The satellite dish has become a ubiquitous fixture of 21st Century living. Office buildings in the city, tract homes in the suburbs, even historic enclaves in the hot spots of the Middle East all share a bumper crop of dishes pointed toward an imaginary point 35,786 kilometers above the equator. Satellite dishes come in varying sizes, but they all share a similar shape. It’s a quirk of nature that a parabolic curve produces the perfect antenna for extremely high frequency, extremely short wavelength, transmissions.
This is not a new discovery. Archimedes helped describe it in mathematical terms. In the 10th Century physicists were playing with parabolic mirrors. Later on, lighthouses used parabolic lenses to take a lantern’s light and turn it into a beam. In the 18th Century parabolic lenses found use in telescopes. The common goal in all these pursuits was the same as what’s accomplished with a satellite dish–you’re taking a weak signal and amplifying it.
We have not repealed the laws of physics. You can’t increase a lantern’s light with a simple lens, but you can concentrate it. Most areas will see less light, but those favored by the lenses geometry will see a great deal more. Technically speaking, the light has been collimated.
What we think of as a satellite dish is actually just a parabolic reflector. The real antenna sits in front of dish at its “focus.” Here’s the science that makes it happen: Any radio waves that strike the dish and are traveling parallel to its axis are reflected to the antenna at the focus. It doesn’t matter where on the parabola the signal strikes. As long as it’s coming in at the right angle it’s going to the focus.
Instead of just picking up what randomly strikes the antenna, you’ve now got the force of everything that strikes the dish–a much larger area. Antenna gain in the direction the dish is pointed can easily exceed 20dB, or 100 times what you’d expect without the parabola. As an added bonus, interference coming from anywhere but where the dish is pointed is attenuated. That’s a pretty sweet trick which allows satellites to use the same frequencies as their neighbors while sitting reasonably close to each other over the equator.
A signal coming down from space then passing through the atmosphere is very weak. Without a dish there is little chance home satellite reception would be practical. And, since the dish itself is a passive participant in the signal path, it’s easy and cheap to build. Thanks Mother Nature. Nicely done.
Original post by Geoff Fox
