12 November 2016
THE HAM‘S WIRELESS WORKBENCH n BY WARD SILVER N0AX
For low frequency control, switching, and DC power, it is possible to get the job done without worrying too much about wiring and cabling practices. If Point A is supposed to be connected to Point B
and the ohmmeter says it is — that’s generally sufficient.
The “fun” usually begins when the operation of Circuit A
starts disrupting the operation of Circuit B, or if the project
involves low-level audio such as from a microphone or
pickup coil. If there is a nearby transmitter of any sort
or perhaps a sensitive receiver, RF gremlins might begin
to appear. Suddenly, a whole new set of cautions and
constraints gets piled on to your “simple” project. In this
column, I’m going to provide an overview of shielding,
including some practices you should know as a defense
against these gremlins.
Most readers will have some sort of shielded cable in
their parts inventory. Maybe it is a shielded audio cable
with one or two conductors inside an outer shield of
thin foil or fine wires wrapped around them. Maybe it’s
a multi-conductor control cable with a braided shield.
Rarely on the schematic, but crucial to good performance!
Shielding and Shielded Cables
Loops: Keep Them Small
Back in the early 19th century, Faraday discovered that a
changing magnetic field enclosed by a wire would induce a voltage
in that wire. This relationship became known as Faraday’s Law.
Any loop formed by conductors — whether they are wires in a
piece of equipment, conductors in a cable, or traces on a printed
circuit board (PCB) — will pick up a signal from a stray magnetic
or electromagnetic field. Similarly, varying current flowing in
a loop will create and radiate a signal that can be a source of
An easy and inexpensive way to keep a circuit from becoming
a source or victim of interference is to simply twist the wires
together so that the area of the loop they form is small. Or, lay out
PCB traces for a signal and its return path next to each other. The
higher the current carried by the loop or nearby conductors (such
as for motors or appliances), the more important this becomes.
n FIGURE 1. Examples of common mode and differential
mode signals for different types of cables. A shows
differential mode signal current in a two-wire unshielded
cable. B shows a common mode signal current with a
return path via the enclosure and the earth ground. In C,
a common mode signal flows on the outside of a coaxial
cable shield with a differential mode signal inside the
cable. D shows the differential mode signal on signal wires
inside a shield with common mode current flowing on the
outside of the shield. (Graphic courtesy of the American Radio