Radiation and Antennas
Accelerating charges and oscillating currents radiate electromagnetic energy, the basis of antennas and the scattering of waves.
Definition
The study of how time-dependent charge and current distributions generate propagating electromagnetic fields that carry energy to infinity, and of the engineered structures (antennas) and scattering processes that emit, receive, and redirect that radiation.
Scope
This area covers the production of electromagnetic radiation by time-varying sources: retarded potentials, the fields of accelerating charges, dipole and multipole radiation, the design and analysis of antennas and arrays, radiation resistance and patterns, and the scattering of electromagnetic waves by objects. It connects Maxwell's equations to practical radiating and receiving systems, while guided propagation is treated under electromagnetic waves.
Sub-topics
Core questions
- How do accelerating charges and oscillating currents produce radiation?
- What determines the radiation pattern and power of a source?
- How are antennas characterized and combined into arrays?
- How do objects scatter incident electromagnetic waves?
Key concepts
- retarded potential
- radiation field
- Larmor formula
- electric dipole radiation
- radiation pattern
- gain and directivity
- antenna array
- scattering cross section
Key theories
- Retarded potentials and radiation fields
- The potentials of time-varying sources depend on the source at the earlier, retarded time; far from the source the fields fall off inversely with distance and carry energy away as radiation.
- Dipole radiation
- An oscillating electric dipole radiates power proportional to the fourth power of frequency with a characteristic angular pattern, the prototype for most radiating systems.
- Antenna theory
- Antennas are characterized by radiation pattern, gain, directivity, radiation resistance, and impedance, and arrays combine elements to shape and steer the radiated beam.
Clinical relevance
Radiation and antenna principles enable radio, television, mobile and satellite communications, radar and remote sensing, radio astronomy, and the radiofrequency coils and exposure assessment used in magnetic resonance imaging.
History
Hertz built the first deliberate radiating and receiving system in 1887-1888, confirming Maxwell's waves. Larmor derived the power radiated by an accelerating charge in 1897, and Marconi turned radiation into practical long-distance wireless communication around 1900.
Key figures
- Heinrich Hertz
- Joseph Larmor
- Guglielmo Marconi
Related topics
Seminal works
- jackson1998
- balanis2016
Frequently asked questions
- What is required for a charge to radiate?
- A charge moving at constant velocity does not radiate; radiation requires acceleration, so oscillating or accelerating charges and time-varying currents are the sources of electromagnetic waves.
- What makes a good antenna?
- An antenna's usefulness depends on matching its size to the wavelength and on its radiation pattern, gain, and impedance match to the feed; arrays of elements let engineers shape and steer the beam.