GEOMETRY OF EXPERIMENT. The HF heater at Platteville is shown by concentric circles indicating approximate beam widths at altitudes of 60 km (solid) and 120 km (dashed). Transmissions from Ft. Collins, North Dakota, and Longmont were received, after passing through the heated volume, at the receiving stations indicated by open circles. On the path from Ft. Collins to Bennett, cross modulation was detected on 4 frequencies from 60 kHz to 5 MHz.
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