Introduction to RF Radiation Safety / 2 / 2.4

2.4 SIGNAL MAGNITUDE AND RF EXPOSURE

The amplitude of an alternating voltage, current, or electromagnetic field can be stated in several ways. The peak value is the greatest magnitude reached relative to zero, while the peak-to-peak value is the difference between the maximum positive and negative excursions. For a symmetrical sinusoid, the peak-to-peak value is twice the peak value.

RF safety measurements and limits commonly use the root mean square (RMS) value. RMS is a power-equivalent measure: it is the steady value that would produce the same average heating in a resistive load as the alternating quantity. For a sinusoid, the RMS value is the peak value divided by √2, or approximately 0.707 times the peak value. Conversely, the peak value is √2 times the RMS value.

This distinction matters because the ordinary arithmetic average of a symmetrical RF electric or magnetic field over complete cycles is zero, even though the field transfers energy. A meaningful average is therefore obtained from the squared field values: square the instantaneous values, average those squares over the specified period, and take the square root when an RMS field strength is required.

In the far field of a plane wave, average power density is related to RMS field strength by S = E²/377 = 377H², where S is in watts per square meter, E is in volts per meter, H is in amperes per meter, and 377 Ω is the approximate wave impedance of free space. These plane-wave relationships must not be assumed in the near field, where electric and magnetic fields may need to be assessed separately.

Peak, RMS, and time-averaged values answer different questions and should not be interchanged. Modulation and duty cycle can make the instantaneous or short-term field much greater than its long-term average. Chapter 3 develops the relevant modulation concepts, while Chapters 6 and 8 apply signal averaging and field quantities to RF exposure and assessment.