EMC Question of the Week: December 20, 2021

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)

Answer

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, 

$P_{DENSITY}=\frac{P_{RAD}}{2\pi r^2}{D}_{MAX}\approx \frac{\left(100\text{W}\right)}{2\pi {\left(3\text{m}\right)}^2}\left(1.6\right)=2.83{\text{W/m}}^{\text{2}}\text{.}$

The rms value of the electric field strength is therefore,

$\left|E\right|=\sqrt{P_{DENSITY}\times {\eta }_0}=\sqrt{\left(2.83{\text{W/m}}^{\text{2}}\right)\left(377\Omega \right)}=32.7\text{V/m.}$

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

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