In this paper, we consider cellular-connected unmanned aerial vehicle (UAV) networks, in which the UAVs are aerial user equipments (UEs) served by the ground base stations (GBSs). A realistic ground-to-air channel model, with line of sight (LOS) and non-LOS (NLOS) transmissions, is incorporated into the analysis. Three dimensional (3D) antenna radiation combing the vertical and horizontal patterns is taken into account for the GBSs. More importantly, 3D antenna patterns are also considered for the UAVs. In particular, we compare the performances of four types of antenna configurations, namely an omni-directional pattern, a doughnut-shaped sine pattern, a doughnut-shaped cosine pattern, a directional pattern with tilting angle toward the serving GBS. Via numerical results, it is demonstrated that the directional pattern with tilting angle provides the best signal-to-interference-plus-noise ratio (SINR) coverage probability. It is also shown that the optimal UAV altitude varies depending on the environment and antenna patterns.