Which device is used to convert one form of energy into another?

$A$ carrier wave of peak voltage $15 V$ is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of $80\%$ (in $V$)?

The maximum distance between the transmitting and receiving antennas for satisfactory communication in line of sight mode is $57.6 \ km$. If the height of the receiving antenna is $80 \ m$,the height of the transmitting antenna is (Radius of earth $= 6.4 \times 10^6 \ m$) (in $m$)

Broadcasting antennas are generally

If a transmitting antenna of height $105 \,m$ is placed on a hill, then its coverage area is (in $\,km^2$)

For the propagation of $10 \ MHz$ signals as sky waves,what should be the minimum electron density in the ionosphere?