Handheld electronic devices are provided that contain wireless communications circuitry. The wireless communications circuitry may include an antenna. The antenna may be formed from a ground plane having a dielectric-filled slot that defines a slot antenna structure and having a planar-inverted-F (PIFA) resonating element located above the opening. The slot antenna structure and the PIFA resonating element may both contribute to the performance of the antenna, so that the antenna exhibits the performance of a hybrid PIFA-slot antenna. The PIFA resonating element may contain multiple antenna resonating element branches. The branches may be configured to operate in different communications bands than the slot antenna structure.