Abstract: A plurality of millimeter wave power sources are arranged in a planar mat array located in a wave-beam resonator including two reflecting surfaces which are large in terms of the operating wavelength. One surface comprises a planar reflector located in relatively close proximity to the source array, while the other reflector is located in front of the array at a relatively larger distance and being partially transparent and curved, with the curvature being expressed by a pair of focal lengths which define a curvature in two perpendicular axial planes. Each source is comprised of an IMPATT or GUNN diode coupled to a short dipole or closed current loop structure, all lying in a plane parallel to the planar reflector and transverse to the longitudinal central axis of the resonator.

Abstract: An improved serpentine feed and method of making same. The invention includes a block of conductive material having a primary microwave waveguide channel and at least one secondary channel. Each secondary channel has at least one broadwall common to the primary channel. A high performance coupler is provided in the common broadwall by which microwave energy communicates from the primary channel to the secondary channel.

Abstract: In a microwave apparatus with an air-tight window plate, first and second coaxial waveguide assemblies are coupled to each other so that a coaxial waveguide is formed. The first and second inner conductor sections of the first and second assemblies have first and second metal blocks. The first metal block is fitted in the second metal block by a shinkage fit and the first outer conductor section of the first assembly is air-tightly welded to the second outer conductor section of the second assembly. An RF matching annular groove is defined between the first and second metal blocks.

Abstract: A local coil for NMR imaging includes a pair of loop-gap resonators which are mounted to a circular cylindrical insulating base. The NMR signal is coupled to a transmission line which terminates at a head connector that mounts to the base and which connects to a coupling loop formed by a conductive foil. The conductive foil forms a closed loop which is mounted to the base between the two loop-gap resonators and which follows the contour of the loops of the loop-gap resonators.

Abstract: Circularly polarized radiation is tapped off from an input waveguide through a input iris into an entry cavity, where it is resolved into two orthogonal linearly polarized components. These respectively proceed along two discrete paths to an exit cavity. In each path six independently tunable resonances--traversed by both direct and bridge couplings--provides enough degrees of freedom for quasi-elliptic filter functions. In the exit cavity the resultants from the two paths are combined to resynthesize circularly-polarized radiation, which traverses another iris to the output waveguide. In one layout, four resonant tri-mode cavities form a rectangular array--with entry and exit cavities at diagonally opposite corners and intermediate cavities for the two discrete paths in the two remaining corners.

Abstract: A waveguide filter is provided which comprises a plurality of waveguide resonators arranged in series in the longitudinal direction of the filter with each contacting tightly with adjacent ones, each of the waveguide resonators being composed of a rectangular dielectric having a length and a permittivity complying with the center frequency of a bandpass filter, on the periphery of which a metal film is provided except on the portions of induction windows of its two sides mutually opposed in the longitudinal direction.

Abstract: A dual mode circular-type waveguide filter (10) has a cylindrical waveguide body (12) separated by septums (16, 18) into a plurality of resonant waveguide cavities (22, 24, 26). Mutually orthogonal electromagnetic fields (37, 81) in adjacent cavities (24, 26) are electromagnetically coupled with each other by an internal coupling element (60) which is mounted on one of the septums (18) and extends into the adjacent cavities. The coupling element comprises an elongate, conductive wire which includes a first portion (62) extending into one of the cavities and forming a magnetic loop, and a second portion (64) extending into the adjacent cavity and forming an electric probe. The coupling element provides an asymmetrical stopband pole (72a) in the frequency response (70) of the filter.

Abstract: An apparatus that utilizes a solid, dielectric-filled waveguide filter in a microstrip circuit is dislosed. The microstrip circuit couples directly into and out from the waveguide filter through slot openings in waveguide filter walls without penetrating the interior of the waveguide. Adjustment of the coupling is accomplished through varying the length of the slot. Resonant frequency tuning of the filter is accomplished using additional slots, or by removing metallic plating from metallized cavities which extend into the waveguide core from a central region of a waveguide broad wall.

Abstract: A cavity resonator coupling type power distributor/power combiner can be used as a distributing amplifier or a combining unit. A first cavity resonator, having a single coupling terminal, and a second cavity resonator having a plurality of coupling terminals, are coupled by a coupling window or a coupling rod. As a result, a wide bandwidth of microwave electric power can be distributed or combined.

Abstract: A compact police radar warning receiver is provided having an antenna in which some of the mixer and oscillator components are secured to the antenna by a cantilever structure formed integral the circuit board overlying a portion of the antenna and to which the components are electrically and mechanically coupled.

Abstract: An output section which takes output by a coaxial line from the output cavity of a microwave tube such as a klystron, and has a structure such that the coaxial line is divided into a coaxial line section continuous with the output cavity, and a coaxial line section including a dielectric air-tight ring which is fastened so as to be vacuum-tight between the internal and external conductors, and with the internal and external conductors of these divided coaxial line sections each joined so as to be air-tight by thin weld rings. The output section obtained is easy to manufacture, has high reliability, and is applicable to high power usage.

Abstract: A microwave filter which is optimized to reduce its dimensions, weight, cost, and manufacturabiity includes a filter body which is preferably manufactured by pressure die-casting of an aluminum alloy. In the filter body are formed a plurality of cavities in which resonators as well as tuning and connection screws are located. The cavities are so located with respect to each other that a line which joins the centers thereof appears as the series of connected line segments which are inclined with respect to each other at angles which exceed 90.degree.. To compensate the filter for the relatively larger coefficient of expansion associated with the use of die-castable alloys, the resonators and tuning screws employ materials which are different than that used in the filter body. Electrical losses are minimized by varying, from cavity to cavity, the ratio of the diameter of the cavity to that of the resonator. The weight of the filter is further reduced by cavities or voids which are formed therein.

Abstract: Disclosed is a coupling arrangement which comprises an enclosed cavity with an aperture in one of its walls. An adjustable probe is positioned within the aperture to allow energy within the cavity to be coupled onto the probe. A microstripline transition is connected at one end to the adjustable probe and at the other end to external circuitry. The arrangement allows variable coupling of energy within the cavity onto the probe without requiring cumbersome procedures for fine adjustment.

Abstract: A microwave oscillator having very low phase noise characteristics has a reflection oscillator circuit coupled to a right-circular cylindrical resonant cavity designed to operate in the TE.sub.011 mode. The resonator length L and inner diameter D are restricted to a range of values such that (D/L).sup.2 is less than 2.5 but greater than 1.7. To suppress the TM.sub.111 resonant modes in the resonator, two techniques are employed. A single tuning post is disposed on either resonator end plate at a point where the TM.sub.111 electric field Z-component is a maximum. The post length is adjustably fixed to maximize the output microwave power at the cavity output port. Each end plate also has a continuous gap separating each plate from the housing to suppress electric field currents of the TM.sub.111 mode flowing on the inner surface of the resonator.

Abstract: The subject invention relates to an improved microwave cavity resonator and internal magnetic shield of small design for use with an atomic frequency standard.

Abstract: A bandpass filter has a plurality of cascade cavities that operate in a dual-mode and are mounted in a planar relationship to one another. Each cavity contains a dielectric resonator and resonates at its resonant frequency in two HE.sub.11.delta. modes. An iris is located between adjacent cavities and contains an aperture that can independently control inter-cavity coupling. The dielectric resonator is cylindrical in shape and an adjustable metallic plunger is located in a wall of each cavity so that the cavity can be tuned over a relatively broad range. The filter has good mechanical and thermal stability as well as being extremely flexible in the arrangement of cavities.

Abstract: A dual mode filter has a hollow cylindrical longitudinal outer body which is divided by transversely extending disk-like separation walls into a plurality of internal cavities. At least one coupling opening in each separation wall permits coupling of energy from one cavity to another. Said coupling opening being preferably an arcuate opening, in each separation wall, which extends along a constant radius at the outer periphery of the separation wall and over a limited angular span. When the openings are two, they are spaced 180.degree. apart, i.e. opposite one another. Adjacent separation walls are rotatably displaced 90.degree. with respect to each other so that their respective coupling opening(s) is (are) misaligned by 90.degree.. One respective adjustment screw is provided for each separation wall which penetrates through a hole in the outer body into one of the coupling openings in the given separation wall.

Abstract: Disclosed is a microwave solid-state amplifier arranged such that a plurality of amplifier units are used in combination of a radial cavity resonator so as to perform power dividing/combining, and that in order to make the microwave solid-state amplifier capable of performing a wide band operation and have less circuit loss, a coupling portion between the radial cavity resonator and each of the amplifier units is formed such that a loop-like conductor constituted by a stripline forming an input/output portion of the amplifier unit is removably inserted through a slot formed in a side wall surface of the cavity resonator.

Abstract: An odd order bandpass filter has at least one cavity resonating at its resonant frequency in three independent orthogonal modes. The filter has at least one feedback coupling that is made to resonate and change sign at a center frequency. When the filter has two cavities, one being a triple cavity and the other being a dual mode cavity, the filter can be operated to achieve an elliptic function response. Also, the filter of the present invention can achieve a weight and volume reduction when compared to six-pole dual mode filters.

Abstract: A transmitter combiner includes a plurality of adjacent square prism filters, each of which shares at least one square wall with another of filters. Tuning of the individual square prism filters is accomplished by adjusting the position of tuning rods located in the high electric field region of each cavity. Tuning of each filter is achieved by rotating a control rod perpendicular to and extending through a rectangular end wall to pivot the tuning rod in that filter. In one embodiment of the invention, the control rod extends through a conductive tube attached along a square wall of the filter and perpendicular to the rectangular wall. In another embodiment of the invention, the control rod extends from the center of the rectangular wall and supports the tuning element near the center of the cavity. Temperature compensation is accomplished by variation in the length of the control rod as a function of temperature to compensate for changes in the dimensions of the cavity with the temperature thereof.

Abstract: A dielectric resonance apparatus for resonating in a TM mode such as TM 110 or the like. The apparatus includes a case having therein at least two TM-mode dielectric resonators, these resonators being oriented in the case so that their magnetic fields intersect each other. The apparatus also comprises means for coupling the magnetic fields. The TM-mode dielectric resonators may be integrally or separately formed. Each adjacent pair of resonators may be magnetically interconnected by an irregularly shaped portion of the case, such as a depressed portion or a projecting portion, for influencing the respective magnetic fields of each resonator by a selected degree, such that different respective degrees of influence are obtained with respect to the even and odd modes to be produced by the two resonators.

Abstract: A superconducting cavity has a long time constant when the coupling factor of its output port is low so as to accumulate input microwave energy therein, and after a certain accumulation of the input microwave, an output microwave pulse with a large power is extracted from the cavity by suddenly increasing the coupling factor of its output port.

Abstract: An electric field probe is disclosed as having an arcual shaped transmission line which is mounted over a substrate having a corresponding arcual shaped transmission line deposited thereon. The two arcual shaped transmission members overlap such that the electric field energy sampled by the probe may be communicated to a monitoring circuit or the like. Rotating the probe, varies the portion of the arcual shaped members that overlap thereby adjusting the physical and electrical length of the transmission line. Accordingly, the phase of the field energy will shift as viewed from the output port.

Abstract: The microwave power head or modular heat applicating unit comprising a resonator, a waveguide, a source of microwave energy and a water load device. The resonator is divided into two chambers by a center wall member. A microwave energy receiving antenna means is operatively connected to the center wall for coupling microwave energy into the resonator. A microwave energy transmitting antenna spaced from said receiving antenna is operatively connected to the center wall for transmitting microwave energy from the resonator to a water load. In this manner, the resonator provides a dual function.

Abstract: A heat applicating device having a resonator divided into a plurality of chambers by one or more energy radiating wall like members. The energy radiating wall member is coupled to a microwave energy source at a point on one edge thereof whereby a magnetic field is generated along the opposite (upper) edge of said wall member. An elongate slot is provided in the resonator generally above the upper energy radiating edge portion of the wall member. The ends of the wall member are sloped to provide a gradual termination of the electrical field.

Abstract: A cyclotron resonance maser system includes coaxial coupling between coaxial wave guides. Such coupling is provided between an outer coaxial waveguide through which a gyrotron electron beam passes in generating or amplifying microwaves and an inner coaxial waveguide terminating in an annular window. The coupling separates the microwaves from the electron beam before the collector region of the gyrotron so that the collector dimensions are not related to, and hence restricted by, the wavelength of the output microwaves. The particular preferred coupling includes substantially axial slots in the common wall between the respective waveguides which slots provide selective coupling between the waveguides in the desired modes and limiting mode conversion. The coupling is preferably between a TE.sub.On or TE.sub.ml mode in the outer input waveguide and a TE.sub.On' mode in the inner, output waveguide, n and n' being integers greater than 1. A preferred resonant cavity limits the generated microwaves to a TE.sub.

Abstract: A solid state microwave power combiner is disclosed having a central combining cavity with a plurality of coaxial oscillator circuits disposed about and coupled to the cavity. The oscillator circuits are oriented such that their longitudinal axes are parallel with the central axis of the combining cavity. They alternate in configuration about the cavity such that adjacent oscillator circuits have their power generating diodes located on opposite sides of the cavity. In one embodiment, the coaxial lines used in the oscillator circuits have two parts of different sizes. The oscillator circuits are oriented about the cavity so that the smaller sized coaxial line part of one is adjacent the larger sized coaxial line part of the next. Both rectangular and circular combining cavities are disclosed.

Abstract: A dielectric resonator frequency selective network. A frequency selective network for microwave circuits is provided whereby a dielectric resonator is coupled to associated circuitry by input and output coupling loops formed in a single circuit board. The two loops are closely spaced, but partially overlapping at a position such that they are substantially decoupled from one another. A dielectric resonator is placed adjacent one of the loops so as to couple one loop to the other through the resonator and to cause the resonator to operate in its dominant mode. The circuit board is constructed by forming conductors separated by insulating material on a ceramic substrate.

Abstract: In a microwave device, for example a microwave local oscillator with a harmonic mixer for feedback control, problems of coupling the mixer circuit to the R.F. cavity are overcome by providing a d.c. blocking capacitor of the mixer on a planar surface of a support extending across an aperture in one wall of the cavity. In preferred embodiments the capacitor is a planar device formed by a plurality of interdigitated fingers. These fingers are oriented to couple magnetically with the R.F. energy at the aperture in the cavity. The support conveniently comprises a printed circuit board having a ground plane on the same surface as the capacitor, which is located in a small opening in the ground plane. A second ground plane is provided on the opposite side of the printed circuit board, overlying the opening. The two ground planes are interconnected by plate-through-holes.

Abstract: Cascaded dual-mode resonance cavities are separated by plates having each a cruciform iris therein, the arms of which have lengths determined by a direct and precise procedure starting from a conventional Butterworth prototype filter. The transfer function parameters are altered to change the filter response and the group delay performance is determined and compared to an ideal flat response for producing a penalty function signal from the difference in order to modify the parameters again in such direction that the penalty function signal is likely to be reduced. The process is repeated until the penalty function signal is minimum, whereby a compact structure is realized which provides an attenuation response having sharp cut-off slopes at the edges of the signal bandwidth together with a close to flat in-band group delay response.

Abstract: A wide-band distributed coupler for coupling rf energy from an input waveguide into a tapered interaction waveguide in a traveling-wave amplifier comprising a plurality of channel filters connecting between the input and interaction waveguides, with each filter coupled to the interaction waveguide at the appropriate cross-sectional position along its tapered length where the interaction waveguide cutoff frequency approximately matches the wave frequency propagated by the filter. Each filter comprises, in one embodiment, a main coaxial cavity tuned to a distinct center frequency, a first simple isolation cavity for coupling rf energy between the input waveguide and the main cavity, and at least one second simple isolation cavity for coupling energy between the main cavity and the tapered interaction waveguide. This coupler is compatible both in bandwidth and geometry with the tapered interaction waveguide.

Abstract: A cavity resonator coupling-type power distributor/power combiner includes a first cavity resonator operatively resonating with a cylindrical T.sub.O,n,O mode, and a plurality of second cavity resonators arranged on the periphery of the first cavity resonator and extending radially and symmetrically with respect to the first cavity resonator. The second cavity resonators each have the same shape and size so that magnetic-field coupling is established between the first cavity resonator and each of the second cavity resonators, for distributing or combining microwave power in a microwave amplifier.

Abstract: A stabilized microwave oscillator having a selected operating frequency range, composed of a rectangular waveguide dimensioned to have a cutoff frequency above the selected operating frequency range, at least one active semiconductor element and components connected for supplying a direct operating voltage to the element, mounted in the rectangular waveguide, a transmission resonator, and a coupling device coupling the rectangular waveguide to the resonator, the coupling device including a waveguide section having a length of about .lambda./2, where .lambda. is the wavelength of the center frequency of the operating frequency range, and dimensioned at least in part to operate as an evanescent mode waveguide and/or a waveguide having a cutoff frequency in the vicinity of the upper end of the selected operating frequency range.

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 coupling arrangement for a cavity resonator (10), in particular for measuring the magnetic resonance of a sample (17) located within the cavity resonator (10) comprises a loop (14) for supplying a radio frequency signal to the cavity resonator (10) and exciting in the latter a specific mode of oscillation. For optimizing the coupling of the radio frequency signal, mechanical means are provided, preferably in the form of an axially adjustable screw (16), which permit the surface of the loop (14) to be varied mechanically, and the loop (14) is capacitively coupled with the resonator housing.

Abstract: A structure comprising a cascade of dual mode resonance cavities wherein each cavity has a pair of tuning screws located at 90.degree. to each other in a sectional plane of the cavity and a coupling screw located at 45.degree. to the tuning screws for coupling the two resonances supported by the cavity. Each cavity is coupled to the adjacent cavity by means of a coupling iris set at a determined angle relative to the angular position of the tuning screws of the cavity and the adjacent cavity is positioned at a determined angle relative to the angular position of the coupling iris which couples said cavity to the former one.

Abstract: A frequency multiplier for electromagnetic waves in which multiples of the input frequency propagate, while the input frequency evanesces. A rectangular waveguide or other suitable wave conveying element with a cut-off frequency above the input frequency but below the output frequency contains the multiplying structure. The input frequency is multiplied using a post located in the waveguide or other element adjacent to the source of input waves. The post may define a multiplier gap between one of its surfaces and a wall of the waveguide or other element, in which case the multiplier gap may contain a multipactor between its surfaces which generates multiples of the input frequency. The multiplier gap may alternatively contain a non-linear element responsive to a high electric field, such as a ferroelectric material, which also generates multiples of the input frequency. Alternatively, the post may be surrounded by a non-linear element of ferromagnetic material responsive to a strong magnetic field.

Abstract: A cyclotron resonance maser microwave amplifier includes coaxial input and output waveguides. Coupling apparatus for cyclotron resonance maser microwave amplifiers and other microwave power systems is provided for transferring microwave power between first and second waveguides. Geometry permits unwanted modes to be effectively attenuated. The coupling apparatus includes first and second coaxial hollow metallic circularly cylindrical members. The first member forms a first waveguide section. A second waveguide section coaxial therewith is bounded internally by the first member, and externally by the second member. The first member has a plurality of apertures for transferring microwave power between the first and second waveguide sections. The apertures are small relative to the radius of the first member and are spaced around the circumference of the first member in a generally symmetrical array.

Abstract: A bandpass filter 8 has a plurality of cascaded wave-guide cavities 11, 12 1N and inter-cavity coupling means 36, 37. One or more of the cavities 12, 1N is rotated relative to an input reference mode 52. The rotation causes quasi-orthogonal coupling structures to be introduced into said cavities 11, 12 1N. The rotation of one or more cavities enables symmetric or asymmetric amplitude and phase responses to be produced as desired.The filter can be used in channel multiplexers for satellite communication systems.

Abstract: A microwave excitation system for coupling microwave energy from a magnetron to a load via a waveguide of relatively compact dimensions open at one end for radiating energy therefrom and enclosed at the other end by a conductive end wall. A magnetron output probe extends axially into the waveguide through a central opening in the end wall of the waveguide. A cup-like member is mounted within the waveguide and inverted over the probe concentric therewith to capacitively couple microwave energy from the probe into the waveguide. The cup-like member is dimensioned to provide a matched load condition for the magnetron.

Abstract: A mm wavelength power combiner comprising an open resonator comprising a r of confronting concave reflectors which can be either spherical or parabolic. The resonator dimensions are many times the wavelength of the energy sources to be combined. A plurality of mm wave energy sources are applied to the resonator in such a way that the great majority of the energy bounces back and forth between the reflectors near the axis thereof in the fundamental or Gaussian mode. The design minimizes multimoding and diffraction losses.

Abstract: An improved microwave cavity resonator for use in vapor-cell atomic frequency standards. The resonator comprises a generally-rectangular hollow metallic body designed to support the TE.sub.101 mode of standing wave. A dielectric element is positioned within the cavity to concentrate electro-magnetic field so that a compact design is achieved and so that a preselected, substantially uniform magnetic field will be coupled into an alkali metal vapor absorption cell within the resonator. Means are provided for mounting elements of an excitation circuit to preferentially excite the desired TE.sub.101 mode.

Abstract: A distributed inductive reactance is coupled to a terminating impedance at the interface of a combiner probe with a resonant cavity. The distributed reactance is implemented by a metal insert having a cavity interface and having a coupling portion, which is coaxial with a cylindrical dielectric and with a cylindrical cavity in an end of the terminating impedance.

Abstract: A resonant microwave cavity 2 has a bi-metal end cap 4 or a tri-metal end cap 26. The end caps 4, 26 include irises 10, 28. The end caps are affixed to walls of the cavity 2 in the usual manner. As temperature varies, the end caps or irises expand into or out of the cavity to compensate for the increase or decrease in length of the cavity walls due to variations in temperature. The internal volume of the cavity is maintained substantially constant. When a bi-metal end cap is used, each layer of metal has a different coefficient of expansion. When a tri-metal end cap is used, the center layer has the highest of coefficient of thermal expansion.

Abstract: A frequency stabilized oscillator includes a rectangular resonant cavity having a ground or short circuit wall, an H wall, and a top wall having an inductive window therein. A solid-state oscillating device such as a GUNN diode is attached at one end thereof to the short circuit wall, and a disk type dielectric resonator is supported from the short circuit wall and is located substantially adjacent the solid-state oscillating device so as to electromagnetically couple with the oscillating device. The inductive window is located so as to be in substantially overlying relationship with the dielectric resonator. A strip conductor is attached to the upper end of the oscillating device and extends through the H wall to the exterior of the oscillator to thereby provide a DC supply terminal for the oscillating device. The H wall of the resonant cavity is isolated from the DC supply applied to the strip conductor by means of a thin insulating layer between the strip conductor and the H wall.

Abstract: An absorptive resonant cavity filter suitable for use on the output of a transmitter power amplifier and capable of substantially constant predetermined resistive input impedance at all frequencies. The structure comprises a bandpass cavity which instead of an input coupling loop employs a conductor coupled from the input and configured along the wall of the cavity to form a transmission line of predetermined impedance and terminated by a resistor of similar impedance value.

Abstract: There is provided a resonator comprising principally a hollow metal finger integral with the wall of a guide or a cavity. A metal plunger, coated with a fine dielectric layer, of fairly low permittivity, is able to slide inside the hollow metal finger to cause the tuning frequency of the cavity or the susceptance coupled to the guide to be varied. This layer may be formed from Teflon.

Abstract: An electromagnetic cavity filter (10) is formed by at least two cavities (12) having electrically conductive walls (40, 15). When more than two cavities (12) are employed, their midpoints do not have to be colinear; rather, it is sufficient that the angle formed by the midpoints of any three successively coupled cavities is an integral multiple of 90.degree.. Thus, a folded "engine block" geometry can be realized such that the filter's input cavity (12) is proximate to the output cavity (12). This allows a canonic filter response. Each cavity (12) is the equivalent to two filter poles because two orthogonal modes of electromagnetic radiation can resonate therewithin. Electrically nonadjacent modes of proximate cavities (12), as well as electrically adjacent modes, can be coupled, permitting elliptic filter functions. Electrically nonadjacent modes are coupled by means of an iris (30) opening between the two cavities (12).

Abstract: A band-reject filter comprises a resonator with an adjustable resonator rod which is mounted in proximity to a co-axial assembly. The coupling rod passes thru the inner conductor of the co-axial assembly and extends into the resonator. At the end of the rod inside the resonator there is a coupling disc. Means are provided to adjust the distance between the resonator rod and the disc. The frequency rejected by the filter can be adjusted by either adjusting the length of the resonator rod or by adjusting the distance between the disc and the resonator rod.

Abstract: An equalizer cavity apparatus which is directionally coupled to a rectangular waveguide utilizes a circular waveguide with an adjustable RF absorber therein to lower the quality factor, Q, of the equalizer cavity.