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AM broadcast
radio sends music and voice in the Medium
Frequency (MF—0.300 MHz to 3 MHz) radio
spectrum. AM radio uses amplitude
modulation, in which the amplitude of the
transmitted signal is made proportional to
the sound amplitude captured (transduced) by
the microphone while the transmitted
frequency remains unchanged. Transmissions
are affected by static and interference
because lightning and other sources of radio
that are transmitting at the same frequency
add their amplitudes to the original
transmitted amplitude. The most wattage an
AM radio station is allowed to use is 50,000
watts and the majority of stations that emit
signals this powerful were grandfathered in;
these include WGN (AM), WJR, and CKLW. In
1986 KTNN received the last granted 50,000
watt license.
FM broadcast radio sends music and voice
with higher fidelity than AM radio. In
frequency modulation, amplitude variation at
the microphone causes the transmitter
frequency to fluctuate. Because the audio
signal modulates the frequency and not the
amplitude, an FM signal is not subject to
static and interference in the same way as
AM signals. FM is transmitted in the Very
High Frequency (VHF—30 MHz to 300 MHz) radio
spectrum. VHF radio waves act more like
light, traveling in straight lines, hence
the reception range is generally limited to
about 50-100 miles. During unusual upper
atmospheric conditions, FM signals are
occasionally reflected back towards the
Earth by the ionosphere, resulting in Long
distance FM reception. FM receivers are
subject to the capture effect, which causes
the radio to only receive the strongest
signal when multiple signals appear on the
same frequency. FM receivers are relatively
immune to lightning and spark interference.
High power is useful in penetrating
buildings, diffracting around hills, and
refracting for some distance beyond the
horizon. Consequently, 100,000 watt FM
stations can regularly be heard up to 100
miles (160 km) away, and farther (e.g., 150
miles, 240 km) if there are no competing
signals.
A few old "grandfathered" stations do not
conform to these power rules. WBCT-FM (93.7)
in Grand Rapids, Michigan, runs 320,000
watts ERP, and can increase to 500,000 watts
ERP by the terms of its original license.
This huge power level does not usually help
to increase range as much as one might
expect, because VHF frequencies travel in
nearly straight lines over the horizon and
off into space. Nevertheless, when there
were fewer FM stations competing, this
station could be heard near Bloomington,
Illinois, almost 300 miles (500 km) distant.
FM Sub-carrier services are secondary
signals transmitted "piggyback" along with
the main program. Special receivers are
required to utilize these services. Analog
channels may contain alternative
programming, such as reading services for
the blind, background music or stereo sound
signals. In some extremely crowded
metropolitan areas, the sub-channel program
might be an alternate foreign language radio
program for various ethnic groups.
Sub-carriers can also transmit digital data,
such as station identification, the current
song's name, web addresses, or stock quotes.
In some countries, FM radios automatically
re-tune themselves to the same channel in a
different district by using sub-bands.
Aviation voice radios use VHF AM. AM is used
so that multiple stations on the same
channel can be received. (Use of FM would
result in stronger stations blocking out
reception of weaker stations due to FM's
capture effect). Aircraft fly high enough
that their transmitters can be received
hundreds of miles (or kilometres) away, even
though they are using VHF.
Marine voice radios can use AM in the
shortwave High Frequency (HF—3 MHz to 30
MHz) radio spectrum for very long ranges or
narrowband FM in the VHF spectrum for much
shorter ranges. Government, police, fire and
commercial voice services use narrowband FM
on special frequencies. Fidelity is
sacrificed to use a smaller range of radio
frequencies, usually five kHz of deviation,
rather than the 75 kHz used by FM broadcasts
and 25 kHz used by TV sound.
Civil and military HF (high frequency) voice
services use shortwave radio to contact
ships at sea, aircraft and isolated
settlements. Most use single sideband voice
(SSB), which uses less bandwidth than AM. On
an AM radio SSB sounds like ducks quacking.
Viewed as a graph of frequency versus power,
an AM signal shows power where the
frequencies of the voice add and subtract
with the main radio frequency. SSB cuts the
bandwidth in half by suppressing the carrier
and (usually) lower sideband. This also
makes the transmitter about three times more
powerful, because it doesn't need to
transmit the unused carrier and sideband.
TETRA, Terrestrial Trunked Radio is a
digital cell phone system for military,
police and ambulances. Commercial services
such as XM, WorldSpace and Sirius offer
encrypted digital Satellite radio.
From
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