DESCRIPTION SPREAD-SPECTRUM HIGH-FREQUENCY HEATING DEVICE TECHNICAL FIELD [0001] The resent invention relates to an RF heating system for use in a microwave oven, for example. BACKGROUND ART [0002] A conventional RF heating system for use in a microwave oven includes a high power direct oscillator device called "magnetron", which uses a vacuum tube, and an antenna (or radiator) for radiating the electromagnetic wave that is generated by the magnetron inside the heating chamber. The frequencies of electromagnetic waves for such RF heating usually fall within an ISM band, and the oscillation frequency of the magnetron is ordinarily defined to be a predetermined value falling within the range of 2.40 GHz through 2.50 GHz. Actually, however, the oscillation frequency of the magnetron will fluctuate according to the voltage applied to the magnetron and the impedance inside the heating chamber. As a result, the spectrum of its oscillation will eventually cover almost the entire 100 MHz range from 2.40 GHz through 2.50 GHz. [0003] To overcome such a problem, a solid-state RF heating system, including an oscillator and a solid-state power amplifier in place of a magnetron, has been researched and developed. Such a system is now proposed because an RF semiconductor device of GaN or SiC (which will be referred to herein as a "semiconductor power amplifier") has become more and more popular these days. An RF heating system that uses such a semiconductor power amplifier receives an RF signal, supplied from an oscillator, amplified by the semiconductor power amplifier and has the electromagnetic waves radiated from a radiator into the heating chamber with a lot of power. [0004] A solid-state RF heating system can radiate electromagnetic waves with a line spectrum and with almost no noise components. In addition, by adjusting the settings of its oscillator, the radiation frequency of the line spectrum can be varied arbitrarily within the range of 2.40 GHz to 2.50 GHz. [0005] However, a semiconductor power amplifier would be easily damaged under the heat when exposed to an intense reflected wave, which is a serious problem that must be solved to actually use it in that field of applications. If such a semiconductor power amplifier is used in the field of telecommunications, electromagnetic waves are radiated into a free space, and therefore, there is little need to keep the semiconductor power amplifier from getting damaged by reflected waves. On the other hand, if such a semiconductor power amplifier is used in a closed environment where intense electromagnetic waves are radiated into the heating chamber of a microwave oven, for example, strong reflected waves are easily produced inside the heating chamber. For that reason, the semiconductor power amplifier must be shielded from such reflected waves in one way or another. [0006] Patent Document No. 1 discloses an example of an RF heating system with such a semiconductor power amplifier. As shown in FIGS. 9(a) and 9(c), such an RF heating system performs a monitor mode operation for measuring the intensity of a reflected wave with the intensity of a radiated wave (i.e., radiation power) kept low and with the frequency sweeping continuously right after the system has been turned ON. In the example illustrated in FIG. 9(c), the frequency is continuously varied in the monitor mode from 2.40 GHz through 2.50 GHz. When the intensity of the reflected wave produced in such a situation is sensed, it can be seen that the intensity of the reflected wave varies significantly according to the frequency of the electromagnetic wave as shown in FIG. 9(b). [0007] By performing such a monitor mode operation, a frequency that will lead to the lowest reflected wave intensity can be determined. After the oscillation frequency has been fixed at such a frequency that has been determined in this manner, the output (i.e., the radiated wave) is raised as shown in FIG. 9(a), thereby starting radiation for the purpose of heating. In the system disclosed in Patent Document No.-l, if the intensity of the reflected wave becomes equal to or higher than a predetermined value for some reason during the heating process, the output (i.e., the electromagnetic waves radiated) is decreased and the heating process is stopped in order to prevent the solid-state amplifier from getting damaged by the reflected wave. [0008] Such a solid-state RF heating system not only can perform an RF heating process at such a frequency at which the radiated waves are hardly reflected and the object can be heated with high absorption efficiency but also can keep the power amplifier from getting damaged by the reflected waves. [0009] On the other hand, Patent Document No. 2 discloses an RF heating system that detects impedance in the heating chamber and controls the oscillation frequency based on the result of the detection. By adjusting the oscillation frequency, the system tries to get impedance matching always done and get the object cooked evenly. [0010] Meanwhile, Patent Document No. 3 discloses a microwave processor that memorizes the relation between the reflection power and the frequency by making the microwave frequency sweep in the range of 2.4 MHz through 2.5 MHz and by detecting the reflected current. Such a microwave processor extracts a frequency that will lead to the lowest reflection power as a heating frequency by reference to the memorized relation between the reflection power and the frequency. CITATION LIST PATENT LITERATURE [0011] Patent Document No. 1: Japanese Patent Application Laid- Open Publication No. 2007-317458 Patent Document No. 2: Japanese Patent Application Laid- Open Publication No. 59-165399 Patent Document No. 3: Japanese Patent Application Laid- Open Publication No. 2008-34244 SUMMARY OF INVENTION TECHNICAL PROBLEM [0012] The RF heating system disclosed in Patent Document No. 1 needs to perform a preliminary monitor mode for measuring the reflected wave intensity with the frequency of the electromagnetic waves, which are radiated with low power, sweeping continuously. Also, if the object being heated in the heating chamber changed its condition during the heating process, then the frequency control could not keep up with the change, thus resulting in poor heating efficiency. On top of that, if the intensity of the reflected wave increased to reach a certain value, the heating process should be stopped to keep the semiconductor power amplifier from getting damaged, which is also a problem. Added to that, as the radiation frequency is fixed during the heating process, the distribution of the electromagnetic field will become steadily non-uniform inside the heating chamber, thus getting the object heated unevenly, which is another problem with that system. [0013] Furthermore, according to the conventional technique disclosed in Patent Document No. 1, if a number of radiation units, each including a power amplifier and a radiator, are provided for a single RF heating system, then those radiation units will radiate electromagnetic waves at the common frequency that has been determined in the monitor mode. That is why the reflected wave of the electromagnetic wave radiated from one radiation unit cannot be distinguished from the electromagnetic wave radiated from another radiation unit. As a result, a significant error could sometimes be caused in the monitored intensity of the reflected wave. [0014] Patent Document No. 2 does disclose an RF heating system that controls the oscillation frequency based on the impedance detected during the heating process but is silent about how to determine the best impedance. The system disclosed in Patent Document No. 2 seems to detect such a frequency that will get the impedance matching done while making the oscillation frequency sweep. If that is the case, however, Patent Document No. 2 will have the same problem as what has already been described for Patent Document No. 1. [0015] Likewise, the microwave processor disclosed in Patent Document No. 3 also needs to make the frequency sweep, and therefore, will cause a similar problem to what has already been described about Patent Documents Nos. 1 and 2. [0016] What is more, the reflection spectrum of the object being heated has a complicated profile and could have multiple ranges where the intensity of the reflected wave decreases locally according to the frequency. On top of that, the profile of the reflection spectrum could often change dynamically during the heating process. For these reasons, according to the conventional techniques disclosed in Patent Documents Nos. 1 and 2, even if such a frequency that will lead to a locally low reflected wave intensity could be detected temporarily but if the condition of the object being heated varied as the heating process advances, the frequency that will lead to the lowest reflection intensity could also change significantly. In such a situation, if the reflection spectrum of the object being heated changed, the best frequency could not be detected so as to keep up with that change. As a result, the solid-state amplifier could get damaged seriously by a reflected wave, of which the intensity has decreased steeply. [0017] It is therefore an object of the present invention to provide an RF heating system that can radiate electromagnetic waves with the reflection power kept low by preventing the solid-state amplifier from getting damaged by the reflected wave and that can control the radiation frequency adaptively according to a change that could occur with time in the condition of the object being heated. SOLUTION TO PROBLEM [0018] An RF heating system according to the present invention includes: a variable-frequency oscillator; a semiconductor power amplifier for amplifying the output of the variable-frequency oscillator; a radiator for radiating an electromagnetic wave for heating based on the output of the semiconductor power amplifier; a reflected wave monitoring circuit for detecting a reflected wave of the electromagnetic wave for heating; and a controller for controlling the oscillation frequency of the variable-frequency oscillator. The controller changes the oscillation frequencies of the variable-frequency oscillator discontinuously, thereby conducting a frequency-hopping spread-spectrum radiation. [0019] In one preferred embodiment, the controller determines the probability of generation of each said oscillation frequency by reference to a relation between the intensity of the reflected wave that has been detected by the reflected wave monitoring circuit and the oscillation frequency. [0020] In this particular preferred embodiment, the controller sets the probability of generation of the oscillation frequency in a frequency range in which the intensity of the reflected wave detected by the reflected wave monitoring circuit is relatively low higher than that of the oscillation frequency in a frequency range in which the intensity of the reflected wave is relatively high. [0021] In a specific preferred embodiment, in an initial stage of a process for heating an object with the electromagnetic wave for heating, the electromagnetic wave for heating is radiated, and the relation between the intensity of the reflected wave detected by the reflected wave monitoring circuit and the oscillation frequency is observed, with the output of the semiconductor power amplifier adjusted to a relatively low value. [0022] In this particular preferred embodiment, during the process for heating the object with the electromagnetic wave for heating, the controller updates the relation between the intensity of the reflected wave detected by the reflected wave monitoring circuit and the oscillation frequency, thereby changing the probabilities of generation of the oscillation frequency dynamically. [0023] In still another preferred embodiment, during the process for heating the object with the electromagnetic wave for heating, the controller changes the oscillation frequencies of the variable-frequency oscillator discontinuously between a number of frequencies including a frequency at which the intensity of the reflected wave becomes a local minimum and a frequency at which the intensity of the reflected wave does not become a local minimum. [0024] In yet another preferred embodiment, the RF heating system includes a heating chamber to be loaded with the object, and the controller changes the oscillation frequencies of the variable-frequency oscillator discontinuously between a number of frequencies that fall within the range of 2.40 GHz to 2.50 GHz. [0025] In an alternative preferred embodiment, the RF heating system includes a heating chamber to be loaded with the object, and the controller changes the oscillation frequencies of the variable-frequency oscillator discontinuously at an interval of at most 1.0 millisecond. [0026] In this particular preferred embodiment, the controller changes the oscillation frequencies of the variable-frequency oscillator discontinuously at an interval of at least 0.01 milliseconds. [0027] In a specific preferred embodiment, the semiconductor power amplifier is a GaN HFET. [0028] In yet another preferred embodiment, the oscillation frequencies are changed discontinuously following a frequency sequence that has been determined by the controller. The frequency sequence is determined by performing time-series filtering on a white random frequency series at a probability of generation associated with a normalized absorption rate, which is represented as a reverse of the relation between the intensity of the reflected wave and the oscillation frequency. [0029] Another RF heating system according to the present invention includes multiple radiation units, each radiating an electromagnetic wave for heating at variable frequencies. Each radiation unit includes: a variable-frequency oscillator; a semiconductor power amplifier for amplifying the output of the variable-frequency oscillator; a radiator for radiating an electromagnetic wave for heating based on the output of the semiconductor power amplifier; and a reflected wave monitoring circuit for detecting a reflected wave of the electromagnetic wave for heating. The RF heating system further includes a controller for controlling the oscillation frequencies of the respective variable-frequency oscillators included in those radiation units. The controller changes the oscillation frequencies of the respective variable-frequency oscillators discontinuously so that there is no correlation at all between the frequencies of the radiation units, thereby getting a frequency-hopping spread-spectrum radiation done independently of each other by the respective radiators included in those multiple radiation units. [0030] In one preferred embodiment, the controller determines the probability of generation of the oscillation frequency of the variable-frequency oscillator included in each radiation unit by reference to a relation between the intensity of the reflected wave that has been detected independently by the reflected wave monitoring circuit included in that radiation unit and the oscillation frequency. [0031] An RF heating system driving method according to the present invention is a method for driving an RF heating system that includes: a variable-frequency oscillator; a semiconductor power amplifier for amplifying the output of the variable-frequency oscillator; a radiator for radiating an electromagnetic wave for heating based on the output of the semiconductor power amplifier; a reflected wave monitoring circuit for detecting a reflected wave of the electromagnetic wave for heating; and a controller for controlling the oscillation frequency of the variable-frequency oscillator. The method includes the steps of: (A) radiating an electromagnetic wave for heating by the radiator while changing the oscillation frequencies of the variable-frequency oscillator discontinuously; (B) detecting the intensity of a reflected wave of the electromagnetic wave for heating by the reflected wave monitoring circuit and finding a relation between the intensity of the reflected wave and the oscillation frequency; and (C) locating a frequency range in which the reflected wave has a relatively low intensity by reference to the relation between the intensity of the reflected wave and the oscillation frequency that has been found in the previous step (B) . The step (A) includes setting the probability of generation of the oscillation frequency in a frequency range in which the reflected wave has a relatively low intensity higher than that of the oscillation frequency in a frequency range in which the reflected wave has a relatively high intensity. [0032] In one preferred embodiment, the step (C) includes determining an oscillation frequency, at which the reflected wave has the lowest intensity, by reference to the relation between the intensity of the reflected wave and the oscillation frequency that has been found in the step (B). ADVANTAGEOUS EFFECTS OF INVENTION [0033] According to the present invention, the frequency that will lead to the lowest intensity of the reflected wave (i.e., the reflection power) is detected by frequency hopping. Thus, there is no need to perform the monitor mode operation with low power, and therefore, the heating process can get done in a shorter time. Also, even if the condition of the object in the heating chamber changed with time, the heating process conditions can always be optimized by performing that frequency hopping during the heating process. That is to say, according to the present invention, the heating process is not performed continuously at a frequency at which the reflected wave has a local minimum intensity but hopping is done from such a frequency at which the reflected wave has a local minimum intensity to another frequency at which the reflected wave does not have a local minimum intensity in the middle of the heating process. [0034] By getting that frequency hopping done a number of times at short intervals, the electromagnetic wave is never radiated for a long time at such a frequency at which the reflected wave has a high intensity, thus preventing the semiconductor power amplifier from getting damaged by an intense reflected wave. On top of that, there is no need to provide an isolator anymore in order to protect the environment from the reflected waves. As a result, the size and cost of the heating system can be reduced. Added to that, since the distribution of the electromagnetic field in the heating chamber can be changed by that frequency hopping, the object can be heated uniformly even without stirring up radio waves or using a turntable. [0035] In an embodiment in which a single RF heating system includes multiple lines of power amplifiers and radiators, the radiation frequencies can be changed so as to have no correlation at all between those lines. As a result, the reflected waves of the respective lines can be monitored accurately. On top of that, the heating process can get done with reduced power dissipation, thus saving a lot of energy efficiently. [0036] What is more, the RF heating system of the present invention does not use any magnetron, and therefore, each radiation spectrum is narrow enough to be adjusted appropriately to its proper permissible radiation range. As a result, the electromagnetic compatibility (EMC) can be improved, and the overall cost can be cut down, too, because a number of parts that would otherwise be needed to improve the EMC can be omitted according to the present invention. BRIEF DESCRIPTION OF DRAWINGS [0037] FIG. 1 illustrates the principle of spread spectrum RF heating according to the present invention, wherein FIG. 1(a) illustrates how the radiated wave intensity (i.e., radiation power) varies with time while frequency hopping is carried out, FIG. 1(b) is a graph illustrating how the reflected wave intensity (i.e., reflected power) varies with time, and FIG. 1 (c) is a graph showing the frequency dependence of the reflected wave intensity. FIG. 2 is a block diagram illustrating a first specific preferred embodiment of an RF heating system according to the present invention. FIG. 3 illustrates an exemplary configuration for the reflected wave monitoring circuit of the first preferred embodiment. FIG. 4 illustrates an exemplary configuration for the controller of the first preferred embodiment. FIG. 5 is a flowchart showing an exemplary algorithm for determining a frequency sequence according to the present invention. FIG. 6 is a flowchart showing how a frequency series is calculated by time series filtering according to the present invention. FIGS. 7(a) through 7(d) illustrate how to get the time series filtering done. FIG. 8 is a block diagram illustrating a second specific preferred embodiment of an RF heating system according to the present invention. FIG. 9(a) is a graph showing how the intensity of a radiated electromagnetic wave (i.e., radiation power) varies with time in the monitor mode disclosed in Patent Document No. 1 and other documents, FIG. 9(b) is a graph showing how the reflected wave intensity varies with time, and FIG. 1(c) is a graph showing how the frequency varies with time. DESCRIPTION OF EMBODIMENTS [0038] An RF heating system according to the present invention changes the oscillation frequencies of a variable-frequency oscillator discontinuously within a particular range, thereby getting frequency-hopping spread-spectrum radiation done. Just like the '"spread spectrum" technology that is extensively used in the field of radio communications, the "frequency hopping spread spectrum" technology to be described herein is a technique for expanding (or spreading) an electromagnetic wave frequency range from a line spectrum into a range with a predetermined band width. [0039] According to the present invention, the frequency of the electromagnetic wave that irradiates the object to be heated is not fixed but changed one after another between multiple discrete values (i.e., discontinuous values) that fall within a particular frequency range. That is to say, the electromagnetic wave is radiated for just a short period of time at each of those discrete frequencies and hopping is done immediately to the next frequency. That is why even if the object to be heated were irradiated with a strong electromagnetic wave at a frequency associated with a high reflected wave intensity (i.e., a low absorption index), the frequencies would be changed to the next value before the solid-state power amplifier gets damaged by that reflected wave. As a result, it is possible to prevent the solid-state power amplifier from getting damaged by the reflected wave. [0040] Before preferred embodiments of the present invention are described, the fundamental principle of "spread spectrum heating" adopted in the present invention will be described. [0041] "Spread spectrum" is a technique that is well known in the field of radio communications and is roughly classified into a technique called "frequency hopping" and a technique called "direct sequence". Unlike radio communications for transmitting information in a free space, the RF heating system of the present invention radiates RF energy into a heating chamber that is a closed space and gets the energy absorbed into the object to be heated, thereby transforming the electromagnetic wave into heat. However, the RF energy is also radiated according to the present invention with its frequency spectrum spread over a particular range as in the spread spectrum technique in the field of radio communications, and therefore, the technique of the present invention will be referred to herein as "spread spectrum heating". [0042] As described above, a magnetron oscillates directly. When regarded as a sort of oscillator, a magnetron has a low Q value. That is why the frequency spectrum of RF waves radiated from a magnetron has a gently rising and falling broad distribution with a lot of noise components. Meanwhile, according to the present invention, as a variable-frequency oscillator and a semiconductor power amplifier are used, the oscillator has a high Q value. For that reason, the frequency spectrum of the output RF signal of the oscillator is a substantially "line spectrum", which has a sharp peak at an arbitrary frequency. The semiconductor power amplifier amplifies an RF signal with such a line spectrum, and a high power electromagnetic wave with a substantially line spectrum (and with a half width of 1 kHz, for example) can be radiated. If the oscillation frequency of the variable-frequency oscillator is varied within the range of 2.40 GHz through 2.50 GHz, for example, the frequency of the electromagnetic wave for heating that has been radiated by the radiator (i.e., radiation frequency) will also vary within the same range according to the oscillation frequency. [0043] According to the present invention, a number of candidate frequencies are set in advance within the particular frequency range in which the electromagnetic waves can be radiated. Specifically, n candidate frequencies Fl, F2, ... and Fn (where n is a natural number that is equal to or greater than three and FKF2, ...