In a communication system,what is used to extend the range of a communication?

An amplitude modulated wave is represented by $C_{m}(t)=10(1+0.2 \cos 12560 t) \sin (111 \times 10^{4} t)$ volts. The modulating frequency in $kHz$ will be ........... .

The electron density of $E, F_1, F_2$ layers of the ionosphere is $2 \times 10^{11} \, m^{-3}$,$5 \times 10^{11} \, m^{-3}$,and $8 \times 10^{11} \, m^{-3}$ respectively. What is the ratio of critical frequencies for the reflection of radio waves?

The maximum amplitude of an amplitude modulated wave is $16 \,V$, while the minimum amplitude is $4 \,V$. The modulation index is

For transmitting a signal of frequency $1000 \text{ kHz}$, the minimum length of the antenna is (in $\text{ m}$)

For sky wave propagation of a $10\, MHz$ signal, what should be the minimum electron density in the ionosphere?