Free Space Path Loss
Whenever an RF signal is transmitted from an antenna, its amplitude decreases as it travels through free space. Even if there are no obstacles in the path between the transmitter and receiver, the signal strength will weaken. This is known as free space path loss.
What is it about free space that causes an RF signal to be degraded? Is it the air or maybe the earth’s magnetic field? No, even signals sent to and from spacecraft in the vacuum of outer space are degraded.
Recall that an RF signal propagates through free space as a wave, not as a ray or straight line. The wave has a three-dimensional curved shape that expands as it travels. It is this expansion or spreading that causes the signal strength to weaken.
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