EMC Question of the Week: February 3, 2025
A current filament in free space carries a sinusoidal current, Ic. The intrinsic impedance of free space is η0. The free-space wavelength is λ. If the length of the current filament is ℓ, which equation for the magnitude of the maximum radiated electric field strength is correct (or at least plausible)?
Answer
The best answer is “a.” This equation (in one form or another) is widely cited in EMC publications. But even if you didn't know the correct equation, the other equations can be eliminated due to problems with their units. The standard mks units on both sides of the first equation are volts per meter (V/m).
The standard mks units on the right side of the second equation are amperes per meter (A/m).
In the third equation, the argument of the sin() function is not unitless. The standard mks unit of length is the meter (m).
The standard mks units on the right side of the fourth equation are volts per meter squared (V/m2).
Checking the units is a good idea when applying any equation. It can be a particularly useful practice when taking a multiple-choice exam.
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