Skip to main content

Short note: Power flux density

Short answer

Power flux density is the power flowing per unit area normal to propagation, in W/m², and equals the magnitude of the average Poynting vector. For isotropic radiation PFD = Pt/4πd²; with gain, PFD = EIRP/4πd² = E²rms/120π. For EIRP 1 kW at 10 km, PFD = 7.96×10⁻⁷ W/m² and E_rms = 17.3 mV/m.

Write a short note on power flux density (PFD).

Power flux density (PFD) is the power flowing per unit area normal to the direction of propagation, in W/m² (or dBW/m²). It is the magnitude of the average Poynting vector.

For an isotropic radiator, power Pt spreads uniformly over a sphere of area 4πd²:

PFD = Pt / (4π d²)
With antenna gain:  PFD = Pt Gt / (4π d²) = EIRP / (4π d²)
In terms of field:  PFD = E²rms / η0 = E²rms / 120π

Example: EIRP = 1 kW, d = 10 km: PFD = 1000/(4π × 10⁸) = 7.96 × 10⁻⁷ W/m² (−61.0 dBW/m²), giving E_rms = √(PFD × 120π) = 17.3 mV/m.

Uses: received power Pr = PFD × Ae; ITU sets PFD limits for satellites to protect terrestrial services. PFD falls as 1/d² (inverse square law).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Propagation and Antenna chapter 2 questions →