Use Your Head

Last week I saw an interesting experiment demonstrating a phenomenon that I had not seen before.  In this video clip from a past episode of Top Gear, Clarkson uses a remote key fob to lock a car from a distance of about 40 yards.  He then walks an additional 10-15 yards away from the car, and demonstrates that the key fob no longer works… until he places the fob against his temple and tries again, and it works!

Some additional online searching suggests that this “trick” of using your head to increase the key fob’s range also seems to work as well or even better by placing the fob under your chin, sometimes accompanied by opening your mouth.  There are a lot of proposed explanations for why this works, along with repeated experiments confirming that it does work.  One of the more interesting explanations is the following quote from a 2009 New York Times article:

The trick turns your head into an antenna, says Tim Pozar, a Silicon Valley radio engineer.  Mr. Pozar explains, “You are capacitively coupling the fob to your head. With all the fluids in your head it ends up being a nice conductor. Not a great one, but it works.”

At this point I was certainly intrigued, but skeptical.  I think the experimental setup is very important here, with the potential for either unintended misinterpretation of results… or even the intentional misdirection of a magician’s trick.  The experiment is reproduced again in this Unplggd article:

One step back beyond the point of being out of range, I confirmed that the remote didn’t work. Then I raised the remote [my emphasis], pressed it to my chin, and squeezed the button. I heard a “chirp!” in the distance and saw the headlights flash. Huh. So I started walking backwards. All in all I was able to back up 42 steps, roughly 85 feet, before I was no longer able to make contact with the car by holding the remote to my chin.

Sometimes– as in the neutrino timing experiment mentioned in my last post– science is hard, being some combination of expensive, difficult, or complicated to carry out.  But in this case, it was relatively cheap, easy, and straightforward to simply try it myself, and see what happened.

It is nice being married to a woman who not only tolerates but enables these projects and excursions of mine.  Last Saturday morning, she and I drove to the reasonably flat, completely empty parking lot at the nearby high school.  She recorded the maximum distance from which I could consistently lock the car using the remote key fob, while holding the fob in several different positions and orientations.

(Distances were measured in increments of 10 feet, conveniently marked by the intersections of painted lines on the lot.  By “consistently lock the car” I mean that I could lock the car with each of three button presses, but 10 feet farther away I failed on at least two of three attempts.  I was somewhat surprised that the range actually dropped off this sharply.)

Holding the fob in “typical” fashion, away from my body at approximately chest height and pointing it at the car, the fob’s range was 80 feet; i.e., at 90 feet it stopped working.  As in the above experiments, I then held the fob under my chin… and the fob’s range increased to 110 feet!  So far, so good, although opening or closing my mouth did not appear to make any difference.  I also tried holding the fob to my right temple, where the range increased to 140 feet.

See where this is going?  I think the problem is that, at least in the descriptions I have read or videos I have seen, this is where the experiment ends.  But I also tried holding the fob at chin height, in the same orientation relative to the car, but away from my body, and the range remained at 110 feet.  Similarly, holding the fob at the height of my temple, but away from my body, yielded a range of 140 feet.  (We did actually observe one success out of three attempts at 150 feet.)

Finally, the most effective method that I tried was holding (and pointing) the fob straight up over my head, which increased the range to 190 feet.  In other words, the higher the fob, the longer the range.  We observed no effect of the fob’s position relative to my head, only relative to the ground.

Myth busted?  It would seem so… but I wonder if others have tried this experiment, also distinguishing height above the ground from position relative to the head as separate controlled parameters, and observed different results.  Maybe I was doing something wrong?  It would certainly make for a much cooler “did you know” phenomenon if there really was something to the “capacitive coupling to your head.”


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2 Responses to Use Your Head

  1. Jessee says:

    I haven’t repeated your experiment yet, but I did discover by accident One day that my panic button works from inside my pocket sitting at my desk next to the door (not the window) in my 3rd floor office to my car in the parking lot below. Maybe there’s some ducting going on.

    • Interesting. I suppose I would not be surprised if the panic button did actually have a longer range than the lock/unlock functions. Longer range is presumably safer, and worth the extra draw on the battery since it is not intended to be used regularly. Let me know if you test that– since I would most likely annoy my neighbors if I tried it tonight :).

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