I was just reading Henry Ott's book "Electromagnetic Compatibility Engineering" and chapter 16.3.3 where he discusses how changing reference planes is detrimental to EMI.
Quoting from the book:
When a signal trace changes from one layer to another, the return current path is interrupted because the return current must also change reference planes
which he explains increases loop area and radiated emissions.
Then I then came across this statement:
At 247 MHz (diamond marker in Fig. 16-9B), the emission is almost 30 dB greater for the case where the signal transitions from the top to bottom layer, versus the case where the signal is routed on a single layer.
That is in perfect accordance with the previous statement, but:
"Above about 2 GHz, the interplane capacitance is sufficient to reduce the impedance of the return path, and hence, the radiation in both cases are about equal"
And indeed by observing the graphs, above 2 GHz emissions in Figures 16-9 A,B are approximately the same.
So my question is:
Does designing for frequencies above 2 GHz have less restrictions, in that you don't have to take such issues as changing reference planes into consideration? That would be counterintuitive to what I've considered so far about higher frequency signals and how they are more susceptible to EMI.