TITLE: "MICROWAVE APPLICATOR"
Technical Field
This invention relates to a microwave applicator for applying microwave radiation to materials by passing the material through a microwave field. Backσround Art
The industrial use of microwave energy has significantly increased in recent times and several devices have been proposed to irradiate various materials. In many applications there is a requirement to substantially uniformly apply microwave energy as a form of heating on a continuous basis. Hitherto, the devices proposed for this application have suffered from several disadvantages.- The most common problem is lack of uniformity due to the multi-mode nature of the radiation used. A device has been proposed in U.S. patent 4,160,145 to uniformly apply microwave energy to the materials however, it suffers from the disadvantage but.also significantly radiating microwave energy in the surrounding area while supplying the microwaves to the material.
One important application of microwave energy is in the heating of sheet materials of a cementitious nature for the purpose of curing surface coatings. Microwave energy is particularly suited to this application since the temperature required to cure some coatings is sufficiently high to damage the substrate. It has been found that microwave energy can be used to cure the coatings without any damaging effect on the substrates. Additionally, the conventional drying apparatus used for continuous curing processes occupies an extremely large area and requires enormous quantities of energy. Disclosure of the Invention
It is an object of this invention to provide a microwave applicator which will overcome, or at least ameliorate, some of the above disadvantages.
Accordingly, this invention consists in a microwave applicator comprising a hollow rectangular applicator waveguide, two centrally disposed longitudinally extending slots respectively formed in the opposing major sides of said applicator waveguide to allow material to be passed through the applicator waveguide; and two antenna means respectively associated with said applicator waveguide and symmetrically disposed on opposite sides of said slots to supply in phase microwave radiation substantially uniformly along the length of said applicator waveguide and excite in said applicator waveguide odd order microwave propagation modes so that a surface current minimum substantially coincides with the centres of said slots.
Preferably, the applicator waveguide and antenna means are configured to excite substantially only the TE propagation mode in the applicator waveguide. For this purpose the dimension of the rectangular waveguide, in the direction of the minor transverse axis is preferably less than half of one wavelength of the microwave radiation in air.
The antenna means are preferably slotted hollow rectangular waveguides of the kind disclosed in United States patent 4,160,145. The antenna waveguides preferably share a common wall with the applicator waveguide, -the energy coupling tuned slots being formed in this wall. In a preferred form of the invention the antenna waveguides and applicator waveguide are formed by a removable septum fitted into a rectangular structure.
Preferably, the opening through the applicator waveguide is sealed from the remainder of the interior of the applicator waveguide by substantially microwave transparent sheet material such as teflon. This acts to prevent the material to be irradiated entering the waveguide or fouling on the edges of the slots. The sealing material is also effective in isolating the material to be irradiated from any arcing that may occur in the waveguides.
Microwave radiation is preferably supplied to the antenna waveguides by way of a power splitter and appropriate waveguides. Phase equalisation is preferably effected by respective path length adjustment, for example by the narrowing of the longer wave guide along an appropriate length for increasing the phase velocity by a predetermined amount along that length.
Preferably, one or more quarter wavelength chokes are provided adjacent the marginal edges of the slots outside the applicator waveguide to reduce radiation leakage. A water load is also preferably positioned adjacent the chokes to further reduce radiation.
The microwave applicator of this invention is suitable for use with microwave radiation of any frequency and in particular the commonly use 2450 Mhz and 915 Mhz frequency bands. It will be appreciated that the waveguides and other transmission components have to be dimensioned to suit the operating frequency. Brief Description of the Drawings
One embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figure 1 is a schematic partly cut away perspective view of a microwave applicator according to this invention; and
Figure 2 is a schematic perspective view showing part of the arrangement of figure 1 in greater detail and including representative graphs of field magnitude and surface current.
Figure 3 is a schematic partly cut away perspective view of a microwave applicator according to the invention including a narrowing of one of the feeder waveguides. Modes For Carrying Out The Invention
Referring to figure 1 and figure 2 the microwave applicator 1 of this invention comprises a two part waveguide housing 2 separated by passage 3. A main applicator waveguide 4 of hollow rectangular form is formed in two parts each respectively disposed in the two parts of the housing 2. Passage 3 passes through the centre of waveguide 4 by virtue of two centrally disposed and longitudinally extending slots (not shown in detail) respectively formed in the opposing sides of the waveguide. The interior of passage 3 is lined by sheet teflon barriers 5 which is substantially transparent to microwave radiation. These barriers seal the passage 3 from the interior of waveguide 4 and are provided mainly to prevent any material entering the waveguide or fouling the waveguide as it passes through passage 3. (Barriers 5 are not shown in Figure 2). Microwave radiation is supplied to waveguide 4 by means of two antenna waveguides 6 and 7 respectively disposed on opposite sides of passage 3 formed by the slots in the waveguide 4. The antenna waveguides share a common wall 8 with the applicator waveguide 4 which wall is formed in two parts. A series of tuned slots 9 (figure 2) spaced apart by three quarters of a wavelength are provided to couple the microwave radiation into waveguide 4. Wall 8 is removable to permit variation in the nature of the tuned slots 9 without reconstruction of the waveguides.
The slots are arranged in the same manner as described in U.S. 4,167,145 so as to provide a substantially uniform distribution of microwave energy along the length of waveguide 4. The waveguide antenna 6 and 7 are symmetrically disposed and the dimensions of the waveguide 4 are chosen to excite only odd order microwave propagation modes in the waveguide 4. For this purpose the dimension of the waveguide 4 in the direction of the minor transverse axis is less than one half of one wavelength or, for 2,450 Mhz radiation, about 60 mm. It is preferred that the configuration is such as to substantially excite only the TE mode by suitable choice of the waveguide dimensions. As a consequence of only the TE mode or other odd order propagation modes being excited the surface current in the waveguide 4 is at a minimum at the centre of the longitudinally extending slots. Thus the surface current at the slot edges is minimal and thus significant radiation leakage from the slots is avoided. Graphs of the E-field and the surface currents for the TE 10 mode form part of Figure 2. It will be apparent that the TE 10 mode the microwave field is also greatest at the centre of the slot which is desirable to ensure maximum irradiation of material passed through passage 3. Additionally, it will be apparent that the undesirable TE 20 mode which technically could be excited is suppressed by the symmetrical spacing of the tuned slots 9 with respect to the longitudinal axis of applicator waveguide 4.
Two one quarter wavelength deep slots 10 which act as chokes short-circuitry are provided adjacent each edge of the slots in the waveguide 4 to reduce radiation leakage. The chokes 10 extend right around the inside of passage 3 although this cannot be seen from Figure 1. It is preferred that a water load (not shown) is positioned adjacent each set of chokes 10 to further reduce radiation leakage. Plates 11 and 12 at each end of housing ports 2 maintain the spaced-apart relationship and plate 11 provides a short circuit to both the applicator waveguide 4 and antenna waveguides 6 and 7. Plate 12 provides a short circuit to applicator waveguide 4 but pierced by feeding waveguides 13 and 14 which couple to antenna waveguides 6 and 7 respectively.
Microwave radiation is provided by substantially conventional magnetron and launcher assembly 15 which provides microwave power at 2,450 Mhz and at a power of approximately 5 kW. The launcher (not shown) conforms to the European R26 waveguide type. Magnetron and launcher assembly 15 is connected to a three port 6kW rated R26 circulator 16 of a substantially conventional type. The return port of the circulator 16 is connected to a 6kW water load schematically shown at 17. The output port of the circulator 16 is directed via an R26 directional coupler 18 having a coupling coefficient of -30 dB to a four screw tuner 19. The directional coupler 18 provides for measurement of forward and reflected power in the microwave system by means of power metering terminations 20 and 21 which are connected to conventional 2,450 Mhz microwave power meters (not shown) . The four screw tuner 19 is of a substantially conventional type and is provided to tune the impedance of the circuit to minimise the amount of reflected power. It should be noted that the reflected power is preferably not however reduced to zero since too high a Q for the circuit can result in tuning mismatch due to small frequency variations in the microwave source. In practice the circuit is preferably tuned to have a forward to reflected power ratio of approximately 8 to 10:1. Four screw tuner 19 is connected by waveguide section 22 to a 3 dB power splitter 23 of known type from which feeding waveguides 13 and 14 extend. Feeding aveguides 13 and 14 are half height guides that is, they have a width equivalent to the R26 standard guide but half the height. A teflon block 24 is provided in waveguide 14 to provide for path length adjustment between the radiation travelling to antenna guide 6 and that travelling to antenna guide 7 so that the two- are in phase upon entering waveguides 6 and 7.
In use the microwave applicator is tuned as described above and the material to be irradiated is passed through passage 3 preferably by a conveyor system. The applicator of this invention is particularly suited to applying microwave energy to sheet material, for the curing of surface coatings. It will be apparent that the establishment of the TE 10 mode in the applicator guide 4 results in a maximum of microwave field at the centre of the slot so that the maximum microwave energy is transmitted to the material passing through the passage. The energy distribution in the longitudinal direction of waveguide 4 is substantially a standing wave pattern and preferably multiple applicators according to this invention are provided offset by one quarter of a wavelength to provide a uniform energy distribution.
The invention has significant advantage in that a controlled amount of substantially uniform heating power can be applied to material passing through the passage 3 and suitably spaced apart applicators can be used to selectively heat at the required rate. The size of the apparatus required for the application of a given amount of power is also considerably less than that associated with conventional drying tunnels. Additionally, the efficiency of a microwave generator for heating is significantly greater than conventional heating apparatus.
It will be appreciated that a conveyor belt of a suitable material such as teflon which is substantially microwave transparent can pass through the passage 3 to convey relatively small items or particulate materials. Furthermore, suitable control circuitry can be provided to monitor and adjust forward and reflected microwave power by adjustment of the magnetron output and the tuner. Additionally, suitable dielectric phase delay structures, for example teflon, can be provided to match the phase velocities of the microwaves in the antenna guides and applicator guide to improve coupling efficiency and uniformity of the microwave field in the applicator guide.
However, referring to figure 3 where corresponding features are denoted by corresponding reference numerals, an alternative phase matching method is illustrated, whereby a length of reduced width waveguide 25 is provided in waveguide 13. Waveguide 25 provides for both length adjustment between the radiation travelling to antenna guide 6 and that travelling to antenna guide 7 so that the two are in phase upon entering the respective waveguides.
It will be appreciated that the foregoing describes only two embodiments of this invention and modification can be made thereto without departing from the scope of the invention.