(Exam level: CBRE)
the April 13 issue of RWEE, we asked:
All stations in the commercial part of the FM band
are protected to their 1 mV/m contours except for:
a. Class A and full Class C
b. Class B and Class D
c. Class B and Class C2
d. Class B and Class B1
e. Class B1 and C1
SBE certification is the emblem of professionalism in
broadcast engineering. To help you get in the exam-taking frame of mind, Radio
World Engineering Extra poses a typical question in each Certification Corner.
Although similar in style and content to test questions, these are not from
past exams nor will they be on future exams in this exact form.
(FM) radio has gone through a number of changes since Major Armstrong and his
supporters pressed to get this revolutionary radio service into existence back
in the 1930s.
FM had many
“fathers,” but no one was more of a driving proponent than Armstrong. He said
his motivation was spawned when he was attempting to listen to and enjoy a
classical music broadcast at his home in the Yonkers area, north of New York
City. A passing lightning storm was wiping out AM reception. Armstrong was a
person who made his best efforts working from a defined problem. The
high-fidelity, static-free performance of FM today is the historic result of
War II the FM band was in the 42 to 50 MHz region. Wartime advances in
components, improved VHF receiver performance and expanding demand for spectrum
(mainly from TV agitators) mandated a move for FM services into higher
Entravision’s 100 kW Class C
station KOFX at 92.3 MHz serves its community of El Paso and neighboring Ciudad
Juarez from high in the Franklin Mountains.
stations started migrating to our present FM band of 88 to 108 MHz. By 1948,
everyone was located in the “new” band.
dominance and simplicity of AM, the band move, the cost of receivers, consumer
apathy and postwar economic factors almost put the kabosh on early FM.
Even so, the
FCC, visionary broadcasters and discriminating elements of the public knew the
intrinsic value of FM and persevered. Today FM is the dominant radio medium. In
2002, for example, Entravision bought a cluster of Los Angeles FMs and paid
roughly $10,000 per watt. While the long-term future of radio is a popular
debate topics, FMs today are seen as extremely valuable, almost unique
vision of Congress and the FCC is that an FM broadcast station is a valuable community
service and resource. Not wanting to duplicate the relatively
chaotic development of the AM band, various schemes were developed to assign FM
facilities equitably across the United States. Starting in the 1950s, a table
of community assignments was generated in the attempt to distribute FM spectrum
evenly by population and community.
Since there was
no spectrum squeeze, facilities were treated on a case-by-case basis with the
result that stations as varied as WHO’s 400,000 watt ERP monster existed on the
same dial with little Class D 10 watters. Power and height restrictions were
put in place in 1962.
As the number
of applications, construction permits and licenses grew, more order was needed.
This led to the adoption of sophisticated “class” descriptors with more exact
facility specifications. An enlarged table of community allocations was adopted
that for years guided the growth and station distribution on the FM dial.
A population density concept also was added. FM
stations in highly populated areas were allowed to cover a marketing footprint
of sufficient population to be economically successful, but somewhat limited to
allow more allocations. Elsewhere, broadcasters were allowed to cover more area
with bigger and more powerful signals to serve isolated areas as well as
accumulate enough of a population to make an economic go of it. Hence the class
difference where we have maximum coverage Class Bs for mainly the dense eastern
areas and Class Cs elsewhere.
WITH SOME RESERVATIONS
A mandatory part of all FM applications pre-computer filing,
the ‘official coverage map,’ with the community of license inside the 3.16 mV/m
contour. Some of the most valuable FM signals are in the New York City area; this
is the signal of one of them, the 1980 facility of 93.5 MHz, New Rochelle.
notable feature on the FM dial is the distinction between the “reserved
section” that lies between 88 and 92 MHz and the commercial portion between 92
and 108 MHz.
Band was a critical component in the creation of the FM broadcast service.
Generally attributed to Major Armstrong, the fundamental concept was to provide
communities with a vehicle for programming outside of the usual commercial
sources. The vision was to give the arts, education and religion the
opportunity for broadcast carriage without commercial necessity.
In the commercial band, all initial
allocations are done by minimum spacing. The spacings have been designed to
minimize interference without having to complete a detailed engineering study.
However, in the Reserve Band the FCC
rules allow allocations to be done by interference method (somewhat similar to
AM methodology). This means that if a proposed station can demonstrate that it
causes no new interference to any existing stations, it is allowed.
for Reserve Band allocation purposes is calculated according to charts
developed by the FCC in the 1950s. The protected service contour (the 1 mV/m
contour) is first determined using the FCC’s 50/50 graph (or formulas that
emulate that graph) on a specific azimuth toward the potential interfering
station being considered. The interfering station’s signal level on the reverse
azimuth is calculated using the FCC’s 50/10 graph. If the two contours overlap,
interference is assumed and the allocation would not be permitted.
Class B FM stations were allowed a maximum of 20,000 watts ERP, as these were
expected to parallel the allocation and service areas of “regional” class AM
stations. Around 1975, a change in the perception of what constituted the
service contours of these allocations moved up the maximum ERP to 50,000 watts.
rationalizing this new power and the intended service area, it was determined
that Class Bs would be protected to their 0.5 mV/m predicted service contour. I
have identified no technical reason for this Class B exception other than the
reasonable anticipation of existing FMs to maintain their established service
area and audience. All other classes received 1 mV/m protection.
correct answer to the question posed at the beginning of this article is “d.”
MORE AND MORE
the suggestion of noted consulting engineer Serge Bergen, some small changes in
required spacing allowed hundreds of new FM stations to be accommodated under
the famous Docket 80-90 rulemaking. Bergen and other proponents noted that
improvements in FM receivers, especially adjacent-channel rejection, would
allow reduced spacing to other stations within 600 kHz. The goal was to allow
hundreds of unserved communities to have their first radio service.
With this new
spacing criterion and in a sort of “AM-ization” of the FM band, in-between
classes of FM stations were created with appropriately adjusted spacing values
so that more stations and an overall maximization of the FM band was achieved.
You can see the table of minimum separations for the range of station classes
Shown here is
the commercial coverage and contour table directly from the FCC website. As you
can see, all classes except for Class B and B1 are protected to their 1 mV/m
contour listed, 3.16 mV/m (70 dBu) is important because this is the coverage
that is required over the city of license.
classic signal levels are very high, corresponding to typical receiver
performance in 1950. The F(50,50) graphs predicted FM service would be present
50 percent of the time at 50 percent of the locations within the service area.
With today’s receivers, full quieting FM reception can be had at a fraction of
these signal levels.
“Buc” Fitch, P.E., CPBE, AMD, is a frequent contributor to Radio World.
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|Strong in Your Field
Question for next time
(Exam level: CBRE)
What factor(s) most affect the predicted or measured far-field signal
strength of a nondirectional AM station?
a. Transmitter power
b. Antenna power input, antenna height
c. Antenna power input, antenna height, radial count
d. Antenna power input, antenna height, radial count, top loading
e. All other factors being equal, ground conductivity