Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> The ground surface affects radiated and received signals of antennas near the surface of ground. By considering spatially extended current distributions, we obtain both positive and negative image results for both horizontal and vertical antennas. We present numerical results for HF with ground surfaces of soil, seawater, and a perfect electrical conductor. To illustrate the angle and frequency-to-height dependence of the constructive and destructive interference, we plot contours (of equal amplitude) of the four different image factors in terms of \theta and height/wavelength ( h/\lambda ). These results provide insight into design considerations involving antennas in the near vicinity of ground. <s> BIB001 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> A comparison between the antenna-pattern comparison technique and the free-space voltage standing-wave ratio technique for evaluating the reflectivity level of radio anechoic chambers is presented. Based on an analysis of the two techniques, it is pointed out which parameters influence the measured value of the reflectivity level. The comparison is illustrated with experimental results and it is explained why inconsistent and uncorrelated results may be found when the two methods are used. Furthermore, it is demonstrated, by introducing improvements in a chamber, how the reflectivity level can be used to measure the improvements. This work is inspired by the current discussion of finding a figure of merit for anechoic chambers. Based on the results, an evaluation procedure for anechoic chambers is indicated. However, it is pointed out and illustrated by examples that further investigations are necessary before a satisfactory procedure can be outlined. <s> BIB002 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> A method of inferring microwave antenna radiation patterns over wide bandwidths from a series of subnanosecond pulse measurements is described. Monochromatic patterns can be inferred from the time-domain measurements with simple apparatus. Results are presented which show that the method is inherently insensitive to errors caused by reflections from the environment. <s> BIB003 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> Anechoic chamber reflection levels, as much as 50 dB down, can be measured at frequencies up to a few gigahertz, with broadband information derived from a single time-domain measurement. The technique has an advantage over frequency-domain methods because it can separate different reflections from each other, allowing any particular area of the walls to be studied in detail. The source of a particular reflection can be identified by its distance from the antennas, as well as direction, so that improvements can be made to the absorber arrangement. <s> BIB004 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for extraneous fields created by reflection and scattering of the range antenna field from fixed objects in the range and by leakage of the range RF system from a fixed location in the range. TZF compensation can be used on fixed line-of-sight (static) far-field, compact, and near-field ranges. Other compensation techniques are seldom used in practical measurement situations because they are limited in the amount of compensation they provide. These techniques do not adequately model the type of extraneous field present in the range or require increased measurement time and equipment necessary to implement the technique. TZF compensation overcomes these limits as follows. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used antenna pattern measurements to compensate for extraneous fields. TZF compensation theory is presented and demonstrated using measured data. > <s> BIB005 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> This paper focuses on the measurement of the antenna radiation patterns when fully anechoic conditions are not available and, as consequence, some undesirable echoes are initially present in the measure. Two techniques are analyzed and compared, which - starting from data measured in the frequency domain - allow the echo contributions to be identified and the antenna radiation pattern to be retrieved. The accuracy of both techniques is evaluated at the frequency of 22 GHz by comparison with measurements obtained in an anechoic chamber. <s> BIB006 </s> Overview of Novel Post-Processing Techniques to Reduce Uncertainty in Antenna Measurements <s> Reflection suppression method <s> In the paper, we have shown that averaging the radiation patterns over several distances between two antennas is useful in order to reduce the influence of reflection waves, using simulation software FEKO. In the conditions with and without reflection waves we simulate the radiation patterns of the biconical antenna (the operation band is from 30 MHz to 300 MHz) in distance between antennas from 315 cm to 615 cm by 50 cm steps. Averaging the radiation patterns is useful for reducing the influence of the reflection waves in an anechoic chamber. We will apply the proposed method to various antennas, and determine the optimum averaging number, distance, and steps. <s> BIB007
Normally, the antenna measurements are carrie d o u t i n a n e c h o i c c h a m b e r s t o r e d u c e unwanted contributions, such as reflections or diffractions from the environment. However, there are special cases in which the use of that kind of measurement setup is not possible, and a semi-anechoic chamber or an outdoor measurement system has to be employed. In fact, reflection waves may also appear in anechoic chambers due to imperfections in the radiation absorbing material (RAM). In any case, when the measurement is not performed in a fully anechoic environment, the unwanted contributions could significantly alter the actual antenna properties, producing a ripple in the radiation pattern. The number of approaches to analyze and cancel the effects of unwanted contributions has increased in recent years. There are methods that are employed not to remove the reflection waves, but rather to characterize the chambers by a figure of merit, usually called the reflectivity level, in frequency domain BIB002 or time domain BIB004 . Another method, based on measurements at several distances, was presented BIB007 . Other solutions within the category of so-called compensation methods can be applied to avoid measuring the AUT several times at different distances with respect to the probe BIB005 and . Time-gating techniques can also be employed as an option to remove reflected contributions, as it has been presented in  and BIB003 . However, the complexity of the pieces of equipment is lower when using frequency-domain measurements as proposed in BIB006 . The proposed method is based on a diagnostic technique. The input data are taken over only one arbitrary surface in the frequency-domain. Moreover, compared with some time-gating techniques or frequency decomposition techniques, it is not necessary to measure more than one frequency. Therefore, the measurement time is reduced considerably and, because the frequency-domain is employed, complicated equipment is not needed. If reflected waves are present in antenna measurements, the radiation properties obtained will be perturbed. These same incorrect results are achieved with an equivalent system where reflections can be viewed as direct waves coming from virtual sources, as shown in Fig. 9 (a) . Such a replacement is explained in detail via image theory, which was also used in BIB001 to study ground reflections as image current distributions. The identification of the virtual sources is not possible with a conventional diagnostic technique, where the field is only reconstructed over the antenna aperture. However, if the field reconstruction is performed over a surface larger than the antenna dimensions, the aforementioned fictitious sources can be found and cancelled with a filtering process. By applying any of the reconstruction methods explained before, the extended reconstructed field can be found. In the reconstructed domain, real sources and virtual sources are not spatially coincident. Therefore, a spatial filtering can be applied to suppress those virtual sources associated to the reflections. This filtering is defined as follows where Z V represents the zone where virtual sources are placed. www.intechopen.com