Effective Radiated Power (ERP)

Effective Radiated Power (ERP) is a measure of the radio frequency (RF) power transmitted in the direction of maximum antenna gain, referenced to a half-wave dipole antenna. It is commonly used in broadcasting to define signal strength, coverage area, and regulatory limits. ERP is typically expressed in watts (W) or kilowatts (kW).

ERP vs. EIRP

ERP differs from Effective Isotropic Radiated Power (EIRP) by its reference antenna:

• ERP is referenced to a half-wave dipole antenna (gain = 0 dBd)

• EIRP is referenced to an ideal isotropic radiator (gain = 0 dBi)

The conversion between the two is:

• ERP = EIRP / 1.64

• ERP (dBm) = EIRP (dBm) − 2.15 dB

This 2.15 dB difference accounts for the dipole’s higher gain compared to an isotropic radiator.

How ERP Is Calculated

In linear terms:
ERP = P_t × G_d

Where:

• P_t = Transmitter power

• G_d = Antenna gain relative to a dipole (unitless)

In decibels:
ERP (dBm) = P_t (dBm) + G_d (dBd)

These calculations allow regulators and engineers to assess coverage and interference potential in a standardized way.

Example: Broadcast System Power

• An FM radio station with 50 kW ERP has an equivalent EIRP of ~82 kW

• A TV transmitter with 10 kW power and 4 dBd antenna gain results in:
  ERP = 10 kW × 2.51 ≈ 25.1 kW

Such values are regulated and licensed by authorities to ensure spectrum integrity.

Use in Regulation and System Design

ERP is a key parameter in:

• FM and AM radio broadcasting

• Television transmission

• Licensing procedures based on ERP and HAAT (Height Above Average Terrain)

Regulatory bodies like the FCC and ITU use ERP values to limit interference, define protected service contours, and allocate spectrum. ERP remains standard in legacy systems, while EIRP is more common in mobile and satellite communication.