For commercial telephonic communication,the frequency range adequate for speech signals is

$A$ $TV$ transmitting antenna is $128 \ m$ tall. If the receiving antenna is at the ground level,the maximum distance between them for satisfactory communication in line-of-sight mode is (radius of the earth $R_e = 6.4 \times 10^6 \ m$):

$A$ wave is represented as $e = 10 \sin(10^8 t + 6 \sin(1250 t))$. The modulation index is:

The layer of the atmosphere which efficiently reflects high frequency waves particularly at night is

The height of a transmitting antenna is $h \ m$ and the height of a receiving antenna is $45 \ m$. The distance between them is $40 \ km$. For satisfactory $Line\, of\, Sight$ $(LOS)$ communication between the two antennas,the height of the transmitting antenna $h$ must be ....... $m$. (Radius of the Earth $R = 6400 \ km$)

Indicate which one of the following systems is digital.