Radio aids used in aviation employ ElectroMagnetic Waves (EM Waves) for their functioning. EM waves are created by movement of alternating current (AC) in a wire. The waves are radiated or transmitted through air if the wire is connected to an ariel which is called as electromagnetic radiation. Some parts of the ElectroMagnetic spectrum are used for Radio and Radar transmission and are hence called as Radio Waves.

### Components of an ElectroMagnetic Wave

When an alternating current is passed through an ariel it has an electrical field in the plane of the ariel. This electrical field creates a magnetic field perpendicular to the transmitted electric field. In short, we can say that EM Radiation consists of electrical and magnetic components. Electrical field in the same plane of the ariel and the magnetic field perpendicular to the electrical field.

### Variations in ElectroMagnetic Spectrum

EM waves can travel through vacuum and has a wide spectrum which vary in their wave length, frequency and energy levels. Visible light is the only part of EM Spectrum seen by a naked eye. At the higher end we have the X Ray spectrum having higher frequencies and lower wavelength. On the other hand, long wave audio spectrum has lower frequencies and higher wavelength. Short waves have high levels of energy which are harmful to the human body. Irrespective of their frequency or wavelength, all EM waves travel at the speed of light in vacuum, which is equal to 1,62,000 nm/sec or 3 x 108 m/sec

### Polarisation of ElectroMagnetic Wave

We are aware that the electrical and magnetic components of EM waves are perpendicular to each other. The electrical component is the one which carries the message. Therefore, polarisation is named after the electrical component of the wave. Polarisation of a wave is the plane of the electrical component of the wave.

### Horizontal and Vertical Polarisation

If the electrical component is horizontal to the earth’s surface, it is called as a horizontally polarised wave. Similarly, if the electrical component is vertical to the earth’s surface, it is called as a vertically polarised wave. Vertical and horizontal ariels transmit vertically and horizontal polarised waves respectively. Similarly, vertically polarised wave can be received only by a vertical ariel and horizontally polarised wave can be received only by a horizontal ariel.

### Circularly Polarised Waves

Circularly polarised waves are created by a helical antenna which has the benefit of being received by many types of antennae. Circular polarisation is used if the polarisation is likely to change during transmission. Circular polarisation has the additional benefit of reducing rain clutter which is used in radars. However, generating circularly polarised waves require high power transmitters.