The Static-y Allure of AM Radio . . . and How it All Works

by John C. Alsedek:

Most of us—particularly those of a certain age—have had one of those moments. For you, maybe it was while you were sitting in your grandparents’ living room in Baltimore on some rainy Sunday afternoon, listening to the Boston Red Sox hosting the Yankees. Maybe it was at 2 a.m. as you were driving cross-country with nothing but the voice of Art Bell to keep you awake as he took calls on “Coast to Coast.” Or maybe it was like it happened to me, an eight-year-old discovering the world of radio drama on a tiny transistor radio late at night.

What I’m talking about is the magic of AM radio—that crackly, distorted vestige of what the medium used to be. In the age of crystal-clear digital broadcasts, AM radio is as old-fashioned as a Victrola—and just as scratchy. Yet it endures and holds a fond spot in the hearts of many. But just what is it about AM that makes it so nostalgically endearing?

Well, to understand that, you have to understand how AM radio works. “AM” stands for “amplitude modulation,” the means by which AM programming is broadcast. Radio waves in general carry energy in the same way that ocean waves do: via up-and-down movement. Only in the case of radio waves, it’s electricity and magnetism, not water, that’s in motion. To broadcast a radio program, the program is added to a radio signal by means of a carrier wave; this is known as modulation and happens with both AM (amplitude modulation) and FM (frequency modulation). The difference between AM and FM is how the carrier wave is altered; in AM broadcasting, it’s the peaks of the carrier wave that are altered. To use a real-life example of how this works, imagine that you’re sitting in a canoe on a lake, gentle waves lapping onto the shore—those are your “radio waves.” You decide that you want to send a message to a friend standing on the beach, so you write a note and put it in a bottle—your “radio program.” To propel your bottle on its way, you begin to rock the canoe to create more impetus for the bottle—that’s your “carrier.”

Okay, now let’s continue this example to show the major problem with AM radio, and what also gives it that signature crackle. As you send your bottle on its merry way to shore, a couple of kids jump into the lake and start splashing around. The splashing breaks up your carrier waves—not enough to keep your bottle from reaching shore, but enough to muck things up a bit. And that’s what happens to AM broadcasts (FM too, but not to the same extent). As they leave the radio transmitter on their way to your home or car radio (the “receiver”), the broadcast runs into all sorts of interference: natural or man-made obstacles, radio signals on nearby frequencies, and other sorts of electrical signals. The result? The “AM sound.”

That explains the sketchy audio quality that AM is known for. But what about the other well-known trait of AM: its long reach? There are plenty of anecdotes of people tuning in to faraway cities at night to listen to a favorite sports team or famous DJs like Cousin Brucie and Wolfman Jack. Well, the secret is in two words: at night. Because what makes it possible to hear AM stations at long distances is a phenomenon known as skywave.

Skywave is the effect that occurs when radio waves are reflected back downwards off the electrically charged layer of our atmosphere, called the ionosphere, instead of traveling near the Earth’s surface (groundwave). Such “bounces” can stretch an AM station’s footprint from less than one hundred miles to hundreds—or occasionally even thousands—of miles; FM stations, since they operate on higher frequencies than their AM counterparts, don’t get the skywave bounce and are pretty much line-of-sight in terms of coverage. But the skywave phenomenon only works at night, which is the reason for this . . .

FCC regulations require AM radio stations to do one of three things at night: lower their power levels drastically, change from an omnidirectional broadcast to a directional one, or cease broadcasting entirely. If not for those regulations, there would be massive levels of interference as stations that were normally well outside of one another’s listening area suddenly found themselves overlapping and overriding each other’s signals.

So, the next time you’re cruising a lonely stretch of road late at night and the only stations you can pick up are over on the AM band . . . now you know why. Have fun trying to figure out where they’re broadcasting from!

Next time, we’ll be going way, way, way back to the very first radio dramas! Till then, thanks for reading!