Realization of electric and magnetic radio-frequency field strength
Retraceability to SI basic units

At PTB electromagnetic fields with a field strength as well defined as possible are used as default for the calibration of portable field strength sensors (known as ‘radiation monitors’). Naturally, these fields cannot be preserved as embodied measurement standards. Instead, these physical entities need to be available on demand (‘realized’) in a standard measurement equipment anytime. This task requires generation methods where the field strength is derived from the physical basic units (‘retraceability’). Here it is assumed that all parts of the test equipment are calibrated and hence retraced. In this case the field can be derived from this data and physical laws, only. In the frequency and field strength range of interest we assume classical electrodynamics (see Jackson, John David) represented by Maxwell’s equations to be valid.

In particular for the magnetic field strength (H, measured in A/m) such a retracement seems to be quite simple since only the basic entities length (in meters) and current (in Amperes) are involved. In practice the realization of the relevant RF field entities – electric and magnetic field strength (E, H) or energy flux density (S) – requires a multitude of intermediate steps and measurement units that are specially suited for RF technology. For the realization of fields the Working Group 2.21 uses methods based on RF real power, scattering parameters (see Michel, Hans J.) and, if applicable, mechanical dimensions, only. For these entities rf measurement equipment (power meters, vector network analyzers) with a small measurement uncertainty is available. It can be retraced to the respective calibration standards of the Working Group 2.22 – High Frequency Measurement Techniques.
For calibration the generated fields need to have certain well defined and reproducible properties:

• the energy transport will be in one direction only as linearly polarized transversal electromagnetic wave (so called ‘TEM’ wave)
  
• the far-field conditions are fulfilled, which means
  
  • electric and magnetic field exist simultaneously
    
  • field vectors are perpendicular to each other and to the propagation direction and oscillate in phase
    
  • their amplitudes are in a fixed ratio (E/H~377 O).
    

Depending on frequency or wavelength, respectively, the electromagnetic waves and the technical equipment have different properties. Especially at lower frequencies emitter antennas are quite large and the performance of absorber cabins is rather poor. The determination of the antenna gain is problematic. In addition, reflections from the cabin walls overlap the calculated field and increase the overall uncertainty. Hence, ‘TEM waveguides’ are used in this range instead, but have a rather limited volume. At higher frequencies the performance of cabin absorbers improves and the gain of horn antennas can be determined with small uncertainty. For the calibration of radiation monitors it is suitable to split the frequency range at approximately 1 GHz. In the lower frequency region the Work Group 2.21 generates waves in special ‘TEM’ and ‘GTEM’ cells, whereas the radiation at higher frequencies is generated via horn antennas in absorber cabins.

Description of methods and available test equipment

The methods applied at PTB are based on common rules described in the guidelines VDI/VDE/DGQ/DKD 2622 Blatt 10: "Kalibrieren von Messmitteln für elektrische Größen - Hochfrequenz-Feldstärke-Messgeräte" (see VDI guideline, also).

The calibration of radiation monitors at PTB yields a correction factor which is given by the ratio of the specified root mean square value of the field and the display value of the measurement instrument. The specified value and hence the result of the calibration are related to the ‘empty field’ - the field magnitude without the perturbation resulting from the radiation monitor.

The user can apply the PTB calibration data directly for his measurements: the best estimate for the empty field magnitude is given by the display value of the measurement instrument multiplied by the correction factor. This requires that the measurement conditions (frequency, orientation etc.) match the calibration conditions (see Metrological Properties of RF Radiation Monitors, also). Hence it is of utmost importance that the customer agrees with PTB on technical details of the calibration task in advance (see order processing, also).

 The following equipment is available at PTB:

-	"GTEM"-Zelle
	The ‘GTEM cell’ generates fields for the exposition of complete radiation monitors from arbitrary directions. This measurement facility is not a calibration standard device because the retracement is based on the adjustment of the electric field strength via a transfer field sensor that has been calibrated in the ‘Micro TEM Cell‘ before.
	Short data	frequency range:	1 MHz bis 1000 MHz
		septum height:	1500 mm
		max. achievable field strength:	150 V/m (1 MHz bis 200 MHz) 80 V/m (200 MHz bis 400 MHz) 50 V/m (400 MHz bis 1000 MHz)
		uncertainty of electric field strength in the empty field:	1,2 dB (k=2)

-	Microwave calibration equipment
	Here, fields with calculable energy flux density are generated for the exposition of complete RF field monitors from almost arbitrary directions. The equipment is a calibration standard since the retracement is based on the measurement of the emitted power, the distance between horn antenna and field sensor and on the antenna gain (which is derived from scattering parameters). Since the far field conditions are fulfilled at the location of the sensor, electric and magnetic field strength can be calculated from the energy flux density.