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Posted by: PhilCo126 May 3 2007, 09:49 AM

Posted my question here as it's about radio transmissions going beyond our solar system...
We all know the excellent opening scene of the hollywood movie 'Contact' which shows radio & television transmissions going beyond our solar system. But as it's Hollywood, there's certainly a catch. Do our regular TV & radio transmissions really travel so far? I have been working in remote areas and it was hard enough to catch TV signals from stations just 75 kilometers away! I'm sure I've read once that the only signals from Earth leaving our solar system are very strong military Radars and bundles of strong (military) satellite uplinks.

Posted by: Analyst May 3 2007, 11:12 AM

With my limited knowlegde of physics:

In principle every electromagnetic signal sent travels at the speed of light (in a vacuum) until it is absorbed or reflected by some kind of matter (e.g. it is interacting with matter). So if a transmission is not absorbed / reflected by earths atmosphere (or other matter in the near vacuum of space) it leaves the solar system and goes on. One big problem is the signal strengh, which decreases by distance squared. So after a specific distance it gets very hard to isolate the signal from other signals and it "becomes" noise. But it is still there. Another problem is interference with other signals, you get a new signal generated from two others and you can't seperate it into the two signals again.

Analyst

Posted by: tasp May 3 2007, 01:21 PM

Wow, 20 years later I get to use my sparky degree . . .


IIRC, the new signal you mention only exists if the 2 (or more) original signals pass through something 'nonlinear'.

I suspect the infinite eternal frozen void of space is pretty 'linear'.

I might suggest a visit to Wikipedia and a review of 'heterodyne', but I haven't really got my morning fog of old age burned off yet . . . .

Posted by: mcaplinger May 3 2007, 01:42 PM

Short answer: maybe. See http://contactincontext.org/cic/v2i1/lucy.pdf for a "yes" and http://www.setileague.org/editor/uhftv.htm for a "no".

Posted by: edstrick May 4 2007, 06:21 AM

In the BIS Daedalus Project study of an unmanned 0.1 c flyby of Barnards Star, they were going to have the hubble-class telescope-spacecraft communicate back to the solar system by laser. Performance wasn't bad given the large diffraction limited optics.

Posted by: Jeffrey Feb 15 2009, 03:02 AM

I read somewhere that the signal would only last out for a lightyear.

Posted by: Andrei Feb 16 2009, 03:16 PM

Well...the receiver is nonlinear to a certain degree, so even if the frozen void is linear you still might get some intermodulation products.

Now back to topic, regarding the distance covered by a radio signal emitted from earth by a regular radio or TV station....well, things are not quite as depicted in Hollywood movies. I'll try to make some calculations.
Lets assume a TV station emitting a power of P0 = 1MW (which is quite a power for a TV station) in phi = 1 degree cone (which means that basically no one on earth will be able to receive it since it is very directional).
Lets also assume that your receiving "TV set" can recover in good conditions a signal having a power to noise ratio of 1 and the only noise source is the thermal noise of your input impedance (lets say R = 50 ohms, there are several good reasons to choose this impedance). Oh...and your first stage in the receiver is cooled to T = 4K (in liquid helium). Your antenna is a d = 10m diameter dish antenna.
If you wish to recover a good TV signal, you'll need around B= 6MHz bandwidth. With all these, the (thermal) noise power of your receiver is Pt = 4 * k * T * R * B (k is the Boltzmann constant), which is about 0.066 pW.
The distance to the emitter for a given received signal power P can be computed with:
L = d / phi * sqrt( P0 / P). If P = Pt => L = d / phi * sqrt( P0 / (4 k T R B ) ).
So, assuming a very good receiver (can work with signal / noise = 1) you can use it up to 2.22 billions km...that is around 14.5 - 15 AUs (i.e. between Saturn and Uranus).
For radio stations, the situation is somehow better since the signal has a bandwidth in the tens of kHz range so you could probably hear it at a distance around 110AUs (well beyond Pluto). That is considering the same emitted power...
If you're targeting regular TV stations, well... not even Mars will be close enough for a good reception.

[EDIT] I guess this is why they used only a 10 frames / second and a 500kHz bandwidth for the first Apollo moon landings [/EDIT]

So I guess it's only in Hollywood movies...

Posted by: helvick Feb 16 2009, 05:36 PM

Excellent Andrei - reminds me of a question in one of my Telecoms exams way back in the dim and distant 80's - if I recall correctly the question was to discuss the physical constraints that would be involved in setting up a telecommunications link between Pluto and Earth. I can't recall if my answer was as succinct and effective as yours, but I only hope it was.

Posted by: mcaplinger Feb 16 2009, 07:04 PM

Note that we are rehashing a thread nearly two years old.

The analysis is fine as far as it goes, but I assume that ETIs would have antennas much larger than 10m. See the references in post #4 for details.

Posted by: lyford Feb 16 2009, 07:30 PM

Perhaps we are practicing valuable "extended conversation" techniques given the long delays for interstellar distances?

Posted by: imipak Feb 16 2009, 08:11 PM

I also remember the scene in the movie where they get a directional fix on Vega, at which point a heated debate erupts - Vega is a very young star, surrounded by a dense disk of dust; therefore the aliens couldn't possibly fly around the system to build the transmitter; therefore, the analysis is wrong. "Well, they could if they blasted the dust out of the way with giant lasers...!!"

The point is that the hypothetical entities capable of detecting the 30s TV broadcast, extracting the signal and then sending it back to us with enough power to be detected by the VLA can be safely assumed to have whatever pseudo-magic NotInventedYettium it takes to make things that are hard or impossible for us, practical (and move the plot along.) Reality doesn't have those constraints. That's why it's called science *fiction*... and now I'll take Mike's hint and zip it

Posted by: Juramike Feb 16 2009, 09:06 PM

Back in the 80's there was a Michael Pare rock opera movie called "http://en.wikipedia.org/wiki/Streets_of_Fire". Although it bombed at the box office, it started with a beautiful introductory caption that allowed the storyline absolute freedom:

"...in another time, in another place..."

Posted by: Okei Aug 1 2009, 04:09 PM

It was a beautiful film, and the first time I saw Michael Pare, Diane Lane, Rick Moranis, Willem Defoe, et al.

I worked in Defense and Aerospace electronics in the 80's as a technician, taking requirements for my BSEE at local community colleges. Some of which were in Silicon Valley.

In the 90's I began my BSEE courses, but it was quite evident that my heart and mind were pre-occupied with Aerospace and Space. I was part of various deployments of wireless networks and the propagation studies in the field. There just wasn't enough people and if you could spell "wireless", you had a good job.

BellSouth PCI in Atlanta had me looking and measuring how well different structures/venues received the 1800-1900 MHz signal from a single test transmitter. At that time, we tested something in RTC.

Yes, I believe that there was, is, and will be "...in another time, in another place..."

Cheers

P.S. to PhilCo126, as a woman I find that "Contact" is my favorite film; they got all of the details right with regard to the VLA, Arecibo. Not that it is used for this, but TRW built a Relay Satellite.