A square loop antenna under even-mode operation is analysed and an approximate model based on equivalent multiple sources is developed to describe its radiation characteristics. The calculated results of the proposed model are validated by a few classical analytical models at first. The proposed model is then applied to analyse traditional linearly polarised (LP) and circularly polarised (CP) loop antennas. Afterwards, it is further used to design a new loop-dipole combined antenna in the step-by-step procedure. It is theoretically exhibited that a loop antenna under even-mode excitation can work together a conventional dipole antenna with odd-mode feeding to form a new complementary antenna with cardiac-shaped radiation pattern. Finally, a prototype antenna is designed, fabricated and measured at the 2-3GHz band. The operating fractional bandwidth of the proposed antenna can achieve up to 30% in both theory and measurement. In addition to that the proposed approximate model for the square loop antenna under even-mode operation is evidently confirmed, a new loop-dipole combined antenna is proposed and its predicted performance is found in good agreement with numerical and experimental results.