EIRP Calculator
Effective Isotropic Radiated Power — the total power radiated in the direction of maximum antenna gain.
What is EIRP?
EIRP (Effective Isotropic Radiated Power) is the hypothetical power that a perfectly omnidirectional (isotropic) antenna would need to transmit to produce the same signal strength in the direction of maximum radiation as the actual transmit system. It combines the transmitter output power with the directional gain of the antenna and accounts for losses in the cable connecting them.
EIRP is the starting point for every link budget — it represents the "effective" signal strength being broadcast into space. A high-gain directional antenna focuses energy, multiplying the effective radiated power in one direction even with the same transmitter output. Conversely, cable loss before the antenna directly reduces EIRP.
Regulatory authorities (FCC, ETSI, Ofcom) typically limit EIRP rather than raw transmit power, because EIRP determines interference potential to other systems and spectrum users.
Why Does It Matter?
- Regulatory compliance — FCC Part 15, ETSI EN 300 328, and similar rules set maximum EIRP limits by frequency band
- Link budget starting point — EIRP minus path loss gives the received signal power before receiver antenna gain
- System design trade-offs — engineers balance transmit power, cable length, and antenna gain to maximize EIRP within legal limits
- Interference assessment — EIRP determines the interference footprint of a transmitter in a given direction
Quick EIRP Calculator
Formula Used by LinkBudgetPro
All values in dB/dBm — logarithmic addition/subtraction · ERP (ref half-wave dipole) = EIRP \(-\) 2.15 dB
This formula is derived directly from the link budget computation. Cable loss is subtracted because it reduces the power reaching the antenna. Antenna gain is added because it focuses the available power into a narrower beam, producing higher effective power in the beam direction.
To convert EIRP from dBm to Watts: \( P_{\text{W}} = 10^{(\text{EIRP}_{\text{dBm}} - 30) / 10} \)
Parameter Explanation
| Parameter | Symbol | Unit | Description |
|---|---|---|---|
| Transmit Power | P_tx | dBm | Output power at the transmitter RF port. 30 dBm = 1 W, 37 dBm = 5 W |
| Antenna Gain | G_tx | dBi | Gain of the transmit antenna relative to an isotropic radiator |
| Cable Loss | L_cable | dB | Insertion loss in the feedline between transmitter and antenna (positive value) |
| EIRP | EIRP | dBm | Effective isotropic radiated power in direction of maximum gain |
| ERP | ERP | dBm | Effective Radiated Power relative to a half-wave dipole: ERP = EIRP − 2.15 dB |
Worked Example
A 5.8 GHz point-to-point link uses a 30 dBm radio, 3 dB of cable loss, and a 24 dBi dish antenna. Find the EIRP:
When Should You Use It?
- Before submitting FCC/ETSI regulatory filings — compute EIRP to verify compliance with spectrum rules
- Comparing radio products — normalize different Tx power and antenna combinations to a single EIRP figure
- Link budget calculations — EIRP is the first term in the received power equation
- Interference analysis — determine the interference radius of a transmitter
- Antenna selection — evaluate whether a higher-gain antenna improves EIRP enough to justify the beam-pointing constraint
Related Calculations
- Free Space Path Loss (FSPL) — path loss that the EIRP must overcome
- Fade Margin Calculator — how EIRP minus losses determines link viability
- Receiver Sensitivity — minimum signal the receiver can detect
- dBm ↔ Watt Conversion — convert EIRP between dBm and Watts
- RF Documentation Index — all RF engineering reference pages
Calculate EIRP and full link budget for your RF system in the free online calculator.
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