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Short note: Antenna, types, properties
Short answer
An antenna is a transducer converting guided electrical signals to radiated electromagnetic waves. Common types include wire (dipole), aperture (horn), reflector (parabolic), array (Yagi), and microstrip. Key properties: radiation pattern (directivity, beamwidth), gain, radiation resistance, polarization, bandwidth, and effective aperture.
Write a short note on antenna, its types and properties.
An antenna is a transducer that converts guided electrical signals (current on a line) into radiated electromagnetic waves in free space, and the reverse. It forms the interface between a transmission line or waveguide and free space.
Types
| Class | Examples |
|---|---|
| Wire antennas | Dipole, monopole, loop, helix |
| Aperture antennas | Horn, slot, waveguide opening |
| Reflector antennas | Parabolic dish, corner reflector |
| Array antennas | Yagi-Uda, phased array, log-periodic |
| Microstrip (patch) | Used in mobile phones, GPS |
| Lens antennas | Dielectric lens at microwave frequencies |
Properties
- Radiation pattern: plot of radiated field/power against direction (isotropic, omnidirectional, directional); has main lobe, side lobes and back lobe.
- Directivity D: ratio of maximum radiation intensity to the average (isotropic) value, D = 4πU_max/P_rad.
- Gain G = ηD: directivity multiplied by radiation efficiency η = P_rad/P_input.
- Radiation resistance R_r: P_rad = ½I²R_r; for a λ/2 dipole R_r = 73 Ω.
- Beamwidth: angle between half-power points of the main lobe (HPBW); narrow beam means high gain.
- Bandwidth: frequency range over which the antenna keeps its impedance and pattern within limits.
- Polarisation: direction of the E-field: linear, circular or elliptical.
- Input impedance: R_r + loss resistance + jX; matched to the feed line to avoid reflection (low VSWR).
- Effective aperture: A_e = λ²G/4π, the area that captures power from an incident wave.
- Reciprocity: the same antenna has the same pattern and gain for transmitting and receiving.
Example: the half-wave dipole has length λ/2, directivity 1.64 (2.15 dBi), HPBW 78° and an omnidirectional pattern in the plane perpendicular to its axis.
✓ Answer checked by Claude Opus 5.5 (high effort)
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