MadSci Network: Astronomy
Query:

Re: television/radio signals escaping earth to space

Date: Thu Apr 20 17:59:43 2000
Posted By: John Dreher, Staff Astronomer, SETI Institute
Area of science: Astronomy
ID: 952896919.As
Message:

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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