Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Techniques to improve the quality of the quiet zone in compact ranges <s> Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The “Two - Level GTD” method has been offered in [1,2] to reduce the reflectivity and improve overall performance for anechoic chambers operating at frequencies >1 GHz, where the characteristic dimensions of the side-walls are normally more than 30λ. It includes the shaping of the side walls of the anechoic chamber along with application of a “fish - bone” absorber layout [3] - both are designed based on GTD principles. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3λ. In this paper the “Two - Level GTD” method is applied to improve the reflectivity in an anechoic chamber with external dimensions as small as 18′(H) × 18′(W) × 33′(L) operating from as low a frequency as 100 MHz. The simulations are performed first to compare the reflectivity results achieved with a “Two-Level GTD” with the reflectivity achieved in a conventional anechoic chamber. The anechoic chamber has been built and tested. The reflectivity results are obtained thru test zone probing and confirm the reflectivity improvement and overall outstanding performance of the chamber. <s> BIB001 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Techniques to improve the quality of the quiet zone in compact ranges <s> A method to analyze parabolic reflectors with arbitrary piecewise rim is presented in this communication. This kind of reflectors, when operating as collimators in compact range facilities, needs to be large in terms of wavelength. Their analysis is very inefficient, when it is carried out with fullwave/MoM techniques, and it is not very appropriate for designing with PO techniques. Also, fast GO formulations do not offer enough accuracy to reach performance results. The proposed algorithm is based on a GO-PWS hybrid scheme, using analytical as well as non-analytical formulations. On one side, an analytical treatment of the polygonal rim reflectors is carried out. On the other side, non-analytical calculi are based on efficient operations, such as M2 order 2-dimensional FFT. A combination of these two techniques in the algorithm ensures real ad-hoc design capabilities, reached through analysis speedup. The purpose of the algorithm is to obtain an optimal conformal serrated-edge reflector design through the analysis of the field quality within the quiet zone that it is able to generate in its forward half space. <s> BIB002
CATR measurement facilities are characterised by a straightforward operation which is able to perform real time antenna measurements along a wide margin of frequencies, whenever reflectors or lenses are used as collimators. In spite of their compactness, CATR facilities operate in far-field conditions. Therefore, the test wave received by the AUT should be a plane wave. The operation of a CATR relies on the planarity of this test wave, inside the "quiet zone" volume. Electromagnetic theory states that the quiet zone region cannot hold a plane wave field distribution, even not with an arbitrarily small volume, for a finite frequency of operation. This is due to the ability of the range's collimator to concentrate the electromagnetic energy within a reduced set of angular directions, following a distribution which is expected to tend to the delta in the angular domain when moving up in frequency. This theoretical constraint must be seen as the first of a set of contributions that affect the quiet zone planarity, and thus, the performance of the CATR which generates it. Usual implementations of the CATR concept focus on minimizing the following quiet zone degradations: 1. Edge diffraction. The collimators are designed so they are able to generate a plane wave inside the quiet zone, while the edge contributions act in the opposite direction. This is minimized in reflector-type collimators through conformal edge treatment: serrated edge and rolled edge geometries (Muñoz-Acevedo, 2012). 2. Collimation scheme. Varied implementations of the field collimation system have been presented, from the classical reflector scheme towards the hologram configuration, going through the dielectric lens. The surface accuracy requirements of a field collimator are increasingly tough when moving up in frequency, being dramatic quiet zone performance reduction the price to pay if they are not fulfilled  . 3. Reflection from chamber walls. Through the use of RAM, sometimes following conformal wall shapes and different size RAM pyramids depending on their location on the chamber  and BIB001 . 4. Time gating techniques. They aim to minimize the contribution of reflected signals from spurious field scatterers inside the chamber: test positioners and various unwanted propagation schemes, such as direct ray from the range's feeder or multiple reflections between the range's elements (feeder antenna and field collimators)  and  . 5. Feeding scheme. The collimator acts on an incident wave, collimating its phase and amplitude pattern towards the quiet zone. Conventional feeding alternatives impress an amplitude pattern over the collimator, being influenced both by the feeder's pattern and the unequal spherical wave propagation over the incident directions. The main effect on quiet zone is the appearance of an amplitude tapper which can be minimized if several collimators are used in cascade, feeder pattern are made conformal, or both of them  and  . Many of these features are used jointly in recent implementations of CATR setups. For a particular facility, its quiet zone distribution can be characterized in diverse ways. The most common are quiet zone probing (Muñoz-Acevedo, 2011) and RCS measurements of canonical objects  . These techniques lead to the knowledge of the quiet zone vector field through the amplitude and phase distributions of the components of interest. The described error depends with the frequency at which the CATR is operated. The collimation capabilities of a CATR increase when the frequency is higher. This is due to the fact that both the field collimators and the edge treatment devices are higher, in terms of wavelengths, and their ability to handle the radiation is, thus, superior. However, besides this fact, the required accuracy in terms of surface distortion and range alignment becomes the limiting factor which degrades the potential field collimation performance of the facility down to the actual one. These notes are summarized in Table 2 . This background makes necessary the use of quiet zone information and correction  . Solid knowledge of the quiet zone fields implies extensive acquisition campaigns which deal with diverse feed-probe configurations, so the vector field distribution is fully known. This task is very time consuming to be performed , becoming even more critical when increasing the frequency. It is possible however, to acquire the quiet zone distribution in a more time efficient way, without loss of information. The assessment of a particular CATR facility is able to lead to sampling grids which are intrinsically less time consuming and also imply an increase in the acquired field's signal to noise ratio figure, as proposed in (Muñoz-Acevedo, 2011). Fig. 18 summarizes the data acquisition setup and the involved subsystems. The proposed CATR approach ensures data robustness both in the sense of quiet zone field planarity and distribution stability. The "Field collimation" block follows the notes drawn in BIB002 . Accordingly, the direct wave contribution acquired by the probe is isolated from the stray signals through a time gating technique, which the instrumentation "VNA" must be able to perform. The anechoic chamber is properly designed so the measurement environment is isolated from the outer radiation sources. Proper absorber for the frequency range of interest is glued on the inner walls, ceil and floor. These solutions focus on the quiet zone planarity.