This paper addresses spectrum sharing issues with regard to High Altitude Platform (HAP)-based fixed service (FS) using Ka band (28/31 GHz). Currently, several strict restrictions have been imposed by the Radio Regulations on the use of the 28/31 GHz bands for the HAP-based FS because of concerns that the HAP-based FS could interfere in other services using the same-adjacent frequency bands. To facilitate the spectrum sharing/compatibility, this paper proposes a new HAP system model different from the model used in the current ITU-R study. Although the service coverage of the proposed HAP model is narrower than the current one, this model results in less interference from/to other services compared with the current model. This paper also deals with radiation patterns of HAP antennas which play an important roll in the interference evaluation. Measured radiation patterns of prototype HAP antennas are compared with the reference patterns which have been used in the ITU-R study. Good performance of the measured patterns is shown.