EMF Exposure Calculator

Introduction to far-field RF exposure estimates

This EMF exposure calculator estimates the radiofrequency energy crossing a square metre at a chosen distance from an antenna. It starts with power delivered at the antenna, antenna gain in the direction of interest, frequency, distance, reflection allowance, and transmit duty factor. It then reports equivalent isotropically radiated power, time-averaged incident power density, electric field strength, magnetic field strength, and magnetic flux density. Where the entered frequency is in scope, it also screens the result against ICNIRP 2020 reference levels and FCC maximum permissible exposure values.

The calculation is intentionally a far-field screening estimate. It is useful when planning access around an antenna, checking a proposed public location in an open main beam, or learning how distance and gain change an RF result. It is not a replacement for a calibrated field survey or a complete compliance assessment. In the reactive near field, electric and magnetic fields can vary independently with location. A single plane-wave power-density number cannot describe that environment, so the calculator identifies when the selected point is too close for this model.

This page addresses RF sources from approximately 100 kHz to 300 GHz. It is not a calculator for 50 Hz or 60 Hz wiring and power-line fields, because low-frequency exposure guidance uses different quantities and physical mechanisms. It is also not suitable for a mobile phone held against the head or body. Handset assessments normally use specific absorption rate, often called SAR, rather than a distant plane-wave power-density estimate.

How to use the RF power-density inputs

Enter average transmitter power delivered at the antenna, after feedline and connector losses. This is not necessarily the transmitter nameplate rating. A radio with a 100 W amplifier may deliver less power after cable loss, and a modulated transmitter may have a lower time average than its stated peak envelope power. Choose watts, kilowatts, milliwatts, or dBm as appropriate. For a dBm entry, the calculator converts the logarithmic value to watts using $P={10}^{(\mathrm{dBm}-30)/10}$.

Gain matters because antennas do not radiate equally in every direction. Use gain for the direction occupied by the person being assessed. Main-beam gain is normally a cautious planning input, while a point behind an antenna or below a downtilted panel may receive much less energy. Select dBi when gain is referenced to an isotropic radiator, dBd when it is referenced to a half-wave dipole, or numeric ratio when the conversion has already been made. Distance is measured from the antenna’s centre of radiation to the possible occupied location, not simply from the base of a mast or edge of a roof.

Frequency selects the reference-level band and determines wavelength, so use the operating carrier or the relevant band edge for a conservative assessment. The optional largest antenna dimension helps estimate a practical far-field boundary. The reflection choice is a broad planning allowance rather than a prediction of a particular wall, roof, or ground path. Duty factor should represent the fraction of the applicable averaging period during which the source transmits. Finally, select general public unless people are informed workers who can understand and control their exposure; occupational or controlled limits cannot be applied merely because a person happens to be at a workplace.

RF power-density formulas and far-field assumptions

In the ideal free-space model, energy from an isotropic source spreads across the surface of a sphere. At distance $r$, that surface area is $4\pi r^2$. The calculator first forms EIRP by multiplying power at the antenna by numeric gain. Dividing EIRP by the spherical area gives the on-axis incident power density:

The basic EIRP relationship and the usual conversion from effective radiated power are:

For a gain entered in dBi, the numeric gain used in the calculation is:

A dBd value is first shifted by 2.15 dB because a half-wave dipole has about 2.15 dBi gain. Power units are likewise normalized before the calculation; for example:

In a far-field plane wave, the electric and magnetic fields are related through the free-space impedance $Z_0$, approximately 376.730 Ω. That relation is why the calculator can show electric field, magnetic field, and flux density from the same modeled power density:

Wavelength is needed for the near-field screening boundary:

The reactive boundary is often written as $\lambda /2\pi$. For an antenna or aperture with largest dimension $D$, OET Bulletin 65 presents useful near-field and far-field distances:

The displayed far-field onset is conservatively based on the largest practical boundary, including half a wavelength beyond supplied antenna extent:

Reflections and intermittent transmission modify the free-space result. A 1.6× electric-field enhancement produces 2.56× power density; a fully constructive 2× electric-field condition produces 4× power density:

Here $d$ is duty factor expressed as a fraction, rather than a percentage:

Combining the adjustment factors with the free-space expression gives the quantity actually shown as the time-averaged screening estimate:

For a selected power-density level $S_{\lim }$, the inverse-square model provides a main-beam screening distance:

At shared sites, individual sources should not be approved one by one. The frequency-specific exposure ratios must be added:

Keep the unit conversion in mind while reading a result:

For logarithmic planning calculations, EIRP can also be stated directly in dBm:

Worked example: a 50 W VHF repeater at 20 metres

Consider a 150 MHz repeater that delivers 50 W to a 6 dBi antenna. A nearby window is 20 m away on the main-beam side, the antenna is approximately 1 m long, and the assessment uses free space with a 100% duty factor. The numeric gain is ${10}^{6/10}=3.981$, so EIRP is approximately 199 W.

The modeled density is 199 divided by 4π × 20², or about 0.0396 W/m². This equals 0.00396 mW/cm². At 150 MHz, the general-public ICNIRP and FCC power-density reference levels are both 2 W/m², so the estimate is about 1.98% of either power-density level. The wavelength is close to 2 m. With the stated antenna dimension, the calculator places its conservative far-field onset near 2 m, making the 20 m point well beyond that screening boundary. This is still a model, not an assertion that a particular window has been measured.

Reading RF exposure results against ICNIRP and FCC limits

The result panel separates physical quantities from comparisons with published reference levels. A value such as 20% means the modeled, time-averaged quantity is one fifth of the selected level. A value at or below 100% is within the selected screening limit under the entered assumptions. A value above 100% means the assumptions need closer review, a more complete calculation, mitigation, access controls, or measurement. It does not by itself predict injury or describe every possible health outcome.

The general-public tier is normally the appropriate selection for accessible areas, visitors, residents, and people without exposure training. Occupational or controlled limits apply only where people are informed and able to control their exposure. The calculator reports both ICNIRP and FCC comparisons because their regulatory histories, frequency bands, averaging periods, and reference-level expressions differ. A planner should follow the rule that governs the specific installation rather than choosing the more favorable output.

Below approximately 30 MHz, ICNIRP does not provide a general incident-power-density reference level because separate electric- and magnetic-field checks are more meaningful. FCC values at those frequencies are plane-wave equivalents and are labeled as reference-only comparisons. At a very low frequency, do not infer that a far-field density result is a complete exposure finding.

Typical RF sources and meaningful comparisons

A Wi-Fi access point, VHF repeater, broadcast antenna, cellular sector, and microwave dish can have very different transmitter powers yet produce similar or very different values at a public location. Distance is powerful because doubling distance reduces this idealized far-field density to one quarter. Directional antennas are equally important: a microwave link may use modest transmitter power, but its dish gain can make EIRP high in a narrow beam. Conversely, an antenna’s rear and sidelobes can be far below main-beam gain.

Buildings, foliage, terrain, polarization, nearby metalwork, antenna downtilt, and other transmitters all matter at real sites. The calculator deliberately does not pretend to know these details. Using main-beam gain with no assumed shielding is often useful for an early conservative estimate, but it cannot show whether a particular person is behind a wall, underneath a panel, or at a local reflection hotspot. Treat the number as a transparent starting point that identifies where more site-specific work may be worthwhile.

Limitations of this RF exposure screening model

This RF exposure screening model assumes one dominant source, a clear main-beam direction, and free-space inverse-square spreading modified only by the selected reflection factor and duty factor. It does not calculate a detailed antenna pattern or model roof edges, terrain, walls, metal railings, absorbing materials, polarization mismatch, multiple paths, or standing waves. The reflection options are broad allowances, not replacements for numerical modeling or an instrument survey. Actual values may be lower because of obstruction or higher at a localized constructive reflection.

The far-field limitation deserves particular attention. When the selected point is inside the reactive near field or transition region, the displayed inverse-square result can be a conservative planning bound but is not a physical field map. A professional review may require measured electric and magnetic fields, numerical electromagnetic modeling, or assessment against applicable basic restrictions. If antenna dimension is unknown, the calculated far-field onset is only a lower-bound estimate; large arrays and aperture antennas can have a substantial transition region.

Duty factor must describe operation during the averaging period used by the applicable standard. Adaptive base stations, radars, burst systems, and shared sites may require traffic records, transmitter logs, or a site-specific averaging method. For multiple sources, add the exposure ratios in their relevant bands rather than treating each individual percentage as independent permission. ICNIRP and FCC reference levels are engineering compliance tools principally linked to established thermal effects. This calculator cannot provide legal, medical, or regulatory advice and cannot substitute for a calibrated survey performed in accordance with the governing rules.