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Old 11-08-2011, 06:40 PM
hdc_bst hdc_bst is offline
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You may find clearer explanations if you take a step back from EM waves, and consider the properties of a one-dimensional time varying signal (which, e.g., can be generated by measuring voltage induced across an antenna with an EM wave)

The property 'frequency' describes a fundamental property of a sinusoid signal - how often it cycles per second. Using a Fourier transform, any signal can be represented as a sum of different sinusoids. The bandwidth of a signal describes the difference between its maximum frequency and its minimum frequency.

If multiple signals share frequency components it can be very difficult to separate them. One solution to this problem is to modulate the signals around a 'carrier' frequency (AM radio, amplitude modulation, is the simplest example of this). This moves the signals to different parts of the frequency spectrum so they can be easily separated.

The carrier frequency must be greater than the original signal bandwidth, and the separation between two different carrier frequencies imposes a limit on the bandwidth of signals that can be transmitted without interference.

The bandwidth associated with a particular frequency is either a) a measurable property of a signal being transmitted or b) (I think this is what you are getting at) a decision by a regulatory body such as the FCC to create a scheme in which people can share the EM spectrum without stepping on each other.