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Date Posted: Mon 26 Jul 2004; 4:59pm
Author: D.James
Subject: Radio transmissions in space

Radioman
I have visited your site and found it very informative and useful especially the link on how to calculate using decibels. I have a question about the propagation of radio waves in space which I have been thinking about for some time. It has to do with the Voyager space craft. OK , so the question is how far can radio waves propagate in space ? Is there any limit . The Voyager mission to Jupiter originally broadcast a distance of a billion kilometers using a 20W transmitter and the Cassini space mission to Saturn is broadcasting from a distance of 1.5 billion kilometers approx. using a 20W transmitter. Using the information on your site and the formula :
L = (c/4pi x d x f ) squared to calculate the Loss in the transmission and where :

1) L = loss
2) C = speed of light
3) D = distance in cm ( Is the speed of light in this also measured in cm.)
4) F = frequency
I got the following L = { 104dB – 11dB + 140dB + 93dB} x 2 = -280dB

Which translates to something like 10 to the power of minus twenty-eight watts. Out of this if take the Gain of the transmitting antenna to be 24dB and the receiving antenna to have a gain of 74dB we get -280dB + 24dB + 74db = 182dB which gives something like 3 multiplied by ten to the power of minus eighteen watts , is this correct. How do we explain the fact that Voyager I transmission from nine billion kilometers out in space are still being received on earth ? Are they linked transmissions i.e from Voayger I to Voyager 2 to Galileo to earth or something like that or is it possible to receive directlky on earth transmissions from such distances. I would appreciate any information on the subject. D. James

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