By geometrical arguments it is possible to explain some of the details of the dependence of Jupiter's decametric radio sources on the position of the innermost Galilean moon Io and Jovian System III longitude. We have adopted the conical beaming model, first presented by Dulk, which is now common to many of the present decameter-radiation theories. Detailed calculations of the beaming directions have been carried out assuming a source region located above the planetary surface along the Io-affected magnetic flux tube at a point where the electron gyrofrequency is approximately equal to the radiated frequency. Agreement with observed morphological structure is critically sensitive to the magnetic field. The octupolar magnetic field models resulting from the Pioneer 11 measurements are the basis for the present analysis. An apparent displacement of the flux tube 20¿ ahead of the orbital position of Io produces good agreement between the calculated and observed positions of the sources. The longitudinal extents of the three main sources can be approximated by limiting radiation to points where the perpendicular angle between the magnetic field vector at the point of emission and the Zenographic equatorial plane is at or near minimum. Other source morphology appears to result from high-frequency cutoff of emission when Jupiter's ionosphere interferes with the radiation process near the cloudtoops. Further cutoffs are possible when the source region is occulted from an earth-based viewpoint by the planetary surface. For lower frequencies the observations indicate that Io-dependent radiation is emitted at all longitudes. The beaming model concurs with this finding when all cutoffs are eliminated.