Need help with the math


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William
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Message 1153639 - Posted: 18 Sep 2011, 17:01:47 UTC

I want to change my approach in the ongoing debate about SETI not hearing anything meaning nothing is out there. I want to put my argument that most signals would be lost in static long before it got here in terms more people can easily understand but I need help with the math. My question is how strong would a signal have to be at the source for SETI to hear it here. I can calculate minimum divergence for a laser but this is beyond me. If anyone can make the calculations I would like to know how strong the transmitter would have to be to broadcast the signal from both a nearby star and from a star an “average” distance away. By average I mean where SETI is looking. I would also like to know the strength for both a general signal like a radio tower and for a focused signal at minimum divergence. Then how does that supposed transmitter compare to what we use on Earth. For anyone willing to challenge this, thank you.

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Message 1153761 - Posted: 19 Sep 2011, 3:00:54 UTC

Well I am not one for the math, but to calculate a logical assumption, there are more than a few dozen variables that exist based on how far and how long a signal can travel through "space." there are obstructions, interferences etc, How long is the signal broadcast for, will it last for seconds, minutes days, years etc...I mean the list goes on. So to get an idea of the math for something like that, you would sort of need to pick on and make a best guess...
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Message 1153793 - Posted: 19 Sep 2011, 5:17:11 UTC - in response to Message 1153761.

I like the idea and wish you the best of luck with it. Some other variables to take into consideration would be the signal strength leaving the antenna(not the power of the transmitter), the type of signal it is. For example, a CW or Morse code transmission mode which occupies a much smaller bandwidth then phone(AM, FM SSB, etc.) can be transmitted with much less power and be heard through much more noise then the above modes. Equally important is the frequency of the transmitted signal. What I mean by that is not how often the signal is being transmitted, but rather on what frequency it is being transmitted on. An AM broadcast signal that is heard on your car radio would never make it the distance, but at the frequency of light it would. A good place to start might be with light pollution and how bright a certain star has to be before it is observed. Then work the frequency and math down for a certain type of antenna and see what you come up with.

There are a lot more things to consider such as the polarization of the signal. I would think it would be circular rather then horizontal or vertical but do not know for sure. The noise in the receiver circuitry and so on.



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Profile Bob DeWoody
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Message 1153865 - Posted: 19 Sep 2011, 12:44:19 UTC

I think a directed signal would be highly unlikely so the most likely source would be omni directional. I'm also fairly confident that someone has already "done the math".
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Message 1153961 - Posted: 19 Sep 2011, 19:01:33 UTC - in response to Message 1153865.
Last modified: 19 Sep 2011, 19:16:41 UTC

I think that we could only detect a directed signal.

"A state of the art radio telescope has a noise temperature of about 25 degrees K. The biggest existing telescope is Arecibo, with a diameter of 305 meters. The feed pattern means this area is about 70 percent utilized. Plugging in the numbers, we find that even for a very marginal signal to noise ratio of 1, we can only detect the carrier out to 0.81 light years. We can only watch the program out to a range of 0.00081 light years, or about 51 AU. So our best technology can’t even detect our strongest television signals out to one measly light year. and can only receive our signals out to a little past the orbit of Pluto." I believe his quote presupposes a transmitter power of 170 kilowatts with an antenna gain of 30 db.

If this quote is correct then one would ask what are we doing looking for persistent intelligence sources ??. THEY couldn't possibly know that we are here--therefore THEY would not be likely to be continuously beaming high power messages our way.

Apparently we could detect a carrier much more easily than decode it's modulation.

A very short laser burst of several giga watts might do the trick.

Notions that we could detect spurious (non-directed) signals from 100 or 1000 light years needs to be re-examined and explained in plain English.

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Message 1153980 - Posted: 19 Sep 2011, 20:17:44 UTC - in response to Message 1153961.

Plain English is what I’m after. I find it unrealistic that any artificially made signal would get here at any strength we could detect unless someone built a transmitter just for that reason. Most of the variables like when and how long don’t matter, just how strong the signal or transmitter would need to be at the origin to get it here and how that compares to what we now have. The only thing about the frequency used that matters is that it needs to be a frequency SETI is listening to, other than that just pick one you like. I believe the answer would be that it would take a transmitter of completely unrealistic power to put out a signal that strong. Remember I am trying to change perspective on an old debate
Lasers on the other hand I know a bit more about. If I were going to build a laser to send a beam to another star I would need to build it as large as I could, the size of NY large, before I even started calculating the power needs. I would need to use a wavelength that is not produced by the stars involved with enough room to spare for Doppler shift. I would then focus the beam where the star will be when the beam gets there. If I was trying to find stars who were listening I would scan the beam across the stars I had selected. The beam would have to be very large diameter so I would want to build it in space and use solar power or a reactor to power it. The catch is the stronger the beam the more it diverges and the weaker it will be at a given distance. I would have to use a very large diameter, weaker beam to reduce divergence and have a stronger beam at the destination. I would have to use a wavelength not produced by the star because at that distance the receiver could not differentiate between the star and laser.
So what is the point of listening? SETI picks up natural sources as well as artificial so they are in a good position to find things we didn’t know were there. Besides, years ago we sent a signal out into space to see if anyone was listening so why should we not hope someone out there did the same thing?

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Message 1154074 - Posted: 20 Sep 2011, 4:51:59 UTC



I have a faint memory of this discussion going round here before and of someone posting a website where you could plug in a transmitter power figure and maybe some other variables and you would get back an estimated signal propogation figure (in light years, I think).

That's been 2 or 3 years ago at least.

A search here or on the Internet might turn up something still.

Lt

Message boards : SETI@home Science : Need help with the math

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