Microwave operating powers

As new applications are identified, improved devices are designed to meet the need. Microwave power tubes manufacturers continue to push the Hmits of frequency, operating power, and efficiency. Microwave technology is an evolving science. Figure 6.1 charts device type as a function of operating frequency and power output. 

Commercial twystrons operate in the S-band (2.6-3.95 GHz) and C-band (3.95 5.85 GHz), and are used for pulsed radar transmitter power amphfiers. The pulsed peak output ranges from 1 to 7 MW with a pulse width of 10 50 /xs. This results in an average output power in the range of 1 28 k W with efficiency of 30-35%. The tube requires a beam voltage of 80-117 kV and a beam current of 45 150 A. The microwave input power to drive the ampHfier twystron is in a range of 0.3-3.0 kW (liao, 1988). 

Figure 9.7 presents, schematically, a pilot vacuum microwave dryer (WZD4S-1, Nanjing, China) with a maximum nominal power of 4.2 kW and an operation frequency of 2450 MHz. The operation power is adjustable between 700 W and 4200 W at 700 W increments. The dimensions of the microwave cavity were 1050 mm, 1080 mm, 800 mm (width, depth and height, respectively). The minimal attainable pressure is 5 kPa. Six material plates in the cavity can move around at 1 rpm during operation. 

Some power tubes can be operated without the need for a protective ferrite isolator. One example is the cooker magnetron (700 W) used in modern microwave ovens (57). At higher power levels, such as 25 kW, it is more common to employ a protective ferrite device, particularly in the form of a circulator (58), as shown in Figure 3. This results in a power loss equivalent to a few percentage points in system efficiency. The ferrite circulator prevents reflected power from returning to the power tube and instead directs it into an auxiHary dummy load. The pulling of tube frequency is thus minimised. 

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). 

A classical setup for microwave conductivity measurements is based on the utilization of the waveguides. A simple installation consists of a microwave generator (typically a gun diode) which, when the Ka-band is used, can be operated in the frequency region of 28-40 Gc/s this is protected by an isolator against back-reflections from the rest of the microwave circuit. The microwave power is conducted by an attenuator across a circulator into the microwave conductor branch at the end of which the electrochemical cell is mounted. The microwave power reflected from the electrochemical sample is conducted via the circulator into the microwave detector. It typically consists of a diode that acts as an antenna, receiving the electrical alternating field, rectifying it, and con-... 

For example, when = 200 mW is attenuated to / ml = 20 mW, then the attenuator indicates an output power P = -10 dB (see also Table 2.2). And when / ml = 2 mW, then P = -20 dB. Every additional attenuation by a factor of 10 gives another -10 dB in P. This type of device is usable to 50-60 dB attenuation, which also corresponds to the lower power limit at which the spectrometer is still operable. Note that, in addition to the relative dB scale, microwave engineers also use an absolute dBm scale. The m in dBm is shorthand for milliwatt and defines the reference point of this scale ... 

A microwave power meter may perhaps be a handy gadget every now and then to check output levels of microwave sources. On the other hand, we have seen that power leveling is used to retain a constant output power over 10000 operating hours (many years), and if the unleveled power has dropped to below the level specification, then the source can be expected to leave for eternal hunting grounds any time now. 

On the other hand, samples can be irradiated at constant microwave power over a certain fixed period, for example at 100 W for 10 min. As there is no control over the resulting temperature or pressure, care has to be taken not to exceed the operational limits of the system and this type of program should only be used for well-known reactions with non-critical limits, or under open-vessel (reflux) conditions. Since in this method only the applied energy and not the resulting temperature is controlled, the quality of reaction control is often superior employing a temperature-controlled program. 

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