EIRP Calculator
Calculate effective isotropic radiated power (EIRP) from transmitter power and antenna gain for RF compliance.
What Is EIRP?
Effective isotropic radiated power (EIRP) is the total power a transmitter would radiate if its antenna were an ideal isotropic radiator. Regulators use EIRP to set limits on wireless emissions for Wi-Fi, cellular, and satellite links.
EIRP Formula
$$EIRP_{\text{dBm}} = P_t_{\text{dBm}} + G_t_{\text{dBi}}$$\(P_t\) is transmit power in dBm and \(G_t\) is antenna gain in dBi relative to an isotropic antenna. In watts:
$$EIRP_{\text{W}} = P_t_{\text{W}} \times 10^{G_t/10}$$Example: 30 dBm (1 W) transmit power with a 3 dBi antenna gives \(30 + 3 = 33\) dBm EIRP, or about 2 W.
Why EIRP Matters
FCC and ETSI rules often cap EIRP rather than raw transmitter power. A directional antenna concentrates energy in one direction, so gain adds to effective radiated power. Always check local regulations before deploying high-gain antennas.
Frequently Asked Questions
What is the difference between dBm and dBi?
dBm measures absolute power relative to 1 mW. dBi measures antenna gain relative to an isotropic radiator. EIRP adds them: EIRP(dBm) = Pt(dBm) + Gt(dBi).
How do I convert dBm to watts?
Use \(P_{\text{W}} = 10^{(P_{\text{dBm}} - 30)/10}\). For example, 30 dBm equals 1 W and 33 dBm equals 2 W.
Does cable loss affect EIRP?
Yes. Cable and connector loss reduce power reaching the antenna. Subtract cable loss from transmit power before adding antenna gain.
What is a typical Wi-Fi EIRP limit?
In the US 2.4 GHz band, FCC Part 15 often allows up to 36 dBm EIRP for point-to-multipoint systems, though limits vary by band and use case.
Can EIRP be negative?
Yes. Low-power devices or high cable loss can produce negative EIRP in dBm, meaning radiated power is below 1 mW.
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