Electric field probe calibration: key for accurate measurements

Kapteos is introducing a breakthrough for electric field probe calibration through an Adiabatic TEM cell (ATEM).

electric field probe calibration

Electric (E)-field probes are of key interest for a wide range of applications: antenna testing, SAR measurement, MRI safety, EMC pre compliance, high voltage measurements, plasma plume dynamics, HPEM…

For most applications, an absolute value of the E-field strength needs to be assessed. Hence, electric field probe calibration is of key importance. For that purpose an E-field applicator generating a perfectly known E field both in direction and magnitude is required.

Major E-field applicators can be classified in three broad categories:

- plane-parallel capacitors for low frequency applications

- GTEM cells for high frequency applications

- TEM and Crawford cells for applications up to a few GHz

Plane-parallel capacitors are easy to produce but they need to be symmetrically powered (positive signal on one electrode while an opposite signal is delivered to the other electrode) and considerable care must be taken for the design of the electrodes in order to homogenize the E field in the gap between electrodes. If such design rules are not followed, large deviations (± 2 to ± 4 dB) could be observed between theory (applied voltage divided by gap width) and practice. Another drawback of this E-field applicator concerns its operating frequency range that is upper bounded in the MHz range.

On the contrary, GTEM cell theoretically doesn’t present any upper bound concerning their operating frequency range. However, the impedance loading of the GTEM cell remains tricky and their standing wave ratio is generally too high to get a well defined E field. E-field strength is usually known within an accuracy of ± 2 to ± 3 dB.

TEM cells and Crawford cells present the key advantage of being two port devices, thus allowing to measure both reflected (S11 parameter) and transmitted (S21 parameter) waves through the use of a Vector Network Analyzer (VNA). However, the abrupt transition between central and tapered sections of the TEM cell leads to a mode mismatch and then to the generation of higher order propagation modes. As a consequence, such cells present a quite low upper bound of their operating frequency range (typically 1 GHz or less).

Kapteos introduces here an innovative concept of ATEM cell that consists in removing the abrupt transition between central and tapered sections that is encountered in a classical TEM cell. Despite the size of this ATEM cell, this latter one exhibits no higher modes up to 6 GHz.

Once having an efficient E-field applicator 6 main issues have to be addressed before being able to carry out accurate electric field probe calibration.

1st issue: Standing wave

In order to illustrate the standing wave issue for electric field probe calibration, a CPW line with a mismatched impedance (80 Ω) has been loaded by 50 Ω and connected to VNA port 1. A near field electric field probe eoProbe™ has been connected to VNA port 2 as shown. Then an E-field map at DUT surface has been carried out, exhibiting nodes and anti-nodes at fixed positions along the Coplanar WaveGuide (CPW) line. Accuracy on E-field strength worsened rapidly with the increase of Voltage Standing Wave Ratio (VSWR). To keep an accuracy on E-field strength better than ± 0.5 dB, VSWR must be lower than 1.12 (S11 < -24.8 dB).

2nd issue: E-field enhancement near metallic parts

In order to illustrate the E-field enhancement issue, 2D ElectroMagnetic (EM) simulations have been carried out considering a 5.5-mm diameter near field electric field probe ET5-air placed between two parallel plates.

In case of a probe with a centered position between the two parallel plates, no significant E-field enhancement (i.e. no enhancement greater than 0.5 dB, i.e. 5.9%) occurs for an inter-electrode distance greater than 3 times the probe diameter. However, when the probe is clamped to one electrode, the E-field enhancement tends towards a low of 15%.

This E-field enhancement phenomenon originates from the Maxwell continuity equation at the interface between two media not presenting the same permittivity: it is growing with the increase of both permi