Wireless communication systems require high-frequency signals for the efficient
transmission of information. Several factors lead to this requirement. For
example, an antenna radiates efficiently if its size is comparable to the signal
wavelength. Since the signal frequency is inversely related to its wavelength,
antennas operating at RFs and microwaves have higher radiation efficiencies.
Further, their size is relatively small and hence convenient for mobile communication.
Another factor that favors RFs and microwaves is that the transmission
of broadband information signals requires a high-frequency carrier signal. In the
case of a single audio channel, the information bandwidth is about 20 kHz. If
amplitude modulation (AM) is used to superimpose this information on a carrier,
it requires at least this much bandwidth on one side of the spectrum.

• Teacher: lee 李旺秋