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Short note: Effective isotropic radiated power

Short answer

EIRP = Pt·Gt/L (or Pt+Gt-L in dB) is power an isotropic antenna would need for same maximum power density. Power flux density = EIRP/(4πd²). Used in link budgets, regulatory limits, satellite footprints. ERP referenced to half-wave dipole: EIRP = ERP + 2.15 dB.

Write a short note on effective isotropic radiated power (EIRP).

Effective Isotropic Radiated Power (EIRP) is the power that an ideal isotropic antenna would have to radiate to give the same power density, in the direction of maximum radiation, as the actual antenna.

EIRP = Pt · Gt / L
EIRP (dBW) = Pt (dBW) + Gt (dBi) − L (dB)

where Pt = transmitter power, Gt = transmit antenna gain relative to isotropic, L = feeder/cable losses.

The power flux density at distance d is then

PFD = EIRP / (4π d²)   W/m²

Example: Pt = 100 W (20 dBW), Gt = 30 dBi, feeder loss = 2 dB EIRP = 20 + 30 − 2 = 48 dBW ≈ 63.1 kW.

Importance:

  • Used in link budget calculations for satellite and terrestrial links.
  • Lets transmitters with different powers and antennas be compared with a single number.
  • Regulators set EIRP limits (e.g. for Wi-Fi), and satellite footprints are drawn as EIRP contours.
  • A high-gain antenna can give a large EIRP with a small transmitter power.

ERP (effective radiated power) is similar but is referred to a half-wave dipole: EIRP = ERP + 2.15 dB.

✓ Answer checked by Claude Opus 5.5 (high effort)

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