Linear-to-circular polarizers can play an important role in the antenna systems for satellite communications.In view of the requirements of K/Ka-band satellite communications，in this paper，a dual-broadband linear-to-circular polarizer is proposed，in which the antenna-filter-antenna （AFA） structure is adopted and the dual-band receiving and transmitting antennas are independent.In particular，the working bandwidth can be expanded by stacking the patches，resulting in a unit cell size of 0.4λ_K×0.37λ_K×0.27λ_K.With the polarizer，an incident x-polarized wave can be converted into a right-hand circular-polarized （RHCP） wave at the K-band，whereas an incident y-polarized wave can be converted into a left-hand circular-polarized （LHCP） wave at the Ka-band.A polarizer prototype is fabricated and tested.The measured results indicate that the operating bandwidth of the polarizer is 9.8% （18.3~20.2 GHz） and 7.8% （28.5~30.82 GHz） within the incidence angle ranging from 0° to 25°.