I worked as a broadcast engineer for decades. When we calculate the amount of power and antenna height needed to send a signal of a certain strength to a certain location, the curvature of the Earth has to be taken into account. If the Earth were flat, then the signal dropoff with distance would perfectly follow the inverse square law (which states that field strength diminishes by 6 dB every time you double the distance from the antenna). But it doesn't follow the inverse square law. If it did, you could pick up FM stations from New York City in Lisbon with perfect clarity. When the curvature of the Earth, based on its sphericity, is included in the equations, then field strength predictions are accurate.