EMC Question of the Week: December 20, 2021

Radiated Immunity Measurement

A biconical antenna transmits 100 Watts of power at 200 MHz in a semi-anechoic environment. What is the maximum field strength 3 meters from the antenna?   

  1. 300 V/m
  2. 30 V/m
  3. 3 V/m
  4. 30 dB(μV/m)


The best answer is “b.” Biconical antennas are omnidirectional, with a maximum directivity of about 1.6. The radiated power density 3 meters away is equal to the radiated power divided by the area of a hemisphere times the directivity, 

P DENSITY = P RAD 2π r 2 D MAX 100W 2π 3m 2 1.6 =2.83 W/m 2 .

The rms value of the electric field strength is therefore,

E = P DENSITY × η 0 = 2.83 W/m 2 377Ω =32.7V/m.

So, at least to an order of magnitude, the radiated field strength at 3 meters is approximately 30 V/m.

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