Abstract: The feasibility of making two antennas work with a shared aperture on a platform is investigated. The aperture is formed by a deep cavity with a dielectric radome. A Quadrifilar Helix Antenna (QHA) is adopted for the platform positioning and a horizontal monopole using the cylindrical wall of the cavity as its ground is adopted for communication. In order to tune the QHA in the cavity, an improvement is proposed by utilizing the bottom of the cavity and the horizontal branches of the four helices. The effect of the cavity on the antennas and the interaction of the two antennas are studied through experiments. The results show that the two antennas of the overall arrangement perform simultaneously well in the cavity and the mutual coupling effect expands the bandwidth of the horizontal monopole.