Efficient Low-Frequency Modal Analysis of Anechoic Chamber

A new method for systematic correlation of measurements in semi-anechoic and anechoic chambers is presented. A signal measured in a semi-anechoic chamber is first processed using the matrix-pencil method which is applied sequentially on small frequency intervals. On each interval the measured signal is decomposed into its propagating wave components. The component corresponding to the wave reflected from the ground is identified and removed to fully retrieve the signal measured in an anechoic chamber. Three examples using log-periodic and Vivaldi antennas in different anechoic and semi-anechoic chambers for varying frequency ranges illustrate the method. The proposed decomposition algorithm can be utilized to characterize measurement test site imperfections.

A GTD model is proposed to discuss the normalized site attenuation (NSA) associated with an open area test site (OATS) with finite metallic ground plane. In this study, the NSA values for three OATS sites, based on GTD model and measurement, are carefully examined and agreement between calculated and measured results supports the usefulness of the proposed GTD model. The GTD model is then adopted to analyze the deviation in NSA values for several OATS structures. The GTD simulation has revealed that the key factor affecting NSA is the width of rectangle metallic plane which is essential in reducing the NSA deviation under the finite test site condition.

The complex fit normalized site attenuation (CFNSA) method was introduced in 2000 as an enhancement to the normalized site attenuation (NSA) model used in ANSI C63.4 and C63.5. The simple dipole NSA model is insufficient to predict the behavior of complex antennas like log periodic dipole arrays (LPDAs) and horns. Thus, NSA measurements made with these antennas using free-space antenna factors can have significant deviations from that predicted by the model. The CFNSA model is a modification to the simple dipole NSA model to account for the variations in antenna pattern and phase center that are possible in complex antennas. This paper evaluates the effectiveness of the phase center/pattern matching (PCPM) method previously proposed, as well as the use of directly measured pattern and phase center information, for a variety of antenna types.

For decades, site validation measurements below 1 GHz have been framed within the concept of Normalized Site Attenuation (NSA) proposed by Smith et al. in 1982. Conceptually, measured results are compared to a theoretical model, which is derived based on a raytracing model between two antennas over a conducting ground plane. It was quickly realized that the theoretical model cannot fully encompass the complexity of real antennas in geometries needed for site validation, and the individual antenna effects cannot be “normalized” entirely from the site attenuations by a simple subtraction (in dB) of the antenna factors. For dipole antennas, correction factors have been included to amend the errors (derived from numerical simulations). For broadband antennas, there had been no simple solution until Geometry-Specific Correction Factor (GSCF) was introduced in ANSI C63.5 in 2000. It may not have been immediately clear to users of the standards that since the introduction of GSCF, the Smith model NSA has been essentially bypassed. Of the two prescribed methods for site validations, the biconical GSCF provided in the form of correction values is based on a numerical model, while the measurement GSCF is a site-to-site comparison method. ANSI C63 can benefit significantly in clarity by removing the pretence of a simple theoretical dipole NSA model, and rewrite the site validation requirements directly in terms of either the numerical or the site-to-site comparison methods.

An antenna is calibrated in an open-field test site (OFTS) and/or an anechoic chamber, which facilities themselves need calibration. The procedures for calibrated test sites (CALTS) are given in the CISPR and ANSI/IEEE standards. The characteristic parameter for calibrated test sites is the normalized site attenuation (NSA). Theoretical calculations and practical measurements of normalized site attenuation are first presented. Antenna-factor (AF) and/or absolute-gain measurement methods are then reviewed. Finally, an alternative antenna calibration approach is given in this tutorial.

In this paper we look into a near-field measurement setup consisting of a cylindrical reflector and a linear array feed. The main application is Random Line-Of-Sight Over-The- Air measurements in anechoic or semi-anechoic chambers (i.e., anechoic chamber with reflecting floor). More specifically, the aim is testing the wireless communication systems on automotive vehicles. The aim is to generate the uniform field distribution of a plane wave over a pre-defined test zone containing the car or at least the part of the car containing the antenna(s). The field intensity variations over the testing zone have been investigated for reference measurements over the testing frequency bandwidth, i.e., 1.6–2.8 GHz.

The measurement of radiated emission (RE) in an anechoic chamber becomes very challenging at high frequencies, up to 60 GHz, because the scanning plane of the receiver is in measurement standard deviation from the actual wavefront. As a result, the RE intensity of the devices may be underestimated, resulting in electromagnetic interference. The deviation between the electric field at the far-field vertical scanning point and the actual wavefront is researched. Then, in an anechoic chamber, a hybrid deep learning amendment model of convolutional neural network (CNN) and transformer is proposed to correct the RE measurement at a 3 m distance. The results indicate that the correction is reliable, with an average error of 6.35% for a 3 m distance in a semianechoic chamber and less than 4.83% for other test scenarios. The proposed method provides a promising solution for RE measurement at a millimeter wave band in an anechoic chamber.

An open test site located at the Telecommunication Lab in Taiwan is characterized by measuring normalized site attenuation (NSA) to determine the suitability of this open test site for antenna measurement. The measured NSA data are compared to the theoretical values. The results show a maximum deviation of less than 3 dB over the frequency range of 30 to 1000 MHz.

In the ANSI C63.4 development of normalized site attenuation (NSA) for vertical polarization the E-field sine pattern generated by short dipoles was used to account for the additional attenuation that occurs as a function of the antenna patterns. A pattern dependent NSA equation is developed incorporating antenna pattern factors for any types of antennas for which mathematical expressions of the patterns exist. For resonant dipoles, biconicals and log-periodics the decibel differences from the existing standard have been found to be small, leading to increased confidence in the utility of the present standard.

The aim of this work is to put forward a new method for systematic retrieval of results obtained in an anechoic chamber from results measured in a semi anechoic chamber. The strategy employed is to identify the different propagating components, viz., the direct and the reflected wave components using the generalised pencil of function method. Once identified, the reflected component can be removed.

The measurement of an antenna in a reverberant or semi-anechoic chamber, or even in an anechoic chamber with non-ideal absorbing material, will include the effects of multipath propagation, which will be evident in a distorted radiation pattern. In those situations, an improvement of the measurement can be achieved through removal or compensation of the undesired contributions. A simple technique to eliminate those contributions, based on the well-known Fourier transform algorithm, is used in this paper. It starts from measurements of the system's response in the frequency domain, followed by Fourier transformation of data to the time domain, detection and gating of undesired echoes in the time domain, back to the frequency domain, and retrieval of the antenna radiation pattern at the frequency of interest. The method is evaluated under different measurement conditions.

This paper discuss the design considerations of a semianechoic chamber (SAC) for Science and Technology Research Institute for Defense's (STRIDE) to cater for electromagnetic compatibility (EMC) measurements in compliance to military and commercial standards. The SAC size is limited to the available space at STRIDE premise of 13.8 meters length, 5.88 meters width and 6.9 meters height. Similarly the size of the control room (CR) and the amplifier room (AR) are restricted to the adjacent smaller space. All of the rooms including the SAC are incorporated with RF shielding, however only the SAC is fitted with microwave absorbers. Three types of test are performed to validate the performance of the SAC which are Shielding Effectiveness (SE), Field Uniformity (FU) and Normalize Site Attenuation (NSA).

It is useful to compute sound field of a source in a virtual environment which is different from the measurement environment. For example, some properties of sound source, such as directivity index and frequency response curve, are required to be measured in an anechoic room or free space, but both of them cannot be always accessible. Consequently, it will be useful to compute sound field of a source in free space when sound field of the source is not measured in the free space. In the aforementioned example, the free space is a virtual environment. Based on reciprocity theorem and modal expansion, a method to predict sound field of a source in a virtual environment is given in this paper when the scattering effect of the source can be neglected. Reciprocity theorem builds the relationship between measured sound field and predicted sound field, which plays an important role in the method. Green’s function in the virtual environment is needed in the method. To restrict measurement points on an enclosed surface, the Green’s function is expanded by a set of modes. A simulation is given to examine the validity of the method.

In 1939, Dr. Dah-You Maa presented a paper entitled "The Distribution of Eigentones in a Rectangular Chamber at Lower Frequency Ranges" [D. Y. Maa, J. Acoust. Soc. Am. 10, 258 (1939)]. Since then, his interest in room acoustics has not diminished. More than six decades later, Maa proposed an idea of adding a monopole solution to the normal modal expansion to improve the accuracy in simulating the near-field sound field in rooms [D. Y. Maa, Acta Acust. (Beijing) 27, 385-388 (2002)]. Following this idea, a hybrid model that combines the free field Green's function and a modal expansion has been proposed by the authors based on a rigorous mathematical derivation [B. Xu et al, J. Acoust. Soc. Am.128, 2857-2867 (2010)]. The hybrid modal expansion can be further extended for complex sound sources by introducing the multipole expansion to the solution. In this paper, the theoretical derivation of the hybrid modal expansion will be reviewed, followed by examples demonstrating the use of the hybrid modal expansion in real-world applications.

For the validation of an open area test site usually the normalized site attenuation (NSA) is considered. However, the model for the calculation of the NSA in the various standards is a point dipole. While this may be a correct assumption for biconical antennas or reference dipoles with a well-defined phase center, it is wrong for the antennas used in an automotive antenna test setup. Measurements of automotive antennas for radio and TV reception generally are done on a dedicated test range consisting of a turntable that can bear the weight of the vehicle and a test antenna that is placed in a considerable distance in order to ensure far field conditions. For broadband measurements often log-periodic antennas are used. In these antennas the phase center is not known exactly for all frequencies. The phase center of the automotive antenna is not known either. This uncertainty in the position of the phase centers in combination with the ground reflection occurring between object under test and test antenna results in a measurement error that cannot be neglected. In this work we show how this error can easily be calculated for all test site geometries and give an exemplary calculation for the test site used at our company.