Thanks Paul that was a great refresher and seems to confirm my general rule of thumb. At night when operating on 10Mhz you would expect a range of approximately 2, 500 miles because 10, 000 ÷ 4 = 2, 500. Where as during the day on the same frequency you wound expect a range of approx 1, 250. It's not the most accurate calculation but it gives you an idea of what frequency you need if you want to reach a specific country for example using sky wave.