I need an answer re: signals sent out into space

[Chaos]

ok.. in the beginning of the movie "Contact" it shows how our tv/radio signals are being sent out into space ..and we later find out *spoilers* that the aliens pick up our signals and send them back to us.

 

WELL HERE'S MY QUESTION..

 

how can these signals go off so far into space and be picked up, yet if a radio station is broadcasting in boston, you don't pick it up on your car radio in los angeles?

 

is it because your antenna isn't good enough in los angeles? or is the transmitting signal from boston too weak? or something else?

 

someone please help clear up my confusion about this.

 

thank you

[Janus]

ok.. in the beginning of the movie "Contact" it shows how our tv/radio signals are being sent out into space ..and we later find out *spoilers* that the aliens pick up our signals and send them back to us.

 

WELL HERE'S MY QUESTION..

 

how can these signals go off so far into space and be picked up, yet if a radio station is broadcasting in boston, you don't pick it up on your car radio in los angeles?

 

is it because your antenna isn't good enough in los angeles? or is the transmitting signal from boston too weak? or something else?

 

someone please help clear up my confusion about this.

 

thank you

 

It's because the curve of the Earth is in the way. Radio signals travel pretty much line of sight. If you draw a straight line from Boston to LA, it will have to pass through a lot of rock.

 

Sometimes, under just the right atmospheric conditions, you can get something called "skip", where the radio waves hit a layer in the atmosphere and bounce back to Earth. This allows the signal to "jump over" the curve of the Earth and allows you to pick up stations that you normally can't. ( this is also why sometimes, at night, with AM radios, you will get distortion on some stations. You are also picking up distant stations that are close in frequency to your own, and their side bands are bleeding over into the station to which you are listening.

Ham radio operators use skip to their advantage by using frequencies that bounce off the Ionosphere, then bounce off the Earth, back up the Ionosphere , etc. Givng them much improved range.

[Jay-qu]

Well in part you are right, they would need to be looking to find it and with sensitive equiptment.