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  Field Sensors Calibration

Calibration of field strength sensors

The calibration of electromagnetic field probes needs to generate a reference field which intensity and spatial configuration are known with good accuracy inside a certain volume where the meter under test will be placed.
PMM TEMs In the frequency range 5 Hz to 400 MHz four Transverse Electromagnetic (TEM) cells are used to produce calculable electric and magnetic field strengths.
They are operated in the transverse electromagnetic mode, so that both the E and H-field components generated between the septum and outer conductor have the characteristics of a wave propagating in free space.
The field strength can be calculated from the dimensions of the cell, its impedance at the measurement plane and the input power.
Over a certain frequency (determined by the size of the cell) higher modes can propagate which give rise to large spatial variations in the field and this limits the operating range of the cell.

 


In the side figure is it possible to see a field distribution inside the TEM.
A two dimensional finite difference model was used to optimise the design of the TEM cell solving the Laplace equation with Leibmann algoritm.

Depending on the site of the necessary uniform volume, Narda STS uses different TEM cells. The transverse section of the biggest one is 1.8m x 1.8m and assures a good field distribution.

Field distribution inside a TEM cell
PMM Anechoic chamber In the frequency range 400 MHz to 40 GHz measurements are carried out inside an anechoic chamber (5,5 m x 4 m x 3,5 m).
The electromagnetic field is obtained by using synthesised signal generators opportunely amplified. The output signal is transferred into adequate aperture antennas. (Open Ended Guide e Pyramidal Standard Gain Horn).

At the opportune distance, so that the far field conditions are guaranteed, an uniform electromagnetic field is created, with a plane wave propagation.
The reference field strength is calculated knowing the net power transferred to the antenna, its gain and the distance from the sensor under calibration.
PMM antennas
Calibration data are supplied in the form of correction factor defined as rapport between true field and indicated reading. Basically the frequency response, linearity and anisotropy checks are evaluated.
The calibrations of the electric field meters at low frequency are carried out inside big TEM cells, parallel plate structures and Helmoholtz coils.
The last ones have dimension adeguate to generate uniform magnetic fields at frequencies even much higher then 50 Hz.
Devices calibrated included:
  • electric field strength probes high frequency
  • magnetic field strength probes high frequency
  • electric field strength probes low frequency
  • magnetic field strength probes low frequency
  • compact analysers for spectrum measurement of low frequency magnetic fields
  • handheld radiation hazard probes and associated meters.
Tipical calibration results
  • frequency response of a electric field strength probe high frequency
  • isotropy
The probe is rotated about the axis of the handle to determine two measurement orientations corresponding to the maximum and minimum sensitivities.

Contacts

For calibration booking please contact Roberto Grego
For technical enquiries please contact Gilberto Basso

 

 

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