Need some help getting started... Two radio transmitters are placed 50 m apart on either side of a runway. The two transmitters will broadcast the same frequency, but out of phase with each other. This will cause a nodal line to extend straight off the end of the runway. As long as the airplane's receiver is silent, the pilot knows she's directly in line with the runway. If she drifts to one side or the other, the radio will pick up a signal and sound a warning beep. To have sufficient accuracy, the first intensity maxima needs to be 60 m on either side of the nodal line at a distance of 3.0 km. What frequency should you specify for the transmitters? I know that there will need to be completely destructive waves. And for that, ΔL/λ=0.5, 1.5, 2.5.... But in order for her to be directly in line with the runway, ΔL=0, so this would not be true. Also, I think this, ΔL/λ=0.5, 1.5, 2.5, is only true when Φ=0, 2π, 4π, etc., but Φ will need to be equal to π for being completely out of phase. I can't see how knowing this helps because ΔL to be directly in line with the runway would still be equal to zero, or is my thinking completely flawed?