The size of the antenna required for a carrier wave of frequency $3 \text{ MHz}$ is: (in $\text{ m}$)

The attenuation in optical fibre is mainly due to

$A$ ground receiver station is receiving a signal at $5\, MHz$ transmitted from a ground transmitter at a height of $300\, m$,located at a distance of $100\, km$ from the receiver station. Given: Radius of Earth $R = 6.4 \times 10^6\, m$,$N_{\max}$ of ionosphere = $10^{12}\, m^{-3}$. Identify the mode of propagation.

In a communication system,a repeater is used to extend the range of transmission. It is a combination of:

Indicate which one of the following systems is digital.

In amplitude modulation,the amplitude of the $.......$ wave varies in proportion to the instantaneous value of the $.......$ wave.