Estimating the Effect of Higher Order Modes in Spherical Near-Field Probe Correction
Authors: Allen Newell, Stuart Gregson
This paper extends a previous simulation study (1, 2) of the effect of higher order probe modes when the spherical numerical software uses the orthogonality approach to solve for the spherical modes of the AUT. In this commonly used approach, the probe is assumed to have only modes for μ = ±1, and if the probe has higher order modes, errors will be present in the calculated AUT spherical coefficients and the resulting far-field parameters. In the previous studies, a computer simulation was developed to calculate the output response for an arbitrary AUT/probe combination when the probe is placed at arbitrary locations on the measurement sphere. The planar transmission equation was used to calculate the probe response using the plane wave spectra for actual AUTs and probes derived from either planar or spherical near-field measurements. The positions and orientations of the AUT and probe were specified by a combination of rotations of the antenna’s spectra and the x, y, z position of the probe used in the transmission equation. The simulation was carried out for rectangular Open Ended Waveguide (OEWG) probes using all of the higher order modes and also for the same probe where only the μ = ±1 modes were used to calculate the probe patterns. The parameter that was used to estimate the error in the measured near-field data was the RMS combination of the complex differences between near-field polarization curves over a χ rotation span of 100º. This RMS combination represented the estimated error signal level relative to the peak near-field amplitude. Using two different AUTs, different measurement radii and a sequence of θ-positions on the measurement sphere, the error signal levels were between -35 and -80 dB and the initial conclusion was that the effect of the higher order modes on typical measurements using OEWG probes would be smaller than other typical measurement errors and therefore have little practical effect on far-field results. In this phase of the study the goal was to develop general guidelines to predict the error signal level for a given AUT/probe/measurement radius combination. The same simulation software was used in this study with the following changes and additions. Rather than use all of the points in the polarization curves to derive an RMS error signal level, only the χ-rotation angles of 0º and 90º were used since these are the only two probe rotation angles used in a typical spherical near-field measurement. In addition to deriving the error estimates for specific spherical angles and measurement radii, complete sets of near-field data were derived for some cases, the far-fields calculated and compared to derive estimates of far-field error levels.
The results of these simulations are presented and guidelines developed to aid in the choice of spherical near-field probes and measurement radii for typical antennas.