Effective Isotropic Radiated Power
EIRP is the total power an isotropic antenna would need to produce the same field strength. It combines transmitter power, cable losses, and antenna gain. Regulations limit EIRP, not just Tx power.
Why This Physics Calculation Matters
Why: Regulators limit EIRP because high-gain antennas with low Tx power can interfere as much as high Tx power. EIRP is critical for link budgets and compliance.
How: Add antenna gain and subtract losses (all in dB). EIRP = P_tx - L_cable - L_connectors + G_antenna. +3 dB doubles power, +10 dB multiplies by 10.
- ●EIRP = Tx Power - Losses + Antenna Gain (all in dB)
- ●Regulatory limits specify EIRP, not just transmitter power
- ●+3 dB = double power, +10 dB = 10× power
- ●EIRP critical for link budgets and range estimation
📡 RF System Examples
⚙️ RF System Parameters
Transmitter
Antenna
Cable
Connectors
⚠️For educational and informational purposes only. Verify with a qualified professional.
🔬 Physics Facts
A satellite ground station with 10W and 45 dBi dish achieves 83 dBm EIRP—equivalent to 200 kW isotropic
— ITU
FCC limits WiFi EIRP to 36 dBm (4W) in the 2.4 GHz band
— FCC
Bluetooth Class 2 transmits at 0 dBm (1 mW) EIRP for short range and battery life
— IEEE
Cable loss increases with frequency—RG-58 loses 1.1 dB/m at 2.4 GHz
— IEEE
📋 Key Takeaways
- • EIRP = Tx Power - Losses + Antenna Gain (all in dB) — represents the power an isotropic antenna would need to produce the same field strength
- • Regulatory limits specify EIRP, not just Tx power — high-gain antennas can exceed limits even with low transmitter power
- • +3 dB = double power, +10 dB = 10× power — decibels compress power ratios into manageable numbers
- • EIRP is critical for link budgets — higher EIRP enables longer range or better signal quality, but must comply with regulations
💡 Did You Know?
📖 How EIRP Calculation Works
EIRP (Effective Isotropic Radiated Power) measures the total power that would be radiated by an isotropic antenna to produce the same signal strength as your actual system. The formula EIRP = Tx Power - Losses + Antenna Gain accounts for all system components.
Decibel System
RF engineers use decibels because power ratios span many orders of magnitude. Adding/subtracting dB is easier than multiplying/dividing large numbers. A +3 dB change doubles power, +10 dB multiplies by 10.
System Losses
Cable loss and connector losses reduce effective power. Each connector typically adds 0.1-0.5 dB. Cable loss increases with frequency and length. High-quality cables minimize losses but cost more.
Antenna Gain
Antenna gain (in dBi) focuses energy in specific directions. A 10 dBi antenna puts 10× the power in one direction compared to an isotropic antenna, but less power in other directions. Gain doesn't create power — it redirects it.
🎯 Expert Tips for RF System Design
💡 Minimize Cable Loss
Mount transmitters close to antennas when possible. Every meter of cable at high frequencies costs significant EIRP. Consider tower-mounted amplifiers for long runs.
💡 Use Quality Connectors
Proper connectors (N-type or better for UHF+) minimize losses. Ensure weatherproof installation and re-torque connections annually. Poor connectors can lose 0.5-2+ dB each.
💡 Verify Compliance
Always calculate EIRP before installation. Regulations vary by frequency band and region. FCC allows 36 dBm EIRP for 2.4 GHz WiFi, but EU limits to 20 dBm.
💡 Measure, Don't Assume
Use a wattmeter at the antenna feedpoint to measure actual power. Catalog specs may not match real-world performance after installation. Check SWR regularly.
⚖️ EIRP Comparison by System Type
| System Type | Typical Tx Power | Antenna Gain | Typical EIRP | Regulatory Limit |
|---|---|---|---|---|
| Bluetooth | 0 dBm (1 mW) | 0-2 dBi | 0-2 dBm | 20 dBm (EU) |
| WiFi AP | 20-27 dBm | 3-6 dBi | 23-33 dBm | 36 dBm (FCC) |
| Ham Radio HF | 50 dBm (100W) | 5-15 dBi | 55-65 dBm | No limit |
| Cell Tower | 43 dBm (20W) | 15-18 dBi | 58-61 dBm | Licensed |
| Satellite Uplink | 33 dBm (2W) | 40-50 dBi | 73-83 dBm | Licensed |
| FM Broadcast | 70 dBm (10kW) | 5-7 dBd | 72-77 dBm | Licensed |
❓ Frequently Asked Questions
What is the difference between EIRP and ERP?
EIRP uses an isotropic reference (0 dBi), ERP uses a dipole reference (2.15 dBi). EIRP = ERP + 2.15 dB. FCC rules may specify one or the other depending on the band.
Why do regulations limit EIRP rather than just Tx power?
EIRP represents actual signal strength in a direction. High-gain antennas with low Tx power can interfere just as much as high Tx power with low-gain antennas. Regulators control interference, not just transmitter power.
Can I use higher gain antennas to extend WiFi range?
Yes, but you must reduce Tx power to stay within EIRP limits. Also remember: the link is bidirectional. Your AP may hear clients that can't hear it back if the antenna pattern is too directional.
How do I measure actual EIRP?
Use a spectrum analyzer with a calibrated antenna at a known distance. Calculate EIRP from received power using the Friis equation: EIRP = P_rx + FSPL - G_rx. Professional testing uses anechoic chambers.
What is the FCC limit for 2.4 GHz WiFi?
FCC allows up to 36 dBm (4W) EIRP in the 2.4 GHz ISM band. Maximum Tx power is 30 dBm (1W). If antenna gain exceeds 6 dBi, you must reduce Tx power by 1 dB for every 3 dBi above 6 dBi.
How does cable loss affect EIRP?
Cable loss directly reduces EIRP. A 10m run of RG-58 at 2.4 GHz loses ~4 dB — that's 60% of your power! Always use the shortest, highest-quality cable possible for your budget.
What is the difference between dBi and dBd?
dBi is gain relative to an isotropic antenna (theoretical sphere). dBd is gain relative to a dipole antenna (2.15 dBi). dBi = dBd + 2.15. EIRP calculations use dBi.
Can EIRP exceed 100% efficiency?
EIRP is not an efficiency measure — it's a power measure. Antenna gain doesn't create power, it redirects it. A 10 dBi antenna puts 10× power in one direction but less in others, conserving total radiated power.
📊 EIRP by the Numbers
📚 Official Data Sources
⚠️ Disclaimer: EIRP calculations are estimates based on theoretical formulas. Actual EIRP may vary due to SWR, antenna misalignment, component degradation, and environmental factors. Always verify compliance with local regulations (FCC, ETSI, etc.) before deployment. This calculator provides estimates for educational and planning purposes only. Professional RF engineering consultation is recommended for commercial installations.