Space wave propagation is possible for which frequency range?

An amplitude modulated signal consists of a message signal of frequency $1 \text{ kHz}$ and peak voltage of $5 \text{ V}$,modulating a carrier frequency of $1 \text{ MHz}$ and peak voltage of $15 \text{ V}$. The correct description of this signal is

An amplitude modulated wave is represented by $C_{m}(t)=10(1+0.2 \cos 12560 t) \sin (111 \times 10^{4} t)$ volts. The modulating frequency in $kHz$ will be ........... .

What is the minimum height of an antenna required to transmit a radio signal of frequency $20 \text{ MHz}$?

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).

Which characteristic of a carrier light wave is modulated for the modulation of a light wave?