New Zealand's link to the world

23:57, Dec 17 2012
Satellite Dish
SATELLITE DISHES: They help us find where we're driving to and determine our location for social networking apps but just how do satellite dishes work?

They are an icon of the age, and for New Zealanders one of our crucial links to the rest of the world. So how do satellite dishes work?

For a start let's get our terms clear. Virtually everyone, apart from the more pedantic telecommunications engineer, refers to them as satellite dishes. After all, they most commonly appear in satellite communications - both attached to satellites in orbit and as the ground link with those satellites.

But they have other uses too where they are usually called dish antennae. A dish antenna forms the basis for most radio telescopes, which is appropriate because what dish antennae most resemble is a light telescope - certainly in function but also, unexpectedly, in form, too.

Unless you're an astronomy buff your first thought on hearing the word "telescope" is probably one of those long metal tube affairs with lenses at either end - known as a refracting telescope because it uses lenses to refract (bend) and focus incoming light. But that's not the only kind of telescope, and for many applications it's not the best kind. The most powerful telescopes are reflecting telescopes which use mirrors rather than lenses to do their work.

In a reflecting telescope - or Newtonian telescope after its inventor Isaac Newton - distant light enters the barrel of the telescope and travels to the far end where a concave mirror reflects and focuses the light at a point in space inside the barrel where a second small mirror mounted at that point reflects the image through an opening in the side of the barrel where it can be observed.

The more light a telescope gathers the better and for the enormous telescopes used in astronomy this means reflecting is the only way to go - it is just impractical to build and mount a lens several metres wide.

A dish antenna works in precisely the same way as a reflecting telescope - indeed it's really just a reflecting telescope for radio waves. The wide dish reflects waves coming from the direction the dish is pointed in and focuses them at a point in front of the dish, where the radio pickup is mounted - that's the bit that sticks out the front. The pickup converts the radio waves into electrical signals.

A dish antenna works the other way, too. Radio signals emitted from the focal point spread out and bounce off the dish as a concentrated beam in one direction. This is exactly the same principle used in car headlights and torches. Using a dish antenna to transmit radio waves from point to point means you can use a lot less power to get the same signal strength at the destination.

The satellite dish may be a symbol of science and technology but it is actually a very simple device and most of its principles are centuries old. Perhaps Isaac Newton wouldn't immediately recognise a satellite dish for what it was - but it would certainly take less explaining than a jet aircraft or an iPhone.


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