Higher order mode probes in spherical near-field measurements

A previous study (1) by the authors reported on a computer simulation using Open Ended Waveguide probe data and actual test antenna data to quantify the effect of using higher order probes with the spherical processing that assumes the probe has only μ = ±1 modes. That study was based on the observation that since the higher order probe's azimuthal modes are directly related to the probe's properties for rotation about its axis, the near-field data that should be most sensitive to these modes is a near-field polarization measurement. Such a measurement would be taken with the probe at a fixed (θ,φ,z) position and the probe is then rotated about its axis by the angle χ over 360 degrees. The amplitude and phase received by the probe would be measured as a function of the χ rotation angle. A direct measurement using different probes would be desirable, but since the effect of the higher order modes is very small, other measurement errors would likely obscure the desired information. A computer simulation rather than a measurement was therefore used where the plane-wave transmission equation was used to calculate the received signal for an AUT/probe combination where the probe is at any specified position in the near-field. The plane wave spectra for both the AUT and the probe were derived from measured planar data for the AUT and spherical near-field data for the probe. The plane wave spectrum for the AUT was the same for all calculations with the main beam along the Z-axis and the AUT at the origin. The receiving spectrum for the probe at each (θ,φ,χ) orientation was determined from the far-field pattern of the probe after it had been rotated by χ. The far-field pattern of the probe as derived from spherical near-field measurements can be filtered to include or exclude the higher order spherical modes, and the near-field polarization data can therefore be calculated to show the sensitivity to these higher order modes. This approach focuses on the effect of the higher order spherical modes and completely excludes the effect of measurement errors. The study indicated that the effect of the higher order probes on the measured near-field data was on the order of 50 dB below the peak of the near-field data and would likely be below the effect of other errors such as scattering, alignment, and total system random errors. Two test antennas were used in the previous study, and a limited number of probe positions were used in the simulation. The results described in this paper extends the initial study to include additional simulation results that are more representative of a spherical near-field measurement. For the current simulations, the AUT pattern is first rotated about the Y-axis to simulate off-axis θ angles on the measurement sphere and the probe z position is varied to simulate different measurement radii.