Advanced spherical antenna measurements

Abstract

Concrete guidelines for effectively performing spherical antenna measurements and for designing multi-probe systems will be provided. The work will mainly be restricted to antennas whose maximum cross-section dimension is in the order of 1-2 λ or less. Specific design guidelines for a very fast radiation pattern measurement system for mobile phone models will be provided. Information on practical aspects related to such a system will be provided by building a demonstrator system and testing it.

Firstly, the errors in the total radiated power and the maximum electric field are illustrated by simulations of near-zone spherical antenna measurements of electrically relatively small AUTs (antennas under test) for various applied truncation numbers and for different measurement distances [P1]. Secondly, a novel iterative matrix method is presented that is shown to provide, for a fixed relatively small number field samples, a lower uncertainty in the determination of the radiated field of an AUT model than the traditional matrix method [P2]. Thirdly, it is shown that, for a fixed relatively small number field samples, the radiation pattern can generally be determined with a lower uncertainty from the complex data than the amplitude-only data [P3]. It is shown in [P4] that a high-order probe correction becomes increasingly significant with an increasing ratio between the radius of the minimum sphere of the AUT and the measurement distance.

It is shown in [P5] that by enclosing the head phantom with a mobile phone inside the minimum sphere, and the calculation of the truncation number for the spherical wave expansion of the radiated field based on the radius of this minimum sphere in wavelengths, leads to an overestimation of the truncation number. It is illustrated by simulations for a mobile phone that by multiplying the truncation number for the mobile phone without a head phantom by a factor of approximately 1.2 leads to a reasonable truncation number for the mobile phone with the head phantom. It is demonstrated in [P6], by building and testing a spherical fully 3-D measurement system for mobile phone models (RAMS), that the radiation pattern of a typically-sized mobile phone model at approximately 1.8 GHz can be determined without its rotation with a relatively small uncertainty from the complex-valued signals gathered from only 32 dual-port probes on a spherical surface. Information on the reflectivity level inside RAMS will be provided. It is shown in [P7] that the complex radiation pattern of a mobile phone model can be determined without taking advantage of the field-disturbing radio-frequency feed cable to the mobile phone model during the measurement.

It is shown in [P8] that, instead of a single spherical wave expansion, the use of multiple spherical wave expansions (MSWE) for the field characterization can lead to a smaller number required spherical modes for reaching a desired level of uncertainty in the determination of the radiation pattern. It will further be shown, that using the MSWE technique can also lead to the smaller number of required measurement locations.