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A wonderful site showing a few different types of radio signal **refraction**. Important to note what you are seeing here is totally different from the effects you get from HF radio frequencies and occurs with VHF and UHF as well. Its why generally line-of-sight frequencies like UHF can sometimes go some ways over the horizon.

dxinfocentre.com/propagation/t

@Science

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@rchrd

Indeed, it also applies to light by the way which is why in the early days there was some dispute as to if the earth was round by a small minority of legitimate scientists... because measurements sometimes actually told us it might be flat or even concave and we didnt understand refraction at the time.

@Science

@freemo is this the same principle as groundwave/skywave differences that make AM radio much better at night?

@rchrd

@khird

No thats actually a very different effect (AM broadcast radio is HF). In that case it has to do with ionosphere ionization that causes pathces that cause reflection. So the reflections happen at a much higher altitude, and the incidence is different too and the effect is not the refraction of the normal path or fair.

The biggest functional differences is with HF bounce you get much larger distances, and when youg et to frequencies as low as am broadcast ~1Mhz you have a very steep incidence angle for reflections to occur. Which means while its still much farther than you'd get with VHF/UHF it wont be the sort of distances you could get with mid range frequencies like ~20MHz.

@rchrd

@freemo @Science

I produce exactly the same types of diagrams for sound waves & rays in atmospheric layers.

Navy acoustics experts also do this for different layers of seawater so that submarines can hide from each other and from ships

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