Effect of Higher Order Modes in Standard Spherical Near-Field Probe Correction

Authors: A.C. Newell, S.F. Gregson

Within the standard scheme for probe-corrected spherical data-processing, it has been found that for an efficient computational implementation it is necessary to restrict the characteristics of the probe pattern such that it contains only azimuthal modes for which μ = ±1 [1, 2, 3]. This first-order pattern restriction does not however extend to placing a limit on the polar index mode content and therefore leaves the directivity of the probe unconstrained. Clearly, when using this widely utilized approach, errors will be present within the calculated probe-corrected test antenna spherical mode coefficients for cases where the probe is considered to have purely modes for which μ = ±1 and where the probe actually exhibits higher order mode structure. A number of analysis [4, 5, 6, 7, 8] and simulations [9, 10, 11, 12] can be found documented within the open literature that estimate the effect of using a probe with higher order modes. The following study is a further attempt to develop guidelines for the azimuthal and polar properties of the probe pattern and the measurement configuration that can be utilized to reduce the effect of higher order spherical modes to acceptable levels. Included in this study are the cases when an Open Ended Waveguide (OEWG) is simulated at a series of measurement distances, a Quad Ridged Horn Probe (QRHP) with very large higher order modes is also simulated, the AUT is offset from the origin of the measurement sphere and the AUT is simulated with its main beam along the equator rather than along the pole. These new simulation cases provide additional guidelines when selecting a probe for spherical near-field measurements and answer some questions that have been raised about generalizing past results.

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