone else?

As KP said, the collimation effect outdoes any absorbtion by the skull. Think about it, most glass lenses transmit only about 90-95%, but they can be used to focus light down to a tiny point and concentrate energy into a small area.

Thus given the proper geometry, the transmitter’s radiated energy into a large solid angle is all redirected toward a target that would only subtend a very small solid angle comparatively.

Therefore, while the human head may have a transmission of only half (?) that of empty space (ahem), it can redirect ten to one hundred times the total energy towards the receiver, giving a signal increase at the receiver of 5 to 50 times more.