Complex sentence: Most transmitters at this were unable to precisely control their output frequencies, thus, signals from two stations operating on the same nominal frequency would combine to make a high-pitched "heterodyne" tone that interfered with the reception of both stations.
Split into simpler sentences: Most transmitters at this were unable to precisely control their output frequencies. Thus, signals from two stations operating on the same nominal frequency would combine to make a high-pitched "heterodyne" tone that interfered with the reception of both stations.