RadioBanter - View Single Post

June 2nd 04, 09:28 PM

(Fred McKenzie) wrote in message ...
....

Working this backwards with the sphere, 4/3 Pi R-squared divided by 8 watts
equals the power (2.7 mW) per square meter at distance R. You can calculate
that the minimum distance R equals 0.072 Meters, which is less than 3 inches.
(Please don't hold any errors against me. Check it yourself!)

Since Fred put that caveat in there, this is obviously nothing against
Fred, but...

We started with 8 watts assumed to be radiated (to find the max
farfield value). Area of a sphere is 4*pi*r^2. (Volume is
(4/3)*pi*r^3.) Seems like we want to say 8 watts/(4*pi*r^2) =
..0027w/m^2, and that yields a radius of about 15.4 meters, which would
actually be a good approximation of far field for 2M wavelength.
Units analysis is always a valuable check; also useful is a quick
mental-arithmetic check: a 1 meter radius sphere has a surface area
of roughly 12 square meters, so 8 watts at 1 meter will be about 2/3
watts per square meter.

OTOH, one would hope that the defib would be considerably less
sensitive than that, else cell phones pose a significant problem.

Cheers,
Tom