HedgeTrimmer
First Officer
United States
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AlphaFlightNW Posted at 2018-7-3 19:30
Yeah... Let me see if I can remember some RF theory.
What I think might be happening is like when your near a lake at night, you can hear sounds better from the other side than during the day. It might be that but Im not sure. Ill check my notes somewhere.
FCC on AM signals:
Because of the way in which the relatively long wavelengths (see Footnote 1) of AM radio signals interact with the ionized layers of the ionosphere miles above the earth's surface, the propagation of AM radio waves changes drastically from daytime to nighttime. This change in AM radio propagation occurs at sunset due to radical shifts in the ionospheric layers, which persist throughout the night. During daytime hours when ionospheric reflection does not occur to any great degree, AM signals travel principally by conduction over the surface of the earth. This is known as "groundwave" propagation. Useful daytime AM service is generally limited to a radius of no more than about 100 miles (162 km), even for the most powerful stations.
However, during nighttime hours the AM signals can travel over hundreds of miles by reflection from the ionosphere, a phenomenon called "skywave" propagation. (Shortwave stations, which operate using AM modulation on several bands between between 2.3 MHz and 26.1 MHz, also use this phenomenon to broadcast still greater distances, up to thousands of miles.) Because of this change in signal propagation from daytime to nighttime, if every AM station kept its daytime operating power at night, massive interference would result.
Why AM signals:
Not all radio waves travel farther at night than during the day, but some, short and medium wave, which AM radio signals fall under, definitely can given the right conditions. The main reason this is the case has to do with the signal interacting with a particular layer of the atmosphere known as the ionosphere, and how this interaction changes from the nighttime to the daytime.
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