Design And Construction Of A 4Km AM Transmitter

CHAPTER ONE

1.1                                                        INTRODUCTION

Radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.

Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels. This technique contrasts with frequency modulation, in which the frequency of the carrier signal is varied, and phase modulation, in which its phase is varied.

AM was the earliest modulation method used to transmit voice by radio. It was developed during the first two decades of the 20th century beginning with Landell de Moura and Reginald Fessenden’s radiotelephone experiments in 1900.^[1] It remains in use today in many forms of communication; for example it is used in portable two-way radios, VHF aircraft radio, citizens band radio, and in computer modems (in the form of QAM). “AM” is often used to refer to mediumwave AM radio broadcasting.

In AM radio broadcasting, the audio signal (music and speech) is used to modulate the amplitude of the carrier wave. In the receiver a rectifier (diode) is used to recover the audio waveform from the tuned signal and convert it to audio.

Some radio-controlled equipment uses AM as it’s method of modulation. Rather than using an audio signal to modulate the carrier, Pulse Width Modulation (PWM) or Pulse Position Modulation (PPM) is often used.

It is also very easy to modulate something like RS232 serial data onto an AM signal and AM was often used in early RF modems in telemetry and control. I have worked with both 27MHz and 35MHz AM telemetry over the years, although FM is more common today.

AM transmitters is very simple devices which is why they were invented before FM. For AM radio broadcasts on medium wave (526.5 kHz to 1606.5 kHz,) or long wave (153 to 279kHz), the simplest working receiver is a ‘crystal set’ which can be made from as few as five components (tuning coil, variable capacitor, germanium diode, small ceramic capacitor and a high impedance earphone).

1.2                             OBJECTIVE OF THE PROJECT

The objective of this work is to construct a transmitter that transmit a radio signal in a frequency of 526.5 kHz to 1606.5 kHz.

1.3                                         SIGNIFICANCE OF THE PROJECT

1. Corverage area of AM transmitter is wider than FM because atmospheric propagation
2. AM is long distance propagation
3. AM Circuit is cheapter and non complex than FM.
4. AM have bandwidth limited

1.4                                            PROBLEMS OF THE PROJECT

1. The only one way to withstand to noise happen is increasing power of transmitter
2. Signal of AM is not stronger than FM when it propagate to obstacle.
3. Only one sideband of AM transmits Information Signal, So it loss power on other sideband and Carrier.
4. Noise mixes AM Signal in amplitude when it propagates in free space that it make difficulty to recover Original Signal at receiver.

1.5                          APPLICATIONS OF THE PROJECT

1. Radio broadcasting
2. Picture transmission in a TV system

1.6                  EFFECT OF NOISE ON AM TRANSMITTER

When the AM wave travels from the transmitter to receiver over a communication channel, noise gets added to it .

The noise changes the amplitude of the envelope of AM in a random manner .

As the information is contained in the amplitude variations of the AM wave, the noise will contaminate the information contents in the AM .

Hence, the performance of AM is very poor in presence of noise .