Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

increased range of a remote



The following Q&A from New Scientist caught my attention:
http://www.newscientist.com/lastword/article.jsp?id=lw962

Or to save you the trouble of clicking on that link:

"Question
My son-in-law discovered a strange property of the remote control
that switches his car alarm on and off. By pressing it firmly to his
chin, he can increase considerably the range at which it works. Not
believing him, we experimented with other chins. It works, and in
some cases the range is doubled depending on the type of car alarm.
How does this effect occur?

Mike Welch , Stockton-on-Tees, Cleveland

Answer
Your son's car remote control has a miniature radio transmitter that
is built into the key ring and which sends a coded message to a
receiver in the car. You don't have to point the device directly at
the receiver in the car, but it will help to increase the range if
you press the remote control against your body. In fact, any part of
the body will do, not just your chin. Arms and legs work too. And you
can have endless fun pointing your bum at your car while pressing the
key ring against your buttocks.

The range over which the remote control operates nearly doubles when
you involve your body because your body picks up the radio signal and
acts like a large aerial, transmitting it more effectively to the car.

The key and the body act like the two plates of a capacitor separated
by an electrical insulator. When an electric charge flows into or out
of one of the plates, the electrostatic effect drives a similar
charge into or out of the other. Although no charge actually crosses
the insulator, a current appears to flow for a while until the
capacitor is fully charged. Applying a varying current to one plate
means that the capacitor never gets fully charged and a current
appears to flow indefinitely through the second plate.

This phenomenon, which is called capacitive coupling, works best with
rapidly varying currents, which is exactly what the radio transmitter
in the key ring produces.

Because it operates at a frequency of 433 megahertz, the current from
the key can cross the insulating barriers of your clothing and skin
to reach the conducting interior of your body. Your body then acts as
a giant aerial when you press your remote control against it.
Editor , London, UK"

My questions for the list:

1. What factor causes the range to "nearly double" (as opposed to say
tripling)? (Are they suggesting that it is because there are now 2
plates?)

2. Would it be fair to roughly estimate the amplitude of the radio
signal broadcast by your body to be proportional to the frequency
divided by the distance between the remote and your skin (ie. the
reciprocal of the capacitive reactance)? Seems like this might be an
easily testable hypothesis.
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/