The two fundamental propagation paths in the spreadsheet are the Free Space path and the Flat Earth path. There are additional entries to account for multipath, obstruction, and penetration losses, which are common inside buildings or on urban streets. Maxim's ISM-RF products are used in parking lots, streets, open areas, and buildings where the radio height above ground is small. This means that the Flat Earth path model is usually best suited for estimating the link budget. In applications where both Tx and Rx are located on towers or rooftops, and the antenna beams are narrow, the Free Space model would be more relevant