[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 19:08:01 -0600

Stupid arithmetic problem, but it does change the answer, at least in an 
emotional way. Here are the corrected numbers:

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 **287** dB.

Assume the receiver is very advanced, capable of demod with 0 dB C/N ratio. So 
the total **287** 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 **287** dB and the 
frequency is 600 MHz.

Max distance = 8.9E+9 Km.

Pluto is between 4.2E+9 Km and 7.5E+9 Km from Earth, which means that in 
principle, this signal could be receivable just within our solar system.

As far as bouncing back to Earth, the highly reflective object would have be 
inside our solar system.


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