1800-2000 KHz (1.8-2.0 MHz ) or the 160 m band
The top band, as it is sometimes caled, suffers from extreme daytime D-layer
absorption. Even at high radiation angles, virtually no signal can pass through the F
layer, so daytime communication is limited to ground-wave coverage. At night, the D layer
quickly disappears and worldwide 160 m communication becomes possible via F2-layer skip.
Atmospheric and man-made noise limit propagation. Tropical and midlatitude thunderstorms
cause high levels of static in summer, making winter evenings the best time to work DX at
1.8 MHz. A proper choice of receiving antenna can often significantly reduce the amount of
received noise while enhancing desired signals.
Source: ARRL Handbook, 1993