Have you ever stopped to think about AM Radio and why some stations can be picked up a long way from their home base and others cannot?
AM radio stands for amplitude modulation. Radio waves are encoded to carry information based signals. Modulation is the actual process of impressing sound on a radio wave. In the case of AM, the radio wave’s “amplitude” or extent of movement is determined by the magnitude of the sound being impressed onto the radio wave. AM radio waves come in two forms: ground wave, which is what most of us hear during the day, and sky wave, which is typically heard only at night from powerful AM stations.
AM radio waves follow the curve of the Earth. But at night, AM radio wave travel differently. In essence, they travel farther. In addition, AM radio waves are subject to interference from solar activity and sunspots, which at the present time is very active.
When AM radio service was first regulated by the FCC in the 1930’s, certain frequencies were designated as “clear channels”, meaning that a station was authorized to cover a wide geographical area and transmit with 50,000 watts, the maximum power allowed by the FCC. Stations on these frequencies transmit an Omni directional signal, or a perfect circle. Here in the DFW area, WBAP Radio is a station that transmits on a clear channel frequency, 820, with 50, 000 watts of power all the time, with an Omni directional signal, all the time. Under a clear channel situation, no other station may broadcast on that frequency at night within 750 miles (as the crow flies) of the clear channel station.
Some AM stations are 50,000 watts all the time but have different signal patterns during the day and at night. For example, one of our radio stations that you see on this website, KRLD Newsradio 1080, operates during the day at 50,000 watts of power with one tower, producing an Omni directional signal. But at dusk, things change. They still operate with 50,000 watts but operate with two towers to create a pattern other than a perfect signal. This is mandated by federal regulations so that KRLD does not interfere with another station at night on the 1080 frequency. KRLD’s nighttime signal is more west at night and pulls in from the northeast. If you are a KRLD listener in the immediate DFW area, you won’t notice any changes in signal strength. But if you drive northeast of the DFW area, at some point you will notice a difference.
Other AM stations are 50,000 watts day but are required to reduce power significantly at night. In our area, KFXR 1190 is an example of that: 50,000 watts day with four towers and 5,000 watts night with 12 towers!! If you drive east toward Rockwall on Interstate 30 at night, you will see the 12 towers all lit up!!!
Other stations have other power authorizations that vary depending on the frequency they are licensed to operate on, ranging from 250 watts to 25,000 watts. Other stations that were previously not licensed to operate at night can do so but at very reduced limits. KHVN 970 and KKDA 730 in our area are examples of that. KKDA 730 had to sign off at dusk years ago to allow a station in Mexico, XEX, to operate at night!
And finally, there are AM stations that must sign off at dusk regardless. Two of them come to mind in our area: KFJZ 870 and KGGR 1040. These stations sign off at dusk as required by their FCC license. But what it cool is what you hear afterwards. On 870, you will hear WWL Radio from New Orleans clear as if it were a local station. On 1040, you will hear WHO Radio from Des Moines IA!!
On our next blog we will talk more about my personal experiences as a kid when I discovered the power of AM Radio during AM’s radio golden years.
See you next time!!