The occurrence probability of the Jovian decameter radio emissions depends on two essential parameters: the central meridian longitude (CML) and the orbital phase of the satellite Io. Four main zones of enhanced occurrence probability emerge from the CML-Io phase diagram: the so-called Io-controlled sources Io-A, Io-B, Io-C and Io-D. We study the compatibility of the location of these sources with the existence of a specific active longitude range, anchored in Jupiter’s magnetic field, and favoring the radio emissions. A theoretical model, based on the cyclotron maser instability (CMI), was proposed a few years ago in order to explain the existence of such active longitudes, assuming that the radiation was emitted at the local gyrofrequency in a hollow cone of constant angle, along a magnetic field line carried away by Io through its revolution around Jupiter. Unfortunately this model was not able to justify the dimension in longitude of all the Io-controlled sources, in particular those located in the Jovian southern hemisphere (Io-C and Io-D). We show that the azimuthal distribution of the four occurrence regions (Io-A, Io-B, Io-C and Io-D) around the gradient of the local magnetic field is not constant so that the emission cone (in each Jovian hemisphere) presents a significant flattening in the direction of the magnetic field vector. Introducing a beaming cone with an elliptical section makes the location and extension in longitude of the sources (in the CML-Io phase diagram) compatible with the existence of an active longitude. A theory of the CMI, acting in an inhomogeneous medium in which the magnetic field vector and the gradient of its modulus are not aligned, shall be required in order to justify the flattening of the emission cone.