MICROWAVE SHIELDING

Abstract: Method of shielding against microwaves for mobile microwave appliances open at the bottom. The radiation path through the small gap between the equipment casing and the surface underneath is lengthened by edge widening. Microwave appliance to be used at the method.

Description: MICROWAVE SHIELDING Microwave technologies bring great advantages for heating of different kinds For that reason it is used to an increasing extent Microwaves are considered to cause danger for damages of many organisms including humans and other warm-blooded animals A fundamental condition at all uses of microwave technology is a reliable shielding against leakage of microwaves If microwaves are used in closed chambers such as ovens, etc the shielding is not a large problem Metals are good reflectors for microwaves and do not release radiation Even where a certain degree of ventilation is required shielding can with relative ease be ananged by perforated plates or nets of a suitable metal The apertures in the plate or net must be dimensioned not to let radiation through The situation becomes more complicated when microwaves are used for the treatment of objects that cannot be enclosed in an oven or similar construction Examples of such use can be found in for instance the Swedish patent application no 9400751-0, PCT/SE95/00218. PCT FI91/00330 and US 4,765,773 Ifthe equipment is to be mobile, an unavoidable air gap will arise between the treated surface and the equipment casing In many publications it is said just that this air gap should be small enough not to allow microwave passage However, information of what is meant is missing When shielding is proposed, it concerns metal brushes, chains, flexible shields of metallized textiles or plastic sheets etc It has been found difficult to get sufficiently reliable shielding, without damages of fnction against the surface or hooking up at it. in these ways Further these types of shielding may damage sensitive surfaces In many applications it is wanted that the shielding does not touch the surfaces treated According to US patent no 4,765,733 microwaves are used for high-speed drying of paint applied at road surfaces The shielding problem is solved by using a nonspecified microwave absorbing material This implies a relatively large distance between the microwave generator and the shielding material Otherwise, the material would be subjected to unacceptable heating According to the Swiss patent no 373491 microwaves are used for the drying of mateπal. for instance wood pieces, placed on a transport chain The drying equipment is designed as a closed box with openings at each end for in and out passage of chain and material As shielding at the openings sloping tlaps, opening in the movement direction, are used The In the figures 1 stands for the outer wall ofthe equipment casing, 2 for an. at the equipment casing applied, edge widening border, 3 for a metal net impervious to microwaves and 3 * for a drawn in border of this net 4 stands for a drawn-in mantle, 5 for an outer stπp of a suitable metal and 6 for the surface that the microwave appliance is working on In the figure 1 a so-called L-profile and in the figure 3 a corresponding bending has been used. However, nothing prevents those other profile forms (for instance an u-profile with the opening downwards) are used ifthe aim to lengthen the radiation path through the gap is fulfilled. The widening can be done by giving the bottom edge a border of a suitable metal for instance aluminium, which is a very good microwave reflector. Altemaiively the plate casing may be bent at the lower edge to get the widening. A similar effect is obtained ifthe plate casing is bent to give a drawn- or drawn-out mantle completed with an outer respectively an inner metal stπp at the bottom. For wheel provided appliances working near walls and similar limitations such designs may be interesting In the figures an inward edge widening is shown. This is from practical reasons the usual design. However, the effect will be the same ifthe edge widening is made outwards with respect to the equipment casing. The same applies to the designs with bending ofthe outer plate. appliances often consist of parallelepipedic plate boxes with one or more magnetrons placed at the top of the box The radiation source may be considered point-shaped and spreading radiation uniformlv in all directions except upwards The plate mateπal reflects the radiation The consequence is that one gets a system of standing waves in the enclosed chamber In a box open at the bottom and with a gap between the lower edge ofthe box and the surface underneath part ofthe reflected waves will hit the gap directly and after that be partly reflected bv the surface just outside the box Part ofthe waves will have an angle of incidence against the surface that approaches the angle of total reflection and be whollv or partly reflected even ifthe surface contains dielectric substances and is microwave absorbing Ifthe reflection occurs near the edge these waves mav, after the reflection, have such direction that they can pass through the gap and propagate themselves outside the box From geometric reasons it is excluded that the waves become completely parallel to the surface The small gap gives, thanks to the edge widening, a longer path for the wave and increases the probability that the spreading will be prevented by reflection against the metal surface and absoφtion in the surface underneath An, inwards or outwards, drawn-out mantle combined with a suitably designed metal stπp decreases in a conesponding way the probability for reflection that propagates radiation outside the equipment casing Even a relatively small increase ofthe edge width decreases the radiation considerably but the best effect is obtained ifthe edge width is as large as or larger than the gap width For wheel provided microwave dryers, such as described in SE9400715-0, standing on rails consisting of plastic pipes of the type use for electπc installations this means an edge width of 10 to 35 mm At designs with inwards respectively outwards drawn out mantles the width may have to be increased still more to give room for the wheels Fig 1 shows a sketch of one side of a microwave device, which has been provide with an aluminium edge border at the bottom Fig 2 shows a corresponding sketch of a microwave appliance with a drawn-in mantle at the bottom and an outer metal stπp Fig 3 shows one side of a microwave appliance where the inner shielding net has been drawn down along the equipment casing and bent to get edge widening

Claims: Patent claims 1 Method of shielding against microwaves for mobile microwave appliances open at the bottom, especially for devices intended for the drying or heat treatment of surfaces limited by walls or similar boundaries where drying respectively heat treatment must be done close to walls or other similar bounding surfaces, characterized in that the radiation path through the small gap between the equipment casing 1 and the surface underneath 6 is lengthened by edge widening with the aid of a border 2, bending ofthe lower part ofthe equipment casing, a drawn-in or drawn-out mantle 4 completed with an outer respectively inner metal strip 5 or bending ofthe shielding net 3' 2 Mobile microwave appliance with reduced microwave leakage meant for the drying or heat treatment of floors, walls, ceilings etc where drying respectively treatment must be done close to bounding surfaces, characterized in that lower edge of the equipment casing at the inner or outer side is widened with the aid of a border 2. bending of lower edge of the equipment casing 1, a drawn-in or drawn-out mantle 4 completed with an outer respectively inner metal stπp 5, or by drawing down the inner shielding net 3 along the inner side of casing and bending 3'

Inventors: ZETTERGREN LIEF GOESTA, ZETTERGREN, LIEF, GOESTA
Applicants: ZETTERGREN LIEF GOESTA, ZETTERGREN, LIEF, GOESTA
Application year: 1996
Main classification: H05B6
IPC codes: H05B6