On Wed, 2002-03-06 at 07:30, John Hasler wrote: > Tom Cook writes: > > Actually electromagnetic radiation falls off inversely with distance,... > > Field intensity is inversely proportional to distance. Power density is > inversly proportional to the square of distance (and power is, of course, > what we can actually detect). > > > ...since no-one has yet devised an antenna which radiates very > > well in all directions... > > That's completely irrelevant. > > > The direction of propagation is perpendicular to the direction of motion > > of the exciting charges (aren't they exciting? ;-) and so the wave > > propagates in the horizontal plane (assuming that your antenna is > > oriented that way. > > The radiation propagates in all directions (though the intensity varies > around the antenna patern). I think you are confounding polarization and > propagation. > > > I agree with you (except about wave energy density); I believe a 1 MW RF > > plane radiator is very difficult to demodulate with our technology from > > the edge of our solar system,... > > Just the other day NASA received a response to a signal that they sent to > Pioneer 10. It is 22 light-hours out and has a low-gain antenna and a > transmitter power of a few watts.
Isn't that incredible? Maybe slide-rules and armies of women on adding machines is worth something after all... According to newscientist.com, the _reply_ came in 22h 6 min, so it would be 11 light-hours out. Still, 24*365 = 8,760, so a 10 light-year star is 87,600 l-h away. That's 7,964x as far away as P10, and given the inverse square law, imagine how small the power density is. http://www.newscientist.com/news/news.jsp?id=ns99991997 -- +------------------------------------------------------------+ | 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. | +------------------------------------------------------------+
