MadSci Network: Engineering

Re: How does a sattelite dish work?

Date: Wed Jan 3 02:19:39 2001
Posted By: Abtin Spantman, , Electrical Engineering, L. S. Research, Inc.
Area of science: Engineering
ID: 976234803.Eg

Hi Jason:

Lets start with the easy ones first.
1) why is a satellite dish shaped the way it is?
The satellite dish is also known as a parabolic dish.  A parabolic dish 
uses the geometric property of parabolas, which is to take the incoming 
rays from all points on the dish and focus them into one spot.  This 
property makes the parabolic dish antenna more efficient in picking up weak 
signals. You can gain some interesting insight if you look up the 
properties of parabolas in a math book.  Or, just hit the web with the key 
word "parabolic dish."  Get some popcorn first.

2) How come there aren't a lot of unwanted signals intercepted?
The dish does pick up unwanted signals.  The electronic circuitry usually 
handles the task of selecting the signals that get through.

3)How does weather affect a signal?
It does, but the effects are small compared to other factors such as 
path-loss.  There is some attenuation (loss of signal power) due to water 
and clouds, but the line of travel of the signal is not affected. The loss 
of signal power is very predictable, and can be compensated for by the 
electronics.  Noise from atmospheric and solar sources are a different 
story. Lightning, and solar flares are things we can not compensate for in 
the circuitry, and do affect the signal integrity.

The signal from a satellite is like the light from a flashlight. Now 
picture this, if you will, that you are across a long, dark hallway, and 
you are shining the flashlight on a fly on the wall at the end of the 
hallway.  The light spreads out as it goes through the hallway, and covers 
a larger area than just the fly.  If there were a hundred flies next to 
each other, they would all see the light from the flashlight.  In much the 
same way, the many parabolic dishes on earth, see the signal from the 
satellite. As you might have guessed, the angle of the cone of light, and 
how far away the flashlight is from the wall, determine how big of an area 
gets lit up. Lets call that the footprint of the flashlight. The 
'footprint' of a satellite is the earth area that the satellite can receive 
from or transmit to. That footprint can be a large portion of the earth, or 
a small portion of the earth, depending on how we design the system. Hmmm. 
I wonder if lasers would be more efficient in communicating from outer 
space?  Something for you to ponder.

Now, let me try to answer some of the questions you may ask later.  We 
sometimes use satellite communication because the curvature of the earth 
prevents us from communicating directly from point to point, in a 
"line-of-sight" fashion.  That means if you can't shine a laser beam from 
your source to your destination, because the earth's curvature gets in the 
way, then your signal is probably not going to get through either.  So we 
send our signal off in a straight line to the satellite, and get it 
reflected back in a straight line to the destination. Using this method, 
the signal has to travel much further, and encounters more losses along 
it's path, but at least it gets to the destination without getting absorbed 
by the earth. 

Good luck with the project.

Abtin Spantman

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