An important variable in the measurement of electromagnetic fields (EMF) is the frequency range. For static fields such as the Earth’s natural magnetic field, the frequency is 0 Hz. Low-frequency fields (e.g. those caused by high-voltage power lines) range from 0 Hz to 100 kHz in the electromagnetic spectrum. High-frequency fields range from 100 kHz to 300 GHz and are used for mobile communications and Wi-Fi, among other things. The measuring device used must therefore be suitable for the frequency range in which measurements are to be taken.

A distinction is made between broadband and frequency-selective measurements. In a broadband measurement, the total field strength is recorded over a wide frequency range. These measurements are particularly fast and therefore suitable for determining the location where the highest total field strengths occur. If you would like to assign field strength values to specific sources or compare the measured values with limit values, you need a frequency-selective measuring device because different limit values apply for different frequencies. A frequency-selective device not only displays field strengths but also allows the field strength values to be assigned to the underlying frequencies.

Improper use of measuring devices can lead to incorrect or distorted measurements. The measurement behaviour can be unpredictable. For example, if fields outside the specification and frequency range of the measuring device are detected or if the device is not designed for the intended measurement purpose.

Measuring devices for the high-frequency range cannot detect electric or magnetic fields caused by power lines. In addition, most high-frequency measuring devices are suitable for measuring only in the far field. This means that incorrect measurement results can occur if the device is too close to a source of exposure. Depending on the type of antenna, the term far-field conditions is usually used when the distance to the source is at least twice the wavelength.

People or objects can distort low-frequency electric fields and thus influence or falsify measured values of the electric field strength that are recorded in their vicinity. Movements of the device during the measurement of low-frequency magnetic fields can also lead to falsified measured values.

Most inexpensive measuring devices determine broadband field strength values or only certain frequency-selective values for individual frequencies or frequency ranges. A detailed frequency-selective field strength measurement for a professional assessment of electric, magnetic, or electromagnetic fields is typically not possible with inexpensive measuring devices. In addition, these measuring devices sometimes have large measurement inaccuracies. The results provided are thus not meaningful or reliable.

In the operating instructions of many measuring devices, values or sometimes also “building biology guide values” are recommended. Based on these, alarm or warning thresholds are often integrated into the devices. Some of these values and thresholds are well below the limits for low- and high-frequency fields applicable in Germany. However, in contrast to the limit values of the 26. BImSchV, these (guideline) values and alarm or warning thresholds are not based on scientifically recognised effects of the fields.

Field strengths measured with these devices are often rated as extremely high – even though the measured values are still far below the legal limits or the EU Council recommendation. In addition, values for the power flux density in the unit W/m², mW/m², or µW/m² are often specified and measured for the high-frequency range. This means that a comparison with the values specified in the 26th BImSchV for electric field strength (in V/m) or magnetic field strength (in A/m) is possible only by means of conversions.

In the high-frequency range, simple measuring devices often display the peak values recorded during the measurement period. This leads to an overestimation of the recorded field strengths when interpreting the measurement results. The 26th BImSchV requires a quadratic averaging of the measured values of the high-frequency fields over a six-minute interval and a power-appropriate summation of the field strengths of individual frequencies.

According to the current state of scientific knowledge, even in the case of long-term exposure, the protection of public health is guaranteed if the specified limit values are complied with. If values other than these limit values are used to evaluate a measurement, it is wise to be a bit sceptical.

The technology built into smart phones is not suitable for either broadband or frequency-selective measurement. Depending on the type of field to be measured, various problems with regard to correct measurement can arise.

Modern smart phones have a magnetic field sensor that is used for navigation. Most smart phone apps advertised for EMF measurements use this sensor to estimate the strength of static magnetic fields. Low-frequency alternating magnetic fields (e.g. those that occur when using electrical devices) can often not be detected or can be detected only inaccurately because of the low sampling rate of the measurement signal. A low sampling rate means that the magnetic field detected is not measured often enough in a certain measuring period in order to be able to recognise an underlying frequency of 50 Hz. A frequency-selective field strength evaluation is not possible. The measured value is therefore a superposition of the field strengths of the natural static geomagnetic field with those of other sources of exposure. In most cases, the Earth’s magnetic field represents the main component of the field strength values. It is not possible to assess the field strengths of individual sources of low-frequency fields such as power lines with a smart phone. Apps for recording magnetic fields therefore provide only limited information regarding the actual exposure.

In the mobile communications frequency range, smart phones receive high-frequency EMF via the antenna and process them further via the downstream electronics in order to obtain information such as voice or data. Smart phones detect only certain mobile communications or Wi-Fi frequency ranges. The electronics of the devices are not designed for field strength measurements. They are therefore not suitable for determining electric or magnetic field strengths in the high-frequency range.