We’re tearing another page off the calendar. Come Monday, it’s August. The days have been getting shorter for over a month now, a minute or two per day. For many AM radio stations, however, Monday will be quite a bit shorter than Sunday.
Certain AM stations are licensed to operate during the daytime only, defined by the approximate sunrise and sunset times in the middle of a given month. For this reason, operating hours will vary throughout the year. Take my company’s daytimer, WHIT. During June, the station signed on at 5:15AM and signed off at 8:45, its longest broadcast day of the year. This month, it gave back 15 minutes on each end of the day. Come Monday, sign-on moves up from 5:30 to 6AM, and sign-off moves back to 8:00 from 8:30. The broadcast day will continue to shorten as autumn and winter approach. In November, after the return of Standard Time, the station broadcasts from 6:45AM to 4:30PM. In December, sign-off stays at 4:30, but sign-on isn’t until 7:15AM. On January 1, however, the first sign of spring appears: although sign-on is delayed until 7:30AM, sign-off is a little bit later: 4:45PM. A station’s sign-on and sign-off times are a function of its latitude and longitude, so they can vary in different places.
Isn’t this variability a competitive disadvantage for radio stations? It can be. For most daytimers, however, variable hours are a simple fact of life you get used to—although I once worked at a company whose AM station signed on at 7AM and signed off at 4:45PM year round, even in the summer when they could have been on far longer, just because it was easier.
Some AM stations operate at night at reduced power. Power will be raised or lowered based on the same set of sunrise and sunset times. Often, a 5,000-watt station will cut back to 1,000, or something like that. The little AM station I worked for was authorized to operate at reduced power after sunset, but was licensed for three watts, or seven watts, or something similarly ridiculous—I forget. That wasn’t enough to get the signal from the transmitter, which was located just outside of town, back to the studios downtown, so we didn’t bother.
Other AM stations “go directional” at sunset. This means their signal propagates in a specific direction, to protect the signal of another station elsewhere in the country on the same frequency. In Madison, for example, WIBA-AM’s signal goes primarily north and east at sundown — you can be a few miles southwest of the tower after sunset and have trouble picking it up, where during the day you’d get it five-by-five. Reduced power or a pattern change is why you would sometimes lose your favorite AM station at sundown.
I was once in the wilds of Minnesota, cruising down a deserted interstate highway, listening to a big AM station out of Winnipeg, Manitoba, early on a winter morning. In the middle of a song, something odd happened. The station’s audio cut out for a second, then cut back in again, briefly distorted, before returning to normal. Then the morning jock keyed his microphone, mid-song, and said, “Good morning, western North America.” In an instant, I realized what I had heard: the morning pattern change. It was officially sunrise in Winnipeg, and it was no longer necessary for the station to propagate its signal eastward to protect some station out west.
I was pretty sure I might be the only person other than the jock who truly understood what had just happened.
The biggest AM stations operate 24/7 at full power, often 50,000 watts, which is as big as AM stations in the United States can be. These signals are called “clear channels.” If you surf around the AM dial late at night, the distant stations you receive will almost certainly be clear channels.
The clear channel designations and all this power-reducing and pattern-changing date back to the dawn of radio, when the Communications Act of 1934 standardized the “wild west” that the AM band had become in the 1920s, to limit interference and make sure local stations serve local areas effectively. The regulations are bureaucratic, but the reasons for them have everything to do with the laws of physics.
If I have made any technical errors in this post, I am certain that those amongst the readership with first-class FCC radiotelephone operator’s licenses will correct me, and thereby educate the whole class.
Read more about why AM stations have to reduce power at night here. And stop back Sunday for a first in the history of this blog: a rerun.