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EIRP Calculator

Effective isotropic radiated power (EIRP) quantifies the peak power density that an RF system radiates in its direction of maximum gain, referenced to a hypothetical isotropic antenna. Engineers and technicians use EIRP calculations to verify compliance with regulatory limits (FCC, ETSI), design radio links, and optimise antenna deployments. This calculator accounts for transmitter output power, cable attenuation at multiple frequencies, connector losses, and antenna gain—all critical variables that determine your system's actual radiated power.

Last updated:

Creators Dominik Czernia, PhD
8,070 people find this calculator helpful

Understanding Effective Isotropic Radiated Power

EIRP represents the equivalent power a lossless, omnidirectional (isotropic) antenna would need to emit to match the peak signal strength of your real antenna system in a given direction. Regulatory bodies such as the FCC and ETSI mandate EIRP limits to prevent interference and protect spectrum users.

The power budget of an RF transmission chain depends on several factors:

  • Transmitter output power — measured in dBmW or watts
  • Cable attenuation — frequency-dependent loss per unit length
  • Connector losses — typically 0.2–0.5 dB per mated pair
  • Antenna gain — measured in dBi (decibels relative to isotropic)

Each component either adds or subtracts from the final EIRP. Understanding how these losses accumulate across your physical installation is essential for meeting regulatory requirements and achieving design margins.

EIRP Calculation Formula

The fundamental EIRP equation combines transmitter power, subtracts all losses, and adds antenna directivity:

EIRP (dBmW) = Tx Power − Cable Loss − Connector Loss + Antenna Gain

Where connector loss is multiplied by the number of mated connectors in the signal path. For systems with known cable length and loss per unit length:

EIRP = Tx Power − (Cable Length × Loss per Unit) − (n × Connector Loss) + Antenna Gain

  • Tx Power — Transmitter output power in dBmW or dBm
  • Cable Loss — Total attenuation of the transmission line, frequency-dependent, measured in dB
  • Connector Loss — Signal loss per mated connector pair, typically 0.2–0.5 dB
  • n — Number of connectors in the transmission path
  • Antenna Gain — Directive gain of the antenna in dBi relative to an isotropic radiator

Isotropic vs. Directional Antennas

An isotropic antenna is a theoretical reference: it radiates power uniformly in all directions with zero loss and no preferred orientation. No real antenna achieves this.

Practical antennas concentrate energy in specific directions, creating gain in some azimuth and elevation angles while suppressing it in others. This directivity does not create additional power; it merely focuses existing power. A 10 dBi gain antenna does not radiate 10 times more power than a 0 dBi isotropic reference—rather, it concentrates the same transmitter power into a narrower pattern.

EIRP references this ideal isotropic radiator, making it a universal metric across different antenna types. A Yagi antenna, horn, patch, or dipole can all be compared fairly using EIRP because the metric normalises for antenna type and pattern shape.

Decibels and dBmW: Units and Ratios

The decibel is a logarithmic ratio, not an absolute value. Power ratio in dB is defined as:

dB = 10 × log₁₀(P_out / P_in)

This logarithmic representation makes large power ranges manageable and aligns with human perception of signal strength. Adding and subtracting decibels is equivalent to multiplying and dividing linear power ratios.

dBmW (decibel-milliwatts) is an absolute reference: 0 dBmW = 1 milliwatt. A 20 dBmW signal is 100 mW; 30 dBmW is 1 W. This differs fundamentally from dB, which is always a ratio and must reference something (e.g., an input or a standard like 1 mW).

In EIRP calculations, transmitter power and antenna gain may be expressed in dBmW and dBi respectively, while cable and connector losses are pure dB (ratios). The decibel arithmetic remains consistent: subtract losses, add gains.

Common Pitfalls and Practical Considerations

Accurate EIRP calculation requires attention to real-world system behaviour and regulatory nuances.

  1. Frequency-dependent cable loss — Coaxial cable attenuation increases with frequency. A 50-foot run of LMR-400 might exhibit 1.5 dB loss at 150 MHz but 4.5 dB loss at 2.4 GHz. Always specify cable type and frequency when calculating cable loss; using a generic or incorrect frequency assumption can introduce 2–3 dB errors.
  2. Connector and junction losses accumulate — Each mated connector pair, T-junction, or splitter introduces loss. A 6-connector installation can easily lose 2–3 dB before the signal reaches the antenna. Document every connector, switch, and passive component. Hidden losses in jumpers or weatherproof seals are a frequent source of budget shortfall.
  3. Antenna gain is directional — Antenna gain specifications apply only in the direction of maximum radiation (typically boresight). At off-axis angles—even modest deviations—gain drops significantly. EIRP limits in regulations typically reference peak gain; verify your actual coverage pattern against regulatory maps and exclusion zones.
  4. Regulatory limits vary by region and application — FCC allows up to 36 dBmW EIRP for many licence-free devices, but WiFi, cellular, and other bands have different thresholds. ETSI and other regional bodies impose different limits. Verify your specific use case and jurisdiction before deployment; exceeding limits incurs substantial fines and potential equipment seizure.

Frequently Asked Questions

How do I compute EIRP when I know only cable length and loss per unit distance?

Multiply the cable length by the loss per unit length to obtain total cable attenuation, then apply the standard EIRP formula. For example, a 100-foot run of cable rated at 0.05 dB/foot yields 5 dB total loss. Subtract this from transmitter power, account for all connectors, and add antenna gain. The calculator handles multiple frequency scenarios automatically, since cable loss is frequency-dependent and provided lookup tables for common frequency bands (150 MHz, 450 MHz, 900 MHz, etc.).

What is the relationship between EIRP and ERP?

Effective radiated power (ERP) references a half-wave dipole antenna as the standard radiator, whereas EIRP references a perfect isotropic antenna. Since a dipole has approximately 2.15 dB gain relative to isotropic, ERP is roughly 2.15 dB lower than EIRP for the same system. To convert: ERP (dBmW) ≈ EIRP (dBmW) − 2.15. This distinction matters in older broadcasting standards and some regional regulations that still use ERP terminology.

Do I need to account for cable loss if my antenna connects directly to the transmitter?

Even very short cables introduce measurable loss. A 1-foot cable at 2.4 GHz can lose 0.1–0.2 dB depending on cable type. More importantly, any connectors between the transmitter and antenna must be included: SMA, N-type, or BNC connectors each add 0.2–0.5 dB. Direct connection (no cables) is rare in practice; always audit your physical interconnects.

Why do regulatory limits specify EIRP rather than transmitter power alone?

Regulators use EIRP because it accurately reflects the actual radiated power reaching the environment, independent of antenna design or cable length. A low-power transmitter with a high-gain antenna can radiate as much power as a high-power transmitter with a poor antenna. EIRP ensures fair comparison and predictable interference levels across diverse system architectures.

How does temperature affect EIRP calculations?

Temperature influences cable attenuation and component losses, particularly in outdoor installations. Coaxial cable loss typically increases 0.3–0.4% per degree Celsius. Over a 40 °C temperature swing, a 5 dB loss budget might increase to 5.8 dB. For critical systems, measure or recalculate EIRP at expected operating temperature extremes, especially in extreme climates or installations with poor ventilation.

Can I use a higher-gain antenna to compensate for cable loss?

Yes, within limits. Antenna gain is frequency-tuned and directional; a gain increase always narrows the radiation pattern. If your application requires wide coverage, a high-gain antenna may leave dead zones. Additionally, antenna costs, installation complexity, and footprint grow with gain. It is usually more cost-effective to reduce cable loss (use larger-diameter or lower-loss cable) than to upgrade antenna gain beyond your coverage requirements.

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