E-field probe calibration
Calibrating an E-field probe remains a delicate issue except below 1 MHz where a simple parallel plate capacitor does the job very well. However it is not straightforward to make the calibration above 10 MHz and even more above 1 GHz.
A few methods exist for calibrating any E-field probe, based on:
either a 2-ports TEM (Transverse ElectroMagnetic) cell up to a ~6 GHz with a ~1 dB calibration accuracy,
either a 1-port GTEM (modified structure of a TEM cell operating in the GHz frequency range) cell up to ~20 GHz with a ~3 dB calibration accuracy,
or a horn antenna in free space with no upper frequency limit but with a calibration accuracy difficult to estimate.
In what follows, we will focus on the calibration of E-field probes in both TEM and GTEM cells using a VNA (Vector Network Analyzer).
Calibration principle with a TEM cell and a VNA
In order to illustrate the calibration principle of an E-field probe using a VNA, we have used our adiabatic TEM cell that allows accurate calibrations up to 6 GHz.
The simplest experimental setup is composed of:
a 2-ports TEM cell (in this case, the adiabatic TEM illustrated above),
a 3-ports VNA,
the E-field probe to be calibrated,
and 3 high grade RF cables.
In the measurement setup as illustrated on the picture, we have considered one of our products, i.e. a transverse optical E-field probe with its optoelectronic converter, the combination of these two devices acting as an interference-free, ultra wide band receiving antenna.
1st step: VNA calibration
The first step consists in the calibration of the 3-ports VNA in order to set the measurement reference planes (Ref 1 to Ref 3 – see figure above) at the end of the 3 RF cables. For that purpose, follow the calibration procedure of your VNA using the calibration kit delivered with your VNA.
2nd step: E-field probe calibration
The calibration of the E-field probe is straightforward as it requires to record only the scattering parameters S21 and S31. Indeed, S21 gives the insertion loss of the TEM cell whereas S31 gives the sum of:
half the TEM cell insertion loss (E-field probe positioned in the center of the TEM cell),
the antenna factor of the E-field probe,
and 20 times the log base 10 of the distance between septum and ground plane at the E-field probe location.
From these two equations, it is straightforward to deduce the expression of the antenna factor of the E-field probe. This expression is given on the figure for the use of eoCal HF6 adiabatic TEM cell for which the distance from septum to ground plane is equal to 17.5 mm. In this last expression, the modulus of the scattering parameters S21 and S31 must be considered in dB.
Case of E-field probe calibration using a GTEM cell
When using a GTEM cell, the procedure is almost the same but with one fewer port. The lack of this port prevents taking into account the GTEM cell insertion loss, thus leading to a systematic underestimation of the E-field probe antenna factor. Another important difference concerns the septum to ground plane distance. Whereas this distance is constant in case of TEM cells it is an effective distance that should be considered when using GTEM cells.
In a TEM cell, septum and ground planes are parallel and the electric field is almost perfectly orthogonal to the septum leading to a precise knowledge of the E field in any TEM cell. On the contrary, the electromagnetic wave propagating in a GTEM cell is spherical and the distance from septum and ground planes varies with the position of the E-field probe leading to a much less accurate assessment of the effective distance from septum to ground plane. Moreover, other elements have to be taken into account to assess calibration accuracy in GTEM cells:
a relatively important Standing Wave Ratio (SWR) leading to E field strength nodes and antinodes,
the size of the E-field probe that leads to a variation of the septum to ground plane effective distance according to whether considering the E-field probe end facing to the GTEM apex or to the opposite direction.
For all these reasons, an E-field probe calibration cannot lead to an accuracy better than ± 3 dB in any GTEM cell.
Please contact us for more information.