RF energy harms body tissue by heating it, as in a microwave oven, which is
continuous
energy. However, my oven on the defrost setting zaps the food only periodically,
like on for X seconds and off for Y seconds. Also, the higher the
frequency, the more the heating. VHF does a poor job, and 1 gHz (transponder)
is better, and that's in the range of a cell phone also.
Transponders are spec'd in peak power, and 250W is typical. RF energy is
transmitted
only when we send a "1 bit" during a pulse train time duration of 21
microseconds,
and how many pulse trains we send is based upon a whole bunch of variables.
Like how many 1-bits we send. How many pulse trains do we send when
our reply lamp flashes? Not really relevant, as we shall see, but it could be
a total of 2 milliseconds worth, as a guess.
So, 2 ms every 2000 ms cuts our avg 250W RF power to an average of only 250mW,
roughly our cell phone's typical power (to save battery, they cut themselves
back
to minimum needed). This comparison is not a good one, because we would need
to multiply the transponder's 250mW times a 50% duty cycle for sending bits,
then by sending typically only 40% of maximum bits, times another % because
of the significant dead time between between each pulse train we send, and
finally
by another % to reflect the fact that a bottom-mounted antenna will not
place our body in a strong part of the radiating pattern at all. And of course,
unlike the cell phone, the distance to our antenna is at least feet away.
There's
a square in the formula for that, so double the distance and we attenuate
the radiated power by 25%.
So, we're talking about an average handful of milliwatts here rec'd by our
body.
Like trying to heat food in the microwave by turning it on for just 2 seconds
each hour. In a week, it will still be room temp.
Fred F.
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http://forums.matronics.com/viewtopic.php?p=124003#124003
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