Microstrip EMI Anaysis
Ansys HFSS Model
Created 3D models in HFSS for Analysizing the radiated energy from routing microstrip trace near the edge of a PCB. Our model controlled for other sources of losses - ie no dielectric and conductor losses. The power losses were analyzed utilizing S-Parameters as well by analyzing the far field emissions.
The conclusion of the analysis was as follows :
If velocity of the signal is a concern, ie timing is really sensitive, maintain about 10 mils from the edge of the return conductor
Radiated Emissions depend heavily on the frequency/bandwidth of the signal. Maximum distance from the edge of the board should be evaluated on a case by case basis if you are worried about radiated emissions.
It was found that EMI from a microstrip being routed too close too the edge a board is not a significant problem until it is closer than two line widths. Any closer than that, radiated power can almost double and become moderately directive.
As a general rule of thumb, for low EMI PCB design - route traces so that there is at least 2 line widths between the edge of your ground plane.
Phase velocity will increase slightly as the trace is brought closer to the edge of the board. This is due to the effective dielectric being slightly smaller
Due to the fact that our model controls for other sources of losses (i.e. PEC (perfect electric conductor), and no loss tangent in the dielectric) we see good agreement looking at the energy losses between the two perspectives