What does energy coupling describe regarding distance?

• A. Energy coupling increases with distance
• B. Energy coupling decreases with distance squared
• C. Energy coupling is independent of distance
• D. Energy coupling increases with distance squared

Answer: (B)

Energy coupling describes how much RF energy moves from a source to a receiving element as distance changes. As you pull the transmitter and receiver farther apart, the radiated energy spreads over a larger area, so the energy that the receiver can capture falls off. In free space, the power density decreases with the square of distance, so the coupling to the receiver scales roughly as 1 over distance squared. That’s why energy coupling gets weaker with increasing distance. Near-field situations can behave differently, but for typical RF propagation, the inverse-square relationship is the guiding idea.