How Everything Works
How Everything Works How Everything Works

Question 931

Why is a satellite dish necessary to receive satellite broadcasts? Why doesn't a conventional radio antenna work? — AW, Karachi, Pakistan
Normal television broadcasts use electromagnetic waves with relatively low frequencies and long wavelengths while satellite broadcasts use waves with relatively high frequencies and short wavelengths. The short wavelength waves from a satellite are known as microwaves while the longer wavelength waves from a normal broadcast station are generally known as radio waves. Since the optimal antenna size for receiving a particular electromagnetic wave is proportional to the wavelength of the wave, you need a smaller antenna to receive the microwaves from a satellite than you do the radio waves from a normal television station. However, the microwaves from a satellite are much weaker than the radio waves from a nearby television station and a small microwave antenna isn't likely to absorb enough of them to produce a useable signal.

The solution to this dilemma is to concentrate the microwaves from a satellite with the help of an optical imaging system. Although it may not look like one, a satellite dish is really a carefully shaped mirror telescope. Just as the curved mirror of the Hubble space telescope can bring light from a distant star to a focus on an optical image sensor, so the curved wire mesh of a satellite dish can bring microwaves from a distant satellite to focus on a small microwave antenna. This microwave antenna sits at the focus of the satellite dish and absorbs the microwaves that the dish collects. The dish's imaging behavior also ensures that microwaves from only one satellite are brought to a focus on the microwave antenna. You must redirect the dish or move the antenna in order to switch from one satellite to another.


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