Physics · Ch 16 — Communication Systems
Propagation of Electromagnetic Waves
16.6
Propagation of Electromagnetic Waves
Propagation of Electromagnetic Waves
Once a transmitting antenna radiates an electromagnetic wave, its strength keeps decreasing as it travels away from the transmitter towards the receiving antenna. How the wave actually propagates depends on several factors, including the structure of the earth's atmosphere. The atmosphere's relevant layers, and how each affects propagation, are summarised below.
Table 15.3 - Different Layers of the Atmosphere and Their Interaction with Propagating Electromagnetic Waves
| Layer (stratum) | Approximate height | Exists during | Frequencies most affected |
|---|---|---|---|
| Troposphere | ~10 km | Day and night | VHF (up to several GHz) |
| D (part of stratosphere) | 65-75 km | Day only | Reflects LF; partly absorbs MF and HF |
| E (part of stratosphere) | ~100 km | Day only | Helps surface waves; reflects HF |
| F1 (part of mesosphere) | 170-190 km | Daytime (merges into F2 at night) | Partly absorbs HF, letting some through to F2 |
| F2 (thermosphere) | 250-400 km daytime, ~300 km at night | Day and night | Efficiently reflects HF, especially at night |