This paper presents a novel circularly-polarized simultaneous transmit and receive (STAR) monostatic antenna system based on the four-arm spiral-helix antenna. The four-arm spiral is configured such that two arms are used for transmitting and the other two arms for receiving. Microstrip feeds with impedance transformers are used to feed the two two-arm spirals and thus eliminate the need for 180° hybrids. Helix termination is used to improve the low-end performance. The symmetric features of the spiral antenna along with the arms orientation and the applied excitation enable a high isolation between the transmitting (TX) and receiving (RX) ports. Isolation>36dB is measured over >8∶1 bandwidth. Symmetric and consistent TX/RX radiation patterns are obtained over the operating bandwidth.