# AM Transmission

This is a sketch of some of the components involved in AM radio transmission. You may click on the illustration for more detail.

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 After the Mixer, the frequency fLocal Oscillator - fCarrier = 455kHz is amplified and then enters the detector stage. There it is rectified and fed into an AM detector circuit. This process amounts to a kind of filter that does not respond to the high frequency variations of the IF, but tracks the low audio frequency "envelope" of the IF signal. This gives an audio frequency output signal which is then amplified and supplied to a loudspeaker to convert it back into sound waves. That sound output is hopefully a faithful reproduction of the original input sound. As a historical note, the reason for choosing 1480 kHz as an example AM frequency out of the range of 535kHz to 1605 kHz was that on one occasion when we were discussing AM radio, we were having a hard time eliminating a 1480kHz signal from some of our electronic equipment in the Department of Physics and Astronomy. For some reason, our whole building seemed to be an effective antenna for that particular radio station. With the help of the engineers from the station and some detective work on our own, we were able to solve the problem.
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# AM Modulation Stage for AM Radio

In order to generate the radio frequency signal for AM radio transmission, the information contained in the original sound signal must be put in a form that can be sent to a great distance. When information from a voice is broadcast on an AM radio station, an electrical image of the sound is made by a microphone. A dynamic microphone is an example. That electrical signal or a signal from another recorded source is used to modulate the amplitude of the much higher frequency carrier wave. The electrical image of the sound information is used to vary the amplitude of the carrier wave by an amount porportional to the strength of the original sound signal. The modulated carrier wave is then amplified by the transmitter, which amounts to a power amplifier which can apply the modulated electrical wave to the conducting element in the transmitting broadcast antenna of the radio station.

The AM band of the Electromagnetic spectrum is between 535 KHz and 1605 kHz and the carrier waves are separated by 10 kHz.

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# AM Transmission Stage

The electrical signal from the AM modulation stage contains the information to be transmitted superimposed on the precisely controlled carrier frequency wave. Neither sound waves themselves nor electrical signals at audio frequencies can be transmitted over large distances. The AM band of frequencies is between 535 KHz and 1605 kHz, corresponding to wavelengths between approximately 200 and 600 meters. These long wavelengths dictate long conducting antennas, so it is customary to see metal towers or antennas on top of mountains or tall buildings.

The radio transmitter is a power amplifier which provides enough power to generate radio frequency electric currents in the conducting broadcast antenna. Such antennae for AM radio are vertical, which leads to transmitted electromagnetic waves which are linearly polarized in the vertical direction. The electromagnetic waves, transmitted at the speed of light, are received by metallic antennae on the radio receivers. These antennae must also be oriented vertically for optimum reception. FM radio signals, by contrast, are transmitted with both horizontally and vertically polarized signals, so vertical antennae are not required.

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