how are the radio waves created ?

Answer 1

By applying an alternating current of suitable frequency to an antenna. The current in the antenna creates a magnetic field, which by its change creates an electric field, and the alternation of energy back and forth between these fields is a radio wave.

Answer 2

Radio waves are sent out by the moving current going through an antenna. Normal sound comes in the form of waves, moving through the air by vibrating molecules. Radio waves, on the other hand, are created when electrons in an antenna vibrate, and send out an electromagnetic wave.

In order for these waves to be picked up by your radio seperate from all the other radio signals flying around, they are modulated into specific frequencies. This can be an amplitude modulation or a frequency modulation (hence am and fm). This simply means that either the amplitude of the wave or the frequency of the wave is stretched/shrunk at a frequency, chosen by the sender.

The radio picks up these waves practically the same way they are sent, but the other way around. Electromagnetic waves stimulate electrons in your radio to vibrate, and these vibrations are interpreted by the radio.

Answer 3

Essentially, any time an electrical current changes (amplitude, phase, frequency) it emits EM waves of photons (photons are much more than just the light spectrum). Thus, changing currents in antennae create the EM radio waves according to the signals carried. (By comparison, DC current, which does not change, does not emit EM waves. This is why transformers cannot be used with direct current.)

As EM waves, they travel at the speed of light (c). These waves are made up of quanta called photons and each quantum has its own particle frequency, which determines where in the EM spectrum it fits in. c = L/t; where c = speed of light, L is the wave length of the wave of photons, and t is the time the photons travel a wavelength.

In general radio waves (of the photon frequencies) are much longer than light waves; which hovers around the L = 1 micrometer = 1 X 10^-6 meter length. Because c = L/t = Lf; where L = the wave length and 1/t = f = wave frequency, and c = the speed of light, we can see as the wavelength goes up, the frequency has to go down in order to preserve the constant c. This means, since energy is directly proportional to frequency, each radio photon carries less energy than any photon in the visible light spectrum.