[opendtv] Re: 47 year old television signals bouncing back to earth
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 19 Dec 2011 18:54:17 -0600
Olivier Houot wrote:
>> I wonder if someone has computed the distance in space an analog TV
>> signal can travel and still be decodable (making reasonable
>> assumptions for the size of the receiving Antenna, or just using
>> Arecibo's diameter).
>
> Well of course, someone has, silly me. Here's one interesting link:
> www.lscheffer.com/tv.pdf
>
> It says an arecibo-like receiver would not even extend the reach of TV
> broadcasts beyond the solar system limits, but suggests some interesting
> improvements, should alien engineers care.
I had done some computations, which also gave very unimpressive results. I've
put them at the bottom, in case you want to see why. One major problem is the
wide bandwidth required for TV.
This puts the whole SETI thing in perspective, when people ask "why haven't we
heard anything?" It's for many reasons. One is the propagation loss due to
distance, and another is time.
The propagation loss was covered in your link, and below, but the time factor
is also important. Our planet is about 4.6 billion years old, and yet "it" has
only been transmitting signals that could potentially be received
extra-terrestrially, i.e. in the VHF band or higher, for only about 60 years.
Which means, these little green men would have to live in a planet no more than
60 lightyears distant. Not very far at all, in astronomical terms. The very
closest star system to us, Alpha Centauri, is already 4.24 lightyears away.
And then there's the signal strength problem.
--------------------------------------------------------
Let's be really optimistic. Assume that the Arecibo dish, designed for 2.38
GHz, were scaled up in size to operate at, say, 600 MHz (UHF band). So that the
gain in UHF remains 70 dBi.
Then assume a state of the art TV receiver, capable of a sensitivity of -120
dBm (cryogenically cooled), for a 5.38 MHz bandwidth.
Then assume, this being analog UHF, a transmitted power of 5 MW. And ignore
absorption by the ionosphere (reasonably low in UHF).
Okay, so the maximum allowable propagation loss, if transmitted power is 5 MW
and receiver sensitivity is -120 dBm, is just under 217 dB.
But our receive antenna has a supposed 70 dBi gain, so we should be able to
tolerate a free space propagation loss of 277 dB.
Assume the receiver is very advanced, capable of demod with 0 dB C/N ratio. So
the total 277 dB loss can be used as free space distance only.
Free space propagation loss is
Loss = 32.45 + 20logbase10(freqMHz) + 20logbase10(distKm)
Solve for distance, since you know the max loss allowable is 277 dB and the
frequency is 600 MHz.
Max distance = 2.814E+9 Km.
Pluto is between 4.2E+9 Km and 7.5E+9 Km from Earth, which means that at its
closest point to Earth, the RF signal would still have to travel too far to be
receivable.
Bert
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