Satellite radio is the open secret of the new media. If you're one of the 17 million Americans who owns a satellite-equipped car or home receiver, you have access to a staggeringly diverse variety of round-the-clock programming that ranges from reggaeton and Howard Stern to Frank Sinatra and "The Shadow." Yet for most of the rest of us, satellite radio is still barely more than a whispered rumor. But now that FCC chairman Kevin Martin has given a thumbs-up sign to the merger of XM and Sirius, the two U.S.-based satellite services, the chances that satellite radio will finally become a major media player have taken an upward tick -- meaning that you may be on the verge of discovering "Theme Time Radio Hour," the most interesting radio show to hit the airwaves in decades.

"Theme Time Radio Hour" is heard on XM's Deep Tracks channel every Wednesday at 10 a.m. EDT, then repeated several times each week on various other channels. The host is none other than Bob Dylan. Yes, that Bob Dylan. Not that he has to vouch for his identity on the air: The raspy, nasal honk of his voice is instantly recognizable to anyone who knows anything about American popular culture. So is the fascinatingly wide-ranging musical sensibility that informs his program, which was launched two years ago and has racked up 75 episodes to date. Each week Mr. Dylan plucks a topic out of the air -- colors, trains, death and taxes, spring cleaning -- and plays recordings of a dozen songs whose lyrics relate to it in some way. In between songs he chats about the music and its makers, interspersing his gnomic mini-lectures with a cornucopia of old radio-station promos, celebrity vignettes and phony phone calls and email readings.

On a recent episode devoted to doctors, Mr. Dylan played, among other things, Jackson Browne's "Doctor My Eyes," B.B. King's "Walking Doctor Bill," Doc Pomus's "Send for the Doctor," the Rolling Stones' "Dear Doctor," the White Stripes' "Girl, You Have No Faith in Medicine," an obscure 1955 calypso song by Lord Lebby called "Dr. Kinsey Report," and "Hadacol Boogie," a jumping ditty recorded in 1949 by Bill Nettles and the Dixie Blue Boys whose subject was the once-celebrated patent medicine touted by its maker as a cure-all for "stomach disturbances, gas, heartburn, indigestion, nagging aches and pains, and certain nervous disorders."

Mr. Dylan's crisp, pungent commentaries were as listenable as the songs he played. Toward the end of the show, he introduced a gospel number by the Five Blind Boys of Mississippi by gently chiding listeners who turn up their noses at songs on religious themes: "Any time people sing about what they believe, it elevates it. You don't have to be a junkie to enjoy the Velvet Underground song 'Heroin.' You don't have to have horns and a pitchfork to enjoy 'Sympathy for the Devil,' but it does help. The thing is, it's all music, and when the people believe what they're singing, it's just better."

Part of what I find so engaging about "Theme Time Radio Hour" is that it flies in the face of the conventional wisdom about radio in the 21st century. Teenagers and college graduates are less likely to listen to radio nowadays, a decline that media consultants attribute to the rise of the iPod, which allows its owners to choose from thousands of previously downloaded songs at will instead of settling for whatever a disc jockey cares to play. The assumption is that under-40 listeners are now choosing to withdraw into gated communities of musical taste, behind whose electronic walls they listen only to what they already know they like. That's how most of the hundreds of existing satellite-radio channels work. Each one is devoted to a narrow stylistic sliver -- show tunes, New Age, old-school hip-hop, even 24/7 Led Zeppelin -- so that when you tune it in, you know just what you're getting. Not so "Theme Time Radio Hour," which gives you what Mr. Dylan thinks you ought to get. Nor is his taste predictable: He likes nothing more than to throw musical curve balls, and if you don't like the song he's playing now, all you have to do is wait three minutes for the next one to come along.