###### Abstract  
This paper proposes an S/X-band single-layer shared-aperture array antenna for the multifunction radars of military ships. A unit cell of the proposed antenna consists of one S-band element and four X-band elements. The S- and X-band elements are printed on the same layer to prevent a blockage effect by upper elements in the stacked shared-aperture antenna. Herein, the S-band element has a mutual complementary configuration for the X-band elements. In addition, the unit cell of the proposed antenna is designed in a symmetrical structure, which can be flexibly extended to a full array configuration. To verify the antenna feasibility, antenna performances are measured in a full anechoic chamber. The fractional bandwidths of the S- and X-band elements are 13.6% and 13.4%, respectively. Moreover, in the 2 \(\times\) 2 array configuration, the S-band array gain in the bore-sight direction varies from 5.4 dBi to 3.5 dBi when the main beam is steered from 0\({}^{\circ}\) to 45\({}^{\circ}\). Under the same conditions, the measured X-band array gain in the bore-sight direction decreases from 13.4 dBi to 11.6 dBi.  
mutual complementary configuration; single-layer antenna; shared-aperture antennas; array antennas +
Footnote †: journal: Applied  
In this paper, we propose an S/X-band single-layer shared-aperture array antenna for the MFRs of military ships. A unit cell of the proposed antenna consists of one S-band element and four X-band elements. The S- and X-band elements are printed on the same layer to prevent a blockage effect by upper elements in the stacked shared-aperture antenna. Herein, the S-band element should have a small radiator to avoid physical interference with the X-band elements, which cause the narrow bandwidth characteristics of the S-band element. Therefore, a mutual complementary configuration for the X-band elements is applied to the S-band element, which maximizes the area for printing the radiator and improves the antenna bandwidth. To further improve the antenna bandwidth, the S-band element is separated into one feeding patch and a pair of parasitic patches. This configuration has small gaps between the feeding patch and the parasitic patches so that the antenna bandwidth can be improved by adjusting the gap size. The X-band elements are rectangular patch antennas, and the dielectric constant and thickness of the antenna substrate are determined to have a low physical interference with the S-band element and a wide antenna bandwidth of the X-band elements. In addition, the unit cell of the proposed antenna is designed in a symmetrical structure, which can be flexibly extended to a full array configuration. The proposed shared-aperture antenna with the single-layer configuration has broad bandwidths while avoiding the blockage effect compared to other previous work. Thus, this antenna design is suitable for the MFR system of military ships. To verify the antenna feasibility, antenna performances such as reflection coefficients, boresight gains [18], and radiation patterns are measured in a full anechoic chamber. The proposed unit cell is extended to a 2 \(\times\) 2 array configuration to examine the array gains and beam-steering performance.