Microwave power

According to the solution of the Bloch equations (Chapter 5), the magnetic resonance absorption, sometimes called the v-mode signal , v, is given by eqn (1.10). 

To simultaneously maximize signal-to-noise ratio (S/N) and minimize distortion, it is best to adjust the microwave power, as measured by the power 

In general it is unnecessary to spend much time adjusting the power level. The general rule is to adjust the power to about 10 db attenuation for organic radicals and to use full power for transition metal complexes and those organometallics where the unpaired electron is primarily located on the metal. 

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One effect of saturation, and the dependence of e on /, is to decrease the maximum absorption intensity of a spectral line. The central part of the line is flattened and the intensity of the wings is increased. The result is that the line is broadened, and the effect is known as power, or saturation, broadening. Typically, microwave power of the order of 1 mW cm may produce such broadening. Minimizing the power of the source and reducing the absorption path length t can limit the effects of power broadening. 

High Frequency Dielectric Strength. Dielectric strength at high frequency is important in microwave power uses such as radar (see Microwave technology). Because SF has zero dipole moment, its dielectric strength is substantially constant as frequency increases. At 1.2 MHz, SF has... 

Frequency Allocations. Under ideal conditions, an optimum frequency or frequency band should be selected for each appHcation of microwave power. Historically, however, development of the radio spectmm has been predominantly for communications and information processing purposes, eg, radar or radio location. Thus within each country and to some degree through international agreements, a complex Hst of frequency allocations and regulations on permitted radiated or conducted signals has been generated. Frequency allocations developed later on a much smaller scale for industrial, scientific, and medical (ISM) appHcations. 

The frequency bands at 915 and 2450 MHz (2375 MHz in the past in eastern Europe) are the most developed bands for microwave power apphcations. Microwave ovens are almost all at 2450 MHz. Many industrial heating appHcations are at 915 MHz. After the 1979 WARC, eastern Europe adopted 2450 MHz in place of 2375. 

Principles in Processing Materials. In most practical apphcations of microwave power, the material to be processed is adequately specified in terms of its dielectric permittivity and conductivity. The permittivity is generally taken as complex to reflect loss mechanisms of the dielectric polarization process the conductivity may be specified separately to designate free carriers. Eor simplicity, it is common to lump ah. loss or absorption processes under one constitutive parameter (20) which can be alternatively labeled a conductivity, <7, or an imaginary part of the complex dielectric constant, S, as expressed in the foUowing equations for complex permittivity ... 

The general engineering task in most apphcations of microwave power to materials or chemicals is to deduce from the geometry of samples and the electromagnetic (EM) environment (appUcator), the internal field distribution, E (r), and hence the distribution, P(r), of absorbed power. From this, the... 

The apphcation of microwave power to gaseous plasmas is also of interest (see Plasma technology). The basic microwave engineering procedure is first to calculate the microwave fields internal to the plasma and then calculate the internal power absorption given the externally appHed fields. The constitutive dielectric parameters are useful in such calculations. In the absence of d-c magnetic fields, the dielectric permittivity, S, of a plasma is given by equation 10 ... 

Various data sources (44) on plasma parameters can be used to calculate conditions for plasma excitation and resulting properties for microwave coupling. Interactions ia a d-c magnetic field are more compHcated and offer a rich array of means for microwave power transfer (45). The Hterature offers many data sources for dielectric or magnetic permittivities or permeabiHty of materials (30,31,46). Because these properties vary considerably with frequency and temperature, available experimental data are iasufficient to satisfy all proposed appHcations. In these cases, available theories can be appHed or the dielectric parameters can be determined experimentally (47). 

The most dramatic evolution of a microwave power source is that of the cooker magnetron for microwave ovens (48). These magnetrons are air-cooled, weigh 1.2 kg, generate weU over 700 W at 2.45 GHz into a matched load, and exhibit a tube efficiency on the order of 70%. AppHcation is enhanced by the avaHabiHty of comparatively inexpensive microwave power and microwave oven hardware (53). The cost of these tubes has consistently dropped (11) since their introduction in the eady 1970s. As of this writing (ca 1995), cost is < 15/tube for large quantities. For small quantities the price is < 100/tube. 

Fig. 3. Basic elements of a microwave power system for processing of materials.

It is common to employ microwave power monitoring by means of a dual-directional coupler in the waveguide transmission system between the power tube and the useful load. Part of the coupled signals may be used for examination with spectmm analy2ers, frequency meters, and other microwave instmmentation for special purposes. Generally, this is not necessary in a practical appHcation. Many microwave measurement techniques have been described (59,60). AvailabiHty of components, plumbing, and instmmentation is weU described in trade journals. 

Food. The most successful appHcation of microwave power is that of food processing (qv), cooking, and reheating. The consumer industry surpasses all other microwave power appHcations. Essentially all microwave ovens operate at 2450 MH2 except for a few U.S. combination range models that operate at 915 MH2. The success of this appHance resulted from the development of low cost magnetrons producing over 700 W for oven powers of 500-800 W (Table 3). 

Microwaves are also used for the rapid inactivation of brain enzymes in rodents (160). Microwave power at high levels of kilowatts is appHed by means of a waveguide appHcator to achieve a rapid sacrifice of the rodent. 

E. C. Okress, Microwave Power Engineering, Vols. 1 and 2, Academic Press, New York, 1968. 

E. L. HoEman, Solid-State Microwave Power Oscillator Design, Artech House, Boston, Mass., 1992. 

Standards," Digest of the 1973 Microwave Power Symposium, IMPI, Manassas, Va., 1973. 

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