How to Measure RMS Value for Narda Broadband Field Meter?

National and international bodies have specified frequency-dependent limit values for electromagnetic radiation to protect us from harmful exposure to non-ionising radiation. In Australia, ARPANSA (Australian Radiation Protection and Nuclear Safety Agency) sets limits on how much EME (Electromagnetic Energy) is safe for people and the ACMA (Australian Communications and Media Agency) ensures telecommunication companies follow the rules defined by ARPANSA.

Measuring the frequency-dependent reference values specified in these standards provides technical validity that the limits are not exceeded. RMS (Root Mean Square) value represent the equivalent average value of the signal power level. The RMS value for a sine wave is equal to the peak RMS value divided by √2 (­­­­­Figure 1). 

Figure 1: RMS (Root Mean Square) value of a sine wave field strength signal versus time

The RMS value is used because it represents the relevant influence quantity for the thermal effects of electromagnetic radiation. Measured field strengths should always be taken as being RMS (Root Mean Square). The NBM Broadband Field Meter always registers the RMS values with its wideband probes regardless of the result type setting (Actual, Maximum, Minimum, Average or Maximum Average). 

How to Derive RMS Values in the Narda NBM Broadband Field Meter

RMS detectors are predominantly used in wideband measurement of high frequency electric or magnetic fields to demonstrate compliance with the exposure limit values in the standards. The RMS detector in the NBM is formed by the probe, which in conjunction with the basic unit produces an overall integration time of about 300 ms. The probe itself delivers a DC voltage proportional to the square of the field strength, but which also still contains some residual ripple.

This residual ripple is smoothed by integration in the basic unit; the signal is then digitised and displayed as the RMS value. This method applies both to diode-based probes and to thermocouple probes. The nonlinearities that inherently occur in diode probes at increasing amplitudes are compensated by the basic unit for each probe individually. To do this, the basic unit reads the linearity data out from the memory module integrated into the probe (figure 2). This method ensures that the highest possible accuracy is achieved for large and small-signal levels alike.

Figure 2: A small memory chip in the NBM probe stores all the major probe data

Averaging the RMS Value

In most of the standards, demonstrating compliance with exposure limit values for frequencies above 100 kHz generally involves an average measurement over a 6-minute period. The RMS field strength values recorded during the measurement interval must be averaged quadratically. The 6-minute interval is based on the absorption characteristics of human tissue, which shows a similar time constant.

However, other averaging times are specified in some standards. The ARPANSA Standard specifies a measuring average time of 6 minutes for frequencies up to 10 GHZ and then decreasing down to 10.2 seconds at 300 GHz. This formulation is consistent with established dosimetry data and it also serves to fully protect against any rapid heating that may occur during exposure to high-level transient fields.

The Narda NBM Broadband Field Meter allows you to set the averaging time in the range 4 seconds to 30 minutes. The NBM uses a floating average to enable standard-compliant measurements to be performed over longer time periods. This uses a continuous memory (figure 3), in which the oldest measurement result is always replaced by the latest value. The highest average value occurring during a long-term measurement can be conveniently displayed on the NBM using the “Maximum Average” result type. This important function is not provided by any other wideband device available in the market.

Figure 3: Formation of the floating average field strength value using a continuous result memory

The Narda NBM Field Broadband Meters detect high-frequency electric and magnetic fields and have been specially designed for personal safety measurements. The NBM devices can provide you with precise, reproducible, and calibrated measurements so they can be traced back to national and international standards. The results are therefore authoritative and conclusive.

Air-Met Scientific is the exclusive distributor of Narda STS range of radiation monitoring devices in Australia. 

If you have any questions about the Narda Broadband Field Meters or would like to learn more on how to measure RMS Values on the NBN Meter, please contact your local Air-Met Scientific representative