Discussion
Why is D layer absorption greater at lower frequencies?
Answer: D
The amplitude of the electron's forced oscillation is inversely related to frequency, so at low frequencies it moves further, collides more with neutrals and transfers more energy to heat. Absorption therefore falls roughly as the inverse square of frequency, which is why daytime high frequency working moves to the higher bands. Penetration depth, electron density and antenna radiation are not the mechanism. A loss that falls with frequency, in a medium whose refraction also falls with frequency, sets up the central trade of high frequency operating.
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