5.9 Necessity to consider the influence of the external environment at the cavity field simulation

The cavity field model (4.13) provides, together with the source introduction (5.9), an efficient method for the simulation of the internal enclosure fields. However, the model has to be extended to consider the influence of the emissions from the open slots at the edges of the parallel planes. Slots can be allocated on an enclosure to enable a cooling air flow, or for connectors which establish the functional interface of the device. Figure 5.19(a) depicts the connector at the enclosure slot of an automotive control device, and Figure 5.19(b) depicts the back plane of a personal computer enclosure with connector and coolant air slots. Coupling of the internal fields to cables and direct radiation from the slots have a significant influence on the internal enclosure field, due to the emission losses. [45] and [101] have shown a rising influence of the radiation loss from the edges of power planes with increasing plane separation distance $h$. An enclosure usually has a much higher cover to PCB ground plane-separation $h$ than power-ground planes, while dielectric losses are much lower. Therefore, the radiation loss from the slots becomes the dominant loss mechanism from enclosures without cables. The coupling to cables introduces additional losses. Both have to be considered in the simulation of the internal enclosure fields in order to obtain accurate field results for a subsequent simulation of the device emissions.
Chapter 6 describes a method to consider these couplings by a new domain decomposition approach. This method is not restricted to slots at the edges of slim enclosures, like that in Figure 5.19(a). It can generally be utilized for every metallic enclosure with apertures, as for example the personal computer enclosure in Figure 5.19(b).

(a) Motor control device with cable harness.

(b) Back plane of a personal computer enclosure.

Figure 5.19: The field inside a metallic enclosure causes direct radiation from the enclosure apertures. Coupling from the internal fields to cables at the connectors causes additional emissions. Large apertures or cable emissions can have a significant influence on the internal enclosure field. Thus, the external environment of the enclosure has to be considered at the internal field simulation.

C. Poschalko: The Simulation of Emission from Printed Circuit Boards under a Metallic Cover