This paper introduces a 90-degree hybrid coupler for feeding a two-port antenna in a printed grating gap (PRGW) substrate to provide right-handed circular polarization (RHCP) and left-handed circular polarization (LHCP) in the frequency range of 27 to 29 GHz in the Ka band. The coupler is proposed for the Riblet model, in which in addition to the dominant mode, a higher-order mode can also be taken, while its general implementation in PRGW is presented, an example of it is designed that is compatible with return losses better than 20 dB and an isolation range better than 17dB. Amplitude response between input port and outputs is 3.2±0.3dB and phase balance is 90±1.5 degrees. The coupler is implemented as a feed of two antenna samples, for the first sample which was a 2x2 array of slot antenna based on the PRGW, the antenna gain is more than 11.9 and the AR is less than 1.8dB for both LHCP and RHCP, the return losses are more than 10 and isolation between inputs are more than 15dB. For the second sample the antenna gain is more than 8.9 and the AR is less than 1.4 for both CP polarizations and the return loss is better than 10 and the isolation between the input ports is better than 15 dB. The target application of both antennas is the frequency range of 27.5 to 28.35 GHz allocated by the FCC for 5G.