An electromagnetic ranging source includes: a tubular body having a circumference; a ferromagnetic core conforming to the tubular body and extending at least halfway around the circumference to define a gap with circumferentially-spaced poles; one or more current-carrying windings around the core to induce a magnetic field between the poles; and a downhole power source that drives the one or more current-carrying windings. A related method includes: equipping a first drill string with such an electromagnetic ranging source; extending a first borehole with the first drill string; assembling a second drill string with sensors; and drilling a second borehole while using said sensors to guide the second drill string relative to the first borehole.