Tightly coupled phased array antenna is a crucial form of ultra-wideband phased array antenna, which mainly consists of radiating elements, coupling capacitors between adjacent radiating elements, a wide-angle impedance matching layer on the array surface, and a reflective ground beneath the array. Its working principle is to decrease the resonant frequency of dipoles and counteract the inductive effect of the ground via the coupling capacitance between units, thereby attaining a low-profile and ultra-wideband impedance matching. Employ an equivalent circuit in conjunction with a Smith chart to elucidate the impedance frequency characteristics and physical significance of the various components within a tightly coupled phased array antenna. The functions and roles of different components of the antenna are deeply analyzed, and the key issues are summarized to provide theoretical guidance for the design of tightly coupled phased array antenna. Domestic and foreign design cases are summarized to provide experience for the theoretical design and engineering practice of this type of antenna.