Radio broadcasters utilize part of the electromagnetic spectrum to transmit their signals,
and they are obliged to pay spectrum fees of
up to $1500 annually (depending on their
size) for this privilege. A natural resource,
the electromagnetic spectrum is composed
of radio waves at the low-frequency end
and cosmic rays at the high-frequency end.
In the spectrum between are infrared rays,
light rays, X rays, and gamma rays. Broadcasters, of course, use the radio wave portion
of the spectrum for their purposes.
Electromagnetic waves carry broadcast
transmissions (radio frequency) from station
to receiver. It is the function of the transmitter to generate and shape the radio wave to
conform to the frequency the station has
been assigned by the FCC. Audio current
is sent by a line from the control room to
the transmitter. The current then modulates
the carrier wave so that it may achieve its
authorized frequency. A carrier wave that
is undisturbed by audio current is called an
unmodulated carrier
The antenna radiates the radio frequency.
Receivers are designed to pick up transmissions AM and FM stations are located at different points in the spectrum: AM stations
are assigned frequencies between 540 and
1700kHz on the Standard Broadcast band,
and FM stations are located between 88.1
and 107.9MHz (megahertz equals 1 million
hertz) on the FM band.
Ten kilocycles (kc) separate frequencies in
AM, and there are 200kc between FM frequencies. FM broadcasters utilize 30kc for
over-the-air transmissions and are permitted
to provide subcarrier transmission (SCA) to
subscribers on the remaining frequency. The
larger channel width provides FM listeners a
better opportunity to fine-tune their favorite stations as well as to receive broadcasts
in stereo. To achieve parity, AM broadcasters
developed a way to transmit in stereo, and
by 1990 hundreds were doing so. The finetuning edge still belongs to FM, because its
sidebands (15kc) are three times wider than
AM’s (5kc).
FM broadcasts at a much higher frequency
(millions of cycles per second) compared
to AM (thousands of cycles per second). At
such a high frequency, FM is immune to
low-frequency emissions, which plague AM.
Although a car motor or an electric storm generally will interfere with AM reception, FM is
static free. Broadcast engineers have attempted
to improve the quality of the AM band, but
the basic nature of the lower frequency makes
AM simply more prone to interference than
FM. FM broadcasters see this as a key competitive advantage and refer to AM’s move to
stereo as “stereo with static.”
Signal Propagation
The paths of AM and FM signals differ from
one another. Ground waves create AM’s
primary service area as they travel across the earth’s surface. High-power AM stations
are able to reach listeners hundreds of miles
away during the day. At night AM’s signal
is reflected by the atmosphere (ionosphere),
thus creating a skywave that carries considerably farther, sometimes thousands of
miles. Skywaves constitute AM’s secondary
service area.
In contrast to AM signal radiation, FM
propagates its radio waves in a direct or
line-of-sight pattern. FM stations are not
affected by evening changes in the atmosphere and generally do not carry as far
as AM stations. A high-power FM station
may reach listeners within an 80- to 100-
mile radius because its signal weakens as
it approaches the horizon. Because FM
outlets radiate direct waves, antenna height
becomes nearly as important as power. In
general, the higher an FM antenna, the
farther the signal travels.
The fact that AM station signals travel
greater distances at night is a mixed blessing. Although some stations benefit from
the expanded coverage area created by the
skywave phenomenon, many do not. In
fact, over 2000 radio stations around the country must cease operation near sunset,
and thousands more must make substantial transmission adjustments to prevent
interference. For example, many stations
must decrease power after sunset to ensure
noninterference with others on the same
frequency: WXXX-AM is 5000W (5kW)
during the day, but at night it must drop to
1000 W (1kW). Another measure designed
to prevent interference requires that certain
stations direct their signals away from stations on the same frequency. Directional
stations require two or more antennas to
shape the pattern of their radiation, whereas
a nondirectional station that distributes its
signal evenly in all directions needs only a
single antenna. Because of its limited direct
wave signal, FM is not subject to the postsunset operating constraints that affect most
AM outlets.
Station Classifications
To guarantee the efficient use of the broadcast spectrum, the FCC established a classification system for both AM and FM stations.
Under this system, the nation’s 10,000 radio
outlets operate free of the debilitating interference that plagued broadcasters prior to
the Radio Act of 1927.
• Class A: Clear channel stations with
power not exceeding 50kW. Their frequencies are protected from interference
up to 750 miles. Among the pioneer, or
oldest, stations in the country are KDKA,
WBZ, WSM, and WJR.
• Class B: Stations with power ranging from
a minimum of 250W to a maximum of
50kW. They must protect Class I outlets
by altering their signals around sunset. As
a Class B station, WINZ-AM in Miami
is required to reduce power from 50kW
to 250W so as not to intrude on other
stations at 940kHz. These stations also
operate on regional channels. If a station
is authorized to operate in the expanded
band (1610–1700), the maximum power
is 10kW.
• Class C: Stations that operate on local and
re
gional channels with power between
250 and 1000W. They may operate
without time restrictions.
• Class D: Stations that operate either
daytime, limited time, or unlimited time
with a nighttime power less than 250W.
Daytime-only stations are Class D.
Section 73.21 of the Code of Federal Regulations, Part 73, provides more details on
AM station classifications.
New AM band space (1605–1705kHz)
is currently being allocated, and the FCC is
encouraging existing AM license holders to
shift to the new space as a means of reducing interference on the clogged band.
FM classifications include the following:
• Class C: The most powerful FM outlets
with the greatest service parameters, these
stations may be assigned a maximum
ERP of 100kW and a tower height of up
to 2000feet. Class C radio waves carry,
on average, 70miles from their point of
transmission.
• Class B: These stations operate with less
power – up to 50kW – than Class Cs and
are intended to serve smaller areas. The
maximum antenna height for stations in
this class is 500 feet, and signals generally
do not reach beyond 40–50 miles.
Class A: The least powerful of commercial
FM stations; they seldom exceed 3 kW ERP
(except in select cases where a ceiling of
6 kW is imposed) and 328 feet in antenna
height. The average service contour for stations in this category is 10–20 miles.
• Class D: Set aside for noncommercial
stations with 10W ERP, this type of
station is most apt to be licensed to a
school or college.
In the 1980s, the FCC introduced three
new classes of FM stations under Docket
80–90 in an attempt to provide several
hundred additional frequencies, and more
subclasses were added later. They are as
follows:
• Class C1: Stations granted licenses to
operate within this classification may be
authorized to transmit up to 100 kW ERP
with antennas not exceeding 984 feet.
The maximum reach of stations in this
class is about 50 miles.
• Class C2: The operating parameters of
stations in Class C2 are close to Class Bs.
The maximum power granted Class C2
outlets is 50kW, and antennas may not
exceed 492 feet. Class C2 stations reach
approximately 35 miles.
• Class C3: These stations operate with
shorter antennas and with power that
typically exceeds 6kW ERP.
• Class B1: The maximum antenna height
permitted for Class B1 stations (328 feet)
is identical to Class As; however, Class
B1s are assigned at least 25 kW ERP. Class
B1 signals carry 25–30 miles