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

The electron density of $E, F_1, F_2$ layers of the ionosphere is $2 \times 10^{11} \, m^{-3}$,$5 \times 10^{11} \, m^{-3}$,and $8 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of critical frequencies for the reflection of radio waves?

The frequency range for commercial $FM$ radio broadcasting is .......

The characteristic impedance of a coaxial cable is of the order of .......... $\Omega$.

In a communication system operating at a wavelength of $800 \,nm$, only $1 \%$ of the source frequency is available as signal bandwidth. The number of channels accommodated for transmitting $TV$ signals of bandwidth $6 \,MHz$ is

Sound waves cannot be transmitted directly after conversion into electrical waves because...