Space wave propagation is possible for which frequency range?

The maximum frequency of transmitted radio waves above which the radio waves are no longer reflected back by the ionosphere is . . . . . . ($N=$ maximum electron density of the ionosphere, $g=$ acceleration due to gravity).

For an amplitude modulated wave,the maximum and minimum amplitudes are found to be $10 \ V$ and $2 \ V$ respectively. Then the modulation index is

In amplitude modulation,

The height of a transmitting antenna at the top of a tower is $25 \; m$ and that of the receiving antenna is $49 \; m$. The maximum distance between them for satisfactory communication in $LOS$ (Line of Sight) is $K \sqrt{5} \times 10^{2} \; m$. The value of $K$ is $\dots$ [Assume radius of Earth is $64 \times 10^{5} \; m$] (Calculate up to the nearest integer value).

An amplitude modulated wave is represented by $V_{AM} = 10[1 + 0.4 \cos(2 \pi \times 10^4 t)] \cos(2 \pi \times 10^7 t) \text{ V}$. The total bandwidth of the amplitude modulated wave is: