Properties of Radio Waves

Introduction

Radio and radar systems are now an integral and essential part of aviation, without which the current intensity of air transport operations would be unsustainable. In the early days of aviation aircraft were flown with visual reference to the ground and flight at night, in cloud or over the sea was not possible. As the complexity of aircraft increased it became necessary to design navigational systems to permit aircraft to operate without reference to terrain features.

The early systems developed were, by modern standards very basic and inaccurate. They provided reasonable navigational accuracy for en route flight over land, but only a very limited service over the oceans, and, until about 40 years ago, flight over the oceans used the traditional seafarer’s techniques of astro-navigation, that is using sights taken on the sun, moon, stars and planets to determine position. Developments commenced in the 1910s, continued at an increasing rate during the 1930s and 1940s and up to the present day leading to the development of long range systems which by the 1970s were providing a global navigation service.

It is perhaps ironic that, having forsaken navigation by the stars, the most widely used navigation systems in the last few years are once again space based, that is the satellite navigation systems we now take as being the norm. Whilst global satellite navigation systems (GNSS) are becoming the standard in aviation and many advocate that they will replace totally all the terrestrial systems, the ICAO view is that certain terrestrial systems will have to be retained to back up GNSS both for en route navigation and runway approaches.

The development of radar in the 1930s allowed air traffic control systems to be developed providing a control service capable of identifying and monitoring aircraft such that aircraft operations can be safely carried out at a much higher intensity than would be otherwise possible. Modern satellite technology is being used to provide a similar service over oceans and land areas where the provision of normal radar systems is not possible.

The Radio Navigation Syllabus

The syllabus starts by looking at the nature of radio waves and how they travel through the atmosphere. This is essential to understand why different radio frequencies are selected for particular applications and also the limitations imposed. The introductory chapters also cover how radio waves are produced, transmitted, received and how information is added to and recovered from radio waves.

Electromagnetic (EM) Radiation

If a direct electric current (DC) is passed through a wire then a magnetic field is generated around the wire perpendicular to the current flow.

If an alternating electric current (AC) is passed through the wire then, because the direction of current flow is changing, the polarity of the magnetic field will also change, reversing polarity as the current direction reverses. At low frequencies the magnetic field will return to zero with the current, but as frequency increases the magnetic field will not have collapsed completely before the reversed field starts to establish itself and energy will start to travel outwards from the wire in the form of electromagnetic radiation i.e. radio waves.

Properties of Radio Waves 1

Waves Radio of Properties 1

The resulting EM energy is made up of two components, an electrical (E) field parallel to the wire and a magnetic (H) field perpendicular to the wire.

Figure 1.1 Vertical polarization