Uncrewed aerial vehicles (UAVs) serving as aerial base stations (ABSs) are expected to extend 6G millimeter-Wave (mmWave) coverage and improve link reliability in urban areas. However, UAV-based air-to-ground channels are highly dependent on height and urban geometry. This paper proposes an ABS height-dependent mmWave channel model and investigates whether urban geometry, beyond the standard built-up parameters, significantly affects line-of-sight (LoS) probability ($P_{\text{LoS}}$) and large-scale fading (LSF). Using MATLAB ray tracing (RT) at 26 GHz, we simulate approximately 10K city realizations for four urban layouts that share identical built-up parameters but differ in their spatial organization. We extract elevation-based $P_{\text{LoS}}$ using a sigmoid model and derive height-dependent path-loss exponents (PLEs) and shadow-fading trends using exponential fits. Results show that PLE for non-LoS decreases toward $2.5$–$3$ at high altitudes, LoS PLE remains near $2$, and shadow fading reduces with height. We also find that geometric layout introduces a modest but consistent change in PLE ($\approx \pm 0.2$), even when built-up parameters are fixed. The proposed model matches RT results and provides a practical, height-dependent LSF model for ABS planning in urban scenarios.