simple AM radio transmitter and test its broadcast range with a radio receiver

Introduction

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. Electromagnetic radiation at frequencies between about 430 tetra hertz (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)). 

Understand the following terms and concepts 

• electromagnetic radiation and waves,
• electromagnetic spectrum,
• wave model,
• speed of light,
• wavelength,
• frequency,
• amplitude,
• crystal oscillator,
• transformer,
• amplitude modulation.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• 2 crystal oscillators

§  Each oscillator should be at a different frequency, within the AM broadcast band (0.53 to 1.71 MHz in North America, 0.53 to 1.61 MHz elsewhere).

§  For use with the solder less breadboard in this project, you want the 'full can' package.

§  Suitable oscillators

1 MHz, part number 520-TCF100-X
1.2288 MHz, part number 520-TCF122-X.

1 MHz, part number 27861
1.2288 MHz, part number 325307.

o    Solder less breadboard (e.g., Radio Shack 276-001).

o    1000 ohm to 8 ohm audio transformer (e.g., Radio Shack # 273-1380),

o    1/8 inch mono phone plug (Radio Shack # 274-286A),

o    a 6 V AA battery holder (holds four batteries),

o    four 1.5 V AA batteries,

o    a set of alligator jumpers,

o    Jumper wires for breadboard.

Building the circuit 

The transformer isolates the music player from the rest of the circuit, couples the music player and the crystal oscillatory, and "steps up" the signal voltage from the music player in proportion to the ratio of 1 kohm to 8 ohms. 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.  

Points for joining the circuit

Connect the terminals of the phone plug to the 8 ohm side of the transformer :

Insert the 1 MHz oscillator across the gap in the breadboard, so that pins 1 and 7 are on one side of the gap, and pins 8 and 14 are on the other. 

  Use the breadboard to connect the positive and negative terminals of the battery holder and the 1000 ohm side of the transformer as shown in the diagram and in Figure below. Note that the 1000 ohm side of the transformer has a center tap which is not used in this project.

 Connect a long jumper wire to the output of the crystal oscillator (pin 8). This will serve as the antenna.

  Double-check to make sure that all of your connections correspond to the circuit diagram.

        Figure, below, shows a photograph of the completed setup including an iPod for generating the music and an AM radio receiving the signal.

Experimenting with the Circuit

Now that we have to built the circuit, here is the fun part—experimenting with it!

1.    Connect the phone plug to the output (headphone) jack of mp3 or CD player and tune AM radio to 1 MHz. Bring the antenna within an inch of your radio antenna. Can we hear the music that we are playing on your mp3 or CD on the radio?

2.    Now tune AM radio to a different frequency say 700 kHz. Can we still hear your music?

3.    Tune the radio back to 1 MHz where we can hear your music. But this time remove the 1 MHz crystal oscillator and in its place put the 1.2288 MHz oscillator. Can we still hear the music?

4.    Without changing the oscillator back to 1 MHz, instead tune your radio now to 1.23 MHz. Can we hear your music?

5.    Use 1 MHz crystal oscillator and tune the radio to 1 MHz. Adjust the volume control of our mp3 or CD player, is there any change in the quality of the sound we hear in our radio?

6.    Until now we have to kept our antenna within an inch of our radio antenna, now move our transmitter's antenna further away slowly and hear what happens. Does the quality of our sound improves or gets worse? Why?

7.    Rotate the radio receiver antenna relative to our transmitter's antenna (or vice versa). Does this affect the quality of the sound? Why?

8.    Try using a longer wire for the antenna. Does this affect the quality of the sound? Does this affect the broadcast range for our transmitter? Why?

Try receiving the signal from your AM transmitter with a crystal radio that you build yourself. You can explore how the relative placement of the receiving and transmitting antennas affects signal strength at the receiver