This methode is very nicely discussed here:

http://www.murata.com/emc/knowh...8to15e.pdf

This discussion shows how extremely low the impedance of a solid ground plane is: Even when only connected via some wires to example board (which must be as short as possible, of course!) the impedance is smaller than the interrupted ground plane of example board!

Nevertheless, the benefit of a solid ground plane decreases the more, the more the ground plane is distanced from signal tracks. So, the suggestion of Andy would yield much better results, than a separate aluminium plate under the PCB.

Why is it so important to have the solid ground plane right under the signal traces?
Because we need the intensive mutual inductive coupling between signal traces and its ground return currents on ground plane. Means, the effective inductance of this arrangement must be so low, that the ground return currents are forced to flow directly under the associated signal traces.

From calculations it can be shown, that even the double sided printed circuit board of about 1.5mm thickness still does not yield enough inductive coupling to guarantee this. Only the multilayer board, where the layers are much closer in contact, will. But for standard applications the double sided PCB already is a huge advantage against single layer PCB. Today, at least a double sided PCB should be used!!!!!!!!!

Now you will mention: And what about the capacitance?
It doesn't really matters. What finally counts is, that with this transmission line arrangement the characteristic impedance of signal traces goes closer to the output impedance of digital gates, means a quasi series termination can be achieved.

Another advantage of a close solid ground plane is, that this guarantees an extremely small capacitive coupling between neighboured copper traces: Without a close ground plane underneath them, there would be a rather big stray capacitance between the two neighboured traces. This stray capacitance is nearly totally eliminated, when a ground plane is underneath the neighboured traces, which's distance to the two traces is smaller than the distance between the two traces!

Fast digital circuits would never work without this close and solid ground plane! It is this ground plane which eliminates the huge influence of stray capacitances between all the many signal traces!!

The advantage and benefit of having a solid and close ground plane cannot be too much focused on!!!!!

Kai