Transmission media

Figure 14 shows a transmitter T and a receiver R which are not in line of sight. Three transmission pathways from T to R are shown.

Explain, for each of the ground wave, sky wave and space wave:

• the conditions necessary for the pathway to operate

• one advantage and one disadvantage of using that pathway.

Figure 7 shows a block diagram for part of a communication system that transmits control information from Earth to a satellite.

Syncom 3 was the first geostationary communication satellite. The satellite was used to broadcast television signals of the 1964 Olympic Games from Tokyo in Japan to Los Angeles in the USA.

State what is required so that de-sensing does not occur in a radio receiver such as that on Syncom 3.

Commentators in Los Angeles interviewed athletes in Tokyo using Syncom 3.

The commentators found that there was a significant time delay between the end of each question and the arrival of its reply.

This time delay was made up of:

• the total time that the signals spent travelling

• the total processing time.

The distance of the geostationary satellite from both Tokyo and Los Angeles was 40 000 km.

The time delay was 900 ms.

Calculate, in ms, the total processing time.

The television signals received in the USA were relayed to the UK using a satellite in low Earth orbit. The availability of live coverage was different for viewers in the USA and viewers in the UK.

Explain why.

For the 2020 Olympics, digital TV signals between Tokyo and Los Angeles used a fibre-optic cable. The cable lies on the seabed.

Explain why a fibre-optic cable rather than a satellite link is now preferred for intercontinental communication.

Figure 8 shows island A, a fully developed island off the mainland coast. The island is connected to the mainland by a fibre optic cable lying along the seabed and it also has a satellite link.

Nobody lives on island B, but it is due to be developed as a major holiday resort over the next 5 years.

Moveable oil rig C is due to explore the four sites marked ‘ X ’ for oil and gas over a 9-month period.

A communications company has been asked to provide solutions for island B which will allow the development to begin immediately and then later to support a fully developed holiday resort.

A communications solution is also required for oil rig C during the 9-month exploration period.

Describe appropriate solutions involving fibre optic cabling and satellite communication systems for each of the two clients, island B and oil rig C.

In your answer you should:

• outline the way each communications system operates

• suggest, with reasons, your choice of system for each solution.

Table 3 shows some communication applications that transmit using different regions of the electromagnetic spectrum.

Explain why each transmission takes the pathway it does from the transmitter to the receiver.

For each of the spectrum regions, you should:

• indicate a frequency range

• refer to the properties of the wave

• name the pathway and outline its properties.

You may use diagrams to help explain your answer.

Space for diagrams

British embassies in Europe are to be connected to a new long-distance communication link. The link, in the form of a land-based cable, will support multiple simultaneous video conferencing as well as the transmission of sensitive government data.

The company installing the link has to consider the choice between using optic fibre or copper wire in the cables.

Compare the advantages and disadvantages of the two options for use in these cables. State which option you would advise the company to use.

For both types of cable refer to their:

• physical properties

• ability to reject external interference

• signal-carrying properties.

Figure 4 shows a block (subsystem) diagram for a radio communication system.

State the letter representing the subsystem in which you might find an induced emf

State the letter representing the subsystem where the audio and radio waves are

The signal strength at stage D must be amplified.

Explain why the signal strength at stage D is weak.

Figure 5 shows graphs of a radio carrier signal and a modulating sine wave.

Complete the graph in Figure 5 to represent the combined amplitude modulated (AM) signal.

Approximately 20 radio stations use amplitude modulation (AM) to broadcast to people living in the London area. Another 35 AM stations broadcast to people outside the London area. However, these broadcasts can still be received in London.

The allocated frequency spectrum for all these broadcasts is in the range 540 kHz to 1600 kHz

Suggest whether all these stations can broadcast hi-fi music using the full audio frequency of 20 kHz

For each link, you should give:

a typical carrier frequency that is used

an explanation of how the signals travel from the transmitter to the receiver

a typical use.

You may use a diagram to help make clear aspects of your answer.

Extra space is available on the next page if needed

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