Q.Which layer of the ionosphere has the greatest effect on a radio signal?

The ionosphere is the electrically charged region of the upper atmosphere that makes sky-wave communication possible. Radiation from the sun, especially ultraviolet rays, strikes the thin gases high above the earth and knocks electrons free, ionising them. This produces a thick but invisible shell of charged particles extending over altitudes of roughly 50 km to 400 km, whose free charges refract and reflect radio waves back toward the ground.

The ionosphere is treated as three basic layers, distinguished by altitude and degree of ionisation:

• D layer — lowest, about 70 km up and roughly 10 km thick; only weakly ionised and it disappears at night. It reflects VLF and LF waves but absorbs MF and HF waves.
• E layer (the Kennelly–Heaviside layer) — about 100 km up and roughly 25 km thick; also weakly ionised and vanishes at night. It helps MF propagation and partly assists HF. …