Make Your Own Low-Power AM Radio Transmitter

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Abstract

Have you ever wondered how an AM radio station works? In this project you will learn the basics of how your favorite songs are transmitted by a radio station. You will learn how you are able to tune to your favorite station and listen to music. This is a very simple project that requires very few materials to make, but it gives a basic knowledge of how a radio transmitter works.

Objective

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

Introduction

Electromagnetic (EM) radiation is pretty much 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 EM 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, EM radiation at frequencies between about 430 THz and 750 THz can be detected by the human eye and are perceived as light. EM radiation at frequencies ranging from 3 Hz to 300 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 kHz in North America, 530 to 1610 elsewhere), and FM radio signals are carried by very high frequency (VHF) radio waves (88 to 108 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, below shows graphically how amplitude modulation works.

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