The successful performance of a high-frequency (HF) over-the-horizon radar (OTHR) is dependent upon the environment in which the radar must operate. This study investigates measured power levels of noise and signal characteristics in the HF environment of an operational OTHR system located in Amchitka, Alaska. These measurements were acquired by the Navy's AN/TPS-71 radar (ROTHR) spectrum monitor and represent a new source of HF environmental measurements for noise characteristics, channel availability, and expected duration of various channel widths as a function of frequency, time of day, and season of year. The analysis and results presented are based on HF environmental measurements obtained over the 5--28 MHz frequency band by a receiver with a very low noise figure, an efficient antenna, high dynamic range, and which was seldom internally noise limited. The analysis of this data has provided information on power levels associated with noise and HF users in the geographic region of the radar. Clear channel availability results are based on a threshold level defined by empirical distribution functions (EDF) of the measured power levels from the ROTHR site in Amchitka, Alaska.