Experiments are reported on a Josephson soliton oscillator designed as an element of a coplanar strip resonator. We have distinguished experimentally between the effects of uniform (electric) and boundary (magnetic) coupling between the junction and the resonator. Detailed structure in the measured current-voltage characteristics and microwave response show that microstrip modes are excited by uniform coupling, while the main resonance is associated with the boundary coupled coplanar mode. Resonances internal to the junction play no role.