On Wed, 2002-03-06 at 01:08, Gary Turner wrote: > On Wed, 06 Mar 2002 16:46:17 +1030, Tom Cook wrote: > > >Ron Johnson wrote: > > > > >This would be true were it an electric field. Actually electromagnetic > >radiation falls off inversely with distance, not with the cube of > >distance, since no-one has yet devised an antenna which radiates very > >well in all directions and there is some fancy explanation of Maxwell > >that shows that it's 1/r, now 1/r^2 or 1/r^3. > > While it is true that the EMF, or voltage is inversely proportional to > the distance, the power is reduced by the square of the distance. > (P=e^2/r, P=i^2*r, or P=e*i) Thus a signal with a power density of 1 > Watt per sq meter at 10 meters distance, will have a power density of > .01 Watts per sq meter at 100 meter's distance (in a lossless system).
Ah. I knew gravity worked that way; I should have realized that EMF works the same way... > >> If most stars need telescopes (even the Hubble) to see them, > >> and they radiate jillions^3 of watts on energy, and still look > >> like pin-pricks, how could the signal from a 50,000 watt radio > >> station, or a 250,000 watt TV station (both of whose signals > >> are absorbed somewhat by the air) reach an antennae 20 light > >> years away, while passing through all that background noise? > Which brings us to power density at the receiving end--there ain't much. Isn't that even more of an understatement than "Hydrogen bombs go 'Boom'."? > Given that the W/sq meter is minuscule, an antenna such as the one the > seti project uses has an effective aperture on the order of 10's of > thousands of sq meters. And that helps. The small beam angle acts to > remove all signals not in the desired direction, so the noise level is > reduced. VLNAs bring sensitivity to a level that a signal energy level > only a few degrees above abs 0 is detectable. Is the project SOL if the alien broadcast is from that part of the galaxy that's only visible from the southern hemisphere? -- +------------------------------------------------------------+ | Ron Johnson, Jr. Home: [EMAIL PROTECTED] | | Jefferson, LA USA http://ronandheather.dhs.org:81 | | | | 484,246 sq mi are needed for 6 billion people to live, 4 ! ! persons per lot, in lots that are 60'x150'. | ! That is ~ California, Texas and Missouri. ! ! Alternatively, France, Spain and The United Kingdom. | +------------------------------------------------------------+
