A wideband simultaneous transmit and receive (STAR) dual-layer circular array is considered to handle the space limitation and efficient use of wireless spectrum. The proposed STAR aperture configuration is composed of upper- and lower-layer circular arrays, where one works as transmitting and the other as receiving antenna. Each array consists of four wideband disc-cone antennas that are equally spaced on the circumference of a circle. The proposed STAR array utilizes both, mode diversity, and self-interference cancellation techniques to achieve "theoretically" infinite isolation over a wide operational bandwidth without any time, frequency, or polarization duplexing. Theoretical study is discussed under both ideal conditions and the presence of circuit components' non-idealities; and the feasibility thereof to significantly mitigate the self-interference is shown computationally and experimentally. Simulated isolation higher than 74dB and measured between 28 and 50dB is obtained over more than 2:1 bandwidth while still maintaining virtually identical TX/RX radiation patterns.