Posted on April 21, 2009
Despite forty years working for radio stations, admittedly mostly FM, I never heard of “critical hours” until messing with the Part 15 hobby. Those are two hours after sunrise and two hours before sundown when the sun angle is relaxing and the ionosphere begins to lift, causing certain stations to skywave across the horizon only to vanish. Critical hours seem to be especially effective on the upper AM band. My main frequency of 1550 KHz is clear daytime and after dark, but during the critical hours no less than three stations come to life at various power levels depending on the day and weather. They all have daytime power of 5 kw, and come from Cape Girardeau, Springfield and Kansas City, all in Missouri. Perhaps they only vanish at night because they drop to a few hundred watts. So I am planning to add a timer which will switch up to 1640 KHz during the critical hours.
At 1640 KHz there’s also somebody two states away that creeps in during the critical hours, but the higher the frequency the more brief is the appearance and disappearance of the distant signal. Sometimes a signal will jump from total static and become as loud as a local, only to vanish within another fifteen seconds.