What is the main factor that limits the range of direct wave transmission?

The primary factor that limits the range of direct wave transmission is Line-Of-Sight (LOS). For direct wave transmission to occur effectively, there needs to be an unobstructed path between the transmitter and the receiver. This means that no obstacles, such as buildings, hills, or trees, should obstruct the wave's path.

When the LOS is clear, the waves can travel without significant interference, allowing for maximum signal strength and range. If the path is disrupted by any obstructions, the signal can weaken or become blocked completely, leading to reduced communication capabilities.

Other factors, such as weather conditions or the frequency of the wave, can also impact transmission, but they are secondary compared to the necessity of maintaining a clear visual or direct line between the transmitter and receiver for successful communication.