On Sun, 27 Nov 2005 09:56:28 -0200, Gerhard Fiedler wrote:

> Vitaliy wrote:
> 
> >> Your 100mW may still be overshadowed by a transmitter a mile away
> >> pumping out 5W....
> > 
> > Wow, you are right! I should have done my homework... this goes against my 
> > intuition:
> > 
> > 100 mW / (10 m)^2 = 1 mW/m^2
> > 5000 mW / (1609 m)^2 = 1.9 mW/m^2
> 
> Maybe your intuition wasn't that wrong, after all... Probably when you
> divided 5000 by 2588881 (that's 1609^2), you took the 10^-3 of the result
> as meaning "milli", but with milli already going in, it's "micro":
> 
>  5000 mW / (1609 m)^2 = 1.9 uW/m^2

Hang on, chaps!  While the ratio of field strengths in the above calculation is correct (1 : 0.0019) it's 
wrong to give them a dimension - the actual field strengths certainly won't be the figures shown above.

If you assume an antenna with no gain (isotropic, giving a perfectly spherical radiation pattern) field 
strength of 100mW at 10m will be about 0.079 mW/m^2 (calculated using the area of a sphere, 4 pi r^2).  Now 
isotropic antennae are kept in the same drawer as frictionless pulleys and light, inextensible string - they 
are theoretical only, but a practical antenna for the appication under discussion is unlikely to have a gain 
over isotropic of more than about 2 or 3.  You'd need a highly directional antenna with a gain of about 12 to 
get 1 mW/m^2, and it's very unlikely that that's what would be used in this case.

Cheers,


Howard Winter
St.Albans, England


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