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Define antenna and EMW propagation

Short answer

An antenna converts electrical signals to electromagnetic waves and vice versa. EM waves consist of oscillating perpendicular electric and magnetic fields traveling at 3×10^8 m/s without a medium. Propagation: ground wave follows Earth surface (<2 MHz); sky wave reflects from ionosphere (2-30 MHz); space wave is line-of-sight (>30 MHz).

Define antenna and electromagnetic wave (EMW) propagation.

Antenna

An antenna is a transducer (a metallic conductor or structure) that converts electrical signals into electromagnetic waves for radiation into free space (transmitting antenna), and converts the incoming electromagnetic waves back into electrical signals (receiving antenna). It is the interface between the transmission line and free space. Examples are a dipole, a monopole (whip), a loop, a Yagi-Uda array, and a parabolic dish.

Its important parameters are gain, directivity, radiation pattern, bandwidth and impedance.

EMW propagation

Electromagnetic waves are fields of mutually perpendicular electric and magnetic components that travel at c = 3 × 10⁸ m/s in free space without any medium. EMW propagation means the way the wave travels from the transmitting antenna to the receiving antenna. There are three modes.

  1. Ground wave: follows the earth's surface, for frequencies below about 2 MHz (AM broadcasting).
  2. Sky wave: reflected back from the ionosphere, for 2 to 30 MHz (short-wave, long-distance).
  3. Space wave (line of sight): straight path between antennas, for above 30 MHz (FM, TV, microwave, satellite).
      ionosphere  ~~~~~~~~~~~~~~
          sky wave /          \
 Tx ===== space wave ======== Rx
  \____ ground wave _________/

✓ Answer checked by Claude Opus 5.5 (high effort)

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