Software-defined radios are great tools for the amateur radio operator, allowing visualization of large swaths of spectrum and letting hams quickly home in on faint signals with the click of a mouse.

WINCHESTER — Tim Angell twists a dial on his thick, black Yaesu radio, and hears a hiss, a crackle and a tone rising and falling like a slide whistle. He twists again, and the radio spits out rapid ...