Fundamentals of field applicators and probes at radiofrequencies and microwave frequencies

In Chapter 1, the first principles of field interactions with materials were discussed. Different effects of high-frequency electric fields and magnetic fields in various types of materials were established. The purpose of this chapter is to introduce the applicator and the probe head, which are devices for establishing those fields within the material. While the electromagnetic principles behind the two devices are the same, the functions are different.

Keywords

Field applicators; material load geometry; electric field applicator; field uniformity; material under test; RF probes; microwave probes; microwave heating applicators; RF heating applicators

Chapter Outline

Introduction

In Chapter 1, the first principles of field interactions with materials were discussed. Different effects of high-frequency electric fields and magnetic fields in various types of materials were established. The purpose of this chapter is to introduce the applicator and the probe head, which are devices for establishing those fields within the material. While the electromagnetic principles behind the two devices are the same, the functions are different.

The evolution and development of applicators and probes in radio frequency (RF) or microwave frequency have been fragmented due to their usage by a wide variety of disciplines and industries. Due to this fact, the terminology developed for these devices is often not standardized. We have made an attempt here to use the most frequently used terms, as long as they suggest accuracy and are not too suggestive of specific applications. For example, some industries use the term “material under test” or “sample” to refer to the material that is subjected to electromagnetic fields. Here, we have avoided such terminology because in many cases, testing is not intended, and the idea is to heat or process the material. Instead, we have used the term “material load” to describe the material subjected to the fields.

2.1 Basic Applicator/Probe Definitions

2.1.1 Distinctions between applicators and probes

The term “field applicator” or, simply, “applicator” is used for a device that applies RF/microwave energy into a material volume at a level sufficient to create either a permanent or temporary change in a material parameter or property. The change could be a rise in temperature, driving out moisture, enhancing a chemical reaction, ablation of a biologic tissue, breakdown of gases to form plasmas, and so on. We use the term “energy deposition systems” to refer to this class of industrial, scientific, or medical (ISM) systems.

The term “probe” or “sensor head” is typically used when the purpose of the field interaction is only to obtain information from the material. For example, in an industrial process, a probe system may be used to apply a field into the process material to obtain information about dielectric properties, which may indicate a material parameter such as moisture level. In that case, the field strength—and therefore the level of energy imparted into the material—is small and is enough for sufficient signal strength over the noise level. Typically, this signal level is far below the level needed to heat the material. There are cases, for example, magnetic resonance (nuclear magnetic resonance, electron spin resonance [ESR]), where the same device can act as both applicator and sensor.

While the fundamental physics are similar, there are practical differences between applicators and probes that make the design considerations quite different. These differences are mostly related to far higher levels of power, voltage, and current in the case of applicators versus signal levels in the case of probes. In addition to size differences, the choice of frequency for probes is far wider because of low power levels, which allow probes to work outside of allowed ISM frequency bands.

2.1.2 Far-field and near-field: antennas versus applicators/probes

Most applicators and probes discussed in this book are near-field, which is generally defined as having the distances between the applicator and the material small compared with the wavelength. In most cases of near-field systems, the material is actually enclosed within the applicator structure; an obvious example is the domestic microwave oven, where the food item is placed within the microwave cavity.

In far-field systems the distances between the antenna and the material can be large compared with the operating wavelength, and instead of applicator/probe terminology, antenna terms are used. In far-field systems, the field interacting with a material is in the form of traveling plane waves, with the ratio of electric field to magnetic field (wave impedance) at the free space level of 377 Ω, while in near-field systems, the fields interacting with the material have impedances that typically deviate greatly from this value. The wave impedance issue was discussed in Chapter 1 and will be covered in the context of applicators and probes in this chapter as well.

Another distinctive feature of near-field applicators/probes is that the presence of the material load typically has a significant impact on the device’s field configuration and some circuit variable such as impedance, voltage, and current. In essence, the subject material, through its electrical properties, becomes a part of the circuit. In far-field (antenna) systems, on the other hand, the effect of the material on the circuit variables is negligible. This book mainly deals with near-field systems. There are, however, excellent references for far-field material interaction systems that deal with microwave and millimeter-wave imaging, sensing, and tomography [1].

Applicators for field–material interactions are somewhat analogous to transmitting antennas in communication systems, and probes are analogous to receiving antennas. In the same manner as in communication systems, the reciprocity theorem [2] holds for applicators and probes.

2.1.3 Electrically small and electrically large applicators

High-frequency applicators can be categorized from the standpoint of electrical size, which is the relative size of an applicator compared with the operating wavelength. Obviously, in most cases, the physical size of an applicator is mainly set by the size of the material load. The electrical size is a very important factor in applicator design and determines the geometry, materials of construction, and method of analysis and design.

Electrically...