Communication Systems: JEE Main 2023 Physics question

A transmitting antenna is kept on the surface of the earth. The minimum height of receiving antenna required to receive the signal in line of sight at $4 \mathrm{~km}$ distance from it is $x \times 10^{-2} \mathrm{~m}$. The value of $x$ is __________. (Let, radius of earth $\mathrm{R}=6400 \mathrm{~km}$ )