A compact rectangular folded antenna for WBAN applications is presented in this paper. The Theory of Characteristic Modes (TCM) has been applied to understand the radiation mechanism of this kind of antenna, and to identify the radiating modes that are excited. This in-depth knowledge of the radiation mechanisms given by TCM has allowed the design of a compact antenna with broadband characteristic and very stable unidirectional radiation pattern for medical applications operating a 5.8 GHz ISM band. Unidirectional radiation is obtained by combining two particular characteristic modes with a phase difference of 90°. The paper demonstrates that it is not necessary to use very complex geometries to obtain antennas with broad bandwidth and unidirectional radiation pattern. The proposed antenna can be easily scaled to any frequency. For the 5.8 GHz band, a −10 dB impedance bandwidth of 45.48%, while keeping the unidirectional radiation characteristic.