Unintentional Antennas in Electric Circuits

Figure 1. Certain design features can introduce
unintentional antennas into electric circuits.

In our last post, we discussed how little radiated emissions it takes for an electronic product to fail an FCC certification test for EMC.

Where do these radiated emissions come from? No one designing an electric circuit board is designing them into their product on purpose. These sneaky antennas do not appear in the schematic. However, we can unwittingly introduce them into our product through certain styles of board and interconnect design features. It is sometimes jokingly said there are two kinds of designers: those who are designing antennas on purpose, and those who aren't doing it on purpose. We’re going to introduce two, basic models of antenna—magnetic dipole and electric dipole (Figure 1)—to reveal a secret source of radiated emissions.

Magnetic dipole antennas are comprised of a complete circular loop. In typical circuit board applications, all transmission lines and signal-return paths, when done correctly, are magnetic dipole antennas with a signal path and a return path making a complete loop. To keep the loop area small and reduce the efficiency of this sort of antenna, we route the return path directly underneath the signal trace.

Electric dipole antennas have a central voltage source with a couple of wires sticking off each end. When the voltage source creates an AC voltage, we get current sloshing back and forth in those wires. The current must make a complete loop, so it returns as displacement current between the plus and the minus ends of the wire, through the fringe electric fields. A perfect example of a dipole antenna occurring in a circuit board is a noisy ground plane with a coax cable’s shield connected to one end of the plane.

Of course, there is no such thing as uniform voltage on the ground plane. There's always going to be noise on that ground plane, typically called “ground bounce.” Ground bounce can be large or small, so that different parts of the plane are different voltages, which means we have a voltage generator. If one part of the ground plane couples to the chassis, and another part to the shield of an external cable, we've created an electric dipole antenna. Differing voltages on the ground plane will drive common currents onto the cable shield, and those common currents are going to radiate and return through displacement current to the plane.

The efficiency of magnetic dipole antennas generally is much, much less than the efficiency of electric dipole antennas. In other words, the same amount of common current flowing and returning through displacement current will be much more efficient at radiating from the electric dipole antenna, and often such structures are going to be the cause of the EMC failures we see. Anytime we generate a voltage source that can drive two conductors on either side, like ground noise on a plane, we will have radiated emissions if there's an external cable attached to the plane, or even if we have a large enough area on the plane. That's fundamentally what a patch antenna is.

Figure 2. Formula for calculating radiated emissions
from common current flowing in a wire.

If we have created an electric dipole antenna, we can use the formula in Figure 2 to calculate the radiated emissions from the current flowing in the wire, given the frequency, the length of the wire, and our distance from the source. How much common current do we need flowing in that external cable at 100 MHz for the electric field strength to be 100 µV/m at 3 m and fail an FCC test? A mere 3 µA. That is not much, which is why we worry about these common currents; they’re the most common source of FCC failures.

And the predominant root cause underlying the generation of common currents is discontinuities in the return path. When most engineers think about discontinuities in the return path, which can be any gap in the return path, they think about the problems caused by reflections from it, but that's generally the least of the problems. The bigger problem is the radiated emissions from large common currents.

That’s also why we worry about ground bounce. It’s the trifecta: it affects signal quality, crosstalk and EMI. Suppose we have some ground bounce noise in our ground plane, and one region of the plane has a different voltage than another region. That's our voltage source. And let’s say we’ve got a little cable sticking out with some common impedance to the return plane, literally the floor. The cable represents a transmission line with the floor as the return. If we had, say, 100 mV of ground bounce noise, assuming a 1 k𝝮 impedance typical of 1 m cables, there would be about 100 µA of common current flowing in the cable, way above the 3 µA FCC limit. That's a 4,000 µV/m electric field at 3 m away. This is why it’s so important to lay out circuit boards in a way that reduces ground bounce noise, less to reduce reflections and crosstalk than to reduce EMI.

Tip: Don’t attach your cable shields to the ground plane, attach them to the chassis.

Watch Dr. Eric Bogatin demonstrate many bench top tests for radiated emissions in the on-demand webinar, Pre-compliance EMC Testing with a Real-time Oscilloscope.