Radio Seems Like Magic, But is Easy to Understand
From Gary Altunian,
Every so often it occurs to me that radio reception is pure magic. When you turn on the radio you hear music, voice or other audio entertainment that is broadcast from hundreds or thousands of miles away. It's really not magic. Radio is actually easy to understand and this article will help de-mystify how radio waves are created and broadcast.
Every so often it occurs to me that radio reception is pure magic. When you turn on the radio you hear music, voice or other audio entertainment that is broadcast from hundreds or thousands of miles away. It's really not magic. Radio is actually easy to understand and this article will help de-mystify how radio waves are created and broadcast.
What are Radio Waves?
Both
AM and FM radio programs are transmitted over the air via radio waves,
which are part of a broad range of electromagnetic waves that include
visible light, X-rays, gamma rays and others. Electromagnetic waves are
all around us in different frequencies. Radio waves are similar to light
waves but are at a frequency our eyes are not sensitive to.Electromagnetic
waves are generated by alternating current (AC), the electrical power
used to run every appliance in our homes from washing machines to televisions. Alternating current in the US is 120 volts at 60Hz,
which means that the current alternates or changes direction in the wire
60 times per second. Other countries use 50Hz as the standard. Both 50
and 60Hz are relatively low frequencies, but even 60Hz alternating
current generates some level of electromagnetic radiation, meaning that
some of the electricity escapes the wire and is transmitted into air.
The higher the frequency of the electricity, the more electricity
escapes the wire and into space. Thus, electromagnetic radiation can be
described as 'electricity in the air'.
The Concept of Modulation
Electricity
in the air is nothing but random noise. To be turned into useful
signals that transmit information (music or voice) it must be modulated,
and modulation is the basis for AM and FM radio signals. In fact, AM
stands for amplitude modulation and FM stands for frequency modulation.Another
word for modulation is change. The electromagnetic radiation must be
modulated or changed to be useful as a radio transmission. Without
modulation, no information is carried in a radio signal. Modulation is an easy concept to understand and is all
around us. Our sense of vision is a good example of how modulation
works. A blank piece of paper is useless unless it is modulated or
changed. Someone must write or draw on the paper for it to communicate
useful information. Hearing is another example; still air must be
modulated or changed with music or a voice to be useful. In radio
broadcasts, the electromagnetic radiation or electricity in the air must
be modulated.
AM Radio Broadcasts
AM radio uses amplitude
modulation and is the simplest form of radio broadcast. To understand
amplitude modulation, consider a steady signal broadcasting at 1000kHz
on the AM band. The amplitude or height of the constant signal is
unchanged or un-modulated, thus no useful information. The steady signal
produces only noise until it is modulated with a voice or music as
shown in figures 1 & 2. Notice how the amplitude or loudness of the
signal increases or decreases to produce useful sound or information.
AM
radio in the Americas operates in a range of frequencies from 520kHz to
1710kHz. Other countries and regions have a different frequency range.
The specific frequency is called the carrier frequency, the vehicle by
which the signal is carried from the broadcast antenna to the receiving
tuner.
AM radio has the advantage of transmitting over greater
differences because AM signals bounce off the upper atmosphere but
suffers from more noise and interference than FM, especially during
thunderstorms. The electricity generated by lightning produces noise
spikes picked up by an AM tuner. AM radio also has a very limited audio
range, from 200Hz to 5kHz, which limits its usefulness to talk radio and
less for music.
FM Radio Broadcasts
FM radio uses frequency modulation, which
changes or modulates the frequency of the unmodulated signal while
keeping the amplitude of the signal constant. When the frequency is
modulated, music or talk is transmitted via the carrier frequency (see
figures 3 & 4).
FM radio operates in the range of 87.5MHz to 108.0MHz, a much higher range of frequencies than AM radio.
The
distance range for FM transmissions are more limited than AM, usually
less than 100 miles, but are better suited for music because the
frequency range of FM is from 30Hz to 15kHz. FM broadcasts are also
commonly in stereo, although a few AM stations also broadcast stereo
signals.
Although FM signals can be subject to noise from
lightning like AM signals, FM broadcasters use a limiter function that
clip-off the noise spikes to produce a relatively noise-free signal.