Q.Explain the three modes of propagation of electromagnetic waves through space.

Step 1. Ground wave (surface wave) propagation, ~2 kHz to 2 MHz. The wave glides directly along the Earth's surface from transmitter to receiver, so both antennas must be close to the ground. It is attenuated by (i) increasing distance (depending on transmitter power, frequency, and the Earth's surface condition), (ii) absorption of energy by the Earth (which behaves like a leaky capacitor as the wave induces currents in it), and (iii) tilting of the wavefront along the Earth's curvature, which eventually kills the wave. High frequencies suffer more absorption, so ground waves stay below ~2 MHz; used for local broadcasting, radio navigation, ship-to-ship/shore communication, and mobile communication.

Step 2. Sky wave (ionospheric) propagation, ~3 MHz to 30 MHz. Waves radiated upward at large angles are reflected by the ionosphere back to Earth. The ionosphere (~50-400 km altitude) is ionised by UV/cosmic/solar radiation, forming a reflecting layer; waves above ~30 MHz penetrate through it instead of reflecting. Multiple reflections let the wave bounce repeatedly between Earth and ionosphere, enabling very long-distance communication (a single reflection covers ~4000 km); used for short-wave broadcast and long-distance radio communication. Skip distance is the shortest surface distance to where the reflected sky wave is received; the skip zone is the region (between where ground-wave reception fails and sky-wave reception begins) where nothing is received at all.

Step 3. Space wave propagation, above ~30 MHz. The wave travels essentially in a straight line directly from transmitter to receiver -- line-of-sight (LOS) communication. Towers must be tall enough that the direct path does not encounter the Earth's curvature. It underlies TV telecast, satellite communication and RADAR, and favours microwaves for their larger bandwidth, higher data rates, better directivity, smaller antenna, and lower power. The maximum coverage distance depends on antenna height via $d=\sqrt{2Rh}$, and for two antennas of heights $h_1,h_2$, the total range is $d=\sqrt{2Rh_1}+\sqrt{2Rh_2}$.

Cover each of the three propagation modes in turn: its frequency range, its physical mechanism, and its characteristic applications/limitations.

• Mixing up the frequency ranges of the three modes.
• Describing only the mechanism without stating the frequency range or applications, both of which examiners look for.