Power Density
Understanding Power Density
Power density connects transmit power and antenna gain to the actual electromagnetic field intensity experienced at a distance. It is fundamental for both link budget calculations and RF safety assessment.
Power Density Calculations
PD = EIRP / (4 pi R^2)
Example: 100 W EIRP at 100 m:
PD = 100 / (4 pi * 10000) = 0.80 mW/m^2
FCC general public limit (> 1.5 GHz):
PD = 1 mW/cm^2 = 10 W/m^2
Occupational limit:
PD = 5 mW/cm^2 = 50 W/m^2
Frequently Asked Questions
What is power density?
Power density is radiated power per unit area (W/m^2). PD = EIRP/(4*pi*R^2). Used for RF safety compliance and link budgets. FCC general public limit: 1 mW/cm^2. Decreases with 1/R^2 distance law.
How do I calculate safe distance?
R_safe = sqrt(EIRP / (4 pi PD_limit)). For 1000 W EIRP and 1 mW/cm^2 limit: R = sqrt(1000/(4 pi * 10)) = 2.8 m. This is the minimum safe distance for the general public in the main beam.
What are the RF safety limits?
FCC (US): 1 mW/cm^2 general public, 5 mW/cm^2 occupational (above 1.5 GHz). ICNIRP (international): 1 mW/cm^2 public, 5 mW/cm^2 occupational. Below 1.5 GHz: limits scale with frequency.