P1g(i) Wireless Signals You will learn about: How Radio waves are reflected and refracted How interference occurs The advantages of wireless technology

Mobile Communications Radio waves and Microwaves are used to send communication wirelessly. Just like all parts of the Electromagnetic Spectrum they can be reflected off objects such as buildings and mountains hours a day seven days a week mobile technology is at our fingertips. In the top image you can see that one signal, the ground wave, is in line of sight – it reaches the boat directly. The sky propagated wave has reflected off the ionosphere to reach the boat after the initial ground wave. In the bottom image you can see that the red wave is in line of sight and reaches the house first. The blue wave shows how a radio wave can be reflected off a large hill to the house. Of course the blue second wave reaches the house later than the first red wave. A reflected signal travels much further than the signal that is in line of sight. An analogue receiver can pick up the two signals. An analogue television shows this as ghosting – the later signal as an image that has been shifted to the right as in the image on the left.

Shadow Zones In this image the aeroplane is flying in the mountain’s shadow. However, the radio signals can still meet the aeroplane even though it is not in direct line of sight. This is because the mountain reflects the radio waves to the aeroplane. The same happens here: the mountain reflects the radio wave so it can reach the receiver. REMEMBER: Waves travel in straight lines. Objects can reflect waves so although they still travel in straight lines they can turn around obstructions to reach a receiver.

Refraction of Radio Waves Radio waves are able to travel vast distances which is why they are useful for communications. A 5 MHz radio wave has a frequency much less than a 100 MHz radio wave. In this image you can see that lower frequency radio waves are refracted more by the atmosphere and returned to the Earth. Whereas higher frequency radio waves are refracted less and hence travel further. Even higher frequency radio waves may even refract through the atmosphere MHz 10 km 200 km Both radio masts transmit a radio wave at the same frequency: 90 MHz. The red signal is in line of sight and is usually the only signal that reaches the house. However, if the weather conditions are poor another same frequency radio signal may be reached from hundreds of kilometres away. When two signals are received together this causes INTERFERENCE.

DAB Digital Audio Broadcasting, DAB, reduces the effects of interference because it utilises digital technology. An analogue radio station needs to be inputted by turning a dial to the relevant frequency. This can be tricky to select the exact frequency you need. With DAB you set a name to your favourite radio station instead of remembering its frequency and it will always play that station. At the moment DAB is not as pure sounding as analogue FM radio but improvements are being made and soon DAB sound quality will exceed FM.

Questions 1.Name three obstacles that can cause ghosting. 2.You are watching television and you notice that you can see two images – one of them is faint and is shifted to the right. What is this called and what has caused it? 3.Heart FM transmits at 107 MHz. Its signal is not reflected back to the Earth. Why not? How could you ensure you could receive its signal? 4.On the South coast of England sometimes you can hear French radio broadcasts. Describe why.

Questions 1.Name three obstacles that can cause ghosting. Tall buildings, tall trees, hills, mountains or poor weather conditions. 2.You are watching television and you notice that you can see two images – one of them is faint and is shifted to the right. What is this called and what has caused it? Ghosting. Caused when a reflected signal reaches the receiver after the line of sight signal does. 3.Heart FM transmits at 107 MHz. Its signal is not reflected back to the Earth. Why not? How could you ensure you could receive its signal? 107 MHz is too high a frequency to be refracted and reflected back to Earth. It refracts through the upper atmosphere and into space. To receive this signal you need to be closer to the radio transmitter or be in its line of sight. 4.On the South coast of England sometimes you can hear French radio broadcasts. Describe why. At lower frequencies radio signals are refracted by the upper atmosphere and are reflected back to Earth, particularly when weather conditions are poor e.g. cloudy or foggy. These signals can travel vast distances so could refract enough to reach the receivers on the south coast if weather conditions are right to do so.