When you look at EMC analysis and prediction you see nearly every possible formulation brought into play depending mostly, I suspect, on the prejudice of the formulation guru. Now that you simply buy an Ajax Storm Door EMI Prediction Program that runs on your PC the situation is even worse because the assumptions used in selecting the algorithms are never stated, and although the same old trash is included, you don’t know which trash.

I don’t mean to say that the formulation is “wrong.” Most of it comes from a text somewhere and ultimately from someone like Kraus who knew exactly what was going on and why but had to get simple answers so that his students could understand his text. So he made carefully described assumptions like you were so far from the source that the waves are plain and have only frequency, amplitude and direction of arrival. If you look at the source you see a point of light. The stars are genuinely in the far field except for one of them – the sun. It is not in the far field. You don’t see a point of light – you see the shape of the source. You’re in the near field. It may be close enough to far field to suit you, but it is near field.

The EMC folks, I hesitate to use the term engineers, settle for having the largest of the near field terms be the same size as the far field terms. This guarantees at least a 6dB difference between the exact solution and the calculated value. Why not just guess based on experience? Why fool around with pseudoscience?

My next favorite stupid machination is using a half-wave dipole to measure the emissions from a distance of 5 meters at say 40MHz. When I drive out of my driveway, I face my neighbor’s house on top of which he has an 11-element two-meter Yagi antenna. It works. He gets about 10dB gain out of the thing because the elements are affecting each other. Each element is close to a half-wavelength long and about a quarter-wavelength from its nearest neighbor. If the whole mass were scaled to 40Mhz, it would be about 15 meters long and every one of the elements would affect the current flowing in every other element. That’s why the darned thing works. If the antenna is an array, not a dipole, you need to be even farther away.

The way we calculate the E-field set up by EMI currents in a culprit circuit is fascinating. First, we figure the current based on Ohm’s law – probably okay if we include the inductance of the wires or clads in the circuit, often the current-limiting factors in circuits above about 30MHz.

But then we fly blithely into never-never land. We get out the antenna book and pull out a formula. The problem is that antenna books are written about standing wave structures – dipoles and short circuit loops – because they work best as antennas. By and large the currents in the culprit circuits or on its circuit boards are traveling wave structures, that is the currents are in conductors connecting a generator to a load and are not standing waves. We almost always use the wrong equations. The equations are technically fine but they are the wrong equations for the situation. It’s like trying to explain the behavior of an octopus based on the known responses of a turkey.

Also, we tend to forget the generator impedance when we are calculating EMI noise. I’ve seen people predict thousands of volts per meter based on curves which supposedly predict E-field based on “noise currents.” Their argument goes like this: The open circuit noise voltage is up-dee-dink volts, so if we connect the generator to a circuit with an inductive impedance of a micro ohm, we get up-de-diddle mega amperes of noise current, and the curves say this produces kilovolts of radiated E-field. Bzzzatt – wrong! The noise generator has an internal impedance. If it is one ohm, then the maximum current that can flow in the circuit is, according to Ohm’s law, one ampere even if the external loop is a short circuit. And by the way, who said Murphy is smart enough to go around matching incidental noise generators to their accidental loads?