Universal (Pseudo-Vector) Field Probes

Chapter 26
Universal (Pseudo-Vector) Field
Probes
26.1
Introduction
Recent years have seen significant progress in near-field evaluation tools with respect to precision,
spatial resolution, etc. Subsequently, near-field scanners are employed more and more for analysis
and optimization of antennas embedded in complex environments. However, current implementations only enable the assessment of the magnitude of electric and magnetic field strengths, i.e.,
polarization and phase information are not available. At the same time, the availability of the polarization would increase the quality of the information with respect to antenna design purposes,
e.g., information on the current flow, distortion of cross-polarization by nearby scatterers, etc.
After successful completion of a research project, SPEAG has included a new probe type in its
product line which, together with an optimized numerical algorithm, enables end users to determine
not only information on the field amplitude but also information on the polarization of the field
at any measured location.
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Universal (Pseudo-Vector) Field Probes
26.2
Application Notes
Concept
For the numerical description of an arbitrarily oriented ellipse in three dimensional space, five
parameters are needed: semi-major axis (a), semi-minor axis (b), two angles describing the orientation of the normal vector of the ellipse (φ, θ) and one angle describing the tilt of the semimajor
axis (ψ). For the two extreme cases, i.e., circular and linear polarization, three parameters only
(a, φ and θ) are sufficient for the description of the incident field.
For the reconstruction of the ellipse parameters out of measured data, the problem can be reformulated as a nonlinear search problem. The semi-major and semi-minor axes of an elliptical field
can be expressed as a function of the three angles (φ, θ, ψ). The parameters can be uniquely
determined in terms of minimizing the error in the least-square sense for the given set of angles
and knowing the measured data. In this way, the number of free parameters is reduced from five to
three, which means that at least three sensor readings are necessary to gain sufficient information
for the reconstruction of the ellipse parameters. However, in order to suppress the noise and increase the reconstruction accuracy, it is desirable to have an overdetermined system of equations.
The solution of using a probe consisting of two sensors angled by γ1 and γ2 toward the probe axis
and performing measurements at three angular positions of the probe, i.e., at β1 , β2 and β3 , gives
an over determination of two. If there is a need for more information or increased accuracy, more
rotation angles can be added.
The reconstruction of ellipse parameters can be separated into a linear and a non-linear part that
is best solved by the Givens algorithm combined with a downhill simplex algorithm.
In order to minimize the mutual coupling, sensor angles are set with a 90◦ shift (γ2 = γ1 + 90◦ ),
and, in view of simplification, the the first rotation angle of the probe (β1 ) can be set to 0◦ .
Schmid & Partner Engineering AG, DASY5 Manual, April 2008
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Universal (Pseudo-Vector) Field Probes
26.3
Application Notes
Probe Design and Calibration
The probe consists of two sensors with different angles (γ1 and γ2 ) arranged in the same plane.
The probes’ sensor configurations have been optimized using a genetic algorithm with the sensor
angles and reconstruction parameters as genotypes.
The calibrations of sensor sensitivity, sensor angles, ConvF and boundary compensation parameters
are performed in the standard setups as defined in our SCS 108 procedures.
Note: Please note that the sensor angles for the E-field probe depends on the medium,
i.e., vector probes are suitable for any medium if calibrated accordingly.
Schmid & Partner Engineering AG, DASY5 Manual, April 2008
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Universal (Pseudo-Vector) Field Probes
26.4
Application Notes
DASY Software Settings for E-Field Probes
Universal probes are only supported by DASY5 AIR, PRO and NEO system level. The probe has
been integrated in the DASY software under E-Field Pseudo Vector Probe (2-D) name.
General, Conversion Factors and Surface Detection properties are the same as for the three-dimensional
E-field probes (see 6.2 Probe).
Mechanical properties page
In the Mechanical page the information on the probe dimensions is stored.
These are then used in the software to
establish the transformation from the
robot’s tool flange to the probe tip and
to align the probe in the light beam
unit. These dimensions are specified by
Manufacturer and can not be modified by the user.
Pseudo Vector page
In the Pseudo Vector page the information on the measurement (roll) and
probe angles is given. Number of steps
defines how many measurements shall
be taken per probe position, whereby
up to 4 angles can be defined. Sensor
mounting angles are probe calibration
parameters and can not be modified by
the user.
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Universal (Pseudo-Vector) Field Probes
26.5
Application Notes
Validation
The probe calibration and settings are validated with our system validation dipoles for air and liquid
by comparing the measured amplitudes and elliptical information with numerical data obtained by
SEMCAD X. This can be repeated by the user at any time, i.e., the amplitudes are checked with
the same procedure as for any other probe and the elliptical information can be compared by the
figures shown below.
Representation of amplitude and polarization of the E-field above the
2000 MHz dipole in head tissue simulating liquid at 4 mm above the phantom
surface (validation setup with 1 W input power and linear visualization with
200 V/m peak): Measurement results.
Simulation results.
26.6
Applications
Vector probes are the most accurate and the only truly universal probes, i.e., vector probes are the
only probes that measure without additional isotropic errors in different media if sensitivity/ConvF,
sensor angles and boundary compensation parameters are calibrated for the corresponding medium.
Furthermore, vector probes provide much useful additional information:
• E- and H-field polarization of the near-field and far-field
• information about amplitude and directions of RF currents inside devices
• information about the cause of parasitic coupling, i.e., what couples from where to what
In summary, vector probes are universal probes and a great analysis tool. They should be used
wherever great precision is needed, field polarization must be validated or parasitic currents detected. The measurement time compared to isotropic probes is only increased by just more than a
factor two (minimally three measurements at different angles need to be taken at each measurement
point).
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BIBLIOGRAPHY
Application Notes
An example is given below. The DUT was a circularly polarized multi-source antenna. Results
assessed using the vector E-field probe show field distributions at 30 mm and 100 mm above the
ground plane. At closer distances from the antenna (plot on the left-hand side) the maximum field
values occur above the four antenna patches, while for the greater distance the maximum radiation
would merge into the center of the multi-patch antenna (plot on the right-hand side).
26.7
References
[1] K. Pokovic, ”Advanced Electromagnetic Probes for Near-Field Evaluations”, Doc. Tech. Sci.
Diss. ETH Nr. 13334, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland, 1999.
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