[en] In this thesis a study of the ion beam behaviour in the central region is described for the AVF cyclotron of the Eindhoven University of Technology. A short survey of experimental results and methods is given. Measurements with the beam diagnostic equipment are described together with extensions of this equipment: with the HF phase measuring system the HF phase of the beam can be measured with an accuracy of 0.5⁰ at a current of 50 nA, with the external HF phase probes the beam position can be determined with an accuracy of 0.5 mm for a current of 10 nA, and the energy can be determined with a time of flight measurement with a relative accuracy of 10^-4. With the time structure system the time resolution can be measured with an accuracy better than 1⁰. With the beam scanners a beam position measurement with an accuracy of 0.1 mm for a minimal current of 1 nA is achieved. (Auth.)