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Radiated Field Strength and Antenna-Link Calculator

Calculate transmitter power, antenna gain, EIRP, and field strength, with an automatic far-field check.

FORMULAv1.0.0
INPUT PARAMETERS

Build a free-space antenna link

Convert between power and field strength, with an automatic far-field check using frequency and antenna size.

CALCULATION RESULT

Calculation result

Calculated locally
∿

Enter the parameters and run the calculation to see results and model assumptions here.

FORMULA & TRACEABILITY

Formulas and applicability

Formulas, variables, and model boundaries are published. References last reviewed on 2026-08-30.

6 formulas · 3 references
CORE EQUATIONS

Core equations

These equations directly produce the primary results and define the tool’s core model.

3
F01

Equivalent isotropically radiated power

EIRP = Pₜ · 10^(−L/10) · G

Combines transmitter power, feed-line loss, and linear gain relative to an isotropic antenna.

Pₜ
Average transmitter output powerW
L
Feed-line loss from transmitter to antennadB
G
Realized antenna gain1
Applicability
  • All quantities correspond to the same frequency
  • Use realized gain in the target direction
F02

Free-space power density

S = EIRP / (4πr²)

In the free-space far field, power density in the target main-beam direction decays with distance squared.

r
Distance from antenna phase center to observation pointm
Applicability
  • Free space, line of sight, and far field
  • Ground reflection and multipath are excluded
F03

Power density and field strength

E = √(Z₀S) = √(Z₀·EIRP/(4πr²))

Calculate steady-state sinusoidal RMS electric field from average power density; the constant is about 29.979 using exact Z₀.

E
RMS electric field strengthV/m
Applicability
  • Free-space far-field plane wave
DERIVED & CONVERSION

Derived and conversion equations

Derive units, levels, and supporting engineering quantities from the core values.

2
F04

Transmitter power required for a target field

Pₜ = 4πr²E² · 10^(L/10) / (Z₀G)

Calculate ideal average transmitter power from a target RMS field strength.

Applicability
  • Excludes amplifier compression, modulation peaks, VSWR protection, and additional margin
F05

Theoretical antenna factor (50 Ω)

AF = √[4πZ₀/(50Gλ²)]

Estimate theoretical antenna factor from realized gain for a reciprocal passive antenna conjugately matched to a 50 Ω receiver.

AF
Theoretical linear antenna factor1/m
Applicability
  • Passive reciprocal antenna
  • 50 Ω conjugate match
  • Not a substitute for calibrated antenna-factor data
BOUNDARIES & RULES

Boundary and rule equations

Check model applicability, measurement conditions, and regulatory rules.

1
F06

Link far-field check

rFF = 2D²/λ；M = r/rFF

Check whether the observation distance reaches the common Fraunhofer boundary before calculating field strength.

D
Maximum radiating dimension of the antennam
Applicability
  • Precision antenna measurements may require a larger distance ratio
REFERENCES

References

01ITU-R · Recommendation P.525-5Calculation of free-space attenuation2024 · Current official reference for free-space links and power-flux-density/electric-field relationships.
02NIST · NBS/NIST measurement methodologyMethodology for Standard Electromagnetic Field Measurements1977 (hosted by NIST) · Gives metrology methods for standard-field measurement, antenna factor, and receiver chains.
03NIST · NIST Technical Note 2159Laboratory Method for Recording AWS-3 LTE Waveforms2021 · The appendix uses 2D²/λ to check the Fraunhofer far-field condition.
Engineering use notice

Results use the models and assumptions shown on this page for design estimates and pre-compliance risk review. Complex structures, dispersion, near-field coupling, and test setup can cause significant deviation.

FAQ

Frequently asked questions

What is the EIRP formula?

EIRP(dBm) = P_tx(dBm) + G(dBi) − L_cable(dB). In the free-space far field, power density S = EIRP / (4π·d²).

How do I back-calculate the required transmit power?

From E = √(30 · EIRP) / d, solve EIRP = (E·d)² / 30 in linear units, then divide by antenna gain and add back feed-line loss to get transmitter output.

When is the link prediction unreliable?

When the observation distance is below the Fraunhofer boundary, ground reflection or multipath exists, or the target direction departs from the antenna main lobe.

PRE-COMPLIANCE SUPPORT

Need accredited testing or pre-compliance support?

These tools support design-stage estimates. For accredited EMC testing, compliance decisions, and troubleshooting, contact us.

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