Do you know what's the difference between different radio stations and newspapers/magazines? The radio we listen but the newspapers we read. Yes, that's right. The radio doesn't let us see pictures, but the papers don't let us get acquainted with the new charts hitting songs. This sounds reasonable too. What else? The papers we buy or subscribe to but the radio we can listen for free. The radio can be a background while we are doing all other things. A newspaper is a background only while we're having breakfast, probably one can say that for some people the breakfast is the background. But that's all isn't it.
Let's speak about the diversity. How many printed mass media are there in your city? Plenty of them. You can't even name them all, if fact you have no idea about the existence of probably more than a half of them. You read a couple of them and that quite enough.
But what about the radio. How many radio stations is there in you radio set. Well there are some. You probably don't know them all but some you can name. If you would feel a sudden need to learn about all the radio stations available on you radio set you could just roll through the tuning and find out: what frequency is occupied by each one radio station. Or in the Web there you'll find for sure a list if the radio stations which are available in your city or town.
The reason is that the amount of radio frequencies is limited.
At first let's see what radio frequency is. Leaving aside all the complicated scientific terms aside one could simply say that the radio frequency is one of the characteristics of an electromagnetic wave tightly correlated with the length of that wave.
Radio transmitter based on a radio station generates those electromagnetic waves spreading around. Each one radio station has it's own frequency that's called bearing frequency. It's bearing the information about the sound which is encoded in it. Here come the AM and FM abbreviations which exactly mean amplitude and frequency modulations. It describes the way information is encoded in the bearing frequency ' either it modulated by the amplitude or by frequency.
So why is the amount of those frequencies limited? Let's first see what are the others purposes of radio frequencies which aren't only supposed to deliver music to our radio set. Well those purposes are plenty. The air is filled with all those electromagnetic signals, and thanks god our senses don't perceive them without special electromagnetic devices. Those wireless devices are ' at first we should name TV-sets, then there are cell phones, satellite communication systems. On the shorter or longer than radio frequencies word such devices as for example TV-set remote-control boxes, cordless keyboards, mice, ear-phones, infrared (as can be seen from the name) ports of those cell phones and other devices.
All the electromagnetic waves are divided in a few groups according to their length. As one understands the longer the wave the lower the frequency and the same on the contrary. The waves of a different frequency ranges are used in different purpose. For long waves are able to spread the signal on the long distances, yet the quality of the signal will be much worse than on the shorter waves.
Local radio stations make use of the short waves and high radio frequencies. Your radio sets do catch the signal of the radio station while the tower of their transmitter is visible. That's why the further you drive in the country the less radio stations are available in your set, and when you are approaching to some other town your set starts catching the local radio stations of that town.
A physicist would tell you that even the range of only short radio waves isn't physically limited. But if all the radio stations would start using any radio frequencies they like it would be probably impossible to listen to any of them because they would be hammering in a signal of one another. That's why people have arranged some rules of broadcasting including the pace in frequency between two neighbor radio stations. And that's why in the big cities all the FM frequencies are already developed, and that's why there exist all these contests for a frequency, procedures with getting a license on the frequency, prolonging that license.