15.1 Which of the following frequencies will be suitable for beyond-thehorizon communication using sky waves?
(a) 10 kHz
(b) 10 MHz
(c) 1 GHz
(d) 1000 GHz


15.2 Frequencies in the UHF range normally propagate by means of:
(a) Ground waves.
(b) Sky waves.
(c) Surface waves.
(d) Space waves.


15.3 Digital signals
(i) do not provide a continuous set of values,
(ii) represent values as discrete steps,
(iii) can utilize binary system, and
(iv) can utilize decimal as well as binary systems.
Which of the above statements are true?
(a) (i) and (ii) only
(b) (ii) and (iii) only
(c) (i), (ii) and (iii) but not (iv)
(d) All of (i), (ii), (iii) and (iv).


15.4 Is it necessary for a transmitting antenna to be at the same height
as that of the receiving antenna for line-of-sight communication? A
TV transmitting antenna is 81m tall. How much service area can it
cover if the receiving antenna is at the ground level?


15.5 A carrier wave of peak voltage 12V is used to transmit a message
signal. What should be the peak voltage of the modulating signal in
order to have a modulation index of 75%?


15.6 A modulating signal is a square wave, as shown in Fig. 15.14.

The carrier wave is given by c t t ( ) 2sin(8 ) = π volts.
(i) Sketch the amplitude modulated waveform
(ii) What is the modulation index?


15.7 For an amplitude modulated wave, the maximum amplitude is found
to be 10V while the minimum amplitude is found to be 2V. Determine
the modulation index, µ.
What would be the value of µ if the minimum amplitude is zero volt?


15.8 Due to economic reasons, only the upper sideband of an AM wave is
transmitted, but at the receiving station, there is a facility for
generating the carrier. Show that if a device is available which can
multiply two signals, then it is possible to recover the modulating
signal at the receiver station.