L$8-1[15$ marks$]$ An airplane flying in the sky uses an antenna to communicate with air traffic control tower locating $20$ km $($point$-$to$-$point distance$)$ away from the airplane. The antenna has a time$-$averaged total power of $50$ kW and works at the frequency of $118$ MHz$.$ Assume the radiation intensity of the antenna is uniform in all directions. Determine the amplitude of the electric field signal received by the control tower.