Calculation Results
EIRP (Effective Isotropic Radiated Power): 0 Watts
Power Density (S): 0 W/m²
Power Density (S): 0 mW/cm²
Understanding RF Exposure and MPE
Radio Frequency (RF) exposure is the measurement of the intensity of electromagnetic radiation emitted by wireless devices like cell towers, Wi-Fi routers, and radio transmitters. Engineers and safety officers use the Maximum Permissible Exposure (MPE) limits to ensure that people are not exposed to potentially harmful levels of non-ionizing radiation.
The Power Density Formula
The calculator uses the standard far-field free space formula to estimate power density ($S$):
S = (P × G) / (4 × π × R²)
- P: Transmitter power (at the antenna input).
- G: Linear gain of the antenna (converted from dBi).
- R: Distance from the center of radiation (the antenna).
- S: Power density (usually expressed in mW/cm² or W/m²).
Calculation Example
Suppose you have a Wi-Fi router transmitting at 0.1 Watts (100mW) with a 5 dBi antenna gain. If you are standing 0.5 meters away, the calculation works as follows:
- Convert dBi to linear gain: 10^(5/10) = 3.16.
- Calculate EIRP: 0.1W * 3.16 = 0.316 Watts.
- Calculate surface area: 4 * π * (0.5)² = 3.1415 m².
- Power Density (W/m²): 0.316 / 3.1415 = 0.1006 W/m².
- Power Density (mW/cm²): 0.1006 / 10 = 0.01006 mW/cm².
FCC & ICNIRP Safety Limits
Most regulatory bodies like the FCC (USA) and ICNIRP (International) set limits based on frequency. For example, at 2400 MHz (standard Wi-Fi), the FCC general population MPE limit is 1.0 mW/cm². This calculator provides a basic reference check against these common standards for educational purposes.
Note: This calculator assumes "Far-Field" conditions and free-space propagation. It does not account for ground reflections, absorption, or specific absorption rate (SAR) which is required for devices used very close to the body (like smartphones).
function calculateRFExposure() {
var p = parseFloat(document.getElementById('transmitterPower').value);
var gDbi = parseFloat(document.getElementById('antennaGain').value);
var r = parseFloat(document.getElementById('distanceMeters').value);
var freq = parseFloat(document.getElementById('frequencyMHz').value);
if (isNaN(p) || isNaN(gDbi) || isNaN(r) || isNaN(freq) || r <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
// 1. Convert dBi to Linear Gain
var gLinear = Math.pow(10, (gDbi / 10));
// 2. Calculate EIRP
var eirp = p * gLinear;
// 3. Calculate Power Density (W/m^2)
// S = EIRP / (4 * PI * R^2)
var sWattsM2 = eirp / (4 * Math.PI * Math.pow(r, 2));
// 4. Convert Power Density to mW/cm^2
// 1 W/m2 = 0.1 mW/cm2
var sMWcm2 = sWattsM2 / 10;
// 5. Update UI
document.getElementById('eirpValue').innerText = eirp.toFixed(4);
document.getElementById('powerDensityW').innerText = sWattsM2.toFixed(6);
document.getElementById('powerDensityMW').innerText = sMWcm2.toFixed(6);
// 6. Compliance logic (Simplified FCC General Population Limits)
// Below 300MHz: 0.2 mW/cm2
// 300-1500MHz: f/1500 mW/cm2
// 1500-100,000MHz: 1.0 mW/cm2
var limit = 0;
if (freq = 300 && freq <= 1500) {
limit = freq / 1500;
} else {
limit = 1.0;
}
var complianceDiv = document.getElementById('complianceNote');
if (sMWcm2 <= limit) {
complianceDiv.style.backgroundColor = "#e6ffed";
complianceDiv.style.color = "#28a745";
complianceDiv.style.border = "1px solid #b7eb8f";
complianceDiv.innerText = "Condition: BELOW general population limits (" + limit.toFixed(2) + " mW/cm² for this frequency).";
} else {
complianceDiv.style.backgroundColor = "#fff1f0";
complianceDiv.style.color = "#cf1322";
complianceDiv.style.border = "1px solid #ffa39e";
complianceDiv.innerText = "Condition: EXCEEDS general population limits (" + limit.toFixed(2) + " mW/cm² for this frequency).";
}
document.getElementById('rfResultArea').style.display = 'block';
// Scroll to result
document.getElementById('rfResultArea').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}