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Radio Bandwidth


As you tune a radio receiver along a radio band, you find different radio stations at various positions along the band, but these radio stations aren't instantaneous blips; they take up space. Some radio stations are wider on the radio band than others, and their width is... bandwidth.

This is an important consideration because it would be nice to get a great many radio stations on the dial and use the frequency spectrum efficiently, but it's not practical to cram them in too close because they need enough elbow-room.signals

Some types of transmission take up more room than others. Morse carrier wave signals don't take up much room at all. Sound, whether it be music or speech, takes up a bandwidth of a few tens of kilohertz. Television signals are much wider, at several megahertz wide.

To see why this is, here's a way to think about it: If a radio transmitter was set up to just exist as a single pure transmission at a particular frequency, just a carrier wave, it could be received, and it wouldn't take up much bandwidth, but its usefulness would be limited. With radio in practicality you want to send signal as well, so what happens is that the signal and carrier are modulated together to be sent. However, as soon as you add the signal to the carrier, it's no longer a pure monofrequency, and instead it's variable within a range. Therefore it takes up some bandwidth. I could say that if you stand still you don't take up much room but if you dance then you take up a lot more room, but with radio signals it's even more notable, as the motion of the signal and the fundamental radio wave are made of the same stuff.

The amount of bandwidth taken up by a radio transmission is of the order of the frequency of the signal. (In practice it's at least double that, for various reasons). If your transmitter frequency is 1MHz and you're sending audio signal, you can expect to occupy about 50KHz of bandwidth. What about higher up the frequency spectrum? The good news on this is that at higher frequencies such as 100MHz, you can still expect to occupy about 50KHz of bandwidth. Even at 10GHz, still only 50KHz of bandwidth. There's a lot more bandwidth available at the higher frequencies.

Satellites offer much better bandwidth than terrestrial transmissions, because satellites use microwave signals. Well you can use microwaves on Earth? Yes, but the signals travel in straight lines, and as the Earth is uncannily round, the signals don't go very far along the surface, whereas in space, satellites have a direct line-of-sight with the dishes that are pointing at them.

Also see the relationship between frequency and wavelength