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This paper considers the bandwidth characteristics of the propagating medium in tropospheric beyond-the-horizon propagation. To study this problem, a frequency-sweep experiment was performed over a 171-mile experimental circuit. A 4.11-kmc transmitter was frequency modulated at a 1000-cps rate over a 20-mc band. The receiver was swept nonsynchronously over the same band at a 30-cps rate. The resultant pulses were displayed on an oscillograph and photographed at the rate of one frame every two seconds. The experiment used a 28-foot transmitting antenna and 8-, 28- and 60-foot receiving antennas. Sequences of selected sweep-frequency pictures are shown for various antenna combinations and transmission conditions. The bandwidths from the experiment are compared with a calculation based on the common volume geometry. Photographs of signals received simultaneously from a twin-feed horizontal diversity system are also shown and discussed.