Radio Principles

General Theoretical Concerns in AMĀ II

Examining the right hand side of the equation we find

Why modulate?

Most of the power in a human voice is at less than 1 kHz in frequency, although harmonics exist as high as 3kHz. This is what gives a voice a unique timbre. Any power radiating through an antenna will, on the alternate half of the cycle, attempt to fall back into the antenna. It is stopped from doing so by the next wave front, which repulses its predecessor and causes dissipation through the air- providing the next wave front arrives fast enough . At frequencies lesser than 15kHz this does not happen. We must modulate the low audio frequency onto the high radio frequency. This is why an early term for a modulating circuit was frequency changer.