EIRP / ERP Regulatory Calculator
Calculate Effective Isotropic Radiated Power (EIRP) and ERP from transmit power, cable loss, and antenna gain. Check compliance against FCC, ETSI, and ISM-band regulatory limits.
Formula
EIRP_{dBm} = P_{TX} - L_{cable} + G_{ant}, \quad ERP_{dBm} = EIRP_{dBm} - 2.15
Reference: FCC Part 15 §15.247; ETSI EN 300 328; IEEE Std 149-1979
How It Works
Equivalent Isotropic Radiated Power (EIRP) is a critical parameter in wireless communications that represents the total power radiated by an antenna system. It accounts for both the transmitter's output power and the antenna's gain relative to an isotropic radiator. EIRP provides a standardized way to measure and regulate the effective radiated power, taking into consideration cable losses and antenna characteristics. An isotropic antenna radiates power equally in all directions, serving as a theoretical reference point. In practical wireless systems, directional antennas can focus energy in specific directions, effectively increasing the radiated power in those regions. The calculation combines transmit power, cable losses, and antenna gain to determine the total effective radiated power. This metric is crucial for compliance with regulatory limits, particularly in unlicensed frequency bands like 2.4 GHz WiFi and ISM bands.
Worked Example
Consider a wireless transmitter with 20 dBm output power, connected to an antenna through a 2 dB loss cable, and using a directional antenna with 8 dBi gain. First, calculate the EIRP: Ptx (20 dBm) - Lcable (2 dB) + Gantenna (8 dBi) = 26 dBm EIRP. To calculate ERP, subtract 2.15 dB from the EIRP: 26 dBm - 2.15 dB = 23.85 dBm ERP. This result is well below the FCC Part 15 limit of 36 dBm for 2.4 GHz devices, ensuring regulatory compliance.
Practical Tips
- ✓Always use calibrated test equipment when measuring transmit power
- ✓Consider environmental factors that might affect antenna performance
- ✓Use low-loss cables to minimize signal attenuation
Common Mistakes
- ✗Forgetting to account for cable losses when calculating EIRP
- ✗Confusing absolute power values with logarithmic (dB) measurements
- ✗Assuming antenna gain is uniform in all directions
Frequently Asked Questions
What does EIRP stand for?
EIRP stands for Equivalent Isotropic Radiated Power, a standardized measure of an antenna's total radiated power.
Why is EIRP important?
EIRP helps regulate wireless transmission power and ensures fair spectrum usage across different communication systems.
How does cable loss affect EIRP?
Cable losses reduce the power reaching the antenna, directly impacting the final EIRP calculation.
What's the difference between EIRP and ERP?
ERP is referenced to a half-wave dipole antenna, while EIRP is referenced to an isotropic antenna, differing by 2.15 dB.
Are EIRP limits the same for all frequency bands?
No, EIRP limits vary by frequency band and are set by regulatory bodies like the FCC.
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