Abstract: A microstrip to dielectric waveguide transition is provided comprising a gth of rectangular dielectric waveguide which has one end tapered in such a manner that the height of the waveguide top surface above the waveguide bottom surface decreases linearly from full height to zero height at the tapered end of the length of waveguide. The bottom surface of the waveguide length is mounted on the top surface of a planar microstrip dielectric substrate having an electrically conductive metallic ground plane on the bottom substrate surface and a length of microstrip conductor on the top substrate surface aligned with the waveguide length and abutting the tapered end of the waveguide length. A second length of microstrip conductor is mounted on the tapered portion and part of the untapered portion of the top surface of the waveguide length and is electrically connected to the first microstrip conductor at the tapered end of the waveguide length.

Abstract: Disclosed is a surface signal launcher for coupling RF signals between a coaxial cable in a single-wire surface wave transmission line. The signal launcher includes a shell-like, electrically conductive launcher horn that is installed at the juncture of the coaxial cable and the surface wave transmission line with the launcher horn concentrically surrunding the portion of the surface wave transmission line that is immediately adjacent the coaxial cable. The coaxial cable outer conductor is electrically connected to the forward end of the launcher horn with the center conductor of the coaxial cable being connected to one end of the surface wave transmission line. To prevent signal reflection at the interface between the coaxial cable and the launcher horn, the diameter of the launcher horn forward end is established to provide an impedance that is equal to the characteristic impedance of the coaxial cable.

Abstract: A very wide pass band polarization diplexer having a power divider, a transition element and a wave guide containing an impedance adapter, and applicable to all devices requiring the use of a wave guide in which two waves in orthogonal modes propagate.

Abstract: A radio frequency coaxial vacuum feedthrough is provided which utilizes a cylindrical ceramic vacuum break formed of an alumina ceramic. The cylinder is coaxially disposed and brazed between tapered coaxial conductors to form a vacuum sealed connection between a pressurized upstream coaxial transmission line and a utilization device located within a vacuum container. The feedthrough provides 50 ohm matched impedance RF feedthrough up to about 500 MHz at power levels in the multimegawatt range.

Abstract: An arrangement for producing a broadband junction between a microstrip line and a coplanar transmission line in the following called "twin band line" provides that the microstrip line and the twin band line (ZL) extend at right angles relative to one another. The ground electrode of the microstrip line and the ground electrode of the twin band line lie immediately one on top of the other. The strip-shaped other electrodes of the microstrip line and the narrower strip-shaped electrode of the asymmetrical twin band line which extends at right angles thereto and coplanar to the wider ground electrode of the twin band line are connected to one another in a broadband manner by one or more ribbons or wires of metal. The arrangement is advantageously employable as a fast integrated optical modulator with cutoff frequencies in the GHz range, whereby the spacing between the coplanar electrodes expediently constricts conically in the longitudinal direction thereof.

Abstract: A transmission line comprising a multi-layer dielectric slab structure including: a dielectric substrate layer (30) having a thickness d.sub.s and permittivity .epsilon..sub.s ; a conductive ground plane (31) on the bottom surface of the dielectric substrate layer (30); a dielectric guiding layer (32) having a thickness h and permittivity .epsilon..sub.g, where .epsilon..sub.g >.epsilon..sub.s, attached to the top surface of dielectric substrate layer (30); at least one elongated and relatively narrow dielectric loading strip layer (33) having a width W, thickness d.sub.1, and permittivity .epsilon..sub.1, where .epsilon..sub.g >.epsilon..sub.1, attached to the top surface of the dielectric guiding layer (32); and a conductive coating (34) on the top surface of the dielectric loading strip layer (32). Such a structure permits single mode propagation over a relatively wide band.

Abstract: A helix type traveling wave tube comprises an evacuated envelope containing an electron gun assembly at one end thereof for forming and projecting a beam of electrons over an elongated beam path to an electron collector electrode at the opposite end of the envelope. A helix slow-wave circuit is arranged along the beam path intermediate the electron gun and the beam collector for electromagnetic interaction with the beam. In order to extract an amplified signal from the downstream end of the slow wave circuit, there is provided an coaxial line including an outer conductor air-tightly fixed at one end thereof to the evacuated envelope and an inner conductor having one end connected to an output end of the slow wave circuit. An output waveguide is air-tightly fixed to the other end of the coaxial line outer conductor in such a manner the other end of the coaxial line inner conductor extending into the output waveguide.

Abstract: A compact adaptor is provided for making transition from ridged waveguide to rectangular waveguide while simultaneously imparting spatial reorientation of associated electric and magnetic fields. The adaptor is especially useful for feeding a phased array of radiating slots (or other structures) where adjacent radiating structures are preferably spaced from one another on the order of a half wavelength and fed with the magnetic field vectors oriented parallel to such inter-element array spacing dimensions. In the exemplary embodiment, a transition from double-ridged waveguide to rectangular waveguide is effected through an electrically short (e.g., 1/8th to 1/4th wavelength) non-resonant cavity using oppositely tapered continuations of the ridged waveguide walls (acting as a TEM parallel transmission line) to opposing walls of a rectangular waveguide port which is spatially oriented transverse to the ridged waveguide.

Abstract: A Super High Frequency (SHF) coaxial cable and connector assembly for connection of the coaxial cable thereof to a relatively low impedance circuit, such as a YIG filter for which the connection impedance is on the order of ten ohms, has a connector providing a port for receiving a conventional fifty ohm coaxial cable plug and includes: a ten ohm SHF coaxial cable having a metal ferrule soldered to the outer conductor at the connector end thereof providing a substantially larger diameter outer conductor at that end than the outer conductor of the ten ohm cable; and a coaxial connector having a coaxial recess in the housing thereof at one end to accommodate the cable ferrule and into which the ferrule is force-fit to make an intimate metal to metal contact therewith, while the ten ohm cable center conductor engages the connector center conductor within the recess; the inside diameter of the metal walls of the ferrule and the coaxial connector housing from the one end of the connector (where the ferrule is atta

Abstract: A waveguide-to-coax-to-microstrip transition includes a rectangular waveguide portion having a ridged impedance transformer attached to the lower wide wall. The waveguide portion is closed off by a conductive wall through an aperture in which the center conductor of a coaxial transmission-line passes. A ridge extension isolated from all four walls of the rectangular waveguide couples the ridge to the center-conductor of the coax. For broadbanding, a glass plate is located between the ridge extension and the upper wide wall. The strip conductor of a microstrip transmission-line is connected to the center conductor of the coaxial transmission-line. The plane of the dielectric plate of the microstrip may be oriented parallel to the narrow wall of the rectangular waveguide to achieve high packing density.

Abstract: A waveguide connection comprising the combination of a rectangular waveguide, an elliptical waveguide having a cutoff frequency and impedance different from those of said rectangular waveguide, an inhomogeneous stepped transformer joining said rectangular waveguide to said elliptical waveguide, said transformer having multiple sections all of which have inside dimensions small enough to cutoff the first excitable higher order mode in a preselected frequency band, each section of said transformer having a transverse cross-section defined by the equation: (2x/a).sup.p +(2y/b).sup.

Abstract: A technique for packaging and integrating of a microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) with a waveguide uses a printed conductive circuit pattern on a dielectric substrate to transform impedance and mode of propagation between the MIC/MMIC and the waveguide. The virtually coplanar circuit pattern lies on an equipotential surface within the waveguide and therefore makes possible single or dual polarized mode structures.

Abstract: The first section of a Marie'-configured TE.sub.10 rectangular mode to TE.sub.01 circular mode waveguide launcher is modified to provide an effectively true linear taper for the injected TE.sub.10 mode wave to the emitted TE.sub.20 mode wave. As a result, the first section is well matched at the low end of the frequency band as well as being resonance free to frequencies above the upper end of the band of interest. According to this modification, rather than have a single taper for the triangular shaped lower portion of the first section, the lower triangular portion has a second taper extending from the intersection at the lower wall of the input to the height of the exit wall at the output. This second taper provides an effective true linear taper for the input section.

Abstract: A microwave direction finding antenna interface is disclosed which is cape of operating over multioctave bandwidth with an upper frequency limit exceeding forty gigahertz. The antenna, feed circuit/transformer and arithmetic network have non-interrupted TEM electrical paths of strip transmission lines (striplines). Further, a process of fabricating the antenna, feed circuit/transformer and arithmetic network in conjunction with the support means is described.

Abstract: A microwave power distributing and synthesizing device includes: a first base and a second base facing the first base at a predetermined distance and having a hole. A coaxial line element including an inner conductive member coaxially penetrates through the hole of the second base and is spaced from an inner wall of the hole by the second base of a predetermined distance. The coaxial line element is supported on the first base at a bottom end and forms an antenna protruding from the hole of the second base into an input waveguide at a top end. At least one electric field distributing and synthesizing member is arranged perpendicularly to the coaxial line element and a plurality of waveguides are connected to the electric field distributing and synthesizing member.

Abstract: Coaxial cables each having a cylindrical electrically conductive outer shield and a central conductive wire insulated from the shield are conductively coupled through a connector to coplanar conductors on a dielectric substrate. The conductors converge toward an opening in the substrate. The coplanar conductors alternately are ground and signal conductors. Each coaxial cable shield is conductively coupled to two ground conductors and the cable wire is conductively coupled to the signal conductor between the two ground conductors. At the substrate opening the ground and signal conductors are conductively coupled to planar stripline waveguide ground and signal blades, respectively. Ground blades between the signal blades are a factor in controlling the signal blade impedance and provide isolation between the signal blades. Each blade is perpendicular to the substrate and has a pitch angle such that the blade extends through the opening below the substrate.

Abstract: Filter comprising a rectangular waveguide extended at both ends by rectangular guide-to-rectangular guide couplers and provided with lateral secondary guides. In order to enable the use of cylindrical secondary guides without introducing unwanted resonances, said couplers are given, at their ends, rectangular internal cross sections both the height and width whereof are smaller at the end coupled to the primary guide than at the end remote from the primary guide.

Abstract: A circular waveguide window between two rectangular wave-guides, having increased bandwidth and increased power handling capability. It uses particular window and an impedance matching structures whose dimensions are related in a particular way to the dimensions of the rectangular waveguides.

Abstract: A phased-overmoded, tapered waveguide transition has a central section which is tapered linearly in the longitudinal direction and two end sections which are tapered curvilinearly in the longitudinal direction. One of the end sections and at least a portion of the other end section are overmoded and, therefore, give rise to higher order modes of the desired microwave signals propagated therethrough. The lineraly tapered central section shifts the phase of higher order modes generated at one end of the transition so that at least a major portion of such higher order modes is cancelled by higher order modes generated at the other end of the transition.

Abstract: An apparatus and method is disclosed for allowing faster and higher fidelity signals to be transmitted from a PC board to an IC by maintaining a constant impedance from the PC board to the IC. A leadless chip connector is used with a ground plane mounted co-planar to the conductor. As the conductor and ground planes extend toward the IC they decrease in width and the distance between the conductor and ground plane varies to maintain a constant impedance.

Abstract: A two-port monolithic microwave amplifier, which uses a distributed negative resistance diode with gain (such as an IMPATT diode) as an active element. The diode is tapered (increasing in width but not in thickness) so that, as the RF signal propagates along the diode, it sees a wider and wider active diode region. This diode is operated in the power-saturated region, so that, as the RF signal propagates along the diode, terminal voltage remains essentially constant, but the RF current increases. This configuration is inherently undirectional.

Abstract: A radio-frequency matching transformer including a wave guide (W) having a fixed length. The waveguide is subdivided into sections (W.sub.1, W.sub.2) which have different characteristic impedances and the lengths of which are interdependently adjustable. A hollow cylinder is provided within the waveguide displaceable therein such that the operating frequency and the transformation ratio can be adjusted within wide ranges without a conversion of the transformer being required. This results in considerable advantages with respect to the known quarter-wave transformer.

Abstract: A wideband power adder-divider for high-frequency circuits including a first conductive transmission line (10) for passing a high-frequency current, second and third conductive transmission lines (20) and (30) over which this high-frequency current is distributed, and a conductive wedge-shaped transition section (40) joining the first to the second and third lines. This section has two arc-shaped outer edges (41) and (42) which are tangentially connected at one end of the section to the first line and at the other end to the second and third lines. The section includes parallel slots extending transversely to the direction of propagation of the current. The parallel slots (51) to (58) have ends which are separated from the arc-shaped edges by a distance which is less than the width of the second and third transmission lines.

Abstract: A dominant TEM mode and a pair of spatially orthogonal TE.sub.11 modes suitably excited at the input ports of a coaxial waveguide commutator portion of a coaxial waveguide commutation feed network are employed to generate a commutatable low-sidelobe amplitude distribution at symmetrically disposed peripheral output ports of the coaxial waveguide commutator. The resulting low-sidelobe amplitude distribution can be used to feed radiating elements of an associated circular phased array antenna. The coaxial waveguide commutator is configured, inter alia, with a linearly tapered inner conductor surrounded by a uniform tube outer conductor to achieve a smoother TEM-dominant mode characteristic impedance transition from the input port plane to the output port plane thereof. Employment of this feed geometry, in conjunction with balanced four-port feeding of the coaxial waveguide commutator which inhibits the higher order TE modes, increases the bandwidth capability while maintaining low insertion loss.

Abstract: A coaxial transmission line to waveguide transition is formed of two waveguide portions disposed on opposing sides of, and enclosing a portion of, a flat plate structure. The enclosed portion of the flat plate structure includes a tapered slot extending through the flat plate structure leaving portions of the flat plate structure protruding into the waveguide as loading ridges which provide impedance matching (transformation) between the coaxial line and the unloaded waveguide. The flat plate structure has a hollow therein and an inner conductor passing therethrough forming a coaxial line. The inner conductor crosses the tapered slot within the waveguide enclosure.

Abstract: A matching section for a spiral antenna used in a monopulse receiver is shown to receive a plurality of coaxial cables at equally spaced points along the circumference of a first circle and to guide the center conductors of such cables to arms of a multi-arm spiral antenna at equally spaced points along the circumference of a second circle, the characteristic impedance along the length of the matching section changing smoothly for the sum and difference modes and impedance matching closely to the antenna sum and difference modes.

Abstract: A TE.sub.01 circular mode waveguide expansion joint, tapered transition dce in which the expansion joint includes a linear tapered section, the greatest diameter of which does not exceed the cutoff diameter for the TE.sub.02 spurious mode. A second taper of the cosine or other suitable type minimizes TE.sub.02 mode generation as the guide diameter increases past the TE.sub.02 cutoff value. The expansion, transition device simultaneously performs the functions of expansion section and waveguide diameter transition for the TE.sub.01 circular mode while minimizing spurious mode generation.

Abstract: A gyrotron cavity resonator is connected smoothly and directly to an output waveguide with a very gradually tapered wall so that values of external Q lower than twice the diffraction limit are obtainable.

Abstract: A radio frequency energy launcher providing efficient energy transfer between a coaxial transmission line having spaced inner and outer conductors and a microstrip transmission line comprising a first relatively thin strip conductor and second wider ground plane conductor separated by a dielectric substrate. The launcher includes a conductive housing providing a passageway forming an outer conductor and a spaced inner conductor angled with respect to the passageway outer conductor forming wall and a surface of the substrate. Such angled inner conductor having an end region connected to the coaxial transmission line and having the other end interconnected to the thin strip conductor of the microstrip transmission line. The inner conductor is angled acutely and/or obliquely to such housing wall and a surface of dielectric substrate.

Abstract: A mode suppressor for use with circular waveguides which are over-sized to provide a low-loss transmission path for signals in the TE.sub.11 mode and which carry an electric field which is perpendicular to the longitudinal axis of the waveguide. The mode suppressor includes resistance cards located in planes which are generally parallel to the longitudinal axis of the circular waveguide and generally perpendicular to the electric field of the TE.sub.11 mode. Resistance cards are also contained in secondary waveguides which are coupled to the sidewalls of the circular waveguide. The resistance cards in the secondary waveguide lie in a plane parallel to the TE.sub.11 electric field and perpendicular to the longitudinal axis of the circular waveguide. The TM and TE.sub.on modes (where n is an integer of 1 or greater) are suppressed by the resistance cards lying parallel to the longitudinal axis. The TE.sub.21 and TE.sub.31 modes are suppressed by the resistance cards contained in the secondary waveguides.

Abstract: An I type (straight) double ridge waveguide rotary joint is provided by connecting input and output sections of double ridge waveguide, oriented along a common axis, to opposite ends of a coaxial line rotary joint that is on the same axis. The input and output sections each connect to the coaxial line rotary joint by a transformer that connects one ridge to the center conductor of the adjacent coaxial line and the other ridge to the outer conductor of the coaxial line, the connections being made in several transitions to avoid an abrupt change of impedance in progressing along the axis through the transformer.

Abstract: A gas-tight, high-frequency permeable window arrangement is provided between the input and output sides of a coaxial line, particularly for traveling wave tubes. The dielectric losses, particularly given high-performance tubes, are held so low that too high a thermal load and, thus, a fracture or, respectively, loss of seal of the window arrangement is avoided. For this purpose, a ceramic hollow cylinder is provided between the coaxial line of the input side and the coaxial line of the output side, the ceramic hollow cylinders surrounding the inner conductor of the coaxial line of the input side in the area of its transition to the coaxial line of the output side as a continuation of the outer conductor of the coaxial line of the input side.

Abstract: An impedance-matching microstrip network for matching a predetermined impedance to a lower impedance over a wide frequency band having a tapered line section for transposing the predetermined impedance to a higher impedance. At least one quarter wave line section is coupled to the narrow end of the tapered line section for transposing the higher impedance to the lower impedance.

Abstract: An electrical coupler for transmitting high frequency electrical signals from a coaxial transmission to a microstrip transmission line. The coupler is adapted to match the characteristic impedance of the two media and the electromagnetic field patterns of the two media at the interfaces therewith. The coupler provides a transition having a very low reflections at frequencies at least up to 18 GHz. The preferred embodiment transitions utilize a cylindrical outer conductor and an inner conductor which is centered relative the outer conductor at the coaxial end of the coupler and gradually shifted offcenter so that it is very near the outer conductor at the microstrip end of the coupler. The characteristic impedance of the coupler is maintained at a constant value by appropriate variation of the inner conductor. Other features are disclosed.

Abstract: A waveguide to microstrip transition for transferring guided electromagne signals from dominant mode rectangular waveguide to microstrip line and vice versa. A microstrip printed circuit card is disposed in parallel to the narrow walls of a waveguide. The printed circuit card includes a microstrip stepped transformer section followed by a linear taper crossover section. The linear taper crossover section leads into a microstrip line conductor and ground plane for completing the transition.

Abstract: A diode modulator type electrical mixer network has at least two hybrids of transmission line construction, interconnected by means of two sets of four or more diodes arranged in bridge circuit configurations. The interconnecting configuration provides a novel means of local oscillator injection that allows the mixer to operate in a mode that substantially reduces the mixer's susceptability to effects caused by reflective external source and load termination impedances differing from an ideal 50 ohm nonreflective condition.

Abstract: The specification describes a dielectric image guide harmonic pumped mixer circuit wherein two dielectric image guide strips of different chosen cross-sectional configurations are mounted on a metallic substrate which forms the image plane for the mixer circuit. These image guide strips have impedance matching tapered ends which extend into a central mixer cavity region on the substrate. A pair of reverse poled mixer diodes are mounted in the central cavity region to receive respectively RF incoming signals from one dielectric image guide and local oscillator signals from the other dielectric image guide. Differential currents generated in the two diodes cause the second harmonic of the local oscillator signal frequency to mix with the incoming RF signal frequency. An intermediate frequency (IF) signal current produced thereby is conductively coupled from the mixer cavity into IF signal processing electronics.

Abstract: This invention relates to the transmission of electric power and in particular provides an electric power-transmission system having reduced vector regulation, voltage drop, and power loss through the inclusion of capacitance in the cable in series between the generator and load by utilizing electric conductors, i.e., connective links, having capacitance distributed along the length of the cable. Such capacitance is achieved by dividing a conductor into two parts which are separated by dielectric material such that the two conductor parts are in capacitive relation along the length of the cable and by connecting one conductor part to the generator and the other conductor part to the load such that the distributed capacitance is in series with the generator and load.

Abstract: A dielectric slab polarizer employs a unique configuration which provides improved impedance matching of the polarizer to the empty space of a waveguide section and, thereby, allows an approximate ninety degree differential phase shift transformation of RF field polarization to be achieved over a wider frequency bandwidth than heretofore attainable. The novel configuration of the slab polarizer is embodied by a flat middle section, conical tapered opposite end sections and reverse tapered intermediate sections which merge with the flat middle section and opposite end sections. The regions of merger between the respective intermediate sections and opposite end sections have a thickness greater than that of the middle section.

Abstract: A diode microwave switch is disclosed having both low power dissipation loss and a low voltage standing wave ratio by the techniques of mounting microwave diodes on pedestals and interconnecting them with air line microstrip connections and utilizing special impedance matching techniques. Voltage standing wave ratios of 1.15:1.00 with 0.3 db loss in a three diode 60 db isolation switch are readily obtained.

Abstract: A double slot balun has two pairs of coplanar, electric conductive sheets in a superposed, spaced, electrically insulated relation on opposite planar surfaces of a dielectric substrate forming two slots disposed in spaced, parallel planes. Unbalanced signals applied simultaneously to common ends of the slots result in balanced in-phase signals at the opposite end of the two slots. A constant impedance balun has two alined, electric conductive strips in a superposed, spaced, electrically insulated relation mounted on opposite planar surfaces of a dielectric substrate with the widths at opposite ends of each conductive strip being chosen for selected parallel-plate impedance characteristics and one conductive strip having a selected taper, preferably following a cosine curve, while the width of the other conductive strip is selected relative to the cosine taper along its length to provide a constant impedance at any point along the length of the two conductive strips.

Abstract: A microwave harmonic absorption filter is disclosed which has an extended single mode pass-band ranging over one or more octaves as well as enhanced attenuation in the stop-band. This filter has a primary, rectangular waveguide having one or more ridges on the inner surfaces of its elongated wall members. Secondary waveguides are provided on the outer surfaces of these wall members and these secondary waveguides serve to diminish propagation of microwave energy in the stop-band. Apertures extend through the wall members of the primary, ridged, rectangular waveguide so that microwave energy within the range of frequencies of the stop-band can be transmitted from the primary waveguide into the secondary waveguides.

Abstract: A clamping device for securing HF fittings to coaxial cables or hollow conductors by means of a clamping sleeve surrounding the cable outer sheath or the hollow conductor sheath. The sheath is radially inwardly deformed by the clamping sleeve in such a manner that a transformation is effected which guarantees a reflection-free transition to the adjoining fitting. In the end portion of the cable or hollow conductor the outer sheath is engaged radially so that it is compressed, giving a reliable hold. The cable sheath is thus deformed in a definite manner so that at the point of entry into the fitting it has a predetermined size, said deformation gradually or abruptly decreasing towards the cable in a transformation portion. This deformation can be effected by suitable means, for example axial tensioning or by a two-part clamping sleeve with tangential tensioning.

Abstract: The characteristic impedance of a planar microstrip or stripline for transmitting signals along a transmission line in the TEM mode is matched by way of a tapered shunt which terminates the line in a dc short circuit to ground. The tapered shunt or termination wraps around the edge of the insulating substrate and electrically connects the microstrip to the ground plane. The geometry of the tapered shunt is incrementally adjusted by means of a series of steps to produce a termination having a selected impedance.

Abstract: Disclosed is an active radial-to-coaxial transmission line wherein an annular configured semiconductor device, such as an IMPATT diode, is embedded in a chosen dielectric material which is in turn positioned between two conductive members forming a radial transmission line. Advantageously, the lower conductive member of the transmission line also serves as a heat sink for the diode, and the upper conductive member of the radial transmission line may be formed as a highly conductive coating which makes good ohmic contact to the diode, thereby minimizing power losses at this contact. The radial transmission line transforms the relatively low impedance of the diode to a substantially higher value at the outer periphery thereof where contact to a coaxial transmission line may be achieved with good impedance matching, thereby maximizing power transfer between the radial and coaxial transmission lines.

Abstract: A multimode waveguide window which provides an RF connection through a me barrier such as the hull of a submarine. RF transmission is effected with a minimum of insertion loss and the structure is capable of withstanding hydrostatic pressures up to 1000 psi. In the event that the incoming waveguide structure is accidentally severed from the ship, the window provides a watertight seal.

Abstract: A transition apparatus for changing from a waveguide to a coaxial connect A tapered or stepped ridge waveguide section is electrically connected to a section of microstrip which is electrically connected to a coaxial connector, the coaxial connector being in line with the waveguide.

Abstract: An impedance standard apparatus for providing a mathematically predictable voltage standing wave ratio between a matched generator and load.

Abstract: A waveguide to microstrip transition apparatus in which a waveguide coupled o a microstrip circuit is provided with a mode filter for suppressing spurious modes of electromagnetic signal propagation. The mode filter is attached to the end of the waveguide that is coupled to the microstrip circuit and the mode filter extends over a portion of the microstrip circuit.