Abstract:
A microwave generator, which includes a housing having two mutually opposite electrodes which are separated via an electrode intermediate space which is filled with a dielectric, and with the electrodes having a spark gap between them, which breaks down in order to emit microwaves when a high voltage is applied. The electrode intermediate space ( 13, 13   a   , 13   b ) can be at least partially filled with a second dielectric ( 15, 15   a   , 15   b ), which has a different dielectric constant compared with that of the first dielectric ( 14, 14   a   , 14   b ) which is located between the electrodes ( 6, 7 ) in order to vary the microwave frequency, whereby the second dielectric ( 15, 15   a   , 15   b ) is held in a reservoir ( 17, 17   a   , 17   b ) which communicates with the electrode intermediate space ( 13, 13   a   , 13   b ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a microwave generator, comprising a housing having two mutually opposite electrodes which are separated via an electrode intermediate space which is filled with a dielectric, and with the electrodes having a spark gap between them, which breaks down in order to emit microwaves when a high voltage is applied. 
     2. Discussion of the Prior Art 
     The operation of a microwave generator such as this is based on the short-circuiting, and thus discharging, of a high-voltage store, for example a capacitor structure, which is charged via a high-voltage supply, for example in the form of a Marx generator, via a spark gap. The short circuit results in highly oscillating discharge or short-circuit currents with a steep flank, which contain a mixture of very high frequencies which, as a rule, are emitted via an antenna as microwave energy with a broadband spectrum that is dependent on the frequency mixture. This broadband microwave spectrum has a sufficiently high energy density that the radio traffic in the area surrounding a microwave generator such as this is at least adversely affected, and input circuits of electronic circuits can be subjected to interference, or even destroyed, as a result of resonance effects. 
     The emission of the microwaves is based on a discharge-dependent resonance affect. The triggering electrodes between which the spark gap is formed are associated with further resonating electrode or conductor structures in which the discharge current pulse that is produced by the flashover leads to powerful sudden oscillations whose frequency mixture is a resonant peak based on the instantaneous electrically effective geometry of the resonator or of the resonator structure. 
     In one known embodiment of a microwave generator of the type mentioned initially, the two triggering electrodes, which are arranged coaxially, of the spark gap are connected to large-area electrodes which are in the form of spherical caps and form the antenna, with their curvatures facing one another. The two flat electrodes or antenna electrodes enclose an intermediate space between them, which is bounded at the side by the generator housing and in which a dielectric, generally a gaseous or liquid dielectric, is provided. This dielectric governs the capacitance and thus the resonant frequency of the antenna electrode structure and, in this way, the emitted frequency or the frequency spectrum of the high-power microwaves. As a result of the capacitively invariant design of the resonator structure comprising the antenna electrodes, this frequency spectrum cannot be varied. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of specifying a microwave generator in which it is possible to vary the frequency spectrum of the emitted microwave pulses. 
     In order to solve this problem for a microwave generator of the type mentioned initially, the invention provides that the electrode intermediate space can be at least partially filled with a second dielectric, which has a different dielectric constant to that of the first dielectric which is located between the electrodes in order to vary the microwave frequency, which second dielectric is held in a reservoir which communicates with the electrode intermediate space. 
     In the microwave generator according to the invention, the effective capacitance between the electrodes which form the antenna resonator can be varied by deliberately changing the dielectric, which influences the capacitance, in the electrode intermediate space, at the same time allowing the resonant frequency and, via this in the end, the microwave frequency, to be varied and set in order to be matched as required to the characteristics of the object which is intended to be subjected to interference by or to be destroyed by the microwaves. For this purpose, a reservoir is provided on the generator side, with a second dielectric which has a different dielectric constant to that of the dielectric which was originally located in the electrode intermediate space. By way of example, a gaseous dielectric, for example SF 6 , is initially located in the electrode intermediate space, with water being located in the reservoir as the second dielectric. This water can now be fed when required out of the reservoir into the electrode intermediate space via a connecting line which leads into it, so that this electrode intermediate space can be partially or completely filled with the substitute dielectric in which case, of course, any filling level between 0 and 100% can be selected by variation of the capacitance as a function of the amount, and thus variation of the emission response. The microwave generator according to the invention thus allows continuous frequency mixing between two end states which are respectively defined by approximately 100% filling with one dielectric or the other. 
     In addition to the reservoir for the second dielectric, one development of the invention also provides a further reservoir for the first dielectric, so that the first and the second dielectrics can be fed reversibly into the electrode intermediate space, that is to say one or the other can be pumped into the electrode intermediate space as required, replacing one another. 
     Repeated variation with multiple use of the microwave generator is thus possible without any problems. 
     Finally, the electrodes may have any desired shape, for example being in the form of flat disks or being hollow-cylindrical. However, it is expedient for them to be in the form of spherical caps and to be connected to the housing at the edge, bounding the electrode intermediate space. This makes it possible to form a compact resonator which nevertheless has a resonance behavior which can be used for this purpose. 
     One particularly advantageous refinement of the invention provides that a flexible stretchable membrane, which bounds the electrode intermediate space towards the housing, is provided and stretches into the electrode intermediate space when pressure is applied. This membrane which, if the resonator is coaxial and rotationally symmetrical, is in the form of an annular collar, on the one hand ensures that it can stretch such that the electrode intermediate space can be approximately completely filled with the second dielectric, with the membrane being sufficiently flexible that it can be stretched into all of the areas and corners of the electrode intermediate space, provided that the pressure used to deform it is sufficiently high. Furthermore, this ensures that, when the second dielectric, for example water, is extracted, it is also fed completely out of the electrode intermediate space, without the electrodes themselves being wetted, and with the membrane relaxing into its initial position, in which it makes close contact with the housing wall. 
     Although there are various design options with regard to the resonator, that is to say with regard to the antenna electrodes and their geometry, one advantageous refinement, as described, provides that the membrane runs in an annular shape along the housing inner wall and, when pressure is applied, is stretched uniformly into the electrode intermediate space towards the encapsulated, centrally arranged spark gap. This ensures that, when pressure (pressure or suction) is applied, that is to say when the aim is to supply the second dielectric, the membrane is stretched symmetrically over the circumference from the outside inwards, that is to say this results in a uniform change at all points. This ensures a homogenous capacitance change over the entire electrode or resonator area, which results in a homogenous variation in the resonant frequency, and thus in the microwave frequency. 
     In order to allow the first dielectric, for example an insulating gas, to be extracted from the electrode intermediate space, it is expedient to provide at least one opening for this purpose on one electrode for extraction and—if reversible supply is intended—for supply of the insulating gas. This opening is expediently located in an electrode area which, by virtue of the design, is only the last to be covered by the membrane, if at all, and which may also be sealed. 
     In order to design the microwave generator to be as compact as possible, it is advantageous to provide the reservoir for the second dielectric immediately behind one of the electrodes, bounded by it. This means that the coaxial design of the generator is also continued in the arrangement of this resonator. In this case, in order to simplify the feeding process, it is also possible to provide a membrane which bounds the reservoir, reduces the size of the reservoir when pressure is applied, and in the process forces the second dielectric out and into the electrode intermediate space, pressing it into and against the membrane which is located there. The membranes themselves are expediently composed of a flexible plastic which can stretch, particularly rubber. 
     Various feed options are possible for the replacement of the dielectric. On the one hand, it is possible for the second dielectric to be fed into the electrode intermediate space by production of a reduced pressure in the electrode intermediate space, by extraction of the first dielectric by means of a pump device. By way of example, the insulating gas located therein is thus sucked out, as a result of which the membrane is drawn by the existence of the reduced pressure into the electrode intermediate space, and with it the second dielectric which is located in the reservoir, possibly assisted by the further membrane, which was stressed when the reservoir was full but has now relaxed. Alternatively, it is also feasible for a pump pressure to be applied to the further membrane from the outside, and thus for the second dielectric to be pressed against the first membrane and thus into the electrode intermediate space, so that the first dielectric located therein is forced out of the electrode intermediate space, possibly controlled by a valve or the like. It is, of course, also feasible for both processes to be actuated simultaneously, that is to say to use the pump device on the one hand for suction and on the other hand to actively reduce the size of the reservoir. In this case, it is feasible to use the first dielectric, which has been pumped around by the pump device and may likewise be connected to a reservoir, to apply pressure to the further membrane in the reservoir. 
     As described, the microwave generator according to the invention allows variation of the capacitance, and as a consequence of this, of the resonant frequency and emission frequency, resulting from replacement of the dielectric. In this case, it is possible to feed the second dielectric into the electrode intermediate space while microwaves are being produced, that is to say it is possible to actively tune the frequency during microwave production operation. The broadband frequency spectrum is thus varied from one microwave pulse to the next. In this case, it is possible to control the supply of the second dielectric as a function of a comparison of a nominal frequency (or corresponding frequency spectra) with the actual frequency which is measured continuously while microwaves are being produced. For example, if a specific frequency spectrum is required for the object that is to be subjected to interference, then the generator can be controlled such that—starting from the basic state—the production of microwaves is initiated, but the replacement of the dielectric also takes place at the same time, to be precise until the desired nominal spectrum has been set. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features and details of the invention will become evident from the exemplary embodiments described in the following text and from the drawings, in which: 
         FIG. 1  shows an outline illustration of a first embodiment of a microwave generator according to the invention, in the form of a section, 
         FIG. 2  shows the microwave generator from  FIG. 1 , after replacement of the dielectric, 
         FIG. 3  shows an outline illustration of a second embodiment of a microwave generator according to the invention, and 
         FIG. 4  shows an outline illustration of a third embodiment of a microwave generator according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a microwave generator  1  according to the invention in the form of an outline illustration. This comprises an insulating housing  2 , which is closed at both ends by closure covers  3 ,  4 . A resonator  5  is formed in the interior, comprising two electrodes  6 ,  7  which are arranged opposite one another and in the illustrated exemplary embodiment are in the form of spherical caps. A spark gap  8  is provided centrally, comprising two triggering electrodes  9 ,  10  which are separated from one another and are accommodated encapsulated in a housing  11  which is filled with a gaseous or liquid dielectric. In order to produce microwaves, a high-voltage generator  12 , for example a generator which operates on the basis of the principle of a Marx surge-voltage circuit, produces a voltage which, in the illustrated example, is applied to the electrode  7 . The two electrodes  6 ,  7  form a capacitor structure which can be charged via the high-voltage source  12 . When the charging voltage on the electrode  7  reaches the breakdown voltage of the spark gap  8 , this results in the ignition of an arc, that is to say in a short circuit between the two electrodes  6 ,  7 , as a result of which radio-frequency discharge currents oscillate in the electrodes  6 ,  7 , thus resulting in a resonant response, and also in the emission of high-energy microwaves via the two electrodes  6 ,  7 , which act as antenna electrodes. The capacitance of the capacitor arrangement comprising the two electrodes  6 ,  7  is governed essentially by the dielectric  14  which is located in the electrode intermediate space  13 . The illustrated exemplary embodiment is based on the assumption that the first dielectric  14  is a gaseous dielectric, for example SF 6 . 
     In order now to provide the capability to vary the resonant behavior and at the same time the frequency of the emitted microwaves, it is possible in the case of the microwave generator  1  according to the invention to replace the first dielectric  14  by a second dielectric  15  with a different dielectric constant. By way of example, the second dielectric is assumed to be water, with a high dielectric constant of ∈ r =80. During pulsed operation, water has a very high withstand voltage. If water is arranged in the electrode intermediate space  13 , then the capacitance is changed and, if it is filled completely, this results in the maximum capacitance and thus the minimum resonant frequency of the antenna structure, comprising the electrodes  6 ,  7  in comparison to the minimum capacity when the electrode intermediate space  13  is completely filled with gas, with this being associated with the highest resonant frequency, with the dielectric constant of gases being ∈ r ≈1. 
     In order to fill the electric intermediate space  13  with a different dielectric  15 , specifically with the second dielectric  15 , the electrode intermediate space is clad on the outside with a flexible membrane  16 , preferably a rubber membrane which is in the form of a ring or collar and closes the coaxial electrode arrangement off from the housing  2 . A connecting line  18  leads from the reservoir  17 , in which the second dielectric is stored, through the antenna housing  2  behind the membrane  16 . The reservoir  17  is itself closed in a sealed manner on one side by the electrode  6 , and on the other side by a further flexible membrane  19 . 
     An opening  20  is provided in the area of the electrode  7  for extraction or supply of the first dielectric, which is located in the electrode intermediate space  13 , to which opening  20 , in the illustrated exemplary embodiment, a pump device P is connected, whose operation is controlled by a control device  21 , and which is connected, for example, to a reservoir  22  for the first dielectric  14 . 
     If it is now intended to replace the dielectric, the first dielectric  14  is sucked out and is fed into the reservoir  22  via the pump device P (which may be any desired pump). This results in a reduced pressure in the electrode intermediate space, which sucks the flexible membrane  16  inwards. This takes place uniformly over the circumference of the electrode intermediate space  13 , because the design is rotationally symmetrical. At the same time, as the membrane  16  is drawn inwards, this results in a reduced pressure towards the reservoir  17 , and this leads to the second dielectric  15  being sucked out via the connecting line  18  and being distributed circumferentially homogenously in the intermediate space between the membrane  16  and the inner surface  23  of the housing  2 . While  FIG. 1  shows the state in which the electrode area  13  between the electrodes  6 ,  7 , which form a dipole, is completely filled with the first dielectric  14 ,  FIG. 2  shows the situation in which the first dielectric  14  has been pumped out approximately completely, and the electrode intermediate space  13  is approximately completely filled with the second dielectric  15  (provided that cladding is possible in the area for encapsulation of the housing  11  over the membrane  16 ). As can be seen, the further membrane  19  is drawn into contact with the electrode  6 . In this case, the volume of the reservoir  17  corresponds essentially to, the volume of the electrode intermediate space  13 . 
     If it is now intended to resume the initial state, then the first dielectric  14  would be forced back via the pump device P from the reservoir  22  into the electrode intermediate space under the membrane  16 , which leads to this being forced back to its initial position, and in the second dielectric  15  being fed back into the reservoir  17  via the connecting line  18  (of which, of course, it is possible to provide a plurality distributed over the circumference) until the state illustrated in  FIG. 1  is resumed. In this case, in principle, it is possible to set any desired intermediate filling level between the end states shown in  FIGS. 1 and 2 . 
     As a consequence of the change in the dielectric located in the electrode intermediate space, this always results in a different “overall dielectric constant”, depending on the filling level with one dielectric or the other, and this governs the capacitance of the resonator structure  5 . Furthermore, as described, this makes it possible to select or tune the resonance behavior and also the frequency spectrum of the emitted high-power microwave pulses. 
       FIG. 3  shows a further embodiment of a microwave generator  1   a  according to the invention, whose design corresponds to the microwave generator  1  shown in  FIGS. 1 and 2 . However, in this case, the pump device P is connected not only to the opening  20   a  for suction of the first dielectric  14   a  which is located in the electrode intermediate space  13   a , but also via a supply line to the housing area  24   a  which is located behind the membrane  19   a  of the reservoir  17   a . When the pump is operated for suction of the first dielectric  14   a , corresponding first dielectric  14   a  is at the same time, for example, forced into the housing area  24   a  from the reservoir  22 . This results in it being actively extracted on one side, with a reduced pressure being produced, while the second dielectric  15   a  is actively forced out of the reservoir  17   a  on the other side. 
     Finally,  FIG. 4  shows a third embodiment of a microwave generator  1   b  according to the invention, whose design corresponds to that of the microwave generator  1  shown in  FIG. 1 . In this case, the pump device P communicates exclusively with the housing area  24   b  which is located behind the reservoir  17   b  and thus behind the further flexible membrane  19   b . If the first dielectric  14   b  located in the electrode intermediate space  13   b  is now intended to be replaced, a gas, for example, is forced via the pump into the housing area  24   b , so that the second dielectric  14   b  is forced out of the reservoir  17   b  via the connecting line  18   b  behind the membrane  16   b , so that it bulges in the electrode intermediate space  13   b . In this case as well, a control device  21   b  is provided, which controls pump operation, in the same way as the control device  21   a  in the embodiment of the microwave generator  1   a  as well. 
     The first dielectric  14   b  which is forced out of the electrode intermediate space  13   b  is held in a reservoir  26   b , which for example is likewise formed above a flexible membrane  25   b . In this case, the membrane  25   b  stretches in a similar manner to a balloon. If the aim is to resume the state shown in  FIG. 4 , the housing area  24   b  is evacuated via the pump device P, so that the membrane  19   b  is sucked outwards again, and the second dielectric  15   b  is sucked back at the same time. The first dielectric  14   b  is fed into the electrode intermediate space  13   b , assisted by the relaxation of the stretched membrane  25   b.    
     At this point, it should be noted that the respective control device  21 ,  21   a  or  21   b  may be designed such that the respective electrode intermediate space is filled during the operation of the respective microwave generator  1 ,  1   a ,  1   b , that is to say with frequency tuning also being carried out during operation. It is also feasible to select a nominal frequency or a nominal frequency spectrum on the respective control device, with the filling being regulated on this basis, that is to say the actual frequency spectrum is measured continuously and is compared with the nominal frequency spectrum, and the dielectric constant is varied by filling of the electrode intermediate space until the actual spectrum corresponds to the nominal spectrum.