An Antenna Efficiency Assessment Method using Reverberation Chamber

Abstract

Due to the growing popularity of personnel mobile communication devices, miniaturized antenna designs are in great demands. However, as the antenna size gets smaller, the input reactance gets larger and the antenna efficiency dwindles as well. Furthermore, conventional antenna performance parameters, such as directivity and polarization, become less apparent since the antenna behaves more like an isotropic radiator. Therefore, it is not quite convenient to measure the antenna characteristics using a traditional far-field chamber. The near-field chamber, on the other hand, is capable of evaluating the antenna efficiency by sampling the radiated power on a spherical grid enclosing the antenna. Nevertheless, the measuring process is time consuming and required relatively expensive equipment. In order to evaluate the antenna efficiency, which is the most critical radiation feature for a small antenna, we proposed a time and cost effective measurement method based on the reverberation chamber (RC) configuration. Unlike conventional far-field or near-field chambers, which attempt to measure the antenna radiation properties in an emulated “free space” environment, the electromagnetic reverberation chamber is a resonant cavity that is rich in propagation paths. The numerous modes of the reverberation chamber provide a more rigorous platform to examine an electronic device with a complex radiation pattern. The reverberation technique is therefore getting more popular for EMC tests and is included in IEC 61000-4-21 standard [1]. An overview and discussion on many aspects about the measurement and simulation of reverberation chamber is provided in [2]. The reverberation chamber technique, which is widely accepted for EMC/EMI testing, is also considered for small antenna performance assessment. Although the RC technique can’t supply radiation pattern information, the antenna radiation efficiency can still be revealed via statistical measures providing that the field uniformity is achieved. The RC technique is particularly attractive for its low cost since no absorber is needed. Via simulations, Krzysztofik and Wolny show that unlike EMC reverberation chamber, which requires an electrically large environment, the chamber size can be greatly reduced for small antenna measurements [3]. In [4], the RC technique is compared with the Wheeler cap method for small antenna radiation efficiency. The measurements on UWB antennas are conducted in [5]. In this work, a small reverberation chamber is implemented to evaluate the efficiencies of small multi-band antennas. Instead of recording transmission levels at different stirrer positions, we attempt to estimate the efficiency based on the averaged level of various stirrer orientations to eliminate stirrer control part. Section 2 describes the formulation of the chamber and the test antennas used. Section 3 presents the derivation for antenna efficiency calculation. In Section 4, efficiency estimations based on RC measurements are compared with efficiencies measured in spherical near field chamber. A short summary is given in the end.