MadSci Network: Engineering

Re: Why does proximity of Car Alarms to human body extend their range?

Date: Wed Sep 12 23:48:13 2001
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Engineering
ID: 999345499.Eg

Hi Tal:

Interesting, isn't it?
In some cases the human body (your hand as well as neck) affects the 
transmission range, some good, some bad, but in this case, I would bet that 
there are other factors that may have a more pronounced effect.

My first response is to claim that what you are seeing may be related to 
"multi-path".  The signal being emitted from the remote control is probably 
electromagnetic in nature, EM waves(think of it as a light bulb making a 
sine wave), and as such, it has sinusoidal characteristics.  These EM wave 
signals can get from the remote to the receiver in the car by taking a 
number of different paths. Think of a light bulb and how it shines in all 
directions. The first and obvious path is a line of sight path between the 
remote and the receiver - that is like the light reaching the target 
directly and with no obstruction. Lets call that the incident wave. But 
there are other paths, as the signal (think light again) also bounces off 
of near-by objects, like walls, ceilings, and ground, and then reaches the 
receiver (you can also think of how many ways you can hit on billiard ball 
with another, either straight on, or bounce the cue ball off of the sides 
of the table).  The signals that get bounce off of nearby objects take a 
longer path, and take a longer time to reach the receiver.  In sinusoidal 
terms, that is equivalent to a time or phase shift.  Given a shift in time 
or phase, the two sine waves, the incident wave and the reflected wave, may 
add constructively (seem like a stronger signal=more range), or 
destructively (seem like a weak signal also referred to as a drop-out).  
You can observe this effect if you walk away from the car to where you 
think is the maximum range, and then step to the left, right, back, and 
front, and repeat the test, always holding the remote in the same exact 
position with respect to your body.  The ground actually is one of the more 
important reflectors out there.  To test that theory, you can hold the 
remote at ground level, waist high, and neck high : )  and repeat the test. 
 Another fun test is to try the above in different directions from your 
car, say, measure maximum range from the front, back, and the sides, and 
see how different they are.

Have fun !

Abtin Spantman

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