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

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

The maximum distance up to which $TV$ transmission from a $TV$ tower of height $h$ can be received is proportional to

The radio horizon of a transmitting antenna of height $39.2 \ m$ is (Radius of the earth $= 6400 \ km$) (in $km$)

For effective transmission of a wave with frequency $3000 \text{ kHz}$,the minimum length of the antenna should be ...... $m$.

The characteristic impedance of a coaxial cable is of the order of .......... $\Omega$.