A guard ring is a sort of electrostatic (and glavanic) screen or shield.
It separates electrical fields (ac and dc) between its one and other side,
when it is connected to a suitable potential (ground or driven), just like
any conductor would. When it forms a closed contour it can act as a
resonator at UHF and above and also as a shorted transformer turn, which
is undesirable (it should be connected to whatever at a single point).

When it acts as a resonator it will 'reflect' any RFI energy that is not
incident at one of its parallel resonant modes. This means that the H
field of incident RFI (perpendicular to the guard ring plane) will be
sharply reduced inside the guard ring, but only at the series resonant
frequencies of the ring. These will be sharp-tuning since the ring has low
resistance and a high Q. E field is reduced also, when incident in the
plane of the ring, such as when coming from other guard ringed circuit
subsytems on the same board. Having guard rings of the same size on the
board is a bad idea (coupled tuned circuits).

A split ring has the same characteristics but it can be tuned (by putting
a small cap across the gap). This helps sometimes. Othertimes the ring is
damped by putting a resistor across the gap. This lowers its Q and causes
the absorbtion effect to be more efficient in a much wider frequency band
but worsens the nulls that can be obtained.

The ring need not be a ring it can be any closed trace shape, and its
circumference should be as short as possible. In theory an un-closed (i.e.
open) shape should work equally well as a shield/absorber/reflector but
I've never see one used like this (it being the open dipole equivalent of
the closed dipole guard ring).

Peter

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