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AM Broadcast Frequency Spectrum

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For many people in the United States, AM radio — a legacy modulation scheme known as amplitude modulation (AM) — is the place to go for talk radio and sports. But AM is more than just commercial broadcasting. Aircraft use AM to communicate with the tower at an airport, and shortwave radio uses AM for international broadcasting.

The transmit signal for wireless communications begins its life as an electrical waveform living inside the transmitter electronics. The transmit antenna launches the signal as a traveling wave, and then electromagnetic wave propagation carries the transmitted signal through free-space to the receiving antenna.

The received signal becomes an electrical waveform again when the receiving antenna performs the opposite conversion. At both ends of the link, the signal may at some point reside in the discrete-time domain.

AM radio broadcasting has assigned channels, ranging from 540 kHz to 1,700 kHz and spaced at 10-kHz intervals. If your desired station is at 750 kHz, adjacent channels lie 10 kHz above and below at 760 and 740 kHz. Each 10-kHz channel contains an upper sideband (USB) and a lower sideband (LSB). Check out the figure for a visual of AM channel spacing.

[Credit: Illustration by Mark Wickert, PhD]
Credit: Illustration by Mark Wickert, PhD

In the language of communications theory, the carrier frequency refers to the frequency of a radio frequency (RF) sinusoid that’s responsible for carrying the communicator’s information or message from the transmitter to the receiver.

The 535- to 1,705-kHz frequency band, which broadcast AM occupies, is classified as a medium frequency (MF) radio band. Medium wave signals follow the curvature of the Earth, using ground wave propagation, but can also bounce off the ionosphere at night, resulting in skywave propagation.

With the addition of skywave propagation, AM broadcast signals can travel great distances — 500 miles or more. If you’ve never tried it, go sit in your car some night and see how many distant AM radio stations you can tune in.

At the heart of broadcast AM is the ability to produce low-cost AM radio receivers. You may have played with a simple crystal detector, which can receive AM stations without the need for even a battery. Go ahead, search the Internet and find out how to build one yourself.


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