P1 Revision Cards- Wireless Signals

• Wireless technology is used by:
  • radio and television
  • laptops
  • mobile phones
• Radio waves are reflected and refracted in the Earth's atmosphere:
  • The amount of refraction depends on the frequency of the wave.
  • There is less refraction at higher frequencies.
• Radio stations broadcast signas with a particular frequency.
• The same frequency can be used by more than one radio station:
  • The radio stations are too far away from each other to interfere.
  • But in unusual weather conditions, the radio waves can travel futher, and the radio waves can interfere.

Interferance is reduced if digital signals are used.

• Digital Audio Broadcasting (DAB) also provides a greater choice of radio stations but the audio quatlity isn't as good as the FM signals currently used.
• DAB eliminates interferance between other radio stations.

• Radio waves are reflected from ionosphere. They behave like light in an optical fibre and undergo total internal reflection.
  • Ionosphere- a region of the Earth's atmosphere where ionisation caused by incoming solar affects the transmission of radio waves; it extends from 70km to 400km above the Earth's surface.
  • Ionisation- The formulation of ions (charged particles).

• Water reflects radio waves but land mass does not.
• Continued reflection by the ionosphere and the oceans allow radio waves to be recieved from an aerial that is not the line of sight.
• Microwaves pass through the ionosphere.
• Microwave signals are recieved by orbiting satellites, amplified and retransmitting back to Earth.
• Communication satellites orbit above the equatorand take 24 hours to orbit Earth.

• Radio waves are diffracted when they meet an obstruction.
• Refraction in the atmosphere needs to be taken into account when sending a signal to a satellite.
• The transmitting aerial needs to send a focused beam to the satellite because its aerial is very small.
• The transmitted beam is slightly divergant.
• Some energy is lost from the edge of the transmitting aerial becuse of diffraction.