QUOTE (tasp @ May 3 2007, 03:21 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 . . . .
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...