P1g(ii) Wireless Signals You will learn about: How radio signals are transmitted around the Earth How radio waves are diffracted

Ionosphere The ionosphere is a layer in our atmosphere. Radio waves can be reflected off the ionosphere as well as by water. Land is not a good reflector of Electromagnetic waves. Similarly to optical fibres, radio waves undergo skywave propagation – that is they totally internally reflect within the atmosphere boundary. In the image you can see how radio waves can reach receivers when they are not in line of sight by reflecting off the ionosphere and the oceans. A radio wave can circumnavigate the entire globe in just seconds permitting there are perfect weather conditions. RECALL: For refraction to occur density must change. The higher the altitude the lower the density. As a radio wave travels higher it bends more and more due to the lower dense medium it is in. This causes it to bend right round and be reflected back to Earth. Radio waves also spread out the further they travel. This spreading out is called diffraction. In this image you can see how the grey radio waves diffract around the hill so that the television in the house can still receive the signal. The red microwaves do not diffract very much at all. They hit the hill and will reflect back. They will not be picked up by the receiver in the house

Microwaves Microwaves have frequencies much higher than radio waves. Microwaves pass through the atmosphere with very little refraction. A communication satellite, 36,000 km above the equator will receive the microwave signals and re- transmit them back to Earth There are thousands of communication satellites in space so it is easy to transmit signals this way.

Communication Issues Radio waves are refracted by the upper atmosphere. When transmitting a signal to a satellite in space the signal needs to reach it otherwise it cannot transmit it back to Earth. The satellite in space only has a small aerial dish. So the signal needs to be sent slightly diverging and with a large focused amount of energy. If the wave reaches the edge of the curved dish it will diffract away and not reach the receiver. The blue arrows represent how a transmitted signal refracts at the edge of the dish. They will not reach the communications satellite. The red signals will. Transmitting aerial

Questions 1.Using the image explain why the person in the house cannot receive mobile phone signals but can listen to some radio stations. 2.Signals transmitted to space are never sent as a parallel beam. Describe why. 3.Why are microwave aerial dishes so much smaller than radio wave dishes? 4.A microwave has a typical frequency of 5 mm. Why would an aerial dish of 20 cm be used to transmit it?

Questions 1.Using the image explain why the person in the house cannot receive mobile phone signals but can listen to some radio stations. Mobile phone signals use short radio waves or microwaves. They will not diffract around obstacles like a lower frequency radio waves will. 2.Signals transmitted to space are never sent as a parallel beam. Describe why. They need to diverge in order to maximise the amount of signal to reach the dish. 3.Why are microwave aerial dishes so much smaller than radio wave dishes? Microwaves have shorter wavelengths than radio waves. 4.A microwave has a typical frequency of 5 mm. Why would an aerial dish of 20 cm be used to transmit it? An aerial of 5 mm would have diffraction effects at the edges. A larger the aerial dish the lower the amount of diffraction. This means the receiver has a greater chance of picking up the signal.