In this paper, millimeter wave end-fire relay antennas for fifth generation (5G) full duplex (FD) selfinterference (SI) suppression are designed. The proposed relay antennas are made of two orthogonally polarized substrate integrated waveguide (SIW)-based antenna elements, namely an SIW horn antenna and an SIW antipodal tapered slot antenna (ATSA). The employed elements not only generate high isolation between transmitting and receiving ports due to the polarization diversity but also maintaining a very low profile, which is very promising for the future vehicular, missile platforms, and etc., where conformal capability is highly desired. Results indicate that the proposed relay antennas operate with a 10.5% impedance bandwidth ranging from 14.4 to 16 GHz. Above all, the minimum isolation between the antenna pairs with a 0.05 λ₀-thickness substrate is 40 dB across the bandwidth.