Which type of medium is used for optical communication?

Radio waves that reach the receiver after being reflected from the ionosphere propagate via .......

$A$ message signal of frequency $5\,kHz$ is used to modulate a carrier signal of frequency $2\,MHz$. The bandwidth for amplitude modulation is $..........\,kHz$.

The function of a detector is to demodulate the modulated carrier wave and the steps for this process are

If the area covered by an $FM$ radio station is to be tripled,the height of the antenna must be .......

In which frequency range are space waves normally propagated?