A New Method for Antenna Efficiency Measurements

天线作为无线电系统的重要设备，有许多重要的参数来表征它的性能。其中之一是天线辐射效率(即天线效率)，它是辐射功率与输入功率的比率。在已知输入功率的情况下，此参数对于计算辐射功率非常有用。由于天线系统(包括材料)的复杂性，该参数的理论值通常不可靠，因此在系统设计(链路预算)和性能评估中经常需要测量天线效率。获取该参数的传统方法是1)。辐射方向图积分方法费时费力，精度不高；惠勒帽法，精度高、成本低，但只适用于电小天线；同样适用于电小天线的Q因子法。第一种方法通常在工作频率较高时使用，而另外两种方法则在工作频率较低时使用。最近，混响室被尝试作为一种新的设备来测量天线的效率，但这种方法需要一个具有已知效率的参考天线，其准确性还有待验证。随着无线电系统向更宽的带宽和更高的频率发展，传统的测量方法已经不能满足需要，例如，超宽带的特性使得惠勒帽法不能用来测量超宽带天线。迫切需要一种更好的方法来高效、准确地测量这些新型天线。该项目包括理论和实验两方面的工作，旨在开发一种基于惠勒帽思想的新的天线效率测量方法，即利用导电腔。然而，这种腔不需要电学上很小，腔内的共振模式可以与混响室中的模式一样多(可以看作是带有旋转搅拌器的过模腔)。在新的方法中，将被测天线(AUT)放置在空腔内的多个战略位置以消除谐振的影响，并测量天线的复反射系数S11。然后，可以使用这些测量的S11来获得天线效率。需要一个校准和数据解释软件。与任何现有的方法不同，空洞的损失将被考虑在内，在某些情况下，这可能是严重的。因此，该方法不仅比现有的方法更准确，而且与Wheeler帽法一样，也是一种经济方便的方法。此外，它将适用于大频率范围内的各种天线。因此，这种创新的方法可以用于测量小型和大型、窄带和宽带天线，满足了行业的需求，为测量和无线界提供了一种替代方法。

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.