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Antenna

EIRP / ERP Regulatory Calculator

Calculate EIRP and ERP from transmit power, cable loss, and antenna gain. Check FCC Part 15 and ETSI regulatory compliance margins. Free, instant results.

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Formula

EIRPdBm=PTX−Lcable+Gant,ERPdBm=EIRPdBm−2.15EIRP_{dBm} = P_{TX} - L_{cable} + G_{ant}, \quad ERP_{dBm} = EIRP_{dBm} - 2.15

Reference: FCC Part 15 §15.247; ETSI EN 300 328; IEEE Std 149-1979

P_{TX}— Transmitter output power (dBm)
L_{cable}— Cable and connector loss (dB)
G_{ant}— Antenna gain (dBi)
EIRP— Effective Isotropic Radiated Power (dBm)
ERP— Effective Radiated Power (vs dipole) (dBm)
M— Regulatory margin (dB)

How It Works

EIRP calculator computes Equivalent Isotropic Radiated Power from transmit power, cable loss, and antenna gain — spectrum regulators, satellite link engineers, and wireless system designers use this to verify transmitter power limits and calculate received signal strength. EIRP (dBm) = P_tx (dBm) - L_cable (dB) + G_antenna (dBi), per FCC Part 15.247, ETSI EN 300 328, and ITU Radio Regulations.

An isotropic antenna radiates equally in all directions; real directional antennas concentrate energy, effectively multiplying power in the main beam direction. A 1 W (30 dBm) transmitter with 20 dBi antenna produces EIRP = 30 + 20 = 50 dBm = 100 W in the peak direction — equivalent field strength to a 100 W isotropic source. ERP (Effective Radiated Power) references a half-wave dipole instead of isotropic: ERP (dBW) = EIRP (dBW) - 2.15 dB.

Regulatory limits vary by band and geography: FCC Part 15.247 (2.4 GHz ISM): 36 dBm (4 W) EIRP for point-to-multipoint, 1 W transmitter with up to 6 dBi antenna, reduced 1:1 for higher gain. ETSI EN 300 328 (EU 2.4 GHz): 20 dBm (100 mW) EIRP maximum. FCC Part 15.407 (5 GHz U-NII): 30-36 dBm depending on sub-band. ITU Region 2 C-band satellite: coordination threshold 45 dBW EIRP toward geostationary arc.

Worked Example

Problem: Determine regulatory compliance and safe distances for a 2.4 GHz point-to-point WiFi bridge with 27 dBm transmitter and 24 dBi parabolic dish antenna.

EIRP calculation:

  1. Transmit power: P_tx = 27 dBm (500 mW)
  2. Cable loss (15 m LMR-400 at 2.4 GHz): L_cable = 15 * 0.115 = 1.7 dB
  3. Connector loss (4 N-type): L_conn = 4 * 0.15 = 0.6 dB
  4. Antenna gain: G_ant = 24 dBi
  5. EIRP = 27 - 1.7 - 0.6 + 24 = 48.7 dBm = 74 W
FCC compliance check (Part 15.247):
  1. Point-to-point rule: For antennas > 6 dBi, reduce P_tx by 1 dB per 3 dB antenna gain above 6 dBi
  2. Antenna gain above 6 dBi: 24 - 6 = 18 dB
  3. Required power reduction: 18/3 = 6 dB
  4. Maximum P_tx: 30 - 6 = 24 dBm (251 mW)
  5. Current P_tx: 27 dBm — NON-COMPLIANT, must reduce to 24 dBm
Corrected EIRP at 24 dBm:
  1. EIRP = 24 - 2.3 + 24 = 45.7 dBm = 37 W (compliant)
ERP calculation:
  1. ERP = EIRP - 2.15 = 45.7 - 2.15 = 43.55 dBm = 22.6 W
RF safety analysis (FCC OET-65):
  1. Public exposure limit at 2.4 GHz: 1.0 mW/cm^2
  2. Safe distance: d = sqrt(EIRP_watts / (4*pi*S_limit))
d = sqrt(37 / (4*pi*0.01)) = 17.1 cm on-axis
  1. In practice, mount antenna > 2 m from public access areas — provides 100x margin

Practical Tips

  • ✓For regulatory compliance, calculate EIRP at the antenna input (after all cable losses) — this is what FCC and ETSI measure; excess cable loss can actually help compliance by reducing effective power
  • ✓Document cable type, length, and connector count in installation records — auditors verify EIRP calculations; having records prevents compliance disputes
  • ✓For multi-sector base stations, calculate EIRP per sector and total for worst-case interference analysis — adjacent sectors may overlap coverage creating zones with combined EIRP

Common Mistakes

  • ✗Forgetting cable losses — a 30 m LMR-400 run at 5.8 GHz loses 5.3 dB; ignoring this overstates EIRP by 5.3 dB and may violate regulatory limits
  • ✗Confusing dBi and dBd antenna gain — dBi references isotropic, dBd references dipole; dBi = dBd + 2.15; mixing them causes 2.15 dB EIRP error
  • ✗Assuming regulatory limits are simple power caps — FCC Part 15.247 has complex rules: 1 W base + 6 dBi antenna, with power-gain tradeoffs for higher-gain antennas; point-to-point allows higher EIRP than point-to-multipoint
  • ✗Using EIRP for non-main-beam calculations — EIRP applies to peak gain direction; sidelobe or back-lobe power is EIRP minus antenna pattern value at that angle

Frequently Asked Questions

Equivalent Isotropic Radiated Power — the power that a theoretical isotropic (omnidirectional) antenna would need to produce the same maximum field strength as the actual antenna system. EIRP accounts for both transmitter power and antenna gain, providing a single number for regulatory limits and link budget calculations. Example: 1 W (30 dBm) with 10 dBi antenna produces 10 W (40 dBm) EIRP — same peak field strength as 10 W into an isotropic antenna.
EIRP serves three purposes: (1) Regulation — spectrum authorities (FCC, ETSI, ITU) set EIRP limits to control interference; comparing EIRP values determines if systems are compliant regardless of their specific power/antenna combinations. (2) Link budgets — receiver signal strength depends on transmitter EIRP, path loss, and receive antenna gain; EIRP simplifies the transmit side to one number. (3) Safety — RF exposure limits are calculated from EIRP using power density formulas; knowing EIRP determines safe distances from antennas.
Cable loss subtracts from EIRP: EIRP = P_tx - L_cable + G_antenna. Higher cable loss means lower EIRP. A 100 mW transmitter with 6 dBi antenna and 3 dB cable loss: EIRP = 20 - 3 + 6 = 23 dBm (200 mW). The same system with 0 dB cable loss: EIRP = 26 dBm (400 mW) — double the EIRP. For compliance near regulatory limits, use accurate cable loss values from manufacturer datasheets at the operating frequency, including all connectors.
EIRP references an isotropic antenna (0 dBi); ERP references a half-wave dipole (2.15 dBi). Relationship: EIRP (dBm) = ERP (dBm) + 2.15 dB. Historical context: ERP was used before EIRP became standard because dipoles were the practical reference antenna. Modern practice: ITU and most regulators use EIRP; some broadcast regulations still use ERP. Always verify which reference is specified — 40 dBm EIRP equals 37.85 dBm ERP; confusing them causes 2.15 dB error.
No — limits vary by band, application, and region. Examples: 2.4 GHz ISM (FCC): 36 dBm EIRP point-to-multipoint, higher for point-to-point with high-gain antennas. 5.8 GHz U-NII-3 (FCC): 36 dBm EIRP maximum. 915 MHz ISM (FCC): 36 dBm EIRP with frequency hopping. 868 MHz (ETSI): 14 dBm ERP (16.15 dBm EIRP) with 1% duty cycle. C-band satellite (ITU): coordination required above 45 dBW EIRP toward geostationary arc. Always check the specific regulation for your frequency, geography, and application.
FCC Part 15.247 rules for 2.4 GHz spread spectrum: Base: 1 W (30 dBm) transmitter power with 6 dBi antenna = 36 dBm EIRP. Point-to-multipoint: Maximum 36 dBm EIRP regardless of antenna gain; reduce transmitter power 1:1 for gain above 6 dBi. Point-to-point (fixed directional): May use higher gain antennas with 1 dB power reduction per 3 dB gain above 6 dBi; effective EIRP can reach approximately 53 dBm with 30 dBi antenna at reduced power. Professional installation required for point-to-point with > 6 dBi antennas. Most consumer APs operate at 20-23 dBm with 3-5 dBi antennas: EIRP = 23-28 dBm, well under limits.
Each 6 dB EIRP increase doubles range (in free space, where path loss follows inverse-square law). Going from 0 dBi omnidirectional to 12 dBi directional adds 12 dB EIRP — 4x range increase in the main beam direction. However, directional gain concentrates energy: a 12 dBi antenna has approximately 60-degree beamwidth, covering 1/6 of the horizon instead of 360 degrees. For point-to-point links (WiFi bridges, backhaul), high-gain antennas are ideal — maximize range in one direction. For area coverage (APs, base stations), omni or sector antennas spread energy across the service area.

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