Calculator Inputs
Example Data Table
| Case | Power | Gain | Distance | Estimated EIRP | Power Density |
|---|---|---|---|---|---|
| Cellular panel example | 25 W | 15 dBi | 30 m | 790.57 W | 0.06990 W/m² |
| Small point to point link | 5 W | 8 dBi | 10 m | 31.55 W | 0.02510 W/m² |
| High gain microwave dish | 100 W | 18 dBi | 100 m | 6309.57 W | 0.05021 W/m² |
Formula Used
1. EIRP from the transmitter chain
EIRP = Ptx × Glinear ÷ Llinear
2. Power density in the far field
S = EIRP ÷ (4πr²)
3. Electric field strength
E = √(S × 377)
4. Magnetic field strength
H = E ÷ 377
This model assumes free-space far-field behavior. It is suitable for first-pass engineering estimates, screening calculations, and distance trend studies.
When the evaluation point is closer than the calculated far-field boundary, measured values or full-wave modeling may be more appropriate.
How to Use This Calculator
- Select whether you want to calculate from transmitter parameters, direct EIRP, or direct ERP.
- Enter the relevant power input and choose the matching unit.
- If using transmitter mode, provide antenna gain and losses.
- Enter the observation distance, frequency, and largest antenna dimension.
- Set duty cycle, environment factor, and your chosen exposure limit.
- Choose graph sweep settings to visualize how density changes with distance.
- Press the calculate button to show the result card above the form.
- Use the CSV and PDF buttons to export the current result summary.
Frequently Asked Questions
What does RF power density mean?
RF power density is the radiated power passing through a unit area. It is commonly reported in W/m² or mW/cm² for exposure and propagation assessments.
What is the difference between ERP and EIRP?
ERP uses a half-wave dipole reference. EIRP uses an isotropic radiator reference. EIRP is 2.15 dB higher than ERP for the same radiated field.
Why does distance reduce the result so quickly?
Far-field density follows the inverse-square law. If distance doubles, the same radiated power spreads over four times the area, so density drops to one quarter.
Why include duty cycle?
Some systems do not transmit continuously. Duty cycle lets you estimate average exposure using the active transmit fraction instead of the peak value alone.
What is the environment factor for?
It scales the basic free-space estimate. You can use it to apply conservative screening margins for reflections, uncertainty, or site-specific engineering assumptions.
Can I use this in the near field?
Use caution. The inverse-square formula is mainly a far-field approximation. Near antennas, the electric and magnetic fields can behave differently and require more detailed methods.
How is electric field strength calculated here?
The calculator converts average power density using free-space impedance. It uses E = √(S × 377), where S is in W/m² and E is in volts per meter.
Are these results enough for regulatory compliance?
They are useful screening estimates, not a substitute for jurisdiction-specific compliance studies. Formal assessments may require averaging rules, reflections, occupancy, and direct measurements.