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Wikipedia says:

... Unlike high frequencies (HF), the ionosphere does not usually reflect VHF waves (called skywave propagation) so transmissions are restricted to the local radio horizon less than 100 miles...

Why does this happen? Is it merely that a VHF signal of say, 100Watts, carries more energy than an HF signal of the same power? Are there any other factors in play here?

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Essentially it comes down to the fact that the ionosphere reflects different wavelengths differently. To understand why we'd have to get into plasma physics but a short version is that the plasma characteristics are a function of density, species, ionization rate and ionization energy. For certain wavelengths the ionosphere can interact with the wavelengths of the RF energy and acts like a mirror, at other wavelengths the frequency is such that the ions don't interact as well and thus don't reflect the energy back.

Of course this is a coarse simplification.

This is similar to how you can have wavelength selective optical filters through interference effects.

Metals, which are typically shiny and mirror like, have their conduction band full of electrons which act as a sea of charge carriers which the light can interact with. But even different metals have different Colors, which indicate a similar phenomenon to the ionospheric interaction.

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    \$\begingroup\$ A more intuitive understanding can be had by considering a prism. A prism can make a rainbow from a beam of white light because the different frequencies (colors) of light are refracted differently. The materials in the atmosphere are different, but one can still easily appreciate that refraction can be dependent on frequency though this analogy. \$\endgroup\$ – Phil Frost Jan 6 '13 at 13:57
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The size of ionized plasma particle is relative to the wavelength of the HF signal and when these signals hit the plasma particles since size is more less eqaul to wavelength these HF signals will be reflected as in case of vhf wavelenths are very small and can penetrate.

One more reason is effect of the refractive index of the medium mostly depends on the wavelength of the signal wchich is passing through that media. At HF frequency this effect is more compared to VHF.

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    \$\begingroup\$ "The size of ionized plasma particle is relative to the wavelength of the HF signal" - no, it's not, by many orders of magnitude. You probably mean the average spacing between them. \$\endgroup\$ – Chris Stratton Sep 23 '14 at 17:26
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The analogy I use to explain, in the most generic and elementary way, is the RF is like a drill bit. The wavelength is the groves in the drill bit and the speed of the drill bit is the frequency and the material is the ionosphere, like wood or metal.

If you have ever tried to drill a hole with a very slow speed and large drill bit, the drill has a very difficult time penetrating the metal or wood. This would represent the ability of HF waves being reflected off the ionosphere. If you use a smaller size drill bit, and drill it at a very high speed, the drill bit goes thru the material very easily, just like VHF or UHF waves penetrate the ionosphere. Don't laugh, my kids got it, and many non- hams also! For all you Amateur Radio academics, hope this doesn't upset you. Lol.

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