Electric-dipole radiation field and pattern
A localized nonrelativistic electric dipole radiates E_rad≈n×(n×p̈(t−r/c))/(4πε₀c²r) and B_rad=n×E_rad/c.
Key knowledge
A localized nonrelativistic electric dipole radiates E_rad≈n×(n×p̈(t−r/c))/(4πε₀c²r) and B_rad=n×E_rad/c. For p(t)=p₀ cosωt along a fixed axis, the cycle-averaged angular power is ω⁴p₀²sin²θ/(32π²ε₀c³) and the total average power is ω⁴p₀²/(12πε₀c³).
Values and equations
E_rad=n×(n×p̈(t−r/c))/(4πε₀c²r); ⟨dP/dΩ⟩=ω⁴p₀²sin²θ/(32π²ε₀c³); ⟨P⟩=ω⁴p₀²/(12πε₀c³)
- E_rad
- Radiation electric field V m⁻¹
- B_rad
- Radiation magnetic field T
- p,p₀
- Dipole and peak magnitude C m
- p̈
- Dipole second time derivative C m s⁻²
- r,a
- Observation distance and source size m
- n
- Radial unit direction
- t
- Time s
- ω
- Angular frequency rad s⁻¹
- P
- Radiated power W
- Ω
- Solid angle sr
- θ
- Polar angle from dipole axis rad
- ε₀
- Vacuum permittivity F m⁻¹
- c
- Vacuum light speed m s⁻¹
Conditions and scope
- Vacuum source size a≪c/ω, radiation-zone r≫a and r≫c/ω, negligible relativistic internal motion and an electric dipole that is the leading radiating moment. θ is measured from its fixed axis and p₀ is peak amplitude, not RMS.
- Near electrostatic/induction fields are excluded. If the electric dipole vanishes or magnetic/quadrupole terms are comparable, this is not the entire source radiation; amplitude and RMS conventions change numerical factors.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin2 sources
Evidence and review
Classical Electromagnetism (2014): Retarded fields: Equations 92–99: far transverse field from the source-current time derivative and B=E/c; charge continuity gives ∫J=d p/dt. The additional electrically small source condition makes internal retarded phases negligible. Classical Electromagnetism (2014): Antenna theory: Equations 1222–1225 give far transverse E/B and Poynting flux; 1232–1234 give the short triangular-current antenna pattern and integrated power. For its I(z)=I₀(1−2|z|/d), peak p₀=I₀d/(2ω) from ∫J=ṗ; substitution converts precisely to the displayed general dipole coefficients. Does not treat that antenna as uniform current.
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