This paper describes several aspects of the physical layer and over the air interface of Loon. Loon utilizes stratospheric balloon-based high-altitude platforms (HAPs) that use Long-Term Evolution (LTE) to connect people with standard User Equipment (UEs) to the Internet. In particular, topics covered include the Loon prototype eNodeB (eNB) antenna pattern, the observed channel, UE battery life, and coexistence with terrestrial networks using the same spectrum. While channel models from a HAP to the ground have been well studied in the past, the use of polarization diversity to establish Multi-Input Multi-Output (MIMO) communication to real UEs below 1 GHz has not. In addition, a theoretical analysis of terrestrial coexistence and an analysis of the estimated impact on UE battery life when communicating with HAPs are presented. Finally, results from several measurement campaigns and from experiments with polarization diversity are presented as a spot check of theory.