The electron density of the ionospheric layer is $10^{11} \ m^{-3}$. Communication waves of frequency up to $f_c$ $MHz$ will be reflected by the ionospheric layer. Find $f_c$.

In which frequency range are space waves normally propagated?

If the maximum and minimum amplitudes of a modulated wave are $25 \ V$ and $5 \ V$ respectively,then the modulation index is

Which frequency range is allocated for commercial $FM$ radio broadcasting?

Consider the following statements regarding satellite communication:
$1$. Frequencies between $1.5 \text{ MHz}$ and $1 \text{ MHz}$ are used in satellite communication.
$2$. The frequencies for uplink and downlink are different.
$3$. The orbit of a geostationary satellite makes an angle of $0^{\circ}$ with the equatorial plane of the Earth.
Which of the above statements are correct?

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