Eric Jacobsen wrote:> > Hehe...I finagled the following out of it: > > So, with some pretty optimistic assumptions, like 1MW output power > through a 3dBi antenna and only 5kHz transmit bandwidth (i.e., a truly > gnarly AM broadcast) at 200MHz. The alien civilization using a 50m > dish, which would have to be pretty carefully pointed right at earth, > and a receiver cooled to 10 degrees Kelvin, they'd have to be within > 0.0013 light years to have a hope of receiving it.Nicely done, Eric. There is one thing which was not considered though: the noise temperature of the space. If I remember it right, it can't be any lower then somewhat 4K, but it can be much higher if a star or similar object falls into the path. So the results are going to be even worse then that. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
OT: TV and radio signals in Space
Started by ●June 18, 2007
Reply by ●June 19, 20072007-06-19
Reply by ●June 19, 20072007-06-19
On Tue, 19 Jun 2007 01:11:03 -0800, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>Eric Jacobsen wrote: > >(snip) > >>>http://www.satsig.net/seticalc.htm > >> Hehe...I finagled the following out of it: > >> Alien transmit frequency GHz = 0.200 >> Alien transmit antenna diameter m = 0.86 >> Alien transmit antenna gain dBi =3.14 >> Alien transmit power W = 1000000 >> Alien transmit EIRP dBW = 63.14 >> Receive antenna diameter m = 50 >> Receive system noise temperature(antenna+LNA) K = 10 >> Receive antenna gain dBi = 38.53 >> Receive antenna G/T dB/K = 28.53 >> Bandwidth Hz = 5000 >> Required overall link carrier to noise ratio C/N dB = 3 >> Path (spreading) loss dB = 280.28 >> Range km = 12284502309.787057 >> Range AU (1AU=Earth to Sun distance) = 82.11565715098301 >> Range light seconds = 40948.341032623524 >> Range light minutes = 682.4723505437254 >> Range light years = 0.0012985731828527545 > >Pretty neat, but there is no option for FM. I used to know >the number, I believe called FM advantage, that indicated the >improvement in S/N ratio due to FM. That is, how much better >the S/N ratio is from the output of the FM demodulator to >that of the input. I remember this being described in the >context of C band satellite TV, where five watts of power >per TV channel can cover much of the US. > >I believe, then, that an FM signal with similar power would >be receivable over a much greater distance. > >-- glenI set the required C/N to 3dB as a generic, probably optimistic, threshold for demodulation of an analog signal. In other words, the analysis is somewhat independent of modulation type. I used a 200MHz carrier frequency to be somewhat generic (and optimistic) as well, so that it covered AM/FM/VHF to some degree. This mostly affects the gain of the alien receive antenna, and does so optimistically. Given that the range is so short (not even twice Pluto's max orbit, which is 49.5AU) even making large changes isn't going to make much difference. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
Reply by ●June 19, 20072007-06-19
On Tue, 19 Jun 2007 01:11:03 -0800, glen herrmannsfeldt <gah@ugcs.caltech.edu> wrote:>Eric Jacobsen wrote: > >(snip) > >>>http://www.satsig.net/seticalc.htm > >> Hehe...I finagled the following out of it: > >> Alien transmit frequency GHz = 0.200 >> Alien transmit antenna diameter m = 0.86 >> Alien transmit antenna gain dBi =3.14 >> Alien transmit power W = 1000000 >> Alien transmit EIRP dBW = 63.14 >> Receive antenna diameter m = 50 >> Receive system noise temperature(antenna+LNA) K = 10 >> Receive antenna gain dBi = 38.53 >> Receive antenna G/T dB/K = 28.53 >> Bandwidth Hz = 5000 >> Required overall link carrier to noise ratio C/N dB = 3 >> Path (spreading) loss dB = 280.28 >> Range km = 12284502309.787057 >> Range AU (1AU=Earth to Sun distance) = 82.11565715098301 >> Range light seconds = 40948.341032623524 >> Range light minutes = 682.4723505437254 >> Range light years = 0.0012985731828527545 > >Pretty neat, but there is no option for FM. I used to know >the number, I believe called FM advantage, that indicated the >improvement in S/N ratio due to FM. That is, how much better >the S/N ratio is from the output of the FM demodulator to >that of the input. I remember this being described in the >context of C band satellite TV, where five watts of power >per TV channel can cover much of the US. > >I believe, then, that an FM signal with similar power would >be receivable over a much greater distance. > >-- glenI set the required C/N to 3dB as a generic, probably optimistic, threshold for demodulation of an analog signal. In other words, the analysis is somewhat independent of modulation type. I used a 200MHz carrier frequency to be somewhat generic (and optimistic) as well, so that it covered AM/FM/VHF to some degree. This mostly affects the gain of the alien receive antenna, and does so optimistically. Given that the range is so short (not even twice Pluto's max orbit, which is 49.5AU) even making large changes isn't going to make much difference. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
Reply by ●June 19, 20072007-06-19
On Jun 19, 8:29 am, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote:> Eric Jacobsen wrote: > > > Hehe...I finagled the following out of it: > > > So, with some pretty optimistic assumptions, like 1MW output power > > through a 3dBi antenna and only 5kHz transmit bandwidth (i.e., a truly > > gnarly AM broadcast) at 200MHz. The alien civilization using a 50m > > dish, which would have to be pretty carefully pointed right at earth, > > and a receiver cooled to 10 degrees Kelvin, they'd have to be within > > 0.0013 light years to have a hope of receiving it. > > Nicely done, Eric. There is one thing which was not considered though: > the noise temperature of the space. If I remember it right, it can't be > any lower then somewhat 4K, but it can be much higher if a star or > similar object falls into the path. So the results are going to be even > worse then that. >Hello Vlad and others, Yes the cosmic background is 2.7 K, but being that the Earth is about 1/3 of the way out on one of the spiral arms of the Milky Way galaxy, anyone looking towards Earth with our galactic core behind us, is going to see a lot of noise (way higher than 2.7K). Even from Earth when ever we look towards the constellation Cygnus (in the the direction of the galactic core) we see all of this noise. Plus from the Earth we also get quite a bit of "sky noise" wich tends to have an effective temperature of anywhere from 10 to 300 kelvins depending on the elevation angle and the frequency band of interest. Clay
Reply by ●June 19, 20072007-06-19
