Radio Propagation and Frequency

by on July 31, 2008 · 14 comments

I’m finally reading Cato’s 2006 Policy Analysis on spectrum property rights. It’s got a lot of good information, but this sentence made me do a double-take:

In free space, radio waves steadily weaken in a very uniform, predictable way and at a rate that depends on frequency. In particular, the higher the frequency, the faster the waves weaken. In the real world—on the earth and in its environs—the situation is much more complicated, and radio links are affected by the earth itself, the atmosphere, and the intervening topography and natural and manmade objects such as foliage and buildings.

It’s been a while since I took physics, but I seem to recall (and Wikipedia seems to agree) that the strength of an electromagnetic wave falls with the square of the distance from the source. Indeed, this result seems to be compelled by the geometry of the situation and the conservation of energy. What am I missing?

Assuming I’m not just confused, one possibility is that they’re talking about propagation in the atmosphere rather than free space. It appears to be true that lower-frequency radio waves travel further along the surface of the Earth because they are affected more by the Earth’s atmosphere.

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