Harv Duvivier

The goal of this project is to build a simple AM radio transmitter and to test its broadcast range with a radio receiver.

Electromagnetic radiation is all around us. For example, light is electromagnetic radiation and so are x-rays. When you listen to an AM or FM radio station, the sound that you hear is transmitted to your radio by the station using electromagnetic radiation as a carrier—radio waves. Electromagnetic radiation is a propagating wave in space with electric and magnetic components. In a vacuum, electromagnetic waves travel at the speed of light.

Electromagnetic waves such as light, x-rays, and radio waves are classified by their frequency or wavelength. For example, electromagnetic radiation at frequencies between about 430 terahertz (THz) and 750 THz can be detected by the human eye and are perceived as light. Electromagnetic radiation at frequencies ranging from 3 hertz (Hz) to 300 gigahertz (GHz) are classified as radio waves. Radio waves are divided into many sub-classifications based on frequency. AM radio signals are carried by medium frequency (MF) radio waves (530 to 1710 kilohertz (kHz) in North America, 530 to 1610 kHz elsewhere), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to 108 megahertz (MHz)).

So how does a radio wave carry sounds such as voice or music to your radio receiver? The radio station broadcasts a carrier wave at the station's assigned frequency. The carrier wave is modulated (varied) in direct proportion to the signal (e.g., voice or music) that is to be transmitted. The modulation can change either the amplitude or the frequency of the carrier wave. The "AM" in AM radio stands for "amplitude modulation," and the "FM" in FM radio stands for "frequency modulation." A radio receiver removes the carrier wave and restores the original signal (the voice or music). Figure 1 shows graphically how amplitude modulation works.

In this project, you will make a simple low-power broadcast circuit, using a crystal oscillator integrated circuit and an audio transformer. You can connect the circuit to the headphone jack of a portable music player (e.g. mp3, CD or cassette tape player). You'll see that you can receive the signal through the air with an AM radio receiver. Although the circuits used in radio stations for AM broadcasting are far more complicated, this nevertheless gives a basic idea of the concept behind a broadcast transmitter. Plus it is a lot of fun when you actually have it working!

Before we get into the step-by-step instructions for building the circuit, we'll first go over the circuit design. Figure 2 shows the connections you need to make to build the circuit. The transformer isolates the music player from the rest of the circuit, couples the music player and the crystal oscillator, and "steps up" the signal voltage from the music player in proportion to the ratio of 1 kΩ to 8 Ω. The stepped up signal from the secondary coil of the transformer modulates the power to the oscillator chip (+ power at pin 14 and - power at pin 7). A wire connected to the oscillator output (pin 8) serves as the antenna for broadcasting the amplitude-modulated radio wave.

To do this project, you will need to know how to use a solderless breadboard. If you have never used a breadboard before you may want to take a look at the Science Buddies reference How to Use a Breadboard for Electronics and Circuits before you start this science project.

To do this project, you should do research that enables you to understand the following terms and concepts:

To do this experiment you will need the following materials and equipment. Most of the electronic components are available from Jameco Electronics.

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