The frequency suitable for beyond-the-horizon communication using sky waves is

What should be the height of a transmission antenna (in $m$) to cover a range of $128 \, km$?

If a receiving station on the ground is receiving a frequency of $10 \text{ MHz}$, the mode of propagation is...

The modulation index used in amplitude modulation of a message signal of peak voltage $10 \,V$ is $0.5$. By how much should the peak voltage of the carrier signal be changed, so as to obtain the modulation index of $0.8$, keeping the peak voltage of the message signal the same as before?

$A$ carrier wave of peak voltage $10 \,V$ 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 $\,V$)?

For an amplitude-modulated wave, the audio frequency is $500 \, \text{cycles/second}$. What is the actual carrier frequency in $\text{cycles/sec}$?