What is the modulation index of an over modulated wave?

If a number of sine waves with modulation indices $n_{1}, n_{2}, n_{3} \ldots \ldots \ldots$ modulate a carrier wave, then the total modulation index (n) of the wave is

In space communication, the information can be passed from one place to another at a distance of $100 \mathrm{km}$ in

The maximum usable frequency in F-layer with the original frequency of $50 \mathrm{MHz}$ will be

In which layer of the atmosphere does the temperature decrease with height?

If the frequency of electromagnetic radiations is halved, then the energy of electromagnetic radiation will become

The sky wave propagation is suitable for radio waves of frequency

Mesosphere reflects which of the following types of wave, particularly at night?