Communication by Air Waves

When a person speaks, his or her vocal chords set up vibrations in the air. A flow of air is required through the voice box to generate these impulses, the particular nature of which depend upon the person's muscle control over their voice box. Air (or some other medium, such as water) must be present to carry these slight fluctuations ofair pressure to the ear of the listener. At the receiving end, the sound waves Impinge upon the ear drums causing them to vibrate at the same frequency. The mechanism in each ear converts these vibrations into signals that are sent to the brain.

It is quite simple to cause minor variations in the atmospheric pressure surrounding you. For example, if you take a steel ruler, hold it on the edge of a table and cause it to vibrate, your ears will pick up the sound waves. By varying the length of the ruler that is allowed to vibrate, you can alter the frequency of these vibrations (i.e. the number of vibrations oscillating per second) and, as a consequence, a different pitch of sound is heard.

Frequency is measured in cycles per second (cps), known as hertz, in honour of the German physicist H. R. Hertz (1857—94), one of the first scientists to investigate radio telegraphy. The human ear can respond to frequencies between approximately 20 and 20,000 cycles (or oscillations) per second, with 20,000 hertz usually expressed as 20 kilohertz (kilo meaning thousand). Frequencies in the 20 hertz to 20 kilohertz range are known as audio frequencies.

he unit hertz is abbreviated to Hz:
One wave per second past a given point is called a frequency of 1 Hz (hertz);
1000 waves per second past a given point is called 1,000 Hz or I kHz (kilohertz); and
1,000,000 waves per second is called 1,000,000 Hz, 1,000 kHz or 1 MHz (mega-hertz).

Note. kHz is normally used up to 30,000 kHz (30 MHz), and MHz is used above this.

Like ripples on a pond, sound waves are attenuated (weakened) quite quickly as they travel away from the source.

Wave motion can be described by its shape and frequency. As you may have noticed when swrmnung, the normal waveshape is a smooth symmetrical shape (referred to in mathematical terms as a sine wave or sine curve).

Water waves consist of water molecules moving up and down (and around) in a regular manner as the energy passes. Sound waves passing through the air consist of the air being compressed and expanded (i.e. pressure variations) in a regular manner. Radio waves belong to an entirely different family to sound waves, and unlike sound waves, radio waves do not require a medium to travel through.

Using Radio Waves to Carry Voice Messages

Radio waves consist of regular variations in the strength of electric and magnetic fields. They can travel through a vacuum, through outer space, and even through walls. However, like sound waves, radio waves are attenuated (weakened) by dense material such as the earth and large buildings:

• sound waves are pressure waves, and they travel at the speed of sound (approximately 340 m/sec, depending upon air temperature); and
• radio waves are electromagnetic waves, and they travel much faster, at the speed Of light (300 million m/sec).

An example of the two different natural speeds (light and sound) is when you see a person some distance off strike a blow with a hammer, say on a metal object; the sound of the blow reaches you some time after you observe the action. Another example is the time interval between an observed lightning flash and its associated peal of thunder during a thunderstorm. The electrical and magnetic signals generated in a microphone from audio signals (such as a voice) are in the audio frequency band from 20 to 20,000 hertz• The frequency spread of radio signals suitable for transmission is much higher than this — from 3,000,000 to 300,000,000 hertz. Radio waves can be illustrated as sine waves that are radiated out and, as can be seen in figure 1-2, the higher the frequency, the more waves per second that are transmitted and the shorter the wavelength.

1: Radio: What It Is & How To Use It

As well as travelling long distances without becoming attenuated, radio waves have the advantage of being able to be tuned selectively. Unlike the various pressure vibrations in the audio range (which our ears hear as a blend of sounds all mixed together), we can actually tune a radio receiver to one particular frequency, allowing it to receive messages on that frequency alone. All unwanted radio frequencies, and the information transmitted on them, are automatically screened out. This useful characteristic allows different radio frequencies to be used for different functions.

Fundamental Radio Terminology

Wave energy is of a continuous nature in that it consists of many waves, one following the other at regular intervals with the wave cycle repeating itself over and over again as the wave motion passes a given point. A cork floating on water would move up and down as water waves pass, not along in the direction of the wave.

Wave motion can be discussed in fairly simple terms, the main ones being:

• wavelength:
the length of one single wave (or of one complete cycle) — it is also the distance travelled by the wave during the transmission of one cycle;
• frequency:
the number of complete waves (or cycles) passing a point per second; and
• amplitude:
the distance from one extremity of the oscillation to the middle point or neutral value.

As we all know, the airwaves are full of millions ofradio messages, and we extract only those on the frequency to which we are tuned. If our desired signal on the selected frequency predominates over unwanted weak signals or noise, then we have good radio reception (or a good signal-to-noise ratio).