Abnormal signal strength depressions on 4 GHz radio paths over a coastal plain

Radio propagation measurements at 4 GHz over two paths on a coastal plain in South Australia have shown occasional periods of prolonged and very severe signal strength depression. The two paths are end-to-end and each about 27 miles long, with effective transmitting and receiving aerial heights of 245 feet. The depression occurs earlier, and more severely, on the west path.