Class A | The SWLing Post

Many thanks to SWLing Post contributor, Marty, who shares the following article by John Schneider in Radio World:

In the early years of AM radio broadcasting, all stations utilized non-directional antennas. Most all of these were wire antennas suspended between towers or buildings. Interference, especially at night, was severe. An interfering signal of 5% or less in signal strength was enough to disrupt reception of the desired station, and if the frequencies of the two stations were slightly separated, there would be a heterodyne beat note. As a result, only a few widely-spaced stations could operate on each of the AM broadcast channels in the entire country at night. This limited the number of stations that could coexist to about 500 nationwide, with many of them sharing time on a single frequency.

As antenna technologies were developed and improved in the early 1930s, a few progressive stations began experimenting with multi-element directional arrays. This approach offered two attractive benefits: 1) It could reduce radiation towards other stations on the same or adjacent frequencies, permitting more stations to share a frequency; and 2) a broadcaster could direct more signal towards the desired coverage area, and away from wasted areas such as open water in the case of coastal stations.

WFLA-WSUN

The first known use of a directional antenna was by a pair of stations in Tampa/St. Petersburg, Fla. In 1927, the Clearwater Chamber of Commerce acquired station WGHB and changed the call sign to WFLA. A companion station, WSUN, was operated by the St. Petersburg Chamber of Commerce. The two stations shared the frequency of 900 kHz, broadcasting on alternate evenings to promote tourism and business opportunities in their respective communities. In reality, they operated with two station licenses, but there was only one transmitter and one antenna.[…]

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(Source: RadioWorld via Bill Patalon)

On AM revitalization, Peter Tannenwald asks, Are we really “revitalizing” AM, or are we walking around in circles?

Late on Friday, October 5, the Federal Communications Commission (“FCC”) released a Second Further Notice of Proposed Rulemaking in a five-year ongoing effort to “revitalize” the AM radio broadcast service. The new proposals continue a trend toward allowing higher power operation by smaller stations, by reducing nighttime signal protection for some 60 Class A AM stations located in the continental United States and 16 stations in Alaska. The end result would be less wide area coverage and more local radio service to the public.

To understand why the FCC is considering this action, it helps to understand a bit of the science behind AM signal propagation. AM radio signals travel through both the ground and through the air. At night, the airborne signal component (“skywave”) is reflected back to the earth from the ionosphere — a layer of the atmosphere extending from about 50 to 600 miles above the earth’s surface. The reflected signals may come back down to earth hundreds, if not thousands, of miles away from a station’s transmitter. Class A AM stations — formerly known as “clear channel” stations (no relation to Clear Channel/now iHeartMedia) — are powerhouses, transmitting with 50 kilowatts of power 24 hours a day – 200 or more times the power of the smallest AM stations.

[…]Signal reflection doesn’t work so well during the day, so the FCC has allowed other stations to occupy the Class A frequencies in other markets. But those stations have to curtail power during “critical hours” (two hours before sunrise and after sunset) and often have to reduce power to nearly nothing or shut down altogether at night. In today’s 24-hour-a-day, nonstop world, not being able to reach an audience at night is a losing proposition; so the FCC has yielded to constant pressure over the years to allow more power and longer hours of operation by those “other” stations, at the expense of long distance reception of Class A signals.

Now the FCC is proposing to go further, rolling back some previous restrictions on non-Class A AM stations and perhaps eliminating whatever remains (and it’s not much) of the protection of far-away reception. Under the proposals, which are sufficiently complicated that you should talk to your engineer if you really want to understand the details, Class A AM stations would be protected only within a higher strength signal contour (and so within a smaller area) than they are now; at least some, if not all, skywave protection would be eliminated.[…]

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