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Limitations of the Simple Model

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EMC Test Scenario

The simple model described above has a number of limitations; the major ones are:

In the antenna near field, the field strength cannot accurately be predicted.
Metallic objects, particularly in the antenna near field, have a significant effect on the radiation pattern; the effect of these cannot be modelled using this approach.
The standard EMC radiated emissions test (EN 55022 [4]) requires the use of a ground plane; the effect of the ground plane cannot be taken into account in the simple model.

Cross-talk

Transmission lines are electrically short when their length is less than λ/10 where λ is the wavelength. λ is deduced from the relationship λ = c/f, where c is the speed of light (3×10⁸m/s) and f is the frequency in Hz.

The main problem with the simple model described previously is that it is only valid whilst the transmission lines are electrically short. As frequency or line length increases, the lines will eventually become electrically long. Once this occurs, standing waves are set up on the lines making the simple model invalid.

To overcome these and other problems, full wave EMC modelling is required.

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References

BS EN 55022:1998, “Information technology equipment - Radio disturbance characteristics - Limits and methods of measurement”

Last Updated: 2004-Oct-21

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