Compact Antenna Test Ranges provide an efficient means of obtaining the plane wave illumination of an Antenna Under Test (AUT) over a specified test zone (“Quiet Zone”) normally provided by a long far-field range in a relatively short distance. This is achieved by conversion of the spherical wave produced by the feed antenna to a high-quality plane wave by a parabolic reflector assembly.
Quiet Zone Characterization
Key to accurate far-field antenna measurements is the shaping of the plane wave in the quiet zone. Under ideal circumstances the antenna under test (AUT) is illuminated by a plane wave. In plane wave conditions the behavior of the AUT for each angle is measured separately by rotating the AUT through the field. In order to achieve ideal plane wave conditions the AUT and feed antenna have to be separated infinitely from each other. Since this is not feasible, shorter distances are determined where the effect of the non-ideal plane wave conditions on the measurement accuracy is acceptable.
In a compact antenna test range (CATR) the distance between range feed antenna and AUT can be much shorter since the plane wave conditions are achieved by a spherical to plane wave conversion. This is typically realized by a parabolic shaped reflector or a dielectric lens. Due to the parabolic shape the wave travels different path lengths to the quiet zone ensuring equal transversal phase of the wave. However only the phase is converted, the amplitude taper of the feed antenna propagates unaffected to the quiet zone. This is the reason for amplitude taper measurements to become a key factor in the characterization of this type of antenna measurement chambers.
Especially for indoor ranges the walls enclosing the measurement environment, although covered with absorbing materials, create further reflections, deteriorating the measurement accuracy. These reflections cause amplitude- and phase-ripple which are also key factors in the characterization of all types of antenna measurement chambers. In a CATR refractions on the reflector edges are an additional source of amplitude- and phase-ripple.
The Voltage Standing Wave Ratio (VSWR) method is a commonly applied procedures to measure the maximum reflectivity inside ATR at a given angle and given direction (longitudinal and transversal). This simple measurement method provide an indication for the quality of the measurement chamber. The IHF has used this method in different measurement campaigns and has programmed evaluation software.
A more comprehensive analysis can be performed by two-dimensional fieldprobing of the quiet zone representing the spatial distribution of amplitude and phase.The focus of this method is the analysis of the desired plane wave behavior. The plane wave is usually described by a taper and a ripple. The IHF has build a measurement setup in order to perform these measurements.
In order to overcome the incompleteness of the gathered information in classical fieldprobing Quiet Zone Spherical Near-Field Scanning can be used. This method measures the amplitude and phase on a closed sphere around the quiet zone. Similar to spherical near-field antenna measurements the field inside the quiet zone can be represented as a sum of orthogonal spherical harmonic waves. The weights of the spherical harmonic waves provide a complete description of the electromagnetic field inside the quiet zone. In addition the measurement can be corrected for the influence of the probe. With this information it is possible to emulate two-dimensional field-probing data and to calculate classical key factors (taper and ripple) as well as new more general key factors (e.g. plane wave to interference ratio).
The IHF participates in different antenna measurement comparison campaigns to investigate and prove measurement accuracies. For this purpose, the same antenna is measured in various institutions, and the results are compared. Current measurement campaigns are:
- SR-40 (extern)
- SH800 (extern)