Question 20.17: A television signal can be transmitted using a coaxial cable...
A television signal can be transmitted using a coaxial cable to pass an analogue signal, using a space wave in the UHF band, using a sky wave linked to a satellite or using fibre optic cable and the internet.
a Explain what is meant by the terms in bold type.
b State approximate values for the frequencies and wavelengths used by the carrier wave in each case.
c Give one advantage and one disadvantage of each of these four methods.
a Coaxial cable contains a central copper core surrounded by an insulator and a mesh, or braid, made of copper wires made into a tube.
An analogue signal can have any value, within limits, and is not restricted to just a few values.
A space wave is an electromagnetic wave above 30 MHz that is not reflected or refracted back to Earth by the ionosphere and may be received by a satellite in space or be detected by line of sight within the Earth’s atmosphere.
A sky wave is an electromagnetic wave that is reflected or refracted back to the Earth by the ionosphere.
b Space wave frequency greater than 30 MHz, wavelength less than 10 m Sky wave frequency between 3 and 30 MHz, wavelength between 10 and 100 m
c
| Method | Advantage | Disadvantage |
| coaxial | less attenuation, noise, interference than wire pair | more expensive than wire pair |
| analogue signal | no ADC and DAC conversion required | more noise, interference than digital |
| space wave | can be received by satellite | satellite use expensive |
| sky wave | worldwide reception possible with multiple reflections | reflection by the ionosphere is not reliable |
Other answers are possible.