Field Strength and Power Density Converter

Electric field, magnetic field, power density, received power and antenna factor at a point — each from whichever one you have. The far-field boundary is worked out first, and the plane-wave conversions are refused inside it, because E = 377·H is not an approximation in the near field, it is wrong.

Last updated 24 Sep 2026

Field strength, power density and received power

Any one field quantity -> all of them

What do you have?Everything else on the page is derived from this one quantity. The first two are the same measurement in different units; the rest need the plane-wave relations, which is why the far-field question below is not optional.

Electric field (dBµV/m)The unit every radiated-emissions limit is written in: decibels above one microvolt per metre. MIL-STD-461G’s RE102 limits sit between 24 and 102 dBµV/m.

Electric field (V/m)The unit immunity requirements are written in. MIL-STD-461G’s RS103 levels are 5, 10, 20, 50, 60 and 200 V/m depending on platform and band.

Magnetic field (A/m)What a loop antenna or a magnetic field probe reads. Converting this to an electric field needs the far field; inside it, the ratio of E to H is a property of the source and not a constant.

Power density (W/m²)The SI unit. Exposure guidelines are usually written in W/m² or mW/cm², and 10 W/m² is 1 mW/cm².

Power density (mW/cm²)The unit most RF safety documents use in practice. 1 mW/cm² is 10 W/m².

Received power (dBm)What a receiver or power meter reads at the antenna connector, before any cable loss. Converting back to a field needs the antenna’s gain and the far field.

Antenna gain (dBi)Used for the effective aperture, the received power and the antenna factor. 2.15 dBi is a half-wave dipole; a biconical is roughly that; a double ridge horn is 6 to 15 dBi across its band. Gain is frequency dependent and belongs on a calibration certificate. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

FrequencyNeeded for the wavelength, which sets the effective aperture, the antenna factor and the far-field boundary. It does NOT affect the E, H and S conversions themselves — those are frequency independent in the far field.

Frequency unit

What kind of source or antenna?This picks the far-field criterion. An electrically small source has its far field beyond λ/2π. An aperture antenna’s far field starts at 2D²/λ, where D is its largest dimension, and that can be much further — a 1 metre dish at 10 GHz has a far field starting at 67 metres.

Largest dimension of the antenna or source (m)Used only for the 2D²/λ criterion. For a double ridge horn it is the diagonal of the aperture; for a dish, the diameter.

Distance from the source (m)Where the field is being measured or specified. Compare it with the far-field boundary printed below — if it is smaller, this page will not convert between E, H and power density, and will say why.

Field quantities at a point, drawn as a geometry. The source is on the left and the measurement point on the right; the dashed boundary between them is where the far field begins — λ/2π for an electrically small source, 2D²/λ for an aperture antenna. Beyond it a wave is a plane wave, E and H are locked together by 377 Ω, and the power density is E²/377. Inside it the ratio of E to H is a property of the source and not a constant, so this page refuses to convert there. The two wave impedances an ideal electric and an ideal magnetic dipole would give at your distance are shown live, so you can see how far from 377 Ω you are.

158.5µV/mExample

a field of 44 dBµV/m at 30 MHz, measured 3 metres from an electrically small source, with a 2.15 dBi antenna

In the far field, and only there

E = η₀ H    S = E²/η₀ = η₀H² = E·H    η₀ = 376.730313 Ω
E(dBµV/m) = 20 log₁₀(E × 10⁶)    S(dBm/m²) = E(dBµV/m) − 115.76
A_e = Gλ²/4π    P_rx = S A_e    AF = E/V = 9.7305 / (λ√G) in 50 Ω
AF(dB/m) = 20 log₁₀f_MHz − G(dBi) − 29.774
far field: r > λ/2π (small source) or r > 2D²/λ (aperture)

η0: the impedance of free space. 376.730313 Ω exactly enough for any measurement; 377 is the working value and 120π is the old one
Ae: effective aperture, the area the antenna appears to collect from. It falls as the square of frequency for a fixed gain, which is why a fixed-gain antenna receives less at higher frequencies
AF: antenna factor: the field divided by the voltage at the connector, in per metre. The constant 9.7305 is √(4πη0/50) and comes straight from the effective aperture and a 50 Ω load — it is not a fitted number
2D²/λ: the Rayleigh distance: where the phase error across an aperture of size D falls to λ/16. For a horn or a dish this is the criterion that matters, and it can be very far

Worked example

a field of 44 dBµV/m at 30 MHz, measured 3 metres from an electrically small source, with a 2.15 dBi antenna
44 dB above a microvolt per metre is 10^(44/20) µV/m = 158.5 µV/m
At 30 MHz the wavelength is 9.993 m and λ/2π is 1.59 m. The measurement is at 3 m, so this IS a far field and the plane-wave relations may be used
The magnetic field is E/η₀ = 420.7 nA/m, and the power density is E²/η₀ = 66.68 pW/m² — which is -71.76 dBm/m², or 44 minus 115.76
A 2.15 dBi antenna at 30 MHz has an effective aperture of Gλ²/4π = 13.04 m², so it would deliver 869.3 pW = -60.61 dBm into a matched receiver
Its antenna factor is 9.7305/(λ√G) = -2.38 dB/m — negative, because a ten-metre dipole collects a lot. So a receiver would read 44 minus that, which is 46.38 dBµV, or 208.5 µV
Had the measurement been made at 0.3 m instead of 3 m, none of the last three steps would be allowed: λ/2π is 1.59 m, and at 0.3 m an ideal electric dipole's wave impedance is 1.929 kΩ and an ideal magnetic one's 73.59 Ω — neither of them 377

The same field in every unit

The first four rows are the extremes of MIL-STD-461G’s RE102 limits; the last three are the range of RS103 test levels. Every row is a far-field conversion and none of them is valid closer than λ/2π to the source. Note that E in dBµV/m to S in dBm/m² is always a fixed offset of 115.76 dB, which is 10 log(377) + 90.
E (dBµV/m)	E (V/m)	H (A/m)	S (W/m²)	S (mW/cm²)	S (dBm/m²)
24	15.85 µV/m	42.07 nA/m	0.6668 pW/m²	0.06668 pmW/cm²	-91.8
44	158.5 µV/m	420.7 nA/m	66.68 pW/m²	6.668 pmW/cm²	-71.8
69	2.818 mV/m	7.481 µA/m	21.08 nW/m²	2.108 nmW/cm²	-46.8
102	125.9 mV/m	334.2 µA/m	42.07 µW/m²	4.207 µmW/cm²	-13.8
120	1 V/m	2.654 mA/m	2.654 mW/m²	265.4 µmW/cm²	4.2
140	10 V/m	26.54 mA/m	265.4 mW/m²	26.54 mmW/cm²	24.2
160	100 V/m	265.4 mA/m	26.54 W/m²	2.654 mW/cm²	44.2

Where the far field starts

The horn diagonals are computed from the aperture dimensions MIL-STD-461G paragraph 5.18.3.2c specifies. Note what this means for RE102, which places the antenna 1 metre from the test setup boundary: at 1 GHz the large horn’s Rayleigh distance is 9.146 m, so the measurement is made well inside the antenna’s own near field. The standard is defining a repeatable measurement, not a free-space field, and that is a distinction worth holding on to.
Source or antenna	Criterion	At 30 MHz	At 1 GHz	At 10 GHz
An electrically small source — a cable, a seam, a short whip	λ/2π	1.59 m	47.71 mm	4.771 mm
A 137 cm biconical	2D²/λ	375.6 mm	—	—
A 69 × 94.5 cm double ridge horn	2D²/λ	—	9.146 m	—
A 24.2 × 13.6 cm double ridge horn	2D²/λ	—	514.1 mm	5.141 m
A 1 m dish	2D²/λ	—	6.671 m	66.71 m

How far from 377 Ω the near field really is

Computed from the exact broadside fields of an ideal dipole, not from the asymptotic formulas. At a tenth of λ/2π the wave impedance is out by 20 dB in one direction or the other depending on the source, and nothing about a field-strength reading tells you which. That is why this page refuses rather than approximating. The two impedances multiply to exactly η₀² at every distance, which is a useful identity and a good check on any implementation.
Distance	Electric dipole	Magnetic dipole	Error if you divide by 377
λ/2π ÷ 100	37.67 kΩ	3.768 Ω	40.0 dB or -40.0 dB
λ/2π ÷ 10	3.73 kΩ	38.05 Ω	19.9 dB or -19.9 dB
λ/2π ÷ 3	1.153 kΩ	123.1 Ω	9.7 dB or -9.7 dB
1 × λ/2π	266.4 Ω	532.8 Ω	-3.0 dB or 3.0 dB
3 × λ/2π	339.3 Ω	418.3 Ω	-0.9 dB or 0.9 dB
10 × λ/2π	373 Ω	380.5 Ω	-0.1 dB or 0.1 dB

The refusal is the point

There are dozens of pages that convert field strength to power density by dividing by 377. The arithmetic is trivial and the answer is frequently wrong, because E = η₀H is a property of a plane wave and a plane wave is what you have in the FAR FIELD of a source. Inside the near field the ratio of electric to magnetic field is set by what the source is: close to a voltage node it rises as η₀/βr and can be kilohms; close to a current loop it falls as η₀βr and can be milliohms. At a tenth of λ/2π that is a 20 dB error in one direction or the other, and nothing about the reading tells you which.

So this page works out where the far field starts before it converts anything, and refuses the cross-family conversions inside it. What it still gives you there are the two wave impedances an ideal electric and an ideal magnetic dipole would produce at your distance, computed from the exact dipole fields rather than from the asymptotes — so you can see how far from 377 Ω you actually are, and decide for yourself.

Two far-field criteria, and the one that bites depends on the antenna. For an electrically small source — a cable, a leaking seam, a short whip — the boundary is λ/2π, which is where the radiation term overtakes the induction and electrostatic terms. For an aperture antenna the boundary is 2D²/λ, the Rayleigh distance, where the phase error across the aperture falls to λ/16; that can be very much further. A 69 by 94.5 cm double ridge horn at 1 GHz has a Rayleigh distance of about 9.1 metres — and MIL-STD-461G’s RE102 places that antenna 1 metre from the test setup boundary. The standard is specifying a repeatable measurement rather than a free-space field, which is a perfectly sound thing to do and worth understanding before quoting an RE102 number as a field strength.

Antenna factor, derived rather than quoted. The familiar constant 9.73 is √(4πη₀/50), and it drops straight out of the effective aperture and a 50 Ω receiver: the power density times Gλ²/4π is the received power, and the voltage across 50 Ω follows. The dB form, 20 log f_MHz − G(dBi) − 29.774, is the same thing. What matters in practice is that antenna factor is a property of an individual antenna at an individual frequency, and the certificate is the source — this page’s figure tells you what a given gain implies, which is a useful cross-check and not a substitute.

What lives elsewhere. Transmitted power, path loss and link margin belong to the link budget page and the free-space path loss page; dBm, watts and volts across a load belong to the dBm converter. This page owns field quantities at a point — which is what every emissions limit, every immunity level and every exposure guideline is written in. For the requirements themselves, the RE102 margin page and the RS103 test field page are the two ends of it.

Frequently asked questions

Why will this page not divide my field strength by 377?

It will, as soon as you are in the far field. Inside it the relation is not approximately true, it is untrue: the ratio of E to H is set by the source, and close to a current loop or a voltage node it is orders of magnitude away from 377 Ω. The page prints the wave impedance an ideal electric and an ideal magnetic dipole would give at your distance so you can see the size of the error you would have made.

Which far-field criterion should I use?

λ/2π for anything electrically small — a cable, a seam, a short whip, a small loop. 2D²/λ for an aperture antenna, where D is the largest dimension. For a large antenna at a high frequency the second is far larger and is the one that matters; for a small horn at a low frequency the first can be the binding one. The page prints both.

What is 115.76?

The fixed offset between dBµV/m and dBm/m², which is 10 log(377) + 90. It comes out of S = E²/η₀ once both are written in decibels, and it applies only in the far field like everything else here.

Where does the 9.73 in the antenna factor come from?

It is √(4πη₀/50), and it falls out of the derivation: received power is the power density times the effective aperture Gλ²/4π, the voltage across a matched 50 Ω receiver follows from that power, and antenna factor is the field divided by that voltage. It is not an empirical constant. In decibels the same relation reads AF = 20 log f(MHz) − G(dBi) − 29.774.

Can I use this to work out an exposure level?

You can convert the units, and the page will do that correctly. It is not an RF safety calculator and it does not know your averaging time, your body-part geometry, your spatial averaging or which set of guidelines applies to you. For reference, the ICNIRP 2020 guidelines’ reference levels in the 30 to 400 MHz band are 139 V/m for occupational exposure and 62 V/m for the general public. Those are reference levels with conditions attached, and your RF safety officer owns the question, not this page.

Does frequency change the E to H conversion?

In the far field, no — η₀ is a constant and E/H is 377 Ω at every frequency. Frequency matters for the wavelength, and therefore for the effective aperture, the antenna factor and the far-field boundary. It is the boundary that makes frequency matter to the conversion: the same 30 cm is deep in the near field at 30 MHz and well into the far field at 10 GHz.

Related calculators

References

Balanis CA. Antenna Theory: Analysis and Design. Wiley. The infinitesimal dipole fields including the induction and electrostatic terms, the reactive, radiating near-field and far-field regions, and the 2D²/λ Rayleigh distance criterion — all of which this page’s near-field wave impedances and far-field boundaries come from.
ITU-R Recommendation P.525, Calculation of free-space attenuation. Gives the field strength produced by an isotropic radiator as e = √(30p)/d, which is this page’s power-density relation written another way; the 30 there is η₀/4π = 29.979, so the rounding costs 0.0030 dB.
MIL-STD-461G, paragraph 5.18.3.2c, which specifies the measurement antennas and their aperture dimensions, and paragraph 5.18.3.3c, which places them 1 metre from the test setup boundary. Both are used in the far-field table above.
ICNIRP, Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz). Health Physics 118(5): 483–524, 2020. Cited only for the 30 to 400 MHz reference levels quoted in the FAQ (139 V/m occupational, 62 V/m general public); this page is not an exposure assessment and the guidelines’ own conditions and averaging rules apply.