The design of X/K-band shared aperture antenna (SAA) single-linear polarized K-band, dual-polarized (linear/circular) X-band 36-elements (6×6) planar array antenna with high gain, and low cost for airborne synthetic aperture radar systems is presented. To reduce the complexity and size of the feed networks, the concept of series-feed centre-fed open stub array with Chebyshev amplitude distribution of - 25 dB side-lobe levels (SLL) are employed for K-band design. Two typical frequencies, X-band and K-band of 9.65 and 21 GHz, are chosen (frequency ratio of 1:2.176) in this proposed SAA design on a single-layer printed circuit board. For K-band design, SLL can be controlled by Chebyshev distribution. The amplitude is controlled by the width of the individual patch in the linear array with left and right symmetries. A 50-Ω microstrip line with 0.7λ spacing is used in between the patches for the ±25° scan angle requirement. The measured results are consistent with the simulations and provide a matching impedance bandwidth of 10.36/ 1.45% at X/K-bands, respectively. The array also has a high aperture efficiency of more than 85%, SLL of (X/K-bands: - 18.9/- 25 dB), and gain of (X/K-bands: 24.2/17.4 dBi). © 2020 The Authors.