A station on Earth transmits a signal of initial power 3.1 kW to a geostationary satellite. The attenuation of the signal received by the satellite is 190 dB.

Question 7

(a) Information may be carried by different channels of communication.

State one application, in each case, where information is carried using

(i) microwaves, [1]

(ii) coaxial cables, [1]

(iii) wire pairs. [1]

(b) A station on Earth transmits a signal of initial power 3.1 kW to a geostationary satellite.

The attenuation of the signal received by the satellite is 190 dB.

(i) Calculate the power of the signal received by the satellite. [2]

(ii) By reference to your answer in (i), state and explain the changes made to the signal

before transmission back to Earth. [3]

Reference: Past Exam Paper – November 2014 Paper 41 & 42 Q12

Solution:

(a)

(i) e.g. satellite communication, mobile phones, line of sight communication, wifi

(ii) e.g. connection of TV to aerial, loudspeaker, microphone (if clearly identified)

(iii) e.g. a.f. amplifier to loudspeaker, landline for phone

(b)

(i)

Attenuation (in dB) = 10 lg (P_IN / P_OUT)

P_OUT is the power received by the satellite and P_IN is the initial power of the signal.

190 = 10 lg (3.1×10³ / P_OUT)

lg (3.1×10³ / P_OUT) = 190 / 10

3.1×10³ / P_OUT = 10¹⁹

P_OUT = 3.1×10³ /10¹⁹ = 3.1×10^-16 W = 3.1×10^-19 kW

(ii) The signal is amplified and its frequency is changed to prevent swamping of the up-link signal by the down-link (signal).