Power Density

Power Density is a physical quantity that describes the amount of power transmitted or received per unit area. It is commonly expressed in watts per square meter (W/m²) and is essential in evaluating electromagnetic field exposure, signal strength, and energy transfer in RF systems.

Power Density Formulas

General Definition

S = P / A

where:

• S: Power density (W/m²)

• P: Total power (W)

• A: Area over which the power is distributed (m²)

Electromagnetic Fields

In free space, power density is also related to the electric field strength:

S = E² / η

where:

• E: Electric field strength (V/m)

• η: Intrinsic impedance of free space (≈ 377 Ω)

This equation assumes a plane wave propagating in a lossless medium.

Example Calculations

• Radio transmission:
  P = 100 W, A = 10 m² → S = 100 / 10 = 10 W/m²

• EM field exposure:
  E = 100 V/m → S = 100² / 377 ≈ 26.5 W/m²

Applications

• Electromagnetic safety: Verifying compliance with public and occupational exposure limits

• Wireless communications: Estimating received power and effective radiated power

• Antenna design: Evaluating radiation characteristics and efficiency

• Satellite and radar systems: Determining signal power at the receiver

• Medical technology: Assessing exposure in imaging and therapeutic devices

• Thermal management: Monitoring energy distribution in high-power electronics