The waves which are reflected back to the earth by the ionosphere are:

The bandwidth of an audio signal is approximately:

In a communication system,the process of superimposing a low-frequency signal on a high-frequency wave is known as:

The total maximum transmitted power in amplitude modulation if the carrier output power is $1 \ kW$ is

The radio horizon of the transmission of an antenna placed on the $20^{\text{th}}$ floor in a shopping mall,where the height of each floor is $2 \,m$,is (Radius of Earth is $6.4 \times 10^6 \,m$): (in $\,km$)

$A$ $TV$ tower has a height of $5 \, m$ in a region of population density $\frac{1000}{\pi}$ per square kilometre. The number of people that can receive the transmission is nearly, (in thousands)