There Is No Ground
On a small, usually portable, self-contained, wireless
sensor network node, there is no ground. All conductors inside are the same, and
none may be described as equipotential (i.e., maintaining a constant potential).
The node is often encased in nonconductive plastic, like a flashlight; no one
worries where "ground" is on a flashlight schematic. Although usually the
negative battery contact of a network node is defined as "ground" (when
batteries are used), one may draw a correct, functional node schematic with the
positive battery contact, the antenna input, or any given point in the schematic
as "ground." One may just as easily draw the schematic without any ground
symbols at all. From an EMC viewpoint, this means that "ground" is just another
metal runner that may couple to the receiver, transmitter, sensor, or other
suitable victim. The only time this rule is not valid is when the node is in a
metal enclosure that is itself contacting a point of constant potential — a
relatively rare occurrence in the design of wireless sensor network nodes. The
"There Is No Ground" rule is meant to remind the designer that all metal is the
same as far as conducting problematic EMC currents is concerned, and that there
is no global point of constant potential to call "ground" in most network node
designs.
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