A New Method for Antenna Efficiency Measurements

点击翻译按钮获取中文摘要

The antenna, as an essential device for radio systems, has a number of important parameters to characterise its performance. One of them is the antenna radiation efficiency (i.e. antenna efficiency) which is the ratio of the radiated power to the input power. This parameter is very useful to calculate the radiated power once the input power is known. Since the theoretical value for this parameter is normally not reliable due to the complexity of the antenna system (including materials), measured antenna efficiency is often required for system design (link budget) and performance evaluation. The traditional methods to obtain this parameter are 1). radiation pattern integration method which is time consuming and not accurate; 2). Wheeler cap method which is accurate and cheap but only suitable for electrically small antennas; 3). the Q factor method which is also just suitable for electrically small antennas. The 1st method was normally used when the operating frequency is relatively high while the other two methods were employed when the operating frequency is relatively low. More recently the reverberation chamber was attempted as a new facility for measuring the antenna efficiency, but this method requires a reference antenna with a known efficiency and the accuracy is yet to be proven. As the trend of radio systems moves towards broader bandwidth and higher frequencies, the traditional methods are no longer adequate, for example, the Wheeler cap method cannot be employed to measure UWB antennas due to the ultra-wide bandwidth nature. There is an urgent need for a better method which can be used to measure these new antennas efficiently and accurately. This project consists of both theoretical and experimental work and is aimed at developing a new antenna efficiency measurement method which is based on the Wheeler cap idea, i.e. utilising a conducting cavity. However, this cavity does not need to be electrically small and the resonant modes inside the cavity can be as many as that in a reverberation chamber (which can be viewed as an over mode cavity with rotating stirrers). In the new method, the antenna under test (AUT) is placed inside the cavity at a number of strategic positions to remove the effects of the resonance and the complex reflection coefficient S11 of the antenna is measured. The antenna efficiency can then be obtained using these measured S11. A calibration and data interpretation software are required. Unlike any existing methods, the loss of the cavity, which could be significant in some cases, will be taken into account. Thus the proposed method should be more accurate than current methods and it should also be cheap and convenient as the Wheeler cap method. Furthermore, It will be suitable for various antennas over a large range of frequency. Thus this innovative method can be used to measure small and large, narrow and wide band antennas, meet the demand from the industry and provide an alternative method to the measurement and wireless community.