EMC Question of the Week: October 31, 2022

The current distribution on a half-ounce copper plane below a microstrip trace at 100 MHz is mostly concentrated under the trace as indicated in the figure. At 1 GHz, we would expect the current distribution on the plane to be 

1. about the same
2. more concentrated
3. more spread out
4. flat

Answer

The best answer is “a.” At low frequencies, where magnetic fields can easily pass through the copper plane, the resistance of the plane causes the current to spread out and the current distribution under the trace is fairly flat. However, at high frequencies where the fields cannot penetrate the plane, the current distribution on the plane in the vicinity of the trace is,

$J_S\left(x\right)=\frac{I_{total}}{w\pi }\left[{\tan }^{-1}\left(\frac{2x-w}{2h}\right)-{\tan }^{-1}\left(\frac{2x+w}{2h}\right)\right]$

where w is the width of the trace, h is the height of the trace, and x is the positive or negative distance from the point on the plane below the center of the trace.

This current distribution is not a function of frequency. The current does not become more concentrated under the trace as the frequency increases. 

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