1
Introduction
1.1 Background
Anechoic chambers (s) and reverberation chambers (s) are two very different types of indoor measurement facilities and have been widely used in acoustics as well as in electromagnetics. It is interesting to note that these chambers share similar phenomena, physical quantities, and mathematical expressions in some ways. This book is about ACs and RCs in electromagnetics. Inside an AC, electromagnetic () waves are absorbed by the absorbing materials at the boundary, while inside an RC, EM waves are reflected by the conducting reflector at the boundary. Over the years, these two different chambers have found some common or complimentary applications in antennas, electromagnetic compatibility (), and radio communication measurements. Each has its advantages and disadvantages. Thus, it makes a perfect sense to bring these two different chambers into one book. They are like two sides of one coin: one is based on deterministic theory and the other is based on statistical theory; people working on RCs can be inspired by those working on ACs, and vice versa. Dual quantities can also be found in absorbing and scattering phenomena. This book is aimed at providing a clear and systematic approach to their design, measurement, and applications. Some latest developments are also included. In this chapter, we present an overview of both chambers while more details are provided in later chapters.
1.1.1 Anechoic Chambers
An ideal AC is a room designed to emulate free space - no radio waves are reflected from the walls, ceiling, and floor. The reason for using an AC is well-known: an ideal free space is required for EM measurement in an indoor environment that is not affected by the weather and interference outside the chamber, thus repeatable results can be obtained. A typical AC is given in Figure 1.1a and a typical measurement scenario with an aircraft is shown in Figure 1.1b.
Figure 1.1 Anechoic chamber: (a) 3D model with a cutting plane and (b) measurement with an aircraft inside an AC (pictures from Rainford EMC Systems, Microwave Vision Group).
In practice, because no ACs can absorb EM waves perfectly and reflections always exist, the performance of an AC needs to be characterised to show how close it is to the ideal free space. Thus, how to design an AC effectively and efficiently becomes an important issue. A problem is how to optimise the performance of such a chamber for a given chamber size using the least amount of radio absorbing materials (s) to minimise the cost and maximise the test volume (i.e. the equipment under the test area). The cost of the RAM depends on its size and type. How to choose the RAMs and arrange them properly is another key problem. Currently, the design of the chamber depends on the designer's experience and sometimes a trial-and-error approach or a large safe margin has to be adopted. Intuitively, a large space with high-performance absorbing materials leads to a good AC, but to quantify the chamber performance a well-defined and accurate mathematical model needs to be created. Thus, a scientific and objective way to find the best solution is required. An analytical solution is almost impossible for such a complex system, which offers an opportunity to bring computational electromagnetics (s) and real engineering problems together.
If an efficient computer-aided design () tool was available to predict the performance of an AC, the designer could design the chamber better, faster, and more accurately with the help of computers, not just relying on experience.
The figures of merit used to characterise the chamber performance in practice are site attenuation () for a full AC (all walls are covered with RAMs) and normalised site attenuation () for a semi-AC (no RAMs on the floor), field uniformity (), and site voltage-standing-wave ratio () [1, 2]. The procedures to measure these figures of merit and acceptable limits are given in relevant standards [1, 2].
It is well-known that the performance of ACs is closely related to the reflectivity of RAMs and how to arrange them [3, 4]. The first patented absorber was used to improve the front-to-back ratio of an antenna in 1936 [5]. During World War II (1939-1945), -20?dB (near normal incident angles) in the frequency range of about 2-15?GHz was obtained as the well-known Jauman absorber [6]. During the war years, Neher [7] demonstrated that the reflection from a long pyramidal shaped structure was much smaller than the reflection from a panel of the same absorber. This demonstrated the important role of geometry in the reflection reduction of RAMs. The first commercially available absorber started in 1953. In the early 1950s, 'dark-rooms' were built at a number of government and commercial organisations [8-10]; at that time, a typical level of reflected signal at S band was about 20?dB below the level of the direct signal. In the late 1950s, a new generation of broadband absorbers was able to produce a reflection coefficient of about -40?dB for near-normal incident angles. In the 1960s, by using ferrite underlayers, the thickness of the absorber was reduced greatly at low frequencies and the tapered chamber was developed, which showed a better performance than the rectangular chamber [10, 11]. The normal reflection coefficient at high frequencies achieved -60?dB. Nowadays, by combing the ferrite tiles and the pyramid absorbers, the reflection coefficient can achieve -25?dB at 30?MHz and -51?dB at 18?GHz (http://www.mvg-world.com/en/system/files/fiche_uh_absorbers_hypyr-loss_en_bd_oct_25th.pdf). More details will be discussed in the following chapters.
Three basic types of AC are used in practice, as shown in Figure 1.2: the rectangular chamber (Figure 1.2a), the tapered chamber (Figure 1.2b), and the compact chamber (Figure 1.2c). Test regions are marked with a circle, waves propagate along the lines ideally and absorbers are plotted as small triangles. In practice, because of the reflection and scattering of the RAMs, and because extraneous signals exist, the field in the test region is not uniform. The tapered chamber normally can provide a better FU than the rectangular chamber at lower frequencies, but the SA of a tapered chamber does not follow the Friis free-space transmission formula because of the multiple reflections from the tapered walls [3]. This should be noted for some special measurements such as using the three-antenna method to measure the gain of antennas. A compact chamber can be used to illuminate a large object with plane wave at higher frequencies because the object under test needs to be placed at the far-field region. When the frequency is high, the far-field condition cannot be satisfied without the use of a reflector. A parabolic reflector is normally used to generate a plane wave at higher frequencies, as shown in Figure 1.2c.
Figure 1.2 Three types of ACs: (a) rectangular chamber, (b) tapered chamber, and (c) compact chamber.
How to obtain an optimised AC has been investigated for many years. A well-known book was written by L. H. Hemming in 2002 [3] that provided an overview of this topic, including RAM characteristics, ACs of different shapes, and measurements in ACs. Geometric optics () was mentioned as a general method to analyse the AC, but the calculation was done by hand and how to implement it using a computer was not given. In a recent book [12], B. K. Chuang reviewed the GO method for AC design in one chapter. Although CEMs have evolved over the years, compared with other CEM methods the GO method is still the most robust and efficient in AC analysis. The most attractive advantage is that no detailed information of the material properties (permittivity, conductivity, and permeability) needs to be known; only the reflection coefficient is enough to describe the RAMs. The simulation time is also short with an acceptable error.
In this book we present a systematic solution for AC design, from theory to measurement. The solution proposed in this book is meant to be general and useful for all types of ACs, that is, not limited to specific shapes; it is also possible to use this solution to explore new chamber shapes with special requirements.
1.1.2 Reverberation Chambers
Unlike an AC, an RC is an electrically large conducting-screened room with electrically large stirrers used to stir the field inside the chamber (https://en.wikipedia.org/wiki/Electromagnetic_reverberation_chamber). The RC is also known in the literature as a reverberating chamber, a reverb, a mode-stirred chamber or a mode-tuned chamber. In this book the term 'reverberation chamber' is used as it is now widely used and accepted. The EM field inside the chamber is expected to be statistically uniform and isotropic. Two RCs are shown in Figure 1.3. In Figure 1.3a, two stirrers at the corner of the RC are used while in...
