The radio horizon of the transmission of an antenna placed on the $20^{\text{th}}$ floor in a shopping mall,where the height of each floor is $2 \,m$,is (Radius of Earth is $6.4 \times 10^6 \,m$): (in $\,km$)

$A$ message signal of frequency $20 \, kHz$ and peak voltage of $20 \, V$ is used to modulate a carrier wave of frequency $1 \, MHz$ and peak voltage of $20 \, V$. The modulation index will be ..... .

If the sum of the heights of transmitting and receiving antennas in the line of sight $(LOS)$ communication is fixed at $160 \, m$,then the maximum range of $LOS$ communication is $.... \, km$. (Take radius of Earth $R = 6400 \, km$)

In short wave communication, waves of which of the following frequencies will be reflected back by the ionospheric layer having an electron density of $10^{11} \text{ per } m^3$ (in $\text{ MHz}$)?

An antenna is a device:

$A$ signal of frequency $20 \, kHz$ and peak voltage $5 \, V$ is used to modulate a carrier wave of frequency $1.2 \, MHz$ and peak voltage $25 \, V$. Choose the correct statement.