What is EIRP?
Effective Isotropic Radiated Power (EIRP) is the combination of a transmitter's RF output power and its antenna gain, expressed as a single figure representing how much power would need to be radiated by a theoretical isotropic antenna (which radiates equally in all directions) to produce the same power density at the receiver as the actual directional antenna system. EIRP is the standard measure of transmitting capability in link budget analysis.
Calculation and units
EIRP (dBW) = Transmit Power (dBW) + Antenna Gain (dBi) − Cable/feed Losses (dB). For example: a VSAT terminal with 5 W transmit power (+7 dBW), 1.2 m Ku-band dish with 44 dBi gain, and 1 dB of feed loss: EIRP = 7 + 44 − 1 = 50 dBW. A GEO satellite downlink Ku-band transponder with 120 W power (+20.8 dBW) and a 3 dBi global beam antenna: EIRP = 20.8 + 3 = 23.8 dBW. The spot beam satellites that replaced global-beam designs dramatically increased EIRP by concentrating power into narrow beams with 30–50 dBi of antenna gain.
Regulatory limits
The ITU Radio Regulations and national regulators set maximum EIRP limits for earth station transmitters to prevent harmful interference to adjacent satellites and terrestrial systems. VSAT terminal EIRP limits are a function of the regulatory coordination zone — terminals near the edge of a satellite's coverage area may need to increase EIRP to close the link, but must not exceed the limit that would interfere with a neighbouring orbital position's receiver. Compliance is verified during terminal type approval (e.g., under ETSI or FCC rules) and through carrier monitoring by the satellite operator's network control system.