When to use: Use to calculate Effective Isotropic Radiated Power (EIRP) — the total power an antenna system appears to radiate in its strongest direction, relative to an isotropic radiator. EIRP = TX Power − Cable Loss − Connector Loss + Antenna Gain. Checked against FCC Part 15, 90, and 24 limits for compliance. Required for FCC applications, NFPA 1221 BDA submittals, and any RF system design. Use with the Link Budget calculator for complete coverage analysis.
The EIRP (Effective Isotropic Radiated Power) Calculator computes the total radiated power of an antenna system in its strongest direction, accounting for transmitter output, cable losses, connector losses, and antenna gain. Engineers use EIRP as the fundamental input to link budgets, coverage calculations, and FCC regulatory compliance checks.
EIRP (dBm) = TX Power (dBm) − Cable Loss (dB) − Connector Loss (dB) + Antenna Gain (dBi). EIRP represents what the antenna system would radiate if all the power were focused in a single direction relative to an isotropic (theoretical perfect spherical) radiator. An antenna with 0 dBi gain is isotropic. A 3 dBi omni doubles effective radiated power in the horizontal plane compared to an isotropic source. A 10 dBi directional antenna effectively amplifies the signal tenfold in the boresight direction.
EIRP in watts = 10^((EIRP_dBm − 30) / 10). For example, 37 dBm EIRP = 5 W EIRP, which is the FCC Part 90 maximum for a portable radio system. The EIRP figure is what regulators check against published limits — not the transmitter power alone.
FCC Part 90.219 governs public safety land mobile radio, with maximum output power limits by frequency band. FCC Part 15.247 limits EIRP for 2.4 GHz ISM spread-spectrum systems to 36 dBm (4 W). FCC Part 15.407 limits 5 GHz UNII-1 indoor devices to 23 dBm EIRP. For public safety BDA systems, the BDA server-side output (measured at the antenna port) must be within the FCC-certified power level for the BDA model. NFPA 1221 BDA submittals must include the computed EIRP at each antenna as part of the RF engineering package submitted to the AHJ.
Cable loss is the most commonly underestimated factor in EIRP calculations. A 100-ft run of LMR-400 at 800 MHz contributes 2.23 dB of loss, reducing a 37 dBm transmitter to approximately 34.8 dBm EIRP before antenna gain. For passive DAS systems with multiple cable runs and splitters, cumulative losses can easily exceed 15–20 dB, requiring a BDA to compensate. Connector loss is typically 0.1 to 0.5 dB per connection — on a long cable run with 8 connectors, this can add 1–4 dB of additional loss. Always use the Cable Loss Calculator to determine total cable and connector losses before entering them here.
Enter the transmitter power in dBm, watts, or milliwatts using the unit selector. Enter cable loss and connector loss in dB (use the Cable Loss Calculator for accuracy). Enter antenna gain in dBi — 0 dBi for an isotropic reference, 2–3 dBi for typical omnidirectional indoor DAS antennas, and up to 12–15 dBi for directional panel antennas. Select the applicable FCC limit from the dropdown to check regulatory compliance. The calculator displays EIRP in dBm and watts, and indicates whether the system is within the selected FCC limit with a compliance margin in dB.
ERP (Effective Radiated Power) is referenced to a half-wave dipole, which has 2.15 dBi gain relative to isotropic. EIRP is referenced to an isotropic radiator. The relationship is EIRP (dBm) = ERP (dBm) + 2.15 dB. FCC Part 15 and Part 90 regulations use EIRP. Broadcast engineering sometimes uses ERP. Always confirm which reference is being used when comparing power levels.
FCC Part 90 limits portable (handheld) land mobile radios to 5 watts (37 dBm) maximum transmitted power. With a typical 3 dBi antenna, EIRP is 40 dBm (10 W). Mobile (vehicle-mounted) units are permitted up to 25–50 W depending on frequency band, with corresponding higher EIRP values. BDA server-side EIRP is governed by the FCC certification of the specific BDA model.
Yes — replacing a standard antenna with a higher-gain antenna increases EIRP even if transmitter power is unchanged. FCC Part 15 rules specifically prohibit substituting higher-gain antennas on certified devices without reauthorization. Part 90 licensed systems have more flexibility, but EIRP must remain within licensed parameters. Always check the resulting EIRP against the applicable FCC limit when upgrading antennas.
EIRP is the direct input to the link budget and coverage radius calculation: Coverage radius increases as EIRP increases. Every 6 dB increase in EIRP approximately doubles the coverage radius in a free-space (n = 2) environment. In indoor environments with n = 3.5, a 10 dB EIRP increase extends coverage radius by a factor of approximately 2.2.
Indoor omnidirectional ceiling-mount DAS antennas typically provide 2 to 4 dBi of gain across 700–2700 MHz. Directional panel antennas used in corridors or stairwells range from 5 to 10 dBi. Ultra-low-profile antennas designed for plenum ceiling installation may have gains as low as 1 dBi. Antenna gain datasheets should be verified at the specific operating frequency, as gain can vary by 2–3 dB across a wideband antenna's frequency range.
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