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A Difference in Protection

Which Class of Station Gets Just a Bit More?

Class Distinction (Exam level: CBRE)

In 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.

Frequency-modulated (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 his work.

Before World 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 frequencies.

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.

Around 1946, stations started migrating to our present FM band of 88 to 108 MHz. By 1948, everyone was located in the “new” band.


The 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 broadcast assets.

The grand 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.


(click thumbnail)
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.

A 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.

The Reserve 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.

Interference 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.


Originally, 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.

In 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.

Thus the correct answer to the question posed at the beginning of this article is “d.”


At 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 at

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.

The other contour listed, 3.16 mV/m (70 dBu) is important because this is the coverage that is required over the city of license.

Actually, these 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.

Charles “Buc” Fitch, P.E., CPBE, AMD, is a frequent contributor to Radio World. Missed some SBE Certification Corners or want to review them for your next exam? See the “Certification” tab under Columns at

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