The main concept to understand is that of a carrier wave. The radio wave is that. A radio wave is electro-magnetic radiation, which, unlike other waves, is not a vibration within a medium, like sound waves for example, which are the propagation of alternating high and low pressure within the air, or other fluid media. Electro-magnetic waves are a self propagating electro-magnetic effect which can travel even in a vacuum. Electro-magnetic radiation is a pulsing stream of energy where the pulses are at a constant frequency. What we perceive as different phenomenon are in fact the same electromagnetic radiation but at different frequencies. Microwaves, Radio, Ultra-violet, Visible Light, and Infra-red, are all electro-magnetic radiation moving down the scale from high to low frequency.
The radio transmitter sends out radio waves at a specific frequency or band of frequencies. The aerial of the receiver develops tiny electric currents in it as the radio wave flows past it. These are pulsing currents at the same frequency as the radio wave, but these pulses are not in themselves the content of speech or music. This content, or signal, is carried in the wave, hence the concept of the carrier wave.
It is easier to understand Amplitude Modulation or AM radio so I will restrict myself to that. If you imagine a radio wave as a very long string with the pulses like beads on it, all spaced at precise intervals, then imagine that the beads slowly getter bigger and smaller along its length. Each bead is a packet of energy in the wave and all occur at the same frequency, but where the beads get bigger there is more energy in the packet. It only takes a little more imagination now to see how the pattern of a sound wave can be represented by the bulging out of the beads, and their getting smaller. This is called the amplitude.
Looking at it in practical terms, at the transmitter end sound is converted in the microphone into a corresponding pattern of changes in the current flowing through the microphone. That is your signal. That signal which is at a low electrical power, is used to create the same pattern but in a circuit of much higher electrical power - that is what an amplifier does. The device used to transfer the pattern is called a transistor, which uses the low powered circuit to control the flow in the high powered circuit.
The amplified signal is fed to the transmitter aerial, so now that pattern of electrical energy, identical to the pattern of the sound picked up by the microphone, comes out of the aerial as radio waves, with the pattern held in the varying amplitude of the radio wave.
Now comes a trick to follow. If there are thousands of transmitters all radiating radio waves all over the place, how come your radio only picks up the one signal. This is where the tuner comes in. The principle of radio tuning depends on the opposite properties of coils and capacitors. I don't know if you know what these are, but it doesn't matter if you don't. Both of these devices have a special kind of resistance to electricity known as impedance. Impedance only operates where the current is varying. The voltages and currents in our radio Ariel will be varying at exactly the same frequency as each radio wave it is exposed to, so if we pass that current through either a coil or a capacitor there will be an impedance in the circuit. Now the impedance of the coil increases as the frequency gets higher and the impedance of the capacitor gets higher as the frequency gets lower. An AM tuning circuit simply passes the signal current through both a coil and a capacitor at the same time, and the capacitor is a mechanically variable one, so by turning a knob you can increase or decrease the capacitance. The trick here is that as the capacitance changes the circuit will show least impedance to a specific frequency compared with all the others, so provided the frequencies are not too close, as the law requires them not to be, you will end up with currents flowing in the circuit from only one radio frequency, ie one transmitter, and one signal.
The signal is then amplified just as before, fed through the loudspeaker and converted back to sound. The sound of course isn't generated from the radio frequency it is generated from the changes in amplitude, as explained earlier.
There's a lot more to it then that, but I hope that helps a bit.
· 1 decade ago