Antenna: RF, need help
It seems you came across an okumura hata (OH) propagation model
Base antenna is height of the antenna converted into ideal isotropic and it should inlcude the height where it will be installed.

OH model is an empirical model, meaning it will work properly if it used inside the model boundaries and you can squeeze 3 sigma standard deviation conformance at best.

however, however how low in the ground does your wireless sensor lie? OH model is bounded by 1 meter height until you encounter fading due to low aperture angles. Your three (3) morphology models may not work properly (terrain categories).

here are the links of the model

http://en.wikipedia.org/wiki/Okumura_model

http://en.wikipedia.org/wiki/Hata_model_for_urban_areas