AM Powerdown explained

A comment was posted on our site recently asking why reception of KROE is worse after 10 PM than it is during the day. I thought the explanation for this might be interesting to other listeners as well so I am posting it as a commentary for this week. The listener who posted the comment noted that most AM stations reduce power at night, but they weren't sure about the reason for this. In this commentary I will attempt to explain the various factors involved in the AM power change.

The short answer to the question is: AM signals travel farther at night, so the FCC requires most AM radio stations to reduce power at night in order to avoid interference. The following paragraphs provide a more detailed explanation about the phenomenon that results in this requirement.

The uppermost part of the atmosphere is ionized by solar radiation. This ionosphere is divided into several layers. The innermost layer, the "D layer," absorbs radio frequencies in the AM band, and is heavily influenced by the sun. During the day, the D layer is very strong, and most of the AM signals that are radiated into it are absorbed, causing AM signals to propagate primarily via conduction over the earth's surface. This limits the coverage of AM stations to about 100 miles during the day. At night, as solar radiation decreases, the D layer dissipates, allowing AM signals to reflect back to the earth. This results in a phenomenon known as "skywave" in which AM signals can bounce over hundreds of miles at night.

In the US there are dozens of radio stations operating on the same frequency as KROE. Some of them are in adjacent states such as Montana (KMPT) and Colorado (KIUP, KYKY). Each of these nearby stations have daytime power of around 5,000 Watts. If they were to continue broadcasting using this power at night, these signals could travel far enough via skywave to interfere with each other.

Therefore, the FCC requires these stations to reduce power by nightfall, and the station's license specifies power up and power down times for each month of the year. This ensures that by the time a signal reaches a location with another station on the same frequency, the local station's signal will be so much stronger by comparison that it will not be adversely affected. The night time power typically limits the coverage of a station to little more than just the city of license, and the difference between the authorized day and night power can be dramatic. KROE operates at 5,000 Watts during the day, and just 117 Watts at night. Because the nighttime power is so much lower than the daytime power, some listeners even within the city of Sheridan may notice a degradation in reception at night.

Not all stations are required to reduce power at night. Some AM stations are granted “clear channel” status. These stations typically have no close neighbors on the same frequency, and are authorized to operate at full power 24 hours a day. KOA in Denver is one example of a clear channel station. While reception of this station in Sheridan is nearly impossible during the day, it typically comes in loud and clear at night, because it broadcasts at 50,000 Watts both day and night. If there were a local station operating at 850 AM, daytime reception would typically be fine, but at night it would experience interference from KOA. Other AM stations are designated as “regional” and can operate at higher nighttime powers provided they can protect against interference of neighboring stations through the use of directional antenna systems. Finally, FM stations are not required to reduce power at night because they operate at frequencies higher than those absorbed by the D Layer of the ionosphere.

Because of the close relationship between the sun and the ionosphere, AM reception can be noticeably affected by a combination of the various sunspot cycles and other solar activity, as well as the seasons and, of course, the time of day. AM reception is typically degraded during periods of high solar activity.