EIRP Calculator
Calculate Effective Isotropic Radiated Power for RF systems
EIRP Calculator
Table of Contents
How to Calculate EIRP
EIRP (Effective Isotropic Radiated Power) is calculated using the formula:
- EIRP = Pt × G × L
- Pt = Transmitter power in watts
- G = Antenna gain (linear, not dB)
- L = Cable transmission factor, 10^(-loss in dB ÷ 10), from 0 to 1
- For dB calculations:
- EIRP (dBW) = Pt (dBW) + G (dBi) - L (dB)
- EIRP (dBm) = EIRP (dBW) + 30
This calculator handles all necessary conversions between linear and logarithmic units automatically.
Example: 1 W transmitter power, 3 dBi antenna gain, and 3 dB passive cable loss produce 1 W EIRP (0 dBW or 30 dBm). A negative antenna gain in dBi is possible; passive cable loss cannot be negative.
Understanding RF Power Units
Common Units:
- Watts (W): Linear power unit, used for absolute power levels
- dBW: Decibels relative to 1 watt
- dBm: Decibels relative to 1 milliwatt
- dBi: Antenna gain relative to an isotropic radiator
Key Conversions:
- dBW = 10 × log₁₀(watts)
- dBm = dBW + 30
- Linear gain = 10^(dB gain ÷ 10)
Practical Applications
EIRP is an input to:
- Comparing transmitter and antenna combinations
- Satellite communications link budgets
- Wireless network planning
- Broadcasting system design
- RF coverage prediction
- EMC/EMI assessments
This result does not establish regulatory approval. Applicable EIRP limits depend on frequency, radio service, location, and operating conditions. The FCC calculation instructions (1995) show the power, antenna gain, and cable loss relationship; they are not a current limit for every service.
Frequently Asked Questions
Why is EIRP important?
EIRP lets you compare radiated power from systems with different transmitter powers, antenna gains, and cable losses. Check the rules for the specific service and location separately.
What is the difference between ERP and EIRP?
EIRP is referenced to an isotropic antenna (theoretical perfect sphere of radiation), while ERP is referenced to an ideal half-wave dipole antenna. EIRP is 2.15 dB higher than ERP.
Why use decibel units?
Decibels (dB) make it easier to work with very large or small numbers and simplify calculations by turning multiplication and division into addition and subtraction.