A quaternary artificial magnetic conductor (AMC) is proposed in this letter, and we design broadband circularly polarized antenna on this basis. The main innovation of this work resides in the way of realizing ultra‐wideband radar cross‐section (RCS) reduction. The quaternary AMC has different scattering mechanisms in the low and high bands. In low band, since the AMCs' reflection phases in diagonal are very close, the quaternary AMC can be regarded as a conventional binary checkerboard AMC. But with the variation of frequency, the different reflection phases in the high band will cause the quaternary AMC to be a surface structure like a coding diffuse metasurface. Ultimately, RCS reduction of more than 10 dB is realized from 7.5 to 21.3 GHz, where the bandwidth approximately accounts for 95.83% of the center frequency. Thanks to the proximity coupling, the AMC array around the original antenna, as the parasitic patch, plays a significant role in regulating and improving the axial ratio. The measurement and simulation results demonstrate that the proposed antenna obtains an axial ratio of <3 dB from 8.95 to 11.07 GHz, the impedance bandwidth (ie, VSWR ≤2) ranges from 8.45 to 11.80 GHz.