Radio Craft - May 1948 - p. 70

But it doesn't account for transmissions of similar frequencies which span distances up to
1,400 miles as they did for many months in 1947 when European v.h.f. transmissions interfered
badly with one 43-46 mc television service.  That has been shown convincingly to be caused by
sporadic E-layer ionization, which is found to be pretty frequent in summer time in these
latitudes.  Sporadic E is a superionization of the lower reflecting layer, which occurs more
often and more widely than was formerly suspected and is influenced by sunspot activity.
Appleton has shown that it can be caused by meteoric dust.  Strong sunlight can also bring it
about.

And now for a quiz question for which no prize is offered.  Two places, A and B, are 1,000
miles apart.  One set of radio waves from A reaches B by reflection from the E-layer at a
height of 65 miles; a second gets there by reflection from the F-layer at a height of 200 miles.
Which set meets the reflecting layer at the larger angle?

Those that travel via the E-layer?  Right!  The bigger the angle the smaller is the chance that
the waves have of going smack through the layer and the greater the chance that they will be
reflected, supposing that each layer is ionized to an equal degree.  It follows that v.h.f.
waves which would escape into space and never come back to earth under ordinary conditions may
be returned to earth by a superionized E-layer.  The observed critical frequencies of
sporadic E are such that the maximum range obtained in this way by v.h.f. is about 1,400 miles.

Well, there you are.  Those who said that there would be no mutual interference between v.h.f.
stations working on the same frequency, so long as they were 40-50 miles apart, were somewhat
off the mark.  Atmospheric ducts can cause such interference up to about 160 miles, sporadic E
up to 1,400, and F-layer super-ionization up to 5,000 miles or more.  And those are the waves
of quasi-optical range!