EMC Question of the Week: November 20, 2023

A circuit board trace routed over a return plane can be a significant source of radiated emissions if it

1. is not matched
2. is not electrically short
3. has 90^o corners
4. none of the above

Answer

The best answer is “d.” For common circuit board dimensions and operating frequencies, a trace routed horizontally over a conducting plane is a very inefficient radiation source. Radiation directly from a microstrip trace is not nearly strong enough to be a factor in a radiated emissions test. This is true whether or not the trace has a matched termination, and it is true even when the trace is electrically long (or resonant length).

In order for the signal in a microstrip trace to be radiated effectively, it must couple from the trace to something else that makes a reasonably efficient antenna. This coupling can occur if the trace is routed too close to another trace, component, or structure that carries power away from the signal trace.

Without the return plane, traces that are not electrically short can become efficient radiation sources. This is true even for differential trace pairs if they carry significant common-mode current.

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