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Field Generators

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. 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:

Micro TEM Cell

A TEM cell is a widened coaxial transmission line which is well known (numerous citations, e.g. Crawford, Kanda, Orr, Neibig). It generates calculable TEM fields. The very compact model used at PTB has also been characterized in detail (Schrader, Elsner, Münter, Spitzer, Glimm) and is free of hollow waveguide modes up to over 1.1 GHz. However, because of its small dimensions calibrations are restricted to special transfer field measurement devices with miniaturized sensors (Münter, Pape, Glimm). These devices are then used for the field adjustment in larger field generators as GTEM cells or antenna radiation fields (‘dissemination‘) that are then retraced, also. The properties of the transfer sensors (detector characteristics, transfer function, temperature influence) are measured for each specimen individually. Since such a full calibration takes more than 24 h due to multiple temperature adjustments, all measurements are fully automated.

Fig.: Temperature-stabilized Micro TEM Cell with transfer sensor.

Fig.: Calibration setup measurement standard based on a Micro TEM Cell

Short data:

Frequency Range:

1 MHz to 1100 MHz

Septum distance:

34.25 mm

max.achievable field strength:

150 V/m

Temperature range:

16 °C to 30 °C

Achievable rel. Measurement uncertainty for calibration of transfer sensor:

5 % (k=2) or

0.45 dB (k=2)

GTEM Cell

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.

Fig.: GTEM Cell.

Short data:

Frequency range:

1 MHz to 1000 MHz

Septum distance:

1500 mm

max.achievable field strength:

150 V/m (1 MHz to 200 MHz)

 80 V/m (200 MHz to 400 MHz)

 50 V/m (400 MHz to 1000 MHz)

Achievable rel. Measurement uncertainty for calibration of a field probe:

12 % (k=2) or

1.1 dB (k=2)

TEM Cell

Due to ist size, the TEM Cell allows for exposition oft he measurement head of a field probe, only. The field probe can be brought into the field region from above or from the side. This allows for an orientation of a field probe with measurement head parallel to the electric field (PE) or parallel to the magnetic field (PH). As in the Micro TEM Cell, the electric field strength is traceable via the measured forward feed power. The TEM Cell is used for linearity measurements. For field probes with separate measurement heads connected to the display unit via optical fibers, a calibration can be performed for lower frequencies. Also magnetic field sensors can be calibrated, provided that the E field suppression is sufficient.

Fig.: TEM Cell.

Short data:

Frequency range:

9 kHz to 180 MHz

Septum distance:

300 mm

max.achievable field strength:

400 V/m

Achievable rel. Measurement uncertainty for calibration of a field probe:

7 % (k=2) or

0.6 dB (k=2)

Microwave calibration facility

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.

Fig.: Microwave calibration facility.

Short data:

Frequency range:

1100 MHz to 18 GHz

Distance to horn aperture:

1000 mm

max.achievable field strength:

400 V/m

Achievable rel. Measurement uncertainty for calibration of a field probe:

17 % (k=2) or

1.6 dB (k=2)