The main problem with short wave operation was fading and weak signals. Fading occurs when the ionosphere spreads a signal into two or more paths which are then received at different times or phases.
In 1935, in an effort to reduce the effects caused by multi-path propagation, Bell Telephone Laboratories commenced work designing a Multiple Unit Steerable Array (MUSA).
The MUSA system provided a steerable beam in the vertical plane and therefore could discriminate between multiple incoming waves to defeat multi-path distortion.
The MUSA antenna employed a phased array of 16 rhombic antennas together with a very sophisticated receiver.