At the stroke of midnight on Jan. 1, KDAN—broadcasting at 91.5 FM to Marshall and the western shore of Tomales Bay—faded into static. Gone was the eclectic lineup of everything from folk to rockabilly to Dixieland to bossa nova, leaving Jeff Cotton, a straight-shooting sound engineer who has built small-town radio stations across the West, to mourn the silence. KDAN’s low-power signal emanated from a pump shed on the Barinaga Ranch, where Mr. Cotton, who…