The Radio Waves:

Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum.

A common use is to transport information through the atmosphere or outer space without wires. Radio waves are distinguished from other kinds of electromagnetic waves by their wavelength in the electromagnetic spectrum.

The family of electromagnetic waves includes radio waves and microwaves as well as visible light, X-rays… The radio frequency spectrum used by radio transmission technologies ranges from a few hertz to 300 gigahertz.

All radio applications like television, radio, mobile telephony, networks of wireless Internet access, military applications, satellites communications, etc… must coexist in this interval closed and inextensible.

The radio spectrum is in the public domain of the state. To enable the coexistence of different radio transmitters, several rules must be respected: Authorized application type (fixed or mobile, civil or military ...), limitation of power (in order to limit the range and level of pollution Radio), location of stations ...

Radio waves are divided up into bands by frequency (and corresponding wavelength) as shown in the radio frequency spectrum table below :

 

 

 

 

 

 

 

 

 

Signal propagation :

Radio waves (rated RF for Radio Frequency) propagate in a straight line in several directions. The speed of propagation of waves is 3x10⁸m/s.
The radio signal can be disrupted by obstacles (trees, mountains) or other nearby station.
The higher the frequency used is low, the range will be better and vice versa.
The picture shows that, after having crossed an obstacle, the radio signal is weakened. That is why in most radios application, repeaters are used to amplify the signal.

 

 

 

 

 

 

 

 

 

The spread depends on weather conditions and especially atmospheric pressure.
One can know the spread in real time through a web site that disseminates forecasts in real time: www.dxinfocentre.com

How it works ?

The first diagram shows the principle of radio-to-radio communication called "simplex" and on the second the principle of a relayed radio system called “half-duplex system”.

> Simplex radio system :

 

 

 

 

 

 

 

 

 

In this system, the transmission is direct; every radio transmits and receives on the same frequency, such as 444.6MHz.
The range is still limited (between 5 and 10 km with 5w power.)
This system is now widely used in business for example by security services, or for connections between buildings...

> Duplex radio system :

 

 

In this evolved radio system, the radios transmit and receive on two different frequencies.
Direct connections are not possible between two radios.

The repeater can be used to increase the range and can be connected to computer equipment to perform advanced applications, such as selective calls, creating a connection between your radio and the classical telephony system, and the management of remote equipment (for example, the screens used by the SNCF.)
The signal range of a radio repeater depends on the transmitted power (generally between 25 and 100W).
Today, most radio systems are relayed.
The system quality and radio coverage are better than in simplex mode.
Before being sent through the air, a radio signal must be modulated. There are two types of transmission systems today: The analog and digital transmission system. This is what we will explore in subsequent sections.