[Elecraft] Thinking out loud again...

[Ron D'Eau Claire]

"Critical frequency" is defined as the lowest frequency at which a
signal beamed straight up will still be refracted straight down by the
ionosphere.  Signals beamed straight up which are at a
higher-than-crticial-frequency will penetrate the ionosphere.

The critical frequency varies with time of day and solar activity, of
course.

73, de Earl, K6SE

That's a new one on me, Earl, since "refraction" is bending, not
reflection. The less refraction, the further away the signal returns to
earth. At some point between the amount of refraction that occurs and
the angle at which the signal interacts with the ionosphere the
refraction still bends the signal, but not enough for a return to the
surface. That is what I have always understood is the "critical
frequency". Signals at higher angles (including 90 degrees or 'straight
up') will pass on through as well. 

Ron AC7AC