One of the layers in the Earth’s atmosphere is called the ionosphere. Radio waves can be reflected from the ionosphere. Water is also able to reflect radio waves, but land masses are not such good reflectors.

The propagation of radio signals from one place to another because of reflection from the ionosphere is known as sky wave propagation. Radio waves in the ionosphere behave in a similar way to light rays in an optical fibre – they undergo total internal reflection.

As a radio wave travels up in the ionosphere, it passes from a denser to a less dense medium. It continuously bends away from its path until it is totally reflected back to Earth. Coupled with reflection from the oceans, radio signals can be received well beyond line of sight.

Microwaves are not reflected back to the surface of the Earth. They pass through the ionosphere and are received by satellites orbiting the Earth. The satellite amplifies the signal and then re-transmits the signal back to Earth.

Radio waves are diffracted when they meet an obstruction. The diffraction is only significant if the size of the obstacle is similar to the wavelength (see Figure 7).

Communications satellites orbit the Earth every 24 hours at a height of 36 000 km above the equator.

Communication problems

The refraction of radio waves in the atmosphere needs to be taken into account, particularly when transmitting a signal to a communications satellite. The size of the aerial dish on the satellite is not very large and a focused beam of energy needs to be transmitted.

There will be some diffraction at the edges of the dish. The wavelength of the microwaves must be small compared to the dish diameter to reduce this diffraction.

Many radios now receive digital signals and this reduces the amount of interference, especially between radio stations.