This month's Challenge Question: Specs & Techs from IEEE GlobalSpec:
Short-wave and AM transmitters are required to lower their transmission power and directivity of the signal, and, in some instances, are forced to go off the air at sunset. The reason for this FCC requirement: At night, the wave signals from these transmitters (like your local AM radio station) travel a longer distance than during the day. Keeping all the station’s parameters the same, why does the night help to increase the range of these radio waves? Does this also happen with FM transmitters?
And the answer is:
AM signals (including short-wave) are sent to space by the transmitter. How far an AM station’s signal travels depends on the station’s power, its transmission frequency, its antenna, the conductivity of the land around the antenna, and the ionospheric refraction. Some of these waves (the ground signals, as shown in the figure) do not go too far because the earth stops them. The signals that are transmitted to the sky (sky signals, in the figure) reach the ionosphere and are refracted back to earth, which in turn refracts them back to the ionosphere; this process lasts until the signal power is exhausted.

The ionosphere is a layer ranging from 30 miles to over 100 miles from the earth’s surface. The ionosphere is a heavily charged layer of ion molecules, and the number of ions is a function of the amount of sunlight received by the ionosphere. During the day, the production of ions increases, but during the night—in the absence of sunlight—the ionospheric ions decrease, thus, decreasing the width of this layer. A smaller ionospheric width means a longer distance from the surface of the earth, and this raises the refracting levels so the radio signal travels farther around the earth. This is a problem because the AM signal from a particular radio station may interfere with a faraway radio station with the same frequency in violation of allowed ranges for each station. This is the reason that at sunset U.S. AM radio stations must: (a) reduce power to decrease the range, and/or (b) directionalize their signals so they are sent in some directions more than others, or (c) go off the air until sunrise the next day.
For FM signals, on the other hand, because of their higher frequencies, ionospheric refraction is negligible. Having a short wavelength (high frequency), FM signals move through the ionosphere without appreciable interactions, as is depicted in the right hand-side figure.
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