"Reg Edwards" wrote in message
...

But is it actually reflection?

no, it is actually a refraction. but it is useful sometimes to
model it as
a reflection from a slightly higher level. that makes computation
of angles
of incidence and height a bit easier.

============================================
The trigonometry is quite simple. Things become complicated when the
reflecting layer is not horizontal, ie., the layer is tilted.

The angle and direction of tilt are very difficult to predict.
Consequently, where on the surface of the Earth a ray returns is
anybody's guess.

This makes the vertical take-off angle, reported by antenna modelling
programs, even less useful.

no, it doesn't make it less useful. as a statistic it is still good, but
you have to remember that it is nothing more than a statistic. and everyone
knows 'you can prove anything with statistics'. the fact that the
ionosphere is more complicated than a horizontal reflection layer model
represents doesn't mean that its usefulness is reduced, just that there are
some cases when it won't be completely accurate... those are the fun things
that happen with propagation that keep it interesting.