A DEVICE FOR PREVENTING LEAKAGE OF ELECTROMAGNETIC RADIATION

Abstract: The invention concerns a device for preventing leakage of electromagnetic radiation between a first (10) and a second (12) part. The first (10) and the second (12) parts are shaped to be arranged against each other such that a first gap (20) is formed between them. Furthermore, the first (10) and the second (12) parts are shaped to engage each other such that at least a second gap (30) is formed between them. The second gap (30) defines a second plane (32) which forms an angle (34) with the first plane (22) which is defined by the first gap (20). Furthermore, the second gap (30) has an extension such that it intersects the first plane (32). The second plane (32) also has an extension in a leakage direction (24) which is defined by the first gap (20).

Description: A device for preventing leakage of electromagnetic radiation BACKGROUND OF THE INVENTION AND PRIOR ART The present invention concerns a device for preventing leakage of electromagnetic radiation. Electromagnetic radiation may be disadvantageous in many ways, for example such radiation may disturb electronic equipment. There is therefore often a need to prevent electromagnetic radiation from leaking out from different apparatuses or products. Conversely, it is often important to prevent electromagnetic radiation from reaching into a device that may be disturbed. Shielding of electromagnetic radiation is thus applicable within a large number of different technical fields. As examples of such fields can be mentioned aviation, automobile, boat, train, buildings, mobile telecommunication, radiotransmission and in connection with other measurement technical or electronic apparatuses. It may thereby be particularly difficult to prevent leakage of electromagnetic radiation at joints between different parts. For example, joints in connection with covers or doors which can be opened, but also between permanently joined parts such as different plates or other parts which are installed to be permanently connected. Whenever such a joint is the case, there is a risk that electromagnetic radiation may leak. Such a joint is the case when two parts are abutting each other. A typical already known device is described below in the preamble of claim 1 , i. e. a device which comprises a first and a second part and which is arranged to prevent leakage of electromagnetic radiation at at least a joint between said first and second parts, wherein the first part comprises at least a first surface section and the second part comprises at least a second surface section, wherein the first and the second parts are shaped to be arranged against each other such that a first gap is formed between said first and second surface sections, wherein leakage of electromagnetic radiation through this first gap is possible essentially along a first plane and in essentially a first leakage direction along said first plane. In a device of this kind, there is thus a risk that electromagnetic radiation may leak primarily along said first leakage direction. In order to prevent such a leakage, according to the prior art, an electrically conductive seal is often arranged between the two parts. The two parts normally consist of electrically conductive materials. Since an electrically conductive seal is arranged in the joint between the parts, leakage of electromagnetic radiation may thus be prevented also in the joint. This known technology involves however different problems. It may be difficult to produce electrically conductive seals, which also prevent humidity from entering through the gap. Known seals consist for example of some rubber-like material with an electrically conductive material embedded in the rubber-like material. The embedding of the electrically conductive material may make the seal more fragile, which reduces the useful life of the seal. Furthermore, corrosion problems may occur in the joint, particularly where the electrically conductive seal abuts the surrounding electrically conductive parts. The problem of a limited durability, caused for example by galvanic corrosion, is particularly critical in exposed environments, such as outdoors and in a humid and warm environment. SUMMARY OF THE INVENTION The purpose with the present invention is to achieve an improved device for preventing leakage of electromagnetic radiation, which device does not suffer from the above mentioned problems. This purpose of the invention is achieved with the above described device according to the preamble of claim 1 , which is characterised in that said first part comprises at least a third surface section and said second part comprises at least a fourth surface section, wherein the first and second parts are shaped to engage each other such that at least a second gap is formed between said third and fourth surface sections, wherein the second gap defines a direction of flow essentially along a second plane, wherein said second plane has an extension in said first leakage direction and forms a first angle with said first plane and wherein said second gap has an extension such that it intersects said first plane. The present invention is based on a kind of polarisation phenomenon. The first gap tends to let through radiation with a first direction of polarisation. Since the second gap is arranged at an angle with the first gap, electromagnetic radiation, which is let through by the first gap, tends to be extinguished by the second gap. Thereby a good shielding of electromagnetic radiation is achieved without the necessity that the joint must include a conductive seal. It should be noted that the electrical and magnetical field components in the electromagnetic radiation may be considered as being perpendicular to each other. For the actual construction according to the invention, it is not necessary to discuss which of these components that is parallel or perpendicular to the respective gaps. It should be noted that it is of course advantageous if the gaps that are mentioned are as small as possible. The gaps may thus in principle be non-existent if the two parts abut each other perfectly. In a practical case there are however normally small gaps at the boundary surfaces between the two parts. According to a preferred embodiment of the invention, the first and the second parts comprise a plurality of said third and fourth surface sections such that a plurality of said second gap and thereby a plurality of said second plane are formed. By this construction, a further improved shielding of the electromagnetic radiation is obtained. According to a further embodiment of the invention, said plurality second plains are essentially parallel to each other. According to this construction, a mechanically and electromagnetically well functional construction is obtained. According to still another embodiment of the invention, each of said planes forms a second angle with at least one adjacent further second plane. The second planes are thus not completely parallel to each other. This construction has been shown to be advantageous when the first part is arranged against the second part. For example, when the joint is formed at a cover that may be opened, a door or similar device this embodiment has appeared to be advantageous. According to another embodiment of the invention, said first part comprises a plurality first protruding portions wherein two side surfaces of one such first protruding portion define two of said third surface sections which are positioned next to each other. Such protruding portions have been shown to further contribute to the prevention of leakage of electromagnetic radiation. According to still another embodiment of the invention, said second part comprises a plurality second protruding portions, wherein two side surfaces of one such second protruding portion define two of said fourth surface sections which are located next to each other. According to this embodiment, the advantage described in the previous paragraph is achieved. According to another embodiment of the invention, said first and second protruding portions are shaped to engage each other when the first part is arranged against the second part. Since the protruding portions engage each other a device which further prevents said leakage is obtained. According to another embodiment of the invention, the first and/or the second part comprises at least one recess which is arranged to receive said second and said first protruding portions, respectively. Since the protruding portions are arranged in such a recess, leakage is prevented even more efficiently. According to still another embodiment of the invention, the distance between two side surfaces that belong to the same protruding portion decreases in a direction from said first plane. This embodiment has as a consequence that the first and the second parts in a simple manner may engage each other. At the same time the gaps that arise are minimised. According to still another embodiment of the invention, said first and/or second protruding portions extend essentially orthogonally to said first plane. Thereby is prevented in an efficient manner that electromagnetic radiation which has leaked in through the first gap also can leak through the second gap and vice versa. According to still another embodiment of the invention, at least end portions of said first and/or second protruding portions are curved. This construction means that the first part in a simple manner may be arranged against the second part. According to a further embodiment of the invention, said second plane is essentially parallel to or forms a small third angle with said first leakage direction. With this construction a further improved device for preventing leakage is obtained. The device is particularly advantageous if at the same time the distance between said second planes are relatively small. According to still another embodiment of the invention, said first angle is between 45° and 135°, preferable between 70° and 1 10°, most preferred essentially 90°. This means that the second gap is essentially perpendicular to the first gap. This leads to an efficient extinction of electromagnetic radiation. According to still another embodiment of the device, the device comprises a sealing member that is arranged between said first and second parts. This sealing member may for example be a soft seal that does not need to be electrically conductive. Such a seal prevents for example humidity from entering through the joint. Since the seal does not have to be electrically conductive, the risk of corrosion is furthermore essentially reduced compared to prior known devices. According to a further embodiment of the invention, said first part constitutes a part of a larger apparatus, machine or other device and the second part constitutes a cover arranged to be provided in an opening in said apparatus, machine or other device. For example, such a cover may be arranged such that it may be opened in order to allow access to instruments or other apparatuses which are arranged in said apparatus, machine or other device. According to still another embodiment of the invention, said apparatus, machine or other device is an aircraft. An aircraft often comprises very sensitive apparatuses. It is therefore important that electromagnetic radiation is not allowed to disturb such apparatuses. SHORT DESCRIPTION OF THE DRAWINGS The present invention will now be explained by means of embodiments given as examples and with reference to the annexed drawings. Fig 1 shows schematically the principal of the present invention. Fig 2 shows a calculated relationship of the attenuation as a function of the frequency. Fig 3a and 3b shows schematically a front view and a view from the side of an embodiment of the invention. Fig 4, 5 and 7 shows schematically perspective views of embodiments of the invention. Fig 6 shows schematically the function of protruding portions which may be included in for example the embodiments according to Fig 4 and Fig 5. Fig 8 shows a perspective view of an example on a first and a second part according to the invention. Fig 9 shows a sectional view of the invention comprising a sealing member. DETAILED DESCRIPTIONS OF EMBODIMENTS OF THE INVENTION The invention will first be described with reference to Fig 1 , 3a and 3b. Fig 1 shows the principal of the invention. The device comprises a first part 10 and a second part 12. It should be noted that Fig 1 only is a schematic representation of the invention. The part marked with 10 to the left and to the right in Fig 1 thus constitutes in a practical case one integrated part. For the sake of simplicity this part is however in Fig 1 shown as if constituting to separate parts. The same is the case for the part 12 below in Fig 1 . Between the first 10 and the second 12 parts, a first gap 20 is formed. The gap is formed between a first surface section 16 (see Fig 3a) and a second surface section 18. The first gap 20 extends essentially along a first plane 22 (see Fig 3) and defines a first leakage direction 24 along the first plane 22. Expressed in other words: leakage of electromagnetic radiation is possible essentially along a first plane 22 and in essentially a first leakage direction 24. The axis 24 represents electromagnetic radiation that enters into the first gap 20 in said leakage direction 24. The electromagnetic radiation may be seen as being divided into two components, 21 , 23, which are perpendicular to each other. The component 21 is parallel to the first gap 20. The axis 24 and the component 21 thus define the first plane 22. The first gap 20 lets through primarily electromagnetic radiation (hereafter also EM-radiation) which oscillates in a plane which is parallel to the gap 20, i. e. which oscillates in the first plane 22. According to the invention, the first part 10 comprises at least a third surface section 26. The second part 12 comprises at least a fourth surface section 28. The first 10 and the second 12 parts are shaped to engage each other such that at least a second gap 30 is formed between the third 26 and the fourth 28 surface sections. The second gap 30 extends essentially along a second plane 32. In other words: the second gap 30 defines a flow path essentially along a second plane 32. The second plane 32 has an extension in the first leakage direction 24. The second plane 32 forms a first angle 34 with the first plane 32. Furthermore, the second gap 30 has an extension such that it intersects the first plane 22. Since the second plane 32, along which the second gap 30 extends, forms a first angle with the first plane 22, along which the first gap 20 extends, linearly polarised EM-radiation which has leaked in through the first gap 20 is at least partly blocked. It should in this context be noted that the first gap 20 does not necessarily have to be positioned upstream in the leakage direction 24 relative to the second gap 30. The positions of the two gaps 20 and 30 may thus be reversed. In the shown embodiment in Fig 1 and 3 the first 10 and the second parts 12 comprise a plurality of third 26 and fourth 28 surface sections. Between these surface sections 26, 28 a plurality of second gaps 30 are formed, wherein each of these gaps defines a second plane 32. This plurality of second planes 32 is essentially parallel to each other. The first part 10 comprises a plurality of first protruding portions 38. Two side surfaces 40 of such a first protruding portion 38 define two of said third surface sections 26. The second part 12 comprises a plurality second protruding portions 42. Two side surfaces 44 of such a second protruding portion 42 define two of the fourth surface sections 28. The first 38 and second 42 protruding portions are shaped such that they engage each other when the first part 10 is arranged against the second part 12 such as is shown in Fig 1 and 3. In the shown embodiment, the first part 10 and the second part 12 comprise a respective recess 46 which is arranged to receive the protruding portions 42, 38. In Fig 3a and 3b some dimensions of the device are given. S-i marks the width of the first gap 20. S 2 marks the distance between an end of a protruding portion 38, 42 and the bottom of the recess 46. S 3 marks the distance between two side surfaces 40, 44. H marks the height of a protruding portion 38, 42. W marks the width of a protruding portion 38, 42. d t marks the extension of the recess 46 in the first leakage direction 24. d marks the extension of the joint in the leakage direction 24. Fig 2 shows calculated relationships between transmitted energy divided by incident energy, i. e. Pt/Pi, as a function of the frequency f of the EM-radiation. In all cases, the calculation has been done with the electrical field component orthogonal to the extension of the first gap 20. Curve A concerns the case with only one straight gap. Curves B and C concerns the present invention according to Fig 3a and 3b. The calculated dimensions for the device concerning the curves A, B and C are given in the table below. As can be seen in Fig 2, with the device according to the invention, i. e. according to the curves B and C, an essentially improved attenuation is achieved than with one single straight joint. In order to obtain a suitable attenuation of the electromagnetic radiation, the device should be dimensioned such that W is essentially less than half the wave length of the EM-radiation which is to be shielded. That is, the higher the frequency of the electromagnetic radiation is, the smaller the dimensions of the device should be. For example, the width W may be less than 10 mm, less than 7.0 mm, less than 3.6 mm or less than 2.0 mm. The height H may for example be dimensioned in a corresponding manner. It can be seen in Fig 2 that resonance phenomena occur in the area where the distance W is in the order of magnitude of half the wavelength. It is of course no purpose in itself that gaps should be the case in the device. The distances S 1 τ S 2 and S 3 should therefore be as small as possible. Furthermore, it can be seen in Fig 2 that the device with somewhat larger dimensions, i. e. corresponding to curve B, gives a better attenuation than the device corresponding to curve C up to about 10 GHz. As had been said above, Fig 2 shows only the attenuation for one direction of polarisation of the incident radiation. For radiation with a direction of polarisation that is perpendicular to the one considered in Fig 2, the attenuation is essentially higher. It is thus the direction of polarisation represented in Fig 2 that is critical for the leakage of the electromagnetic radiation. In the remaining figures, the same reference signs as have been used in Fig 1 and 3 are used for the corresponding parts. It should be noted that highest up in Fig 4, 5 and 7 a portion of a first part 10 is shown separately, with the purpose of making clear how the first part 10 is constructed. Fig 4 shows an embodiment of the invention. The device comprises even here thus a first part 10 and a second part 12. The first part 10 and the second part 12 are shaped to be arranged against each other such that a first gap 20 is formed between a first 16 and a second 18 surface section (the first surface section 16 is hidden in Fig 4). The first part 10 comprises also in this case a plurality first protruding portions 38. Also the second part 12 comprises a plurality second protruding portions 42. The distance between two side surfaces 40, 44 that belong to the respective protruding portion 38, 42 decreases in the direction from the first plane 22. This is also clearly shown in Fig 6. This also means that a second plane 32, which is defined by the second gap 30 that is formed between the third 26 and the fourth 28 surface sections, forms a second angle 36 with an adjacent further second planes 32. The second angle 36 is shown is Fig 6. Since the second planes 32 are inclined in this manner, the first 10 and the second 12 parts may in an efficient manner engage each other such that the gaps 30 are minimised. The second angle 36 may for example be within the interval 5-45, 10-35 or 15-25 degrees. Such as has been shown in the embodiments in Fig 1 and Fig 3, the second angle may also be 0 degrees. Fig 6 shows in different steps how the first 10 and the second 12 parts engage each other when the two parts 10, 12 are moved towards each other. To the left in Fig 6, the first 10 and the second 12 parts thus completely abut each other. Since the protruding portions 38, 42 are wedge-shaped, the second gaps 30 are minimised. Leakage of EM-radiation is thereby prevented in an efficient manner. Fig 5 shows a construction that is similar to that in Fig 4. In order to further prevent leakage of EM-radiation, the first 10 and the second 12 parts comprise recesses 46 that are arranged to receive the first 38 and the second 42 protruding portions. In the embodiments in Fig 1 , 3, 4, 5 and 6, the protruding portions 38, 42 extend essentially orthogonally to the first plane 22. Fig 7 shows an embodiment where end portions 48 of the first 38 and second 42 protruding portions are curved This makes it easier to bring the two parts 10, 12 together. In Fig 7 the first gap 20 is positioned behind the second gaps 30 which are formed between side surfaces 40, 44 of the protruding portions 38, 42. In all the embodiments showed up to now, the second planes 32 are essentially parallel to the first leakage direction 24. The first angle 34, which the second plane 32 forms against the first plane 22, is according to the embodiments in Fig 1 and 3 essentially 90°. This first angle 34 is however somewhat different than 90° in the embodiments shown in Fig 4, 5, 6 and 7. Fig 9 shows a side sectional view of an embodiment of the invention which is somewhat similar to the embodiment according to Fig 7. In the embodiment according to Fig 9, the device comprises a sealing member 50 that is arranged between the first 10 and the second 12 parts. Such a sealing member 50 may for example prevent leakage of a liquid or a gas. In the embodiments described so far, it is primarily shown one single leakage direction 24 between the first 10 and the second 12 parts. However, often there are several such leakage directions between the first 10 and the second parts. For example, if one of the two parts 10, 12 constitutes a cover which is to be attached to a larger body, joints with different leakage directions are often formed. Fig 8 shows for example a rectangular first part 10 an a corresponding rectangular second part 12. The first part 10 may for example constitute the outer part of a cover, which is to be attached in a recess in a larger body. The second part 12 may thereby form a part of the larger body to which the cover is to be attached. In such a rectangular embodiment, there are four different leakage directions: one leakage direction corresponding to each side in the rectangular shape. Fig 8 thus shows an overview perspective of the design of a first 10 and a second part 12 that prevent leakage in all these four leakage directions by means of a device according to the invention. In the shown embodiments, there is only one row of said second gaps 30. In for example Fig 4, a number of protruding portions 38, 42 are shown between which a row of second gaps 30 (see Fig 6) is formed. Within the scope of the invention is also the possibility that several rows of second gaps 30 are arranged after each other. With reference to for example Fig 4, it would thus be possible to arrange a second row of protruding portions 38, 42 after the row shown in Fig 4 (i. e. further to the left in Fig 4). The distance between adjacent second gaps 30 in such a second row could thereby differ from the distance between adjacent second gaps 30 in the first row. In this way, the shielding of EM-radiation could be optimised for different frequencies in the different rows of second gaps 30, such that together a very good shielding is achieved. The first 10 and second 12 parts mentioned in the above described embodiments may either constitute one integrated part together with the larger bodies which are to be arranged relative to each other or these parts 10, 12 may be produced separately and attached in a reliable manner to the respective body which are to be joined together. A particular advantage with the invention is that an electrically conductive contact between the first 10 and the second 12 parts is not necessary according to the invention. The two parts 10, 12 may thus for example be painted or protected in another way without this fact influencing the properties of the device concerning shielding of EM-radiation. The two parts 10, 12, or the two bodies of which these parts 10, 12 constitute a part, should be formed in a materiel that does not allow EM-radiation of the frequency which is to be shielded to pass. Such a material is suitably electrically conductive. The two parts 10, 12 may for example be produced in the same conductive material, for example aluminium, which reduces the risk for corrosion in the joints compared to the case that they were produced in different materials. This means that the need for rust preventive treatment, such as for example through painting, is not so important. It should be noted that the distance between adjacent second planes 32 suitably, but not necessarily is constant. Furthermore, the second planes 32 do not need to have the same inclination 34. The present invention is not limited to the shown embodiments but may be varied and modified within the scope of the following claims.

Claims: Claims 1. A device comprising a first (10) and a second (12) part and arranged to prevent leakage of electromagnetic radiation (14) at at least a joint between said first (10) and second (12) parts, wherein the first part (10) comprises at least a first surface section (16) and the second part (12) comprises at least a second surface section (18), wherein the first (10) and the second (12) parts are shaped to be arranged against each other such that a first gap (20) is formed between said first (16) and second (18) surface sections, wherein leakage of electromagnetic radiation through this first gap (20) is possible essentially along a first plane (22) and in essentially a first leakage direction (24) along said first plane (22), characterised in that said first part (10) comprises at least a third surface section (26) and said second part (12) comprises at least a fourth surface section (28), wherein the first (10) and second (12) parts are shaped to engage each other such that at least a second gap (30) is formed between said third (26) and fourth (28) surface sections, wherein the second gap (30) defines a direction of flow essentially along a second plane (32), wherein said second plane (32) has an extension in said first leakage direction (24) and forms a first angle (34) with said first plane (22) and wherein said second gap (30) has an extension such that it intersects said first plane (22). 2. A device according to claim 1 , characterised in that the first (10) and second (12) parts comprise a plurality of said third (26) and fourth (28) surface sections such that a plurality of said second gap (30) and thereby a plurality of said second plane (32) are formed. 3. A device according to claim 2, characterised in that said plurality second planes (32) are essentially parallel to each other. 4. A device according to claim 2, characterised in that each of said second planes (32) forms a second angle (36) with at least an adjacent further second plane (32). 5. A device according to any one of the claims 2-4, characterised j_n that said first part (10) comprises a plurality first protruding portions (38), wherein two side surfaces (40) of one such first protruding portion (38) define two of said third surface sections (26) which are positioned next to each other. 6. A device according to any one of the claims 2-5, characterised in that said second part (12) comprises a plurality second protruding portions (42), wherein two side surfaces (44) of one such second protruding portion (42) define two of said fourth surface sections (28) which are located next to each other. 7. A device according to claim 5 and 6, characterised in that said first (38) and second (42) protruding portions are shaped to engage each other when the first part (10) is arranged against the second part (12). 8. A device according to any one of the claims 5-7, characterised jn that the first (10) and/or the second (12) part comprises at least one recess (46) which is arranged to receive said second (42) and said first (38) protruding portions, respectively. 9. A device according to any one of the claims 5-8, characterised [n that the distance (dS) between two side surfaces (40, 44) that belong to the same protruding portion (38, 42) decreases in a direction from said first plane (22). 10. A device according to any one of the claims 5-9, characterised in that said first (38) and/or second (42) protruding portions extend essentially orthogonally to said first plane (22). 1 1 . A device according to any one of the claims 5-9, characterised in that at least end portions (48) of said first (38) and/or second (42) protruding portions are curved. 12. A device according to any one of the preceding claims, characterised in that said second plane (32) is essentially parallel to or forms a small third angle with said first leakage direction (24). 13. A device according to any one of the preceding claims, characterised in that said first angle (34) is between 45 and 135 degrees, preferably between 70 and 1 10 degrees. 14. A device according to claim 13, characterised in that said first angle (34) is essentially 90 degrees. 15. A device according to any one of the preceding claims, characterised in that the device comprises a sealing member (50) which is arranged between said first (10) and second (12) parts. 16. A device according to any one of the preceding claims, characterised in that said first part (10) constitutes a part of a larger apparatus, machine or other device and the second part (12) constitutes a cover arranged to be provided in an opening in said apparatus, machine or other device. 17. A device according to claim 16, characterised in that said apparatus, machine or other device is an aircraft.

Inventors: BERGMAN LARS, BREVEMARK DANIEL, BERGMAN, LARS, BREVEMARK, DANIEL
Applicants: BERGMAN LARS, BREVEMARK DANIEL, SAAB AB, BERGMAN, LARS, BREVEMARK, DANIEL
Application year: 2000
Main classification: H05K9
IPC codes: H05K9