Criteria for selecting a professional portable measurement equipment

With the very extensive range of electromagnetic measurement equipment on the market, it can be difficult to decide which equipment is suitable for a given purpose, since some devices are inexpensive and lack the features required for professional measurement, or they simply fail to meet existing international standards.

By way of help for selecting a measurement device, the following is a summary of the indispensable characteristics needed for obtaining reliable measurements that comply with the different international standards.

It is important to check those factors in the equipment’s specifications, since in many cases they are lacking and sometimes no information is provided at all (either because of deficient features or simply because the features are absent).

1. Probe isotropy.

Isotropy represents the independence of measures with regard to polarisation and the direction of propagation of the waves being measured. For that reason, an isotropic probe must perform the measurement simultaneously on all three axes.

Directional probes (or near-isotropic probes as they are sometimes called documents) do not measure simultaneously on all three axes. These probes make it necessary to measure each direction separately with manual orientation of the probe in each case, making it impossible to measure the contribution to the total field level simultaneously from all directions.

A triaxial probe is not necessarily isotropic, since the device might measure on each axis separately, in some cases at different times, and then give the average of the measured values. Unless isotropy is stated, this type of probe does not meet the criterion of simultaneous measurement on all three axes, either.

Wavecontrol WPF8 probe. Isotropic and RMS

Among isotropic probes, it is important to ascertain the isotropic deviation of the probe, in other words the maximum error of the measurement depending on the direction of the field being measured. This parameter must be less than 2 dB.

2. RMS (root mean square).

International standards on field measurements require the use of an RMS measurement device.

The main advantage of this type of detection is that the value of the field being measured is independent of the wave form, i.e. the modulation of the signal. Other types of detection are affected by the wave form and so signals with the same RMS strength but different modulation will be detected as having different strengths.

Equipment that does not state RMS measurement fails to comply with international regulations on the measurement of electromagnetic fields.

The measurement range indicates the minimum field level (sensitivity) and the maximum level that the probe can detect correctly. Levels obtained outside that measurement range cannot be considered correct, so the measurement range must be suitable for the measurement requirements:

The sensitivity of the probe must be sufficiently precise to measure the lowest levels. For example, in the measurement of mobile telephone band widths, levels of under 1 V/m are often found and so the sensitivity of the probe must be lower than that value. A probe with a sensitivity of 1V/m is not suitable in this case.

The upper range of the probe must exceed the maximum regulated limits for the frequencies being measured, since it is necessary to check that any measurement taken falls within the allowed levels. For example, in the case of mobile telephones, the measurement probe must have a measurement range of over 41 V/m (a probe with an upper range limit of 20V/m would not be suitable).

Professional measurements of a mobile telephone base station with an SMP device and WPT probe.

4. Measurement uncertainty

When choosing a measurement device it is important to assess the measurement uncertainty (in other words, the possible error in the measurement results obtained). That factor is defined by two parameters, namely frequency response and linearity.

Frequency response is an indicator of the possible variation in the measurement of a single field level depending on the frequency. It is necessary to check that this response is suitable over the full frequency range that we wish to measure and so we need to know its value (dB) and the frequency range to which that value is applicable. The value for frequency response must be ±2 dB at most.

Below is an example of a frequency response graph comparing the response of a professional device and a lower quality device.

Linearity is an indicator of the accuracy of the measurements of a single frequency depending on the field level. Information is needed as to the linearity value (dB) and the minimum to maximum field range (e.g. V/m) to which that value is applicable. The recommended linearity must be lower than ±1 dB.

It is easy to note in both the example of linearity and the example of frequency response that using a low quality device will give measurement results that are totally incorrect.

5. Device memory

Existing regulations require the end results to be the average of successive measurements taken over a given period (normally with measurements taken every second over a period of 6 minutes). Therefore the device must calculate that average automatically or store all the measurements so that the average can be calculated after the data is downloaded.

In order to obtain those results the device needs to have sufficient memory capacity to store all the data.

6. Accredited calibration

The calibration certificate of the measurement device issued by an ILAC (International Laboratory Accreditation Cooperation) accredited laboratory is needed to confirm the accuracy of the parameters supplied in the specifications of the probe (e.g. linearity and frequency response). Without calibration and the pertinent certificate, the measurements obtained cannot be confirmed to be correct.

LabCal. Wavecontrol Calibration Laboratory with accreditation by ENAC (member of ILAC and EA)

Conclusion

We recommend that you check all these points before choosing a professional electromagnetic field measurement device in order to be able to provide professional and reliable measurements.

Where those standards are not met or the information is not given, the device in question should not be considered valid, since it may not take measurements in accordance with international regulations or the margin of error in the measurements may be unacceptable.