The maximum usable frequency $(MUF)$ in the $F$-region layer is $x$,when the critical frequency is $60 \, MHz$ and the angle of incidence is $70^\circ$. Then $x$ is ....... $MHz$.

Radio waves of constant amplitude are produced by a/an . . . . . . .

$A$ $TV$ transmitting antenna is $128 \ m$ tall. If the receiving antenna is at the ground level,the maximum distance between them for satisfactory communication in line-of-sight mode is (radius of the earth $R_e = 6.4 \times 10^6 \ m$):

$A$ carrier is simultaneously modulated by two sine waves with modulation indices of $0.3$ and $0.4$; then the total modulation index is

Which of the following statements is incorrect regarding Amplitude Modulation $(AM)$?

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?