In the modern society, radio is the most widely used medium of
broadcasting and electronic communication : it plays a major role in many areas
such as public safety, industrial manufacturing, processing, agriculture,
transportation, entertainment, national defense, space travel, overseas
use the radio waves which can be both microwaves and longer radio waves. These
are transmitted in two ways: amplitude modulation (AM ) and frequency modulation
( FM ). These two kinds of wave have many differences.

Radio waves are among the many types of electromagnetic waves that
travel within the electromagnetic spectrum. Radio waves can be defined by their
frequency (in hertz, after Heinich Hertz , who first produced radio waves
electronically), which is number of times they pass through a complete cycle per
second; or by their wavelength, which is determined by the distance (by meters)
that is traveled from the crest of one wave to the crest of the next.

Radio frequencies are measured in units called kilohertz, megahertz, and
gigahertz. (1 kilohertz = 1000 hertz : 1 megahertz = 106 hertz, 1 gigahertz =
109 hertz). All radio waves fall within a frequency range of 3 kilohertz, or
3000 cycles per second to 30 gigahertz. Within the range of frequencies, radio
waves are further divided into two groups or bands such as very low frequency (
VLF 10-30 kHz ), low frequency (LF 30-300 KHz), medium frequency ( MF 300-3000
KHz), high frequency ( HF 3-30 MHZ) and very high frequency ( VHF 30-300MHZ).

Amplitude modulation is the oldest method of transmitting voice and
music through the airwaves is by amplitude modulation. This is accomplished by
combining a sound wave from a microphone, tape, record, or CD with a "carrier"
radio wave. The result : a wave that transmits voice or programming as its
amplitude ( intensity ) increases and decreases. Amplitude modulation is used by
station broadcasting in the AM band and by most international short wave
stations.

Frequency modulation is another way to convey information, voice , and
music on a radio wave is to slightly change, or modulate, the frequency. The
main advantage of FM broadcasting is of it is static free. But the drawback to
FM is since the frequency is varied, station takes up more room on the band.
Frequency modulation is, of course, used on the FM band. And it is used for
"action band" and ham transmission in the VHF/UHF frequency range.

In amplitude modulation, what is modified is the amplitude of a carrier
wave on one specific frequency. The antenna sends out two kinds of Am waves :
ground waves and sky waves. Ground waves spread out horizontally from the
antenna. They travel through the air along the earth\'s surface. Sky waves spread
up into the sky . When they reach the layer of atmosphere called the ionosphere,
they may be reflected back to earth . This reflection enables AM radio waves to
be received at great distances from the antenna.

Frequency modulation station generally reach audiences from 15 to 65
miles ( 24-105km) away. Because of frequency of the carrier wave is modulated,
rather than amplitude, background noise is reduced. In FM transmission, the
frequency of the carrier wave varies according to the strength of the audio
signal or program. Unlike AM , where the strength of the carrier wave varies,
the strength of the carrier wave in FM remains the same , while its frequency
varies above or below a central value broadcast. FM transmission have a
broadcast waves ( 88-108 MHZ) are shorter than AM broadcast waves (540 - 1600
kHz) and do not go as far.

In AM transmission, the amplitude of the carrier waves varies to match
changes in the electromagnetic waves coming from the radio studio. In FM
transmission, the amplitude of the carrier waves remains constant. However, the
frequency of the waves changes to match the electromagnetic waves sent from the
studio.

Two types of radio waves are broadcast by AM transmitter : ground waves,
which spread out horizontally from the ground and travel along the earth\'s
surface, and air waves, which travel up into the ionosphere, allows AM
transmission to travel great distances. AM radio stations with powerful
transmitters can reach listeners as far as 1000 miles ( 1600 km ) away.

FM radio waves also travel horizontally and skyward. However , due to
the higher frequency of the carrier waves, the waves that go skyward are not
reflected. They pass through the atmosphere and into space. Although AM waves
can be received at greater distances than FM waves , FM. waves do have