wireless communications- radio waves...

HideShow resource information
View mindmap
  • wireless communications- radio waves...
    • long wavelengths travel well through earth atmosphere...
      • radio waves and microwaves are good at transferring information over long distances
      • this is because they dont get absorbed by the earths atmosphere as much.
    • uses of radio waves
      • radio waves are used for broadcasting radio and tv programmes
    • properties of radio waves
      • radio waves have lower frequencies and longer wavelengths than microwaves.
      • television uses higher frequencies than radio
      • A radio programme does not need to be directly in view of the transmitter to receive signals
      • for low frequency, radio waves, diffracion allows waves to be bent around tall buildings and obstacles.
    • radio waves are used mainly for communications
      • radio waves are EM radiation with wavelengths longer than 10cm
      • different wavelengths of radio wave, refract and diffract in different ways
      • long wave radio (wavelegnths of 1-10km) can be diffracted around the curved surface of the earth
        • Key Information long wave signals can be diffracted around the earth!!
      • radio waves that are used for TV and FM radio, are very short... (10cm -10m) to get reception,, you must be in direct sight of the transmitter
      • short waves radio signals (about 10m-100m) can be recieved at long distances from the transmitter because of the waves being reflected off the ionosphere
    • diffraction makes a difference to signal strength
      • Diffraction-when waves spread out/apart when the waves pass through a gap or past an object....
        • the amount of diffraction depends on the wavelegnth of the wave relative to the size of the gap
      • longer wavelegnths can encounter a lot of diffraction, this means that the waves can bend around tall buildings ect.
        • as a result of this, waves with longer wavelegnths can travel FURTHER between the transmitter and reciever.
          • Because of this they DO NOT have to be in line of sight with each other...
      • shorter wavelength radio waves, do not diffract as much...
        • ... as a result, transmitters need to be located high up in order to avoid obstacles.
      • some area have trobule recieving shorter wavelegnth radio signals
    • refraction is also affected by wavelength
      • when a wave come up against something of a different density, it changes speed... if the wave hits the new substance at an angle, it changes direction..... this is called rerfraction
        • when this happens high up in the atmosphere, it can travel further for long distance communications
      • radio waves travel faster through ionised parts of the atmosphere than non ionised parts. this causes refraction...
      • short waves and medium wave radio signals are refracted mostly in the ionosphere- these are effectively reflected back to earth...
      • the amount that a wave is refracted in the ionosphere depends on its frequency and angle of elevation...
        • >short wave radio doesnt refract as much as medium wave...
      • radio waves bounce off the ionosphere
      • refraction is not always good... it can disrupt a signal by bending it away from the receiver dish...
    • digital radio helps reduce interferance
      • radio stations often broadcast using similar frequencies because there's a limited number of radio wave frequencies that can be used to transmit a good digital signal
      • as a result, many analogue signals suffer from interference because of this
      • noise-similar waves covering a similar area combine
      • DAB(digital audio broadcast) works different to traditional radio- the signal is digital from the start
    • DAB
      • with DAB, many different signals are compressed and transmitted as a single wave.... this is called multiplexing...
      • many signals are transmitted along a small frequency bandwidth, and separated out at the end by receivers
      • DAB suffers less interference than normal radio, and thanks to multiplexing many stations can broadcast at the same frequency
      • even if you receive DAB, often the sound quality is affected because of signal compression


No comments have yet been made

Similar Science resources:

See all Science resources »See all Physics resources »