End Launched Microstrip Or Stripline to Waveguide Transition with Cavity Backed Slot Fed by T-Shaped Microstrip Line Or Stripline

Europaisches Patentamt 19 European Patent Office

Office europeen des brevets © Publication number : 0 684 658 A1

EUROPEAN PATENT APPLICATION

(2j) Application number : 95303420.4 (sT) Int. CI.6: H01P 5/107

(22) Date of filing : 23.05.95

@ Priority : 23.05.94 US 247732 (72) Inventor : Park, Pyong K. 5847 Colodny Drive Hills, CA 91301 @ Date of publication of application Agoura (US) 29.11.95 Bulletin 95/48 Inventor : Holzman, Eric L. 411 Eugenia Drive Medford, NJ 08055 (US) @ Designated Contracting States : CH DE FR GB LI (74) Representative : Jackson, Richard Eric et al Carpmaels & Ransford, (n) Applicant : Hughes Aircraft Company 43 Bloomsbury Square 7200 Hughes Terrace, London WC1A 2RA (GB) P.O. Box 80028 Los Angeles, California 90080-0028 (US)

(54) End launched microstrip or stripline to waveguide transition with cavity backed slot fed by T-shaped microstrip line or stripline.

(57) A low profile, compact microstrip-to- waveguide or stripline-to-waveguide transition. FIG. 1. The end of the waveguide is terminated in a cavity backed slot defined in a groundplane formed on a dielectric substrate. The slot is excited by a microstrip or stripline conductor defined on the opposite side of the substrate. The conductor is terminated in a T-shaped junction including two opposed arms extending along the slot, each having a length equal to one-quarter wavelength at the center frequency of operation. A cavity covers the substrate on the conductor side, and is sized so that no cavity modes resonate in the frequency band of operation. The transition is matched by appropriate selection of the length of the slot and the length and position of the microstrip.

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LU Jouve, 18, rue Saint-Denis, 75001 PARIS 1 EP 0 684 658 A1 2

TECHNICAL FIELD ation does not lend itself well to constructing compact layered phased arrays. This invention relates to transitions between a waveguide and a microstrip line or stripline. SUMMARY OF THE INVENTION 5 RELATED APPLICATION A compact microstrip-to-waveguide transition is described, and comprises terminating elements for This application is related to commonly assigned terminating an end of the waveguide. The terminating application serial number , elements comprise a dielectric substrate having op- filed , Attorney Docket Num- 10 posed first and second surfaces, wherein a layer of ber 92663, "END LAUNCHED MICROSTRIP OR conductive material defines a groundplane on a first STRIPLINE TO WAVEGUIDE TRANSITION WITH surface thereof facing the interior of the waveguide. CAVITY BACKED SLOT FED BY OFFSET MICRO- The conductive layer has an open slot defined therein STRIP LINE," by P.K. Park and E. Holzman. characterized by a slot centerline. A microstrip con- 15 ductor is defined on the second opposed surface, BACKGROUND OF THE INVENTION transverse to the slot. The microstrip conductor ter- minates in a T-shaped microstrip junction comprising Microstrip-to-waveguide transitions are needed first and second opposed arms, which extend from an often in microwave applications, e.g., radar seekers. end of the microstrip conductor parallel to the length Modern millimeter wave radars and phased arrays 20 of the slot. The arms have an effective microstrip have a need for a compact, easy to fabricate high per- electrical length substantially one-quarter wave- formance transition. Usually, the antenna and its feed length at a center frequency of operation of the tran- are built from rectangular waveguide, and the trans- sition. mitter and receiver circuitry employ planar transmis- A conductive cavity covers the microstrip con- sion lines such as microstrip line or stripline. The mi- 25 ductor side of the terminating elements, and is sized crostrip-to-waveguide transition plays a critical role in to prevent cavity modes from resonating in the fre- that it must smoothly (i.e., with minimal RF energy quency band of operation. loss) transfer the energy between the transmitter or The dimensions and placement of the slot and receiver and the antenna. Traditional microstrip-to- placement of the microstrip conductor are selected to waveguide transitions are bulky, and they require that 30 match the respective waveguide and microstrip char- the microstrip line directly couple with the waveguide acteristic impedances. For example, the slot width is the penetrating its broadwall; such transitions are not preferably at least one third the waveguide height. very compatible with the thin planar structures of The edge of the T-shaped microstrip junction is flush state-of-the art radars. with a longitudinal edge of the slot. The conventional microstrip-to-waveguide tran- 35 sition employs a microstrip probe, and is difficult to BRIEF DESCRIPTION OF THE DRAWING fabricate because the microstrip probe must be in- serted into the middle of the waveguide. A hole must These and other features and advantages of the be cut in the waveguide wall for the probe to pene- present invention will become more apparent from trate. A backshort must be positioned precisely be- 40 the following detailed description of an exemplary hind the probe, about one-quarter wavelength. Fabri- embodiment thereof, as illustrated in the accompany- cating the transition with the backshort placed accu- ing drawing, in which: rately is difficult. Furthermore, the transition does not FIG. 1 is a simplified isometric view of a T-shaped provide a hermetic seal, and it is difficult to separate microstrip-to-waveguide transition in accordance the waveguide structure which leads to the antenna 45 with this invention. and the microstrip. A separate set of flanges must be FIG. 2 is a schematic diagram illustrating the si- built into the antenna to allow separation of the anten- nusoidal electric field profile excited by the microstrip na and transmitter/receiver. line of the transition. Another type of transition is the end launched mi- FIG. 3 is a simplified isometric view of an exem- crostrip loop transition. This transition is difficult to 50 plary embodiment of the transition. fabricate because the end of the loop must be attach- FIG. 4 shows an exemplary waveguide to stri- ed Physically to the waveguide broadwall. It is difficult pline transition in accordance with the invention. to Position the substrate precisely and to hold it in FIG. 5 shows a simplified illustration of an air-to- place securely. There is no hermetic seal, and to sep- air missile having an RF processor including a transi- arate the waveguide and microstrip line requires 55 tion in accordance with the invention. breaking the microstrip line for this transition also. FIG. 6 shows a simplified RF processor of the Further, the substrate is aligned parallel to the wave- missile of FIG. 5. guide axis instead of perpendicular; such a configur- 2 3 EP 0 684 658 A1 4

DETAILED DESCRIPTION OF THE PREFERRED end of the arm transforms to an RF short circuit at the EMBODIMENT junction. Thus, maximum current flows at the junction of the T while no current flows at the end of each arm. This invention is a low profile, compact micro- This current amplitude profile over the length of the strip-to-waveguide transition which utilizes electro- 5 arms 58B, 58C of the T-shaped microstrip line junc- magnetic coupling instead of direct coupling. An ex- tion 58 excites a similar electric field profile in the slot emplary embodiment of a transition 50 for transition- 56. The invention employs electromagnetic coupling ing between a rectangular waveguide 52 and a micro- between the edge of the T and the edge of the slot. If strip line is shown in FIG. 1. The end 54 of the wave- the end of a rectangular or circular waveguide is guide 52 is terminated in a cavity backed slot 56 which 10 placed adjacent to the slot, as shown in FIG. 1 , the mi- is excited by a T-shaped microstrip line junction 58. crostrip energy will couple to the slot electric field and The slot 56 and microstrip line junction 58 and micro- into the waveguide. The transition 50 exploits this en- strip conductor 58Aare etched on the opposite sides ergy transfer property. of a dielectric substrate 62, fabricated of a dielectric The slot 56 also can couple the microstrip energy material such as quartz. Thus, in the conventional 15 to unwanted modes such as the parallel-plate and di- manner, the opposite sides of the substrate 62 are ini- electric surface wave modes; such energy would be tially covered with a thin film of conductive material wasted in that it does not couple to the waveguide and such as copper. Using conventional thin-film photoli- increases the transition energy loss. Moreover, in the thographic etching techniques, the dimensions of the event the transition is used in a larger, more complex slot and microstrip and their positions can be fabricat- 20 circuit employing a plurality of similar microstrip to ed precisely, easily and inexpensively. The slot 56 is waveguide transitions, there can be interference be- defined by removing the thin copper layer 64 within tween transitions. the slot outline. The layer 64 extends across the end To eliminate the coupling to these unwanted of the waveguide. To define the microstrip line junc- modes, a rectangular cavity 70 can be used to cover tion 58, the thin conductive layer is removed every- 25 the transition on the side of the microstrip line junc- where except for the material defining the microstrip tion 58. The cavity 70 is essentially a four sided elec- conductor. A backshort placed one-quarter wave- trically conductive enclosure, having a closed end length behind the microstrip line (required in conven- parallel to the substrate 62. The cavity 70 includes a tional transitions) is not required in this transition. small opening 72 defined about the microstrip trans- In this embodiment, the slot 56 is centered on the 30 mission line to permit the line 58A to exit the cavity end 54 of the waveguide 52, in that the center axis 68 without shorting to the cavity walls. If the opening of the slot is coincident with a center line parallel to the maintains a width equal to about three times the width long dimension of the waveguide end which places of the line, typically no capacitive loading will occur. the slot centered along the short dimension of the wa- Smaller openings may require use of known meas- veguide 52. The slot is also centered along the long 35 ures to adjust for the effects of the capacitance. The dimension of the waveguide. This placement will de- cavity dimensions must be chosen so that no cavity pend on the type of waveguide for which the particular modes resonate in the transition's frequency band of transition is designed. For example, the slot will be operation. The selection of cavity dimensions to ac- centered at the end of a circular waveguide. The mi- complish this function is well known in the art. crostrip conductor 58A is disposed transversely to the 40 To maximize the amount of energy transferred slot center axis 68. from the microstrip line junction 58 to the waveguide In the typical application, the substrate 62 com- 52, the transition 50 is matched by appropriate selec- prises a portion of a larger substrate, in turn compris- tion of the length and width of the slot, the length and ing a larger microwave circuit comprising a plurality of width of the arms 58B, 58C of the microstrip line junc- microstrip lines defined on the substrate, and with 45 tion 58, and the T penetration depth into the slot. The other waveguides having their own transition in the T penetration depth D (FIG. 2) measures the overlap same manner as illustrated for waveguide 52 and of the arms 58B, 58C over the slot 56. Typical wave- transition 50. guide characteristic impedances are of the order of When the microstrip conductor 58A is excited, 1 00 to 350 ohms depending on the waveguide height. currents flow in the microstrip line 58 and the ground so On the other hand, the characteristic impedance of plane 64 directly below it. If a slot is cut in the ground the microstrip line is usually 50 ohms for most appli- plane in the path of the microstrip line junction, e.g., cations. One way to match these impedances is to slot 56, the current is disturbed, and an electric field use quarter wavelength impedance transformers on is excited in the slot 56 having a magnitude distribut- either the microstrip side or the waveguide side or ed as shown by curve 76, as shown in FIG. 2. The in- 55 both. These transitions add length and complexity to put current flows into the two arms 58B and 58C of the transition. This invention eliminates the need for the microstrip line junction 58. Each arm is about one- these transformers by taking advantage of the natural quarter wavelength long, so an RF open-circuit at the transforming characteristics of the slot. 3 5 EP 0 684 658 A1 6

FIG. 2 shows the electric field profile 76 of the tion with a cavity backed slot. This transition is similar slot 56 when its length is resonant. The slot length is to the microstrip to waveguide transition 50 of FIG. 1 , resonant when the input impedance seen at the slot except that the stripline conductor 156 is sandwiched centerline 68 is pure real valued. This resonant be- between two layers of dielectric. As in the transition havior is well understood: the voltage profile along 5 50, a dielectric substrate 160 is disposed at the end the slot is sinusoidal, while the current remains con- 1 54 of the waveguide 1 52. The substrate surface fac- stant. Thus, the first step in the design of the transi- ing the interior of the waveguide is covered with a con- tion is to determine the resonant length of the slot 56 ductive layer 164, in which the slot 166 is defined by at the center frequency of operation. The impedance selectively removing the conductive layer within the of the slot measured at the slot centerline or at any 10 slot outlines. On the opposite surface 168 of the sub- multiple of a half wavelength from the centerline will strate 160, the stripline conductor 156 and T junction be purely real at the resonant length. Next, the length 1 70 is defined by selectively removing the conductive of each arm 58B, 58C is set to be roughly one-quarter layer covering the surface. In contrast to the wave- microstrip wavelength at the transition's center fre- guide to microstrip transition 50, the transition 150 in- quency of operation. The characteristic impedance of 15 eludes a layer of dielectric 162 adjacent the stripline each arm should be about 100 ohms since the junc- conductor surface 168 of the first substrate 160, so tion impedance of the microstrip line junction 58 is 50 that the surface 168 is sandwiched between the di- ohms. The slot width should be wide enough so that electric substrate 160 and the dielectric layer 162. there is no interaction between the far edge of the slot One particular application to which the invention and the microstrip line junction. It has been found that 20 can be put to use is in the RF processor of a missile, a width of at least one third of the waveguide height e.g., an air-to-air missile having a seeker head to is sufficient; making the slot 56 any wider has a neg- guide the missile to a target. One such missile 200 is ligible effect on the match. shown in simplified form in FIG. 5. The missile in- The penetration depth D of the arms 58B and cludes an antenna section 202, a transmitter section 58C over the slot is a very sensitive parameter. The 25 204, a receiver module 210 including an RF proces- match is very dependent on the fringing of a portion sor, and a seeker/servo section 206. The receiver of the slot electricf ield through the substrate 62 to the module is shown in further detail in FIG. 6, and in- microstrip T junction 58. As the penetration depth cludes a module chassis 212 which supports several changes, so do the fringing fields. The best results active devices including low noise amplifiers 214. have been achieved when the upper edge 58D of the 30 The module includes an LO input port 216 and a re- T junction 58 is nearly flush, i.e., within a few mils, ceive signal port 218. The LO and receive signals are with the lower edge 56A of the slot 56. delivered to the respective ports via waveguides (not The transition can be constructed without the shown) connected at the back side of the housing. A cavity 70 backing the slot, and it can still be matched quartz substrate (not shown) carries microstrip or to the waveguide and operate well. However, if the 35 stripline circuitry (not shown in FIG. 6) used to define transition is part of a more complex assembly includ- the waveguide to microstrip transition or waveguide ing a plurality of transitions, then energy from one to stripline transition in accordance with the inven- transition can interfere with energy from anothertran- tion. The cavity backing the transition is defined by sition. If, however, such isolation is not required in a sides of the chassis channel 217 and 219 and the particular application, the transition can omit the cav- 40 module cover 220. In this example, the microstrip or ity 70. stripline conductor leading away from the LO port 21 6 FIG. 3 is a simplified line drawing of an exem- is connected to a mixer/control circuit located in area plary embodiment of a Ka-band half-height- 222 of the chassis, and the microstrip or stripline con- waveguide-to-microstrip transition 100 in accordance ductor leading away from the receive signal port 218 with the invention. The waveguide 102 has a rectan- 45 is connected to the low noise amplifiers 214. The re- gular cross-sectional configuration which is 70 by 280 ceiver module 210 is sealed hermetically at the two mils. The quartz substrate 112 is 200 by 186 mils, input ports 216 and 218 by the quartz substrate cov- with a thickness of 10 mils. The slot 106 is centered ering the port openings and being sealed to the chas- within the end of the waveguide, and is 124 mils in sis around the perimeter of the openings. The partic- length by 30 mils in width. The microstrip conductor 50 ulars of the waveguide to microstrip or stripline tran- 1 08A is 21 .4 mils in width, and the microstrip line junc- sitions are as shown in FIG. 1 and FIG. 4. tion is 108 mils wide, with a width of 5 mils. The cavity Current trends in RF seeker design emphasize 1 20 has a depth of 60 mils. A channel 1 30 for the mi- the reduction of cost and volume while achieving high crostrip line is provided, which is 99 mils high, by 130 performance. For millimeter wave radars and phased mils deep, and 65 mils wide. 55 radars, the packaging of the seeker is a significant FIG. 4 shows a waveguide to stripline transition problem. In some cases, although the components 150 for transitioning between a rectangular wave- can be designed and built, they all cannot be placed guide 152 and a stripline, employing a stripline T junc- physically within the seeker envelope. To integrate 4 7 EP 0 684 658 A1 8 the antenna with the transmitter/receiver circuitry is ing in a T-shaped microstrip junction comprising a difficult task with conventional, bulky microstrip-to- first and second opposed arms, said arms ex- waveguide transitions. A typical active phased array tending from an end of said microstrip conductor can easily require hundreds of these transitions. This and along said slot, said arms having an effective invention provides tremendous cost savings and vol- 5 microstrip electrical length substantially one- ume reduction and can make presently unreal izeable quarter wavelength at a center frequency of op- radar designs feasible. eration of said transition. This invention provides a low profile end launched microstrip-to-waveguide transition which has the 2. The transition of Claim 1 wherein said waveguide following advantages compared to existing micro- 10 is characterized by a waveguide characteristic strip-to-waveguide transitions: impedance, said microstrip conductor, dielectric 1 . Amicrostrip line does not have to penetrate the substrate and ground-plane define a microstrip waveguide. transmission line characterized by a microstrip 2. A backshort does not have to be placed one- characteristic impedance, and wherein said di- quarter wavelength behind the microstrip line. 15 mensions and placement of said slot and place- 3. The transition is compact and easy to fabricate ment of said microstrip conductor are selected to from a single piece of dielectric substrate. match said impedances. 4. The transition is compatible with the planar structure of standard transmitter and receiver 3. The transition of Claim 2, wherein said slot has a modules used in phased arrays. 20 slot width dimension along a waveguide height 5. Often, to physically separate the antenna and dimension which is at least one third said wave- transmitter or receiver assemblies is necessary guide height dimension. for testing of the components. Performing this separation with conventional transitions usually 4. The transition of Claim 2 wherein said T-shaped requires that one break the microstrip line. This 25 microstrip junction comprises an edge which is transition provides a natural flat surface (the sub- essentially flush with a longitudinal edge of said strate 58 with the slot in FIG. 1) to easily separate slot. the assemblies without breaking any circuitry. 6. The transition substrate 62 or 160 automatical- 5. A microstrip-to-waveguide transition, compris- ly creates a hermetic seal for the transmitter and 30 ing: receiver assemblies, typically located on a micro- terminating means for terminating an end strip circuit board. In particular, the receiver cir- of said waveguide, said terminating means com- cuit typically has delicate wire bonding and active prising a dielectric substrate having opposed first semiconductor elements which need the protec- and second surfaces, wherein a layer of conductive hermetic seal against corrosion. 35 tive material is defined on said first opposed sur- It is understood that the above-described em- face thereof facing an interior region of said wa- bodiments are merely illustrative of the possible spe- veguide, said conductive layer having an open cific embodiments which may represent principles of slot defined therein characterized by a slot cen- the present invention. Other arrangements may read- terline, and a microstrip conductor defined on ily be devised in accordance with these principles by 40 said second opposed surface disposed trans- those skilled in the art without departing from the versely relative to said slot, said microstrip con- scope and spirit of the invention. ductor terminating in a T-shaped microstrip junction at said slot, said junction comprising first and second opposed arms extending transverse to Claims 45 said microstrip conductor and along said slot, said arms having an effective microstrip electri- 1. A compact microstrip to waveguide transition, cal length of substantially one-quarter wave- comprising terminating means for terminating an length at a transition center frequency of opera- end of said waveguide, said terminating means tion; and comprising a dielectric substrate having opposed so means for defining a conductive cavity be- first and second surfaces, wherein a layer of con- hind said second opposed surface, and wherein ductive material defines a groundplane on said dimensions of said cavity are such that no cavity first opposed surface thereof facing an interior re- modes resonate in a frequency band of operation gion of said waveguide, said conductive layer of said transition. having an open slot defined therein characterized 55 by a slot center-line, and a microstrip conductor 6. The transition of Claim 5 wherein said waveguide defined on said second opposed surface and is a rectangular waveguide, and said means for transverse to said slot, said conductor terminat- defining a conductive cavity defines a rectangu- 5 g EP 0 684 658 A1 10

lar cavity. slot.

7. The transition of Claim 6 wherein said waveguide 14. An airborne missile, comprising an RF processor is characterized by a waveguide characteristic section, said processor section including a micro- impedance, said microstrip conductor, dielectric 5 strip circuit, a port for coupling to a waveguide, substrate and ground-plane define a microstrip and a microstrip to waveguide transition dis- transmission line characterized by a microstrip posed at said port, said transition comprising ter- characteristic impedance, and wherein said di- minating means for terminating an end of said mensions and placement of said slot and place- waveguide, said terminating means comprising a ment of said microstrip conductor are selected to 10 dielectric substrate having opposed first and sec- match said impedances. ond surfaces, wherein a layer of conductive material defines a groundplane on said first opposed 8. The transition of Claim 6, wherein said slot has a surface thereof facing an interior region of said slot width dimension along a waveguide height waveguide, said conductive layer having an open dimension which is at least one third said wave- 15 slot defined therein characterized by a slot cen- guide height dimension. terline, and a microstrip conductor defined on said second opposed surface and transverse to 9. The transition of Claim 6 wherein said T-shaped said slot, said conductor terminating in a T-shap- microstrip junction comprises an edge which is ed microstrip junction comprising first and sec- essentially flush with a longitudinal edge of said 20 ond opposed arms, said arms extending from an slot. end of said microstrip conductor and along said slot, said arms having an effective microstrip 10. A low profile, compact stripline to waveguide electrical length substantially one-quarter wave- transition, employing electromagnetic coupling, length at a center frequency of operation of said comprising terminating means for terminating an 25 transition. end of said waveguide, said terminating means comprising a dielectric substrate having opposed 15. The missile of Claim 14 wherein said waveguide first and second surfaces, wherein a layer of con- is characterized by a waveguide characteristic ductive material defines a groundplane on said impedance, said microstrip, dielectric substrate first opposed surface thereof facing an interior re- 30 and groundplane define a microstrip transmis- gion of said waveguide, said conductive layer sion line characterized by a microstrip character- having an open slot defined therein characterized istic impedance, and wherein said dimensions by a slot centerline, and a stripline conductor de- and placement of said slot and placement of said fined on said second opposed surface disposed microstrip conductor are selected to match said transversely relative to said slot, said stripline 35 impedances. conductor terminating in a stripline T junction comprising first and second opposed arms ex- 16. The missile of Claim 14 wherein said slot has a tending from an end of said stripline conductor slot width dimension along a waveguide height along said slot. dimension which is at least one third said wave- 40 guide height dimension. 11. The transition of Claim 10 wherein said waveguide is characterized by a waveguide character- 17. The missile of Claim 14 wherein said T-shaped istic impedance, said stripline conductor, dielec- microstrip junction comprises an edge which lies tric substrate and ground-plane comprise a stri- slightly inside a longitudinal perimeter edge of pline characterized by a stripline characteristic 45 said slot. impedance, and wherein said dimensions and placement of said slot and placement of said stri- 18. The missile of Claim 14 wherein said transition pline conductor are selected to match said impe- further comprises means for defining a conduc- dances. tive cavity behind said second substrate side, 50 and wherein dimensions of said cavity are such 12. The transition of Claim 11, wherein said slot has that no cavity modes resonate in a frequency a slot width dimension along a waveguide height band of operation of said transition. dimension which is at least one third said waveguide height dimension. 55 13. The transition of Claim 11 wherein said T-shaped microstrip junction comprises an edge which is essentially flush with a longitudinal edge of said 6 EP 0 684 658 A1

MICROSTRIP CURRENT

FIG. 2. EP 0 684 658 A1

FIG. 3.

FIG. 4.

164

152

172

\ \ \ V 156 s >0 s

8 EP 0 684 658 A1

FIG. 5.

9 EP 0 684 658 A1

European ratent EUROPEAN Application Number Office SEARCH REPORT EP 95 30 3420

DOCUMENTS CONSIDERED TO BE RELEVANT T — — category iraaon oi document witn indication, where appropriate, Relevant CLASSIFICATION OF THE | of relevant passages to daim APPLICATION (Int.Cl.6) JS-A-2 877 429 (SOMMERS ET AL.) 1-3,5-8, H01P5/107 10-12, 14-16,18 the whole document *

PATENT ABSTRACTS OF JAPAN 1-3,5-8, vol. 6 no. 151 (E-124) ,11 August 1992 10-12, St JP-A-57 075002 (HITACHI LTD) 11 May 14-16, 1£ 1982, * abstract *

IEEE TRANSACTIONS ON MICROWAVE THEORY AND L.5,10, TECHNIQUES, L4 vol. 34, no. 3, March 1986 NEW YORK US, pages 321-327, DAS ET AL. 'Excitation of waveguide by stripline- and microstrip-1 ine-fed slots 1 * figure 1 *

JS-A-5 198 786 (RUSSELL ET AL.) L4 * column 2, line 49 - line 51 * TECHNICAL FIELDS * column 3, line 49 - line 53 * SEARCHED ant.CI.6) H01P EP-A-0 384 777 (THE MARCONI C0MP. LTD.) L.5,10, L4 * column 2, line 45 - column 3, line 48; figures 1,2 *

ine present search report has been drawn up for all claims UUt 01 COajNtHM OT IK ItMXM THE HAGUE 14 August 1995 Den Otter, A T : theory or principle underlying the invention E : earlier patent document, but published on, or K : particularly relevant if taken alone after the filing date If : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons 4 : technological background D : non-written disclosure ft : member of the same patent family, corresponding P : intermediate document document