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The results of a number of short-term oversea measurements of the variation of received signal level with range from a 10Gc/s transmitter are presented. It is shown that the variation of signal level within the horizon was rarely that expected for propagation through an atmosphere having a uniform refractive-index gradient. Signal losses of from 5 to 30dB frequently occurred well within the horizon, these losses being recovered when the range between transmitter and receiver was sufficiently reduced. A well-defined interference pattern usually occurred in the region of reduced signal level. Some data on the variation of refractive index with height up to about 700ft above sea-level were gathered using a radio sonde and a captive balloon, but the detail was not sufficiently fine to enable a direct relationship to be established between signal losses within the horizon and the occurrence of irregularities in the refractive-index profile at low elevations. A direct relationship was found to exist between the signal level within the horizon and that propagated well beyond the horizon into the extra-diffraction region.