An institution of the airwaves in Los Angeles and beyond, he capitalized on the freedom the FM band offered in the 1970s to blaze his own path.Jim Ladd, a maverick Los Angeles disc jockey who helped pioneer free-form FM radio in the 1970s, and who went on to become a rock institution and an inspiration for Tom Petty’s song “The Last DJ,” died on Dec. 17 at his home near Sacramento, Calif. He was 75.The cause was a heart attack, his wife, Helene Hodge Ladd, said.With his laid-back manner and his considerable equestrian skills, Mr. Ladd was known to longtime listeners as the Lonesome L.A. Cowboy, after a 1973 song by the New Riders of the Purple Sage. His expansive musical knowledge, saucy humor and outspoken political views made him a celebrity in rock circles — not only in Los Angeles, where he had storied runs at KLOS and KMET, but also nationally, thanks to his long-running hourlong syndicated series, “Innerview.”“Innerview,” which made its debut in 1974, featured interviews with countless rock luminaries, including the Beach Boys, Led Zeppelin and Elton John. It was heard on some 160 stations around the country.The same class of rock deity could often be found lounging around Mr. Ladd’s treehouse-like home perched on the wooded hillsides of Laurel Canyon. His house drew friends like Stevie Nicks, George Harrison and Roger Waters of Pink Floyd, who featured Mr. Ladd on his second solo album, “Radio K.A.O.S.” (1987).More interested in challenging listeners with new sounds than spinning the same old chart-toppers, Mr. Ladd was well suited to the early days of free-form radio, which was made possible by a 1964 Federal Communications Commission rule preventing AM stations from repeating more than 50 percent of their formats on commonly owned FM stations in a single market.Mr. Ladd was said to be an inspiration for the Tom Petty song “The Last DJ,” an indictment of commercial radio.This allowed countless D.J.s like Mr. Ladd, on stations around the country, to shatter the Top 40 format on FM and take control of their own programming in an era when experimentation in rock was ascendant and rock itself was hailed as a force for social change.“Free-form radio was an approach to the music, and the show itself, which resulted in a highly personal and completely spontaneous new art form,” he wrote in his 1991 memoir, “Radio Waves: Life and Revolution on the FM Dial.”“Most of us never thought of it as a job,” he wrote. “A job was something ‘straight people’ did to earn ulcers. For us, it was more of a calling. We were guerrilla fighters for a generation of creative explorers, inmates who took over the asylum for just one purpose — to play with the public address system.”Mr. Ladd got his first access to this public address system in the late 1960s at KNAC in Long Beach, Calif., where he challenged listeners’ ears by playing the latest underground tunes and challenged authorities with his political passions, for example by stacking songs like “Universal Soldier” by Donovan, “The Unknown Soldier” by the Doors and “I Don’t Wanna Be a Soldier Mama I Don’t Wanna Die” by John Lennon as a musical protest against the Vietnam War.“The music at that time was filled with radical new ideas and a unique generational perspective,” Mr. Ladd wrote. “Alternative points of view not heard on the six o’clock news came through the music loud and clear. Songs about the peace movement, civil rights, Vietnam, drugs and the generation gap — and massive quantities of sex.”James William Ladd was born on Jan. 17, 1948, in Lynwood, Calif., the oldest of three children of Obie and Betty Ladd. His father was a bank loan manager who won three bronze stars as a medic in World War II; his mother was a banker.Mr. Ladd was honored with a star on the Hollywood Walk of Fame in 2005.Lucy Nicholson/ReutersHis family moved to Vacaville, Calif., near Sacramento, when he was a child. After graduating from Vacaville High School, he returned to Southern California to study at Long Beach City College before joining KNAC.Mr. Ladd spent the early 1970s at the powerhouse Los Angeles rock station KLOS before moving to a rival station, KMET, where he remained until 1987, when the station changed its format and began showcasing smooth jazz. In his book, he derided the new sound as “a computer-programmed Valium tablet, dentist-office music for yuppies.”Even as FM rock stations moved toward more rigid playlists in the 1980s, Mr. Ladd fought to maintain his independence, in both music and message, often running afoul of station management. With his outspoken ways, he was said to be an inspiration for the 2002 Tom Petty song “The Last DJ,” an indictment of commercial radio that featured lyrics like “Well, the top brass don’t like him talking so much/And he won’t play what they say to play.”In the liner notes for the album of the same name, Mr. Petty thanked Mr. Ladd for “his inspiration and courage.” “Let’s say it may have been partially inspired by me,” Mr. Ladd said in a 2015 video interview.“I don’t want to say it’s about me,” he added, “but I am very, very honored, obviously.”Mr. Ladd made stops at multiple stations over the years. In 2011 he joined SiriusXM satellite radio, where he was a host on the Deep Tracks channel. He remained there until his death.In addition to his wife, Mr. Ladd is survived by a brother, Jon, and a sister, Veronna Ladd.In a 2000 interview with The Los Angeles Times, when Mr. Ladd was back at KLOS, he broke out a handful of papers: the station’s playlist schedule, which mapped out the songs to be played over the course of the day — until his slot at 10 p.m., which remained blank. As in the old days, he could play what he chose. The only thing listeners could count on was Mr. Ladd serving up his trademark catchphrase, “Lord have mercy.”When asked why he was allowed to follow his own muse when other D.J.s at the station were not, Mr. Ladd responded, “Stubbornness, stupidity, doggedness.”The station’s program director, Rita Wilde, quoted in the article, offered a different take: “Not that many people, if you gave them the freedom, would know what to do with it.” More