A built-in antenna apparatus for a electronic device is provided. The antenna apparatus comprises a PCB with conductive and non-conductive areas. An antenna radiator is disposed at the non-conductive area of the PCB; the antenna radiator has a feeding portion and at least a first radiating portion configured in a first pattern branched from the feeding portion and has an end portion electrically connected to the conductive area. At least one capacitor is electrically connected in series within the first radiating portion. A resonant frequency of the first radiating portion is a function of a capacitance value of the at least one capacitor. The antenna can be provided in a smaller size for a given frequency band due to the capacitance. A second antenna radiator branched from the feeding portion can also be provided for operation at a different frequency band.