The maximum usable frequency (MUF) for paths from WWV to selected locations is calculated from the model ionosphere using the secant law and a single-hop path. The F region MUF (blue trace) is based on the model foF2 value (green trace) and height of the peak density. The E region MUF (red trace) is the maximum frequency supported in the 90-120 km range (which may not have a well-defined peak.) The overall MUF is the greatest of the E and F region MUFs. Note that the geometrical MUF generally overestimates the actual MUF due to refraction effects.