There are over 100 channels in the AM broadcast band. During the day, you can hear signals on only a few of these channels. At night, you can hear signals on almost all of them. Why?
The AM broadcast band, also known as Medium Waves, is available all over the world. In North America, it stretches from 540 to 1700 kHz with channels spaced 10 kHz apart. Broadcasters use vertical antennas (towers) which vary in height from 150 feet at the high end of the band, up to 460 feet at the lower frequencies. In Canada, these AM radio towers are almost always located south of the city being served. I will tell you why shortly.
AM radio signals are emitted from these towers and generally travel horizontally along the surface of the earth. This is called ground wave. Ground wave AM radio signals generally reach out a few hundred miles over flat ground, far less if blocked by mountains. Almost always, when you are listening to your local AM radio station, you are listening to ground wave.
But some of the AM radio signal transmitted leaves the tower at a bit of an angle and heads up into the sky. This is called sky wave. You would not expect to hear sky wave signals, as they are gone forever, right? That would be true if not for the magic of a certain part of the atmosphere called the ionosphere. The ionosphere is made up of ionized (charged) gases that can act as a reflecting surface for radio waves. So, AM radio signals transmitted as sky waves can actually bounce off the ionosphere and come back to earth. And, they do so at a much greater distance than the ground wave covers. Often thousands of miles.
Why AM radio signals only bounce at night
The ionosphere is comprised of several different layers. They work because they are charged by solar radiation. Each of several layers has its own personality. The higher layers (E and F) are really good at refracting (reflecting) AM radio signals. The lower layer (D) is really good at absorbing AM radio signals. This D layer only exists during sunlight. It forms around local sunrise and dissipates around local sunset. It acts as a barrier which prevents sky wave AM radio signals from getting to the higher E and F layers during the day. And that is why you can only hear a few AM radio signals during the day, and lots at night.
Which brings us back to why Canadian AM radio transmission sites are almost always located to the south of their coverage areas. AM Radio transmitters usually have between 1-5 towers. If there is only one tower, the signal spreads out evenly in all directions. This is called omnidirectional. With two or more towers, the signal is shaped into a pattern. This is called directional. Directional patterns send more of an AM radio signal in certain directions, and less (or none) in others.
AM radio stations generally do a pattern change at sunset and sunrise. They call these night pattern and day pattern. Because AM radio signals travel so much further at night by sky wave, there is a greater risk of interfering with other stations sharing the same frequency. Night pattern is designed to limit sky wave transmission towards other other stations sharing the same AM frequency.
In Canada, night pattern is designed to make sure the AM radio signal is not transmitted towards the south. Why? Because that is where most of the other stations on the same frequency are located, i.e. in the United States. And that is why in almost every Canadian city, you will see the AM transmitting towers located south of the city. To send their power north towards their audience, while limiting interference towards the south.
Two last points.
- Clear, Regional and Local Channels. Generally, most regulators allow different types of power levels, antenna patterns and hours of operation according to the classification of the channel. Certain frequencies are meant for either clear (long range), regional or local services. Not all frequencies are used in the same way, as described here.
- Look at AM radio signal patterns. The Radio Locator web site is designed to show the geographic coverage areas and day/night patterns of all AM radio stations in Canada and the United States. In these maps, you can see how the pattern of your favorite AM station changes at sunrise and sunset, often dramatically.