Q.In sky wave mode of propagation, why is the frequency range of transmitting signals restricted to less than 30 MHz?

Part (a)

Sky-wave communication relies on the ionosphere reflecting the signal back to Earth, which happens only while the frequency stays below the ionosphere's (angle-adjusted) plasma/critical frequency. The critical frequency of the reflecting layers is only about 10–12 MHz, and even at oblique incidence reflection fails above roughly 30 MHz — such waves penetrate the ionosphere and escape into space instead of returning. …

Part (a): Why Sky-Wave Frequencies Are Below ~30 MHz

The ionosphere acts as a frequency-selective mirror. A wave of frequency $f$ is reflected only if $f$ is below the local plasma frequency $f_p=9\sqrt{N_e}$ (adjusted upward for oblique incidence), where $N_e$ is the electron density.

1. The reflecting layers have critical frequencies of only about 10–12 MHz for vertical incidence.
2. At oblique incidence the maximum usable frequency rises (secant law) to around 30 MHz.
3. For $f\gtrsim30$ MHz the free electrons cannot respond fast enough; the wave passes through the ionosphere and escapes into space rather than returning to Earth. …