Field strength converter
Field strength is the electric field a passing radio wave carries, measured in volts per metre or dBµV/m. This converter turns that reading into power density in W/m² or mW/cm², and into the power a receiving antenna delivers to a 50 Ω input in dBm. That last step needs the frequency, because a longer wavelength collects more energy.
Pick what you measured. Everything else on this page follows from it.
Sets the wavelength, and with it the antenna’s effective aperture.
0 dBi is an isotropic antenna, a half-wave dipole is 2.15 dBi.
A plane wave arrives from the left and drives the receiving antenna behind it; the sine is the electric field, the dashed patch the area the antenna effectively collects from. The three cards below carry the same numbers as the results: the field you entered, the power density it implies after dividing by Z₀, and the power at the 50 Ω input after multiplying by that aperture. Amplitude, wavelength and patch follow the inputs but are sketched, not to scale.
Far field, plane wave, free space. The receive antenna is assumed lossless, matched to 50 Ω and aligned with the wave. Close to a transmitter, indoors, or on a reflective site, E and H are no longer tied by Z₀, and a real measurement needs a calibrated antenna factor plus cable loss.
Reference levels
| dBµV/m | Field strength E | Power density S | What the level is |
|---|---|---|---|
| 0.0 | 1 µV/m | 2.65e−15 | Reference point of the dBµV/m scale: 1 µV/m |
| 54.0 | 501 µV/m | 6.67e−10 | FM stereo, lowest usable field in rural areas (ITU-R BS.412) |
| 60.0 | 1 mV/m | 2.65e−9 | 1 mV/m, the worked example below |
| 66.0 | 2 mV/m | 1.06e−8 | FM stereo, lowest usable field in urban areas (ITU-R BS.412) |
| 120.0 | 1 V/m | 0.00265 | Exactly 1 V/m, which makes 120 dBµV/m a convenient anchor |
| 148.8 | 27.5 V/m | 2.01 | FCC exposure limit for the general population, 30–300 MHz (0.2 mW/cm²) |
| 155.8 | 61.4 V/m | 10 | FCC occupational exposure limit, 30–300 MHz (1.0 mW/cm²) |
Power density follows from the field alone, S = E² / Z₀. The FM planning levels are median values measured 10 m above ground. The FCC limits are averaged over 30 minutes for the general population and 6 minutes at work.
How it works
- 01
Read the field in volts per metre
Field strength is quoted in dB above one microvolt per metre: E (V/m) = 10EdB/20 · 10−6. So 60 dBµV/m is 1 mV/m, and every 20 dB is a factor of ten in volts per metre.
- 02
Divide by the impedance of free space
In the far field the wave is locally plane, so E and H are locked together by Z0 = 376.73 Ω. Power density then follows from the field alone: S = E2 / Z0. It is the same arithmetic that turns 27.5 V/m into 0.2 mW/cm².
- 03
Multiply by the effective aperture
An antenna intercepts an area, not a length. Its effective aperture is Ae = G·λ2 / (4π), so the received power is P = S · Ae. Ten times the frequency at the same field strength means one hundredth of the power.
- 04
Or go straight there with the antenna factor
Measurement receivers skip the middle step. The antenna factor AF = 20·log10(9.73 / (λ·G)) converts field to terminal voltage in a single subtraction, and in a 50 Ω system P (dBm) = E (dBµV/m) − AF − 107.
Formulas
- S — power density, W/m²
- E — electric field strength, V/m (RMS)
- Z0 — impedance of free space, 376.73 Ω (the textbook 120π approximation gives 376.99)
- P — power into a matched receiver, W
- Ae — effective aperture, m²
- G — receive antenna gain, linear (G = 10GdBi / 10)
- λ — wavelength, m (λ = c / f)
- AF — antenna factor, dB/m
- 9.73 = 4π · Z0 / 50
- 107 ≈ 120 − 10·log10(20), the dBµV → dBm offset at 50 Ω
Worked example
- E = 60 dBµV/m = 1060/20 · 10−6 V/m = 1 mV/m
- S = E2 / Z0 = 10−6 / 376.73 = 2.65×10−9 W/m2 = 2.65×10−10 mW/cm2
- λ = c / f = 2.998 m, so Ae = 1 · 2.9982 / (4π) = 0.715 m2
- P = S · Ae = 2.65×10−9 · 0.715 = 1.90×10−9 W = −57.2 dBm
- AF = 20·log10(9.73 / 2.998) = 10.2 dB/m
- cross-check with the 50 Ω shortcut: 60 − 10.2 − 107 = −57.2 dBm
FAQ
- What is the difference between dBµV/m and dBµV?
- dBµV/m is a field strength out in space, microvolts induced per metre of antenna. dBµV is a voltage at a connector, normally across 50 Ω. The antenna factor bridges them: V (dBµV) = E (dBµV/m) − AF (dB/m). Without an antenna factor the two numbers are not comparable.
- Why does received power depend on frequency for the same field strength?
- Because the antenna’s collecting area does. Effective aperture is Ae = G·λ2 / (4π), so at a fixed gain it shrinks with the square of frequency. A field of 1 mV/m delivers −57.2 dBm to an isotropic antenna at 100 MHz, but only −92.5 dBm at 5.8 GHz. The field is identical. The antenna simply intercepts less of it.
- What is an antenna factor?
- The ratio of the field strength at the antenna to the voltage it delivers into the receiver, in dB per metre. EMC test antennas ship with a calibrated antenna-factor table, and the measurement is then the receiver reading plus the antenna factor plus cable loss. For a lossless antenna it is AF = 20·log10(9.73 / (λ·G)).
- Do these formulas hold close to the antenna?
- No. They assume a plane wave, which needs the far field: beyond roughly 2D2 / λ for an aperture of size D, and at least a few wavelengths away. In the near field E and H are not tied together by Z₀. Power density cannot be derived from the electric field alone there. It has to be measured separately.
- What field strength do the exposure limits allow?
- In the United States, 47 CFR § 1.1310 caps general-population exposure at 27.5 V/m (0.2 mW/cm²) between 30 and 300 MHz, and occupational exposure at 61.4 V/m (1.0 mW/cm²). Enter those numbers here and the power-density readouts reproduce the table’s own values.
A field strength always belongs to a place. Waveshed computes coverage over real elevation data, so you can see where a transmitter still delivers the level you need, free in your browser.
Map the coverage →Related guides
- Plan radio coverage (RF / ITM) — the simulator works in received dBm, which is what the third scale here gives you
- Antenna patterns & the sensor panel — gain is a direction, not a single number, and the pattern decides what the receiver really sees
Sources & further reading
- ITU-R Recommendation P.525 — Calculation of free-space attenuation ↗ — relates radiated power, field strength and power flux-density in free space
- ITU-R Recommendation BS.412 — Planning standards for terrestrial FM sound broadcasting at VHF ↗ — the usable field-strength levels in dBµV/m used in the reference table
- 47 CFR § 1.1310 — Radiofrequency radiation exposure limits ↗ — FCC MPE table in V/m, A/m and mW/cm², the cross-check for this converter
- Antenna factor — Wikipedia ↗ — derivation of AF = 9.73 / (λ·√G) for a 50 Ω load
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