MadSci Network: Astronomy |
Standard TV does indeed escape the surface of the Earth. You are correct in supposing that 1) the signal would be too weak for a standard TV and 2) multiple signals from different cities would interfere. Typical TV stations broadcast a signal that is just strong enough to provide adequate reception over their assigned service areas -- typically only some 10's of miles from their transmitter site. The antennas used for TV transmission typically beam their signals toward the horizon with a vertical width of roughly 10-15 degrees, but some power is radiated in almost all directions. TV channels (frequency allocations) are reused from city to city, relying on distance to prevent signals from differnt service areas from interfering with one another. Thus a standard TV would receve a very noisy, jumbled signal from earth orbit. Both problems can be remedied by using a large antenna. This antenna would both collect more signal and provide enough directivity to allow clear reception of TV signals from orbit. Here is what a "hypothetical" (;-) antenna might look like: http://www.fas.org/spp/military/program/sigint/rhyolite.htm The atmosphere does block radio emissions at relatively low frequencies, below around 10 MHz, and at relatively high frequencies, well above 10 GHz. But the entire VHF, UHF, and microwave band escapes into space and propagates essentially forever. In particular, if one viewed the earth from a great distance in the band assigned to TV and FM transmitters (VHF/UHF), one would see the limb of the Earth "sparkling" with broadcast transmitters as city after city rotated into the best position for radiating these broadcast signals into the direction of view. In principle, other inhabitants of our galaxy could observe these radio emissions and infer that a technological civilization resides on the Earth. Of course, at present only civilizations within about 50 light years of the Earth would be able to "see" us, since the first strong radio broadcasts in the VHF band commenced only about 50 years ago. In the film "Contact" it is imagined that aliens only 20 ly away have received early TV broadcasts and beamed them back to us as a beacon signal. In real life, that hasn't happened ... yet. See http://www.seti.org for more details.
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