The careful design of applicators is essential if one is to avoid the type of localised over- heating described previously for multimode applicators or to achieve a good impedance match in single mode systems. However, the current design process for many systems is very empirical, often relying entirely on the designers experience, trial and error and the construction of expensive prototypes. The reason for this approach lies in the fact that multimode applicators are notoriously difficult to analyse. Single mode resonant cavity applicators on the other hand, where there is only one dominant field pattern can be described in relatively simple terms by analytical expressions. Even so, when they are
1.3 Applicator Design 11
coupled to a waveguide feed via an aperture, or are heavily loaded so that higher order modes start to become significant, the use of analytical expressions becomes increasingly difficult. The purpose of this thesis is to present a numerical method for analysing dif- ferent applicator structures rather than to present novel designs for microwave systems.
These techniques will hopefully allow the designers of microwave applicators to remove some of the trial and error in the design process.
The field pattern in a multimode applicator is determined by both the nature of the load and the size of the cavity. It is therefore impossible to predict the exact field pattern for an arbitrary load without carrying out a full solution of Maxwell’s equations for the loaded cavity. Since it is often the case that no two loads are identical, especially when dealing with food products, this solution will apply only to the load that has been modelled. The position of the load in the cavity will also effect the field pattern, a change of only a few millimeters being sufficient to alter the distribution. Given these difficulties it is still possible to gain useful insights from simulations of loaded multimode cavities. One can, for example, determine the degree of sensitivity of the system to various parameters, such as the source frequency, feed position, load position or dielectric properties. It is also possible to test different applicator designs against a set of loads, varying the dimensions and observing the effects. The feed system is another area that can be addressed since small changes in the feed can produce large changes in the way the cavity modes are excited. Simulation can then allow the designer to experiment with various changes to a feed design, while keeping all other factors constant. This would be difficult to do in an experimental situation. The behaviour is also dependent on the source of the microwaves. It will be seen from results presented in Chapter 6 that a small change in frequency can lead to large changes in the field distribution inside the applicator, especially when loaded with a low loss dielectric. A further complication arises because the behaviour of the source is generally dependent upon the load to which it is connected. Fortunately, many industrial systems employ an iso-circulator, as shown in Figure 1.1 that isolates the source from the applicator allowing for this effect to be discounted in many applications.
Microwave heating attempts to efficiently couple power from the source of microwaves into the load being heated. It is therefore of obvious interest to determine the amount
of power dissipated inside the load. When the system is fed via an iso-circulator the load impedance will determine the fraction of the supplied power, P0, absorbed by the cavity, P and the amount reflected back into the water load of the iso-circulator, since
P =P0(1− |ρ|2), (1.2) where the reflection coefficient,ρ, is a function of the impedance. Determination of the impedance of the loaded applicator as seen from the waveguide feed is an extremely important aspect of the calculation which is often neglected in numerical simulations. If the magnetron is coupled directly to the cavity then this impedance will be seen by the magnetron and will therefore determine its output power and the efficiency with which it will operate. A knowledge of the absolute value of the power absorbed is necessary to correctly determine the temperature rise. The calculation of the impedance and the reflection coefficient are discussed in Chapter 4. Comparisons of the calculated values and experimentally determined ones for single mode cavities are then given in Chapter 5.
Frequency of Operation
The microwave frequencies allocated for industrial use are 896 MHz (915 MHz in the USA) and 2.45 GHz. The sources that produce the microwave energy, invariably a mag- netron for heating applications, will not operate exactly at these frequencies. The regu- lations allow a tolerance of±10 MHz at 896 MHz and ±50 MHz at 2.45 GHz [Metaxas
& Meredith, 1983]. Variations in construction and in the power supply to the mag- netron mean that every magnetron will operate at a slightly different frequency. The frequency of operation is also dependent on the impedance of the load to which it is supplying power. This thesis will give results that show how very small changes in frequency (<0.5 %) can significantly alter the field distributions with some load con- figurations. This variation in frequency is therefore an important consideration when modelling microwave applicators, a fact which is appreciated by very few workers [Ma et al., 1994]. One of the advantages of the scheme that will be presented in this thesis is its ability to provide information over a range of different frequencies. This immediately provides an indication to the systems sensitivity to frequency and to the nature of the changes that are likely to occur.