Table of Contents
What is EIRP?
EIRP (Effective Isotropic Radiated Power) is the total power that would need to be radiated by a theoretical isotropic antenna to produce the same signal strength as the actual antenna system in its direction of maximum gain. It accounts for the transmitter power, any losses in cables and connectors, and the gain of the antenna, providing a single number that characterizes the system's radiated power capability.
EIRP is the standard measure used by regulatory agencies to set power limits for radio transmitters. It is more meaningful than transmitter power alone because a low-power transmitter with a high-gain antenna can produce the same signal strength as a high-power transmitter with a low-gain antenna. Telecommunications engineers use EIRP for link budget calculations, coverage planning, and regulatory compliance.
Formula
Regulatory Limits (Examples)
| Service | Frequency | Max EIRP |
|---|---|---|
| WiFi 2.4 GHz | 2.4 GHz | 36 dBm (4W) |
| WiFi 5 GHz | 5 GHz | 30 dBm (1W) UNII-1 |
| Amateur VHF | 144 MHz | 1500W PEP (USA) |
| Cell tower | Various | Varies by license |
Applications
- Cellular network coverage planning and optimization
- Satellite communication link budget analysis
- WiFi access point deployment and compliance
- Broadcasting transmitter power allocation
- Radar system range calculation
FAQ
What is the difference between EIRP and ERP?
EIRP is referenced to an isotropic (0 dBi) antenna, while ERP (Effective Radiated Power) is referenced to a half-wave dipole (0 dBd = 2.15 dBi). EIRP is always 2.15 dB higher than ERP for the same system. Most modern specifications use EIRP, but some older regulations and broadcasting standards use ERP. Always check which reference is specified.
How does antenna gain affect coverage?
Higher antenna gain concentrates energy in a narrower beam, increasing EIRP and range in that direction but reducing coverage in other directions. A 3 dB increase in antenna gain doubles the EIRP and extends the range by about 41% in the direction of maximum gain. This is why high-gain antennas are used for point-to-point links while low-gain antennas are used for broad coverage.
Why do cable losses matter so much?
Cable losses directly reduce the power reaching the antenna. At microwave frequencies (above 1 GHz), coaxial cable losses can be several dB per meter, making cable selection and length critical. Using low-loss cable or mounting the transmitter directly at the antenna (with a short jumper) can significantly improve system EIRP without increasing transmitter power.