Ulf Carlberg

Calculated and measured absorption cross sections of lossy objects in reverberation chamber

Reverberation chambers can be used to measure radiation efficiency of small antennas when these are located close to lossy objects. The lossy objects represent a heavy loading of the chamber. This loading is characterized by the mean absorption cross section of the lossy objects. This paper describes how this mean absorption cross section can be calculated from the scattered far field of an object by using the forward scattering theorem, or from a more laborious near-field evaluation. Results for lossy spheres and cylinders are calculated by using three different codes, based on spherical mode expansion, finite difference time domain techniques, and moment methods, respectively. The results for the cylinder are compared with measured levels in a reverberation chamber.

Study of antennas in reverberation chamber using method of moments with cavity Green's function calculated by Ewald summation

A numerical model of wires in rectangular metal cavities is introduced for computing antennas in a simplified theoretical reverberation chamber. The code is based on the method of moments, and it uses the Ewald summation for efficient calculation of the cavity Greens function. The Q-value of the chamber is accounted for in the model by a homogeneous material filling the chamber. The S-parameters of two dipoles placed in many random positions in the cavity are computed and averaged to provide the average transfer function of the chamber. This represents mode stirring by moving the antennas, referred to as position stirring. The computed results are compared with theoretical statistical values as well as experimental results. The discrepancies are discussed.

Investigation of Heavily Loaded Reverberation Chamber for Testing of Wideband Wireless Units

Traditionally, the reverberation chamber has been used as a low-loss cavity to gain high signal levels. For testing of the radiated properties of wireless terminals and their antennas, this is not an issue, and lossy objects like for instance a head phantom can be introduced in the chamber without problem. However, for testing of active units, the bandwidth of the system itself plays an important role. In order to avoid distortion of the signal transmitted in the chamber, the bandwidth of the chamber must be larger than the bandwidth of the signal. This is one reason for loading the reverberation chamber. Another reason is a potential increase in the measurement accuracy, since an increased bandwidth makes more modes excited at a particular frequency. The purpose of this paper is to investigate the effect of loading on the statistics of the chamber and its bandwidth

Numerical Study of Position Stirring and Frequency Stirring in a Loaded Reverberation Chamber

A numerical model of a loaded reverberation chamber is used to study the effectiveness of 3-D position stirring and frequency stirring. The numerical model is based on thin wires, the moment method, and a cavity Greens function. The average power transfer level between two dipole antennas is compared to the average power transfer level between a dipole antenna and a loop antenna. The two transmission levels should ideally be the same, as both the dipole and the loop are treated as being lossless and impedance matched. The standard deviation of the level differences is a measure of the accuracy of the chamber, and this is used to estimate the stirring effectiveness. It is shown that frequency stirring must be done over a larger bandwidth than the average mode bandwidth to be effective. The 3-D position stirring is also shown to be much more efficient than expected.

Extracting electrical material parameters of electrically large dielectric objects from reverberation chamber measurements of absorption cross section

Reverberation chambers can be used to measure the absorption cross section of a dielectric object. The absorption cross section of a dielectric object depends on its size, shape, and electrical material parameters. By comparing with a theoretical model of the absorption cross section, material parameters can be extracted from measurements. A model based on a plane wave approach of incident fields is used here, valid for electrically large material samples in an isotropic environment such as that in a reverberation chamber. Which material parameter can be extracted depends on the properties of the material sample. The presented method combines the accuracy of cavity methods with the flexibility of being able to measure samples of arbitrary size and shape. Because both the reverberation chamber and the material sample are electrically large, the method is particularly useful at millimeter-wave frequencies.

Designing reverberation chambers for measurements of small antennas and wireless terminals: Accuracy, frequency resolution, lowest frequency of operation, loading and shielding of chamber

The last five years the reverberation chamber has been developed to an accurate instrument for measuring the performance of small antennas and active mobile terminals in Rayleigh fading. The present paper gives an overview of the research done by the authors in relation to achieving an accuracy of 0.5 dB RMS or better, when measuring efficiency and radiated power. The accuracy has been verified by comparison with measurements in anechoic chambers, and between reverberation chambers of different size, and from participation in benchmarking of measurement ranges done within ACE (Antenna Center of Excellence, a European Network of Excellence). There have also been developed procedures for measuring quantities that are specific for the Rayleigh environment, such as diversity gain, MIMO capacity, and static as well as dynamic receiver sensitivity at certain data error rate (BER or FER). The paper will describe important topics related to the mode stirring and loading of the chamber, such as the accuracy, frequency resolution, average transfer function, and the system bandwidth when measuring active terminals. Numerical simulations play an important role in controlling the chamber performance.

Definition of Antenna Diversity Gain in User-Distributed 3D-Random Line-of-Sight

The present paper defines diversity gain for stationary users. This deals in particular with gathering the received signal statistics over possible user positions and orientations in space rather than over time, and to define a meaningful diversity gain related to the cumulative improvement of the performances of the 1% users with the worst receiving conditions. The definition is used to evaluate diversity gain for some typical small antennas in an extreme environment with only line-of-sight (LOS). The LOS environment is regarded as user-distributed 3D-random LOS caused by the statistics of an ensemble of stationary users with arbitrary orientations in the horizontal plane (2D), and with arbitrary orientations of their wireless devices in the vertical plane. Thus, an overall 3D-random distribution of user orientation is assumed.

A fast mode analysis for waveguides of arbitrary cross section with multiple regions by using a spectrum of two-dimensional solutions and asymptotic waveform evaluation

A fast mode analysis for waveguides of arbitrary cross section with multiple regions is presented in this paper. The analysis is based on a spectrum of two-dimensional (2-D) solutions with application of asymptotic waveform evaluation, which requires only several 2-D solutions in searching for the propagation modes and calculating the field distribution of the modes in waveguides. The boundaries of waveguides can be modeled by perfect electric conductors, perfect magnetic conductors, dielectric materials, and asymptotic strip boundary conditions. By this modeling, one can also analyze the waveguides with the soft and hard surfaces. The method is excitation dependent, which provides a tool to analyze the response of different modes to different excitations. No existence of spurious modes is experienced by using this method. The verification and comparison of the numerical results with analytical, published data, and measurements are also presented in this paper.

Calculation of absorption cross section of lossy objects used when measuring antennas in reverberation chambers

Reverberation chambers can be used to measure radiation efficiency and input impedance of small antennas. In particular, they can be used to measure these characteristics for small antennas located close to lossy objects, such as a mobile phone antenna located in different positions relative to a head phantom. It is therefore of interest to know the absorption cross section of different lossy objects used when measuring antennas in reverberation chambers. We show how the absorption cross section of a lossy sphere and a lossy cylinder can be calculated from the scattered far field by using the forward scattering theorem and the moment method. The results are in agreement with mean absorption cross sections of a lossy cylinder found from the measured mean transfer level of a reverberation chamber used for measuring small antennas close to a cylinder. They validate the formula that relates the net average transfer level of a reverberation chamber to the mean absorption cross section of a lossy object (see Hill, D.A. et al., IEEE Trans. on Electromagnetic Compatibility, vol.36, no.3, p.169-78, 1994). The absorption cross section of the cylinder varies with angle of incidence. This is important to know, as it may affect the accuracy of measured radiation efficiencies in reverberation chambers.

Diversity gains in random line-of-sight and rich isotropic multipath environment

Antenna diversity gain for theoretical as well as measured antennas is studied in two extreme environments, the rich isotropic multipath environment (RIMP) and the random line-of-sight environment. The RIMP diversity gain was previously defined based on improved fading performance, here we equivalently consider it as a metric for the cumulative improvement of the 1% worst users randomly distributed in the RIMP environment. Similarly, we consider the diversity gain in the random line-of-sight environment to be the performance improvement of the 1% of the users which receives the weakest signal relative to a theoretical Rayleigh distribution of the signal levels among the users. The random line-of-sight environment is regarded as being caused by the statistics of an ensemble of users (or terminals) with arbitrary 3D orientations.