the distinction being that you are closer than a wavelength and a true EM field isn't relevant, rather it's entirely a magnetic coupling of the source and receiver with no significant electrical field generated that close, and by the time you are a wavelength or several away the power has dropped off to well below background noise, hence no detectable electro-magnetic field ever forms and you are only detecting a change in the local magnetic field. with some of the other services that operate at high powers of course there is a true electro-magnetic wave formed within the detection range. i believe some of these are/were used for navigational purposes long before gps (i.e. in tube days, when even the oscillators were tube type). i've seen pictures of some of the huge, huge tubes and coils used in the final stages of these monsters, think very large copper tubing in coils that are bigger than you are! Gerhard Fiedler wrote: >=20 > > ... which transmit a burst signal over up to a few metres at 5 kHz. = This > > is near field communications rather than true RF. End result is abou= t > > the same. >=20 > Why do you say "rather than /true/ RF"? Aren't both about elecromagneti= c > fields, and the difference is just in the reach and other properties ------ --=20 =93Cowardice asks the question: is it safe? Expediency asks the question: is it politic? Vanity asks the question: is it popular? But conscience asks the question: is it right? And there comes a time when one must take a position that is neither safe, nor politic, nor popular- but one must take it simply because it is right.=94 : Martin Luther King Jr. 1929-1968 _______________________________________________ http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist