Standard Gain Horn Calibration
Spherical near-field testing is used for the calibration of Standard Gain Horns (SGH). Since directivity is inherently provided by the spherical near-field method, gain can be obtained by subtraction of computed values of the losses in the horns. Polarisation data are measured using a three-antenna method. The influence of reflections and multiple reflections is taken into account in the calibration procedure.
Besides the calibration of individual standard gain antennas for customers, DTU-ESA Facility also keep the sets of SGH's. Each set of SGH's consists of three similar horns, one of which is kept continuously at DTU-ESA Facility for reference, while the two others are available to ESA projects on a loan basis. Detailed information on each horn, as well as directions for its use, are contained in calibration report.
Near-Field Probe Calibration
The probes for spherical near-field measurement are usually designed as rotationally symmetric conical horns with bandwidth limited by the requirement that only TE11 mode may propagate in the circular waveguide. Due to the mentioned properties, the radiated far-field from the probe is of simple nature. It can be shown that the complete knowledge of the probe pattern only requires the following three measurements:
- The co-polar E-plane pattern and the co-polar H-plane pattern measured in both amplitude and phase.
- The on-axis linear (or circular) polarization ratios of the two ports.
- The amplitude-phase factor describing the ratio between co-polar components of the two ports on the probe axis.
The mentioned data for the probe are accurately measured and the probe correction file is produced which then can be used as input for the SNIFTD software. In the case when the probe does not satisfy mentioned properties the calibration is carried out as regular antenna measurement with sampling the radiated field over the whole sphere around the probe.
General Antenna Measurements
General antenna measurements can be carried out in the frequency range 0.8-40 GHz.
Patterns: Co-polar and cross-polar patterns can be computed at any distance (e.g. far-field) from near-field measurement results. The reference direction for the definition of co- and cross-polar field components can be chosen independently of the coordinate system.
Phase can be measured relative to a well-defined point inside the antenna, allowing determination of possible phase centres. Arbitrary cuts can be plotted in a spherical coordinate system for constant elevation or azimuth values. The coordinate system can be defined relative to the antenna's flange or some other mechanical reference (e.g. an optical cube) on the antenna. Electrical definition of the coordinate system is also possible, but this takes more time since the far-field cannot be observed directly. Contour plots of patterns can also be produced.
Polarisation: Axial ratio, tilt angle and sense of the electromagnetic field can be determined for specified directions. A dual-polarised probe is used to measure the two polarisation components simultaneously, such that errors due to drift between the two measurements are eliminated. Correction for the polarisation of the probe itself is applied to increase the final accuracy.
Directivity: When a full sphere of near-field data has been measured, the ratio between peak radiation and total radiated power can be calculated. This peak directivity is accurately determined because both the peak level and the power are rigorously calculated from a large number of near-field values.
Gain: Gain is measured by comparing the signal at a point in the near field of the test antenna to the signal received from a calibrated SGH. By transforming from the near-field to the far-field for both the test antenna and the SGH, the far-field gain of the test antenna can be determined. Probe correction is necessary to calculate the near-field coupling between antenna and probe.
Measurement time
The overall time needed for spherical near-field testing of a given antenna depends upon a number of factors. Mechanical alignment usually take several hours, while setting-up of an antenna depending largely upon its weight. Usually a calibrated probe is available at the frequency in question; if not an extra day is needed for probe pattern and polarisation determination. The measurement time required for a single complete spherical near-field scan of the test antenna depend on the test-antenna diameter in wavelengths. Fully automatic test programmes with many measurements/ transformations and perhaps several days/nights of continuous operation may be executed. By accepting very little degradation in the accuracy of the results, the measurements outside the main lobe and first few sidelobes can be truncated in cases in which a short turnaround time is essential. The time needed for a complete series of measurements is typically one to three weeks.