aircraft communication topic 2 modulation and propagation of radio waves

AV2220 - Aircraft Communication Systems Chapter 1 1

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In a communication transceiver, the carrier wave is a device that carries the information from the transmitter to the receiver.

The carrier has a frequency high enough to produce electromagnetic waves that radiate from the antenna. This frequency is accurately

controlled so that a sensitive receiver can select the carrier from a specific transmitter and reject the carriers from all other transmitters.

The carrier itself serves no function other than to carry the signal from the transmitter to the receiver, and the carrier is routed to ground after the intelligence is removed from it.

The process of placing intelligence on a carrier is called modulation, and there are several ways to do it.

Three ways most often used in aviation communication equipment are amplitude modulation (AM), frequency modulation (FM), and single-sideband (SSB).

Modulation



When the Italian inventor Guglielmo Marconi was developing the first practical radio system in 1896, there was no way to modulate the radio wave in order to transmit voice so he used a method of switching

the transmitter on and off to transmit Morse code signals.

This simplest form of modulation is called CW or radio telegraphy since it borrowed the Morse code from the telegraph industry.

Within ten years, new inventions permitted voice and music to be transmitted by radio using improved types of modulation.

The simplest form of transmitting data with radio waves is with Morse code dots and dashes or CW:

Modulation (cont’d)



Amplitude modulation, or AM, is a method of modulation in which the voltage of the carrier is changed by the audio signal. The voltage of the resulting carrier varies with the voltage of the modulating audio frequency. Audio frequencies (AF) are those of 20,000 Hz or less. They are called audio because

these are approximately the frequencies of sound waves that can be heard by the human ear.

Amplitude Modulation (AM)

Amplitude modulation



An example of the use of amplitude modulation (AM)

Amplitude Modulation (cont’d)



Man-made interference cause amplitude-modulate all radio signals in their vicinity. Man-made interference caused

by electric motors and ignition systems, and natural interference (caused by lightning in the atmosphere).

Frequency modulation (FM) is used to obtain interference-free communication. The voltage variations of the audio frequency signal produced by a microphone are used to change the frequency of the carrier.

The frequency of the carrier wave is changed when frequency modulation is used. In FM: as the voltage of the AF rises in a

positive direction, the frequency of the carrier increases

as it goes negative, the frequency of the carrier decreases.

One of the advantages of FM is that it is less affected by atmospheric noise from thunderstorms and other disturbances.

Frequency Modulation



The amplitude of an FM carrier is held constant by limiter circuits, and any interference, which amplitude-modulates the carrier, is clipped off so it does not appear in the output.When an FM signal is received, the deviations in frequency are changed into amplitude variations in an audio-frequency voltage that is amplified and used to drive the speaker.

Frequency Modulation (cont’d)

Frequency modulation (FM)



Both AM and FM are limited in that they require a wide band of frequencies for their transmission. If a 25-MHz carrier is modulated

with an AF signal that contains frequencies up to 5,000 hertz, the transmitted signal occupies a band of frequencies from 24.995 to 25.005 megahertz.

This band includes: the carrier, the lower sideband, which is

the carrier frequency minus the modulating frequency;

and the upper sideband, which is the carrier frequency plus the modulating frequency.

Single-Side Band (SSB)

Advantage SSB over AM



The advantages of SSB over AM: The upper illustration shows the

bandwidth required for an AM signal

The lower illustration shows the bandwidth required for an SSB signal.

The carrier and the upper sideband have been removed.

All the information needed is carried in either one of the sidebands, and it is inefficient use of energy to transmit the carrier and both the upper and lower sidebands.

Single-Side Band (cont’d)




Removing the carrier and one of the sidebands and using all of the available energy for transmitting the other sideband give the transmitter a much greater range.

Radio in the United States typically uses the lower sideband, but the upper sideband is used overseas.

At present, SSB is the primary type of transmission for communication in the high-frequency (HF) band.

Single-Side Band (cont’d)




When a radio wave is transmitted from the antenna it moves out along three paths, depending primarily upon its frequency.

These paths are surface waves, sky waves, and space waves.

The lower frequencies such as VLF, LF, and MF normally follow the curvature of the earth in surface waves. These waves travel great

distances and are used for very long-distance communication and navigation.

Radio Wave Propagation

Radio waves propagation

Commercial broadcast signals follow this path in the daytime.



Radio waves at frequencies below the HF band (below 3 MHz) are also called as ground waves because they will follow the curvature of the earth and bend.

HF (band from 3 MHz to 30 MHz) communication and commercial broadcast at night are carried primarily by sky waves. They tend to travel in straight

lines and will not follow the curvature of the earth.

Radio Wave Propagation (cont’d)

The propagation characteristics of ground waves, sky waves and space waves



This energy tries to radiate into space, but it bounces off the ionosphere and returns to the earth at a distance from the transmitter. It is called “skip distance“.

The “skip distance" varies and is responsible for the fading of many signals heard from a long distance.

Frequencies in the VHF and higher bands follow a straight line from the transmitting antenna to the receiving antenna and are said to travel by space waves.





RF propagation characteristics are complex because the earth appears different to radio waves at various frequencies.

The ground acts as a dielectric at frequencies above 5 MHz and as a conductor below 5 MHz.

At low and medium frequencies, the wave formation follows the curvature of the earth, as a ground wave.





Another illustration of radio waves propagation characteristic is shown in Figure below:


Radio waves propagation characteristics



VHF and above radio waves allow only line of sight communication. ‑ ‑

At these higher frequencies, the radio wave is not reflected by the ionosphere, but passes right through it.

VHF communications are ideal in that they minimize interference with distant unrelated stations operating on the same frequency.

A VHF communication system requires a much smaller antenna than an HF system. As the frequency increases, the wavelength decreases The antenna length is usually sized to an even fraction of the operating

wavelength.




There is a definite relationship between the length of the wave and its frequency. The higher the frequency, the shorter the distance between the ends of the

wave.


The speed of electromagnetic wave propagation is also 300,000,000 meters per second. Above 3,000 MHz, coaxial cable is replaced with waveguides and tuned circuits take the form of resonant cavities.

aircraft communication topic 2 modulation and propagation of radio waves

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