Answer 8—Depending on the frequency (particularly in the UHF and microwave bands) and the size and geometry of the PCB trace, the guard ring may resonate at the radio frequency, causing unexpected side effects.

In some cases, this can be used on purpose: a guard ring will have extremely little RF field inside at its resonant frequency, particularly if the E-field lies in the plane of the ring (and if the H-field is perpendicular). A gap can be cut in the ring with a capacitor and resistor placed across it to vary the frequency or sharpness of the resonance.

Contributor: David Tweed

Published February 2004