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

$A$ transmitting antenna has a height of $20 \,m$. What will be the height of the receiving antenna if the maximum distance between them for satisfactory communication in line-of-sight $(LOS)$ mode is $40 \,km$ (in $\,m$)? (The radius of the Earth is $6400 \,km$.)

The government has divided the frequency range from $30\ MHz$ to $60\ MHz$ between $20$ stations,which are independently beaming their signals. Which of the signals can be beamed by each of the stations?

If the maximum and minimum voltage of an $AM$ wave are $V_{max}$ and $V_{min}$,then the modulation index is given by:

Basically,the product modulator is

The attenuation in optical fibre is mainly due to