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High-frequency oblique transmissions made for a short period of time during the maximum phase of the past solar cycle are compared with the frequencies predicted utilizing the ITS-78 and the IONCAP (ionospheric communications analysis and prediction) ionospheric prediction programs developed by the Institute for Telecommunication Sciences. The external atmospheric noise levels predicted by the ITS-78 program are also compared with experimental data. The measurements conducted in the northeastern U.S. reveal that, for undisturbed solar-geophysical conditions, the experimental data are in good agreement with the theoretical predictions.