Where there is activity on the sun causing HF blackouts, aurora and Es, it will affect 600 meters. I keep an eye on this site for solar activity: http://dx.qsl.net/propagation/propagation.html Years ago I used to keep notes in my log for A & K, particle density as well as solar winds and how they affected the 600m signals. There is a definite pattern. Mike-WE0H /16