Abstract: A resonator is provided for electron spin resonance spectroscopy. It comprises a resonant element comprising a cavity, and a predetermined oscillation mode is capable of propagating in the cavity. The resonator comprises further a coupling element comprising a waveguide section. Finally, the resonator is equipped with a tuning arrangement for coupling an electromagnetic wave in the waveguide section with the oscillation mode in the cavity. The tuning arrangement comprises at least one conductive wafer, which is arranged at the free end of a tuning rod projecting into the waveguide section and is adapted for displacement in the direction of a longitudinal axis of the tuning rod. The wafer further coacts with a coupling element disposed between the waveguide section and the cavity. Two wafers are arranged on either side of the coupling element, in the direction of the longitudinal axis, and can be displaced in opposite senses relative to the coupling element.

Abstract: A method of electron spin resonance enhanced magnetic resonance imaging of a paramagnetic substance-containing sample wherein an image of at least part of the sample is generated from a data set produced by manipulation of enhanced free induction decay signals detected under at least two different sets of operational parameters. The operating parameters which are varied to produce the different sets include the timing of the exposure of the sample to magnetic field gradients and spin resonance stimulating radiations and, optionally, also the amplitude and frequency distribution of the electron spin resonance stimulating radiations. The manipulation of the detected signals produces, for individual image elements, a normalized signal representative of the difference between signals for that element under the different sets of operational parameters.

Abstract: There is provided an improved magnetic resonance imaging procedure which essentially combines electron spin resonance enhanced MRI with echo planar imaging MRI.

Abstract: An electron spin resonance spectrometer which is dedicated to a specific narrow range of tasks is reduced in size such that, as contrasted with the usual laboratory ESR spectrometers, it is easily and readily portable and can be carried to various locations to conduct tests. It uses a much smaller magnet structure than usual, a self contained single board computer, and a microwave source which is adjustable by the computer. The microwave source operates in the 2 GHz range and is connected through a circulator with a matching circuit which inductively couples the microwave signal to a sample chamber/resonator positioned in the magnetic field of the magnet structure. An oscillator provides a audio frequency signal which is combined with a slow ramp signal from the computer to modulate the output of the magnet structure.

Abstract: The present invention describes a method of and apparatus for performing electron spin resonance enhanced magnetic resonance imaging (ESREMRI) at ultra-low fields of up to 20 Gauss, optionally dispensing with a primary magnet and using the earth's ambient field to provide the primary magnetic field for the magnetic resonance imaging procedure.

Abstract: A resonator is provided for electron spin resonance spectroscopy. The resonator comprises walls defining a cavity. In the cavity, a predetermined oscillation mode of a preselectable frequency, in particular in the Q band, is capable of propagating. The walls are provided with an opening for irradiating a radiation, in particular light rays. The cavity has a cylindrical shape. An oscillation mode of the type TE.sub.01n is excited, with n being greater than one. The opening is configured as an annular slot extending substantially all around the cylindrical jacket, which defines the cavity, in a radial plane at a distance of m L/4 from a radial end wall delimiting the cavity, and m is an odd number and greater than one (FIG. 2).

Abstract: An electron spin resonance system comprising a magnetic circuit in which a pair of permanent magnets are respectively attached to opposing surfaces of a pair of stationary yokes arranged opposite to each other by way of a gap, a movable yoke is disposed to at least one of the pair of the stationary yokes for continuously changing the distance between opposing surfaces of the stationary yokes, so that the magnetic resistance of a magnetic path formed by the permanent magnets, stationary yokes and movable yokes is varied continuously thereby forming a continuously varying magnetic field in the gap between the opposed permanent magnets, and a modulating magnetic field application coil for applying a modulating magnetic field and/or a sweeping magnetic field application coil for applying a sweeping magnetic field to the gap, a cavity resonator disposed in the gap in which a specimen to be measured can be inserted, and a microwave generator and a microwave detector disposed to the cavity resonator.

Abstract: A magnetic resonance image generating apparatus comprising a first radiation source capable of emitting a first radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in a sample being imaged and means for detecting free induction decay signals from said selected nuclei, characterized in that said apparatus further comprises a second radiation source capable of emitting a second radiation of a frequency selected to excite electron spin transitions coupled to the nuclear spin transitions of at least some of said selected nuclei.

Abstract: A magnetic resonance imaging apparatus comprising device for applying a magnetic field to a sample to be imaged, device for applying pulses of MR transition-exciting electromagnetic radiation to the sample, an NMR signal coil for detecting the resulting radiation from the sample, and device for appling pulses of electromagnetic radiation to the sample for stimulating coupled ESR transitions, in which a pulse generating device is coupled to the NMR signal coil and antennas are coupled to the coil and disposed at plural positions along the coil, whereby pulses for stimulating coupled ESR transitions may be transmitted from the pulse generating device through the coil to the antennas and thence into the sample, the antennas being arranged to give a predominantly magnetic coupling to the sample.

Abstract: Vanadium in marine diesel lubricating oils resulting from raw fuel dilution is detected and quantified by way of an analytical technique employing electron spin resonance (ESR). Dilution of the lubricating oil with residual fuel oils results in the presence of vanadium in the lubricating oil in a form which is capable of being observed by ESR; by contrast, vanadium resulting from the presence of fuel oil combustion products (pentavalent vanadium) in the lubricating oil is not observed by ESR. The method therefore enables fuel vanadium, typically present as vanadyl porphyrins, to be differentiated from vanadium resulting from blow-by in the engine so that the operating condition of the engine may be monitored.

Abstract: The invention relates to a method and apparatus for the examination of a moving component by the application of NMR methods and dynamic nuclear polarization (DNP). The saturation of electron spins required by DNP is adapted to occur within such area of an object in which the polarizing magnetic field has a different strength from the area of object which is subjected to NMR operations.

Abstract: The invention relates to a method of authenticating an object by electron paramagnetic resonance spectroscopy, the method being characterized in that at least one authenticating substance is added to the object, said substance having an EPR peak whose width is not greater than about 1.5 gauss, with the spectroscopy being performed in a static field of an amplitude such that the substance has a resonance frequency in the radio-frequency domain, and with the width of the EPR peak of the substance being used as a criterion for discrimination.

Abstract: An apparatus for the examination of an object, such as a human body, by means of ultrasound and NMR methods in such a manner that both examination methods are adapted to be effected substantially simultaneously and on the same target area. The apparatus is provided with a device for radiating the magnetization of an electron spin system in the said target area in order to achieve a dynamic nuclear polarization and to amplify an NMR signal through a so-called Overhauser-phenomenon.

Abstract: There is provided an electron spin resonance enhanced magnetic resonance imaging (ESREMRI) apparatus having means arranged to generate a primary magnetic field of a first value during periods of nuclear spin transition excitation and magnetic resonance signal detection for the generation of ESREMRI images of a subject and means arranged to generate a primary magnetic field of a second and higher value during periods of nuclear spin transition excitation and magnetic resonance signal detection for the generation of native MR images of a subject, and preferably also means for combining, e.g. superimposing, ESREMRI and native MR images so generated.

Abstract: An electron spin resonance system comprising a magnetic circuit in which a pair of permanent magnets are respectively attached to opposing surfaces of a pair of stationary yokes arranged opposite to each other by way of a gap, a movable yoke is disposed to at least one of the pair of the stationary yokes for continuously changing the distance between opposing surfaces of the stationary yokes, so that the magnetic resistance of a magnetic path formed by the permanent magnets, stationary yokes and movable yokes is varied continuously thereby forming a continuously varying magnetic field in the gap between the opposed permanent magnets, and a modulating magnetic field application coil for applying a modulating magnetic field and/or a sweeping magnetic field application coil for applying a sweeping magnetic field to the gap, a cavity resonator disposed in the gap in which a specimen to be measured can be inserted, and a microwave generator and a microwave detector disposed to the cavity resonator.

Abstract: The invention relates to a method and apparatus for applying dynamic nuclear polarization and magnetic imaging to studying an object. As the saturation of an electron spin system required by dynamic nuclear polarization is proceeding, the intensity of a polarizing magnetic field over the object is different from that of a polarizing magnetic field lying over the object during the detection of an NMR signal. The apparatus is provided with means for changing the intensity of a polarizing magnetic field in a manner such that the difference between magnetic field intensities is achieved.

Abstract: The EPR instrument includes an X-band klystron used to generate microwave power that is transmitted to a superconductive waveguide cavity resonator. The cavity is placed between the pole pieces of a large d-c magnet and small a-c magnet coils energized by a modulation signal source. A microwave crystal detector detects RF energy outputted from the cavity. The detector output is amplified, rectified and filtered in a phase sensitive filter that uses the modulation signal source as a reference. The resultant signal is recorded by an amplitude vs. time recorder. The superconductive cavity includes an inner wall of high-temperature superconductive material, a liquid coolant and an insulating sleeve that passes through the cavity. The specimen to be analyzed is placed in the sleeve.

Abstract: There is disclosed a method of accurately calculating the magnetic field strength at an arbitrary position in an ESR (electron spin resonance) spectrum using Mn.sup.2+ marker. The spectrum contains an ESR signal arising from an unknown sample and an ESR signal arising from a reference sample containing Mn.sup.2+ marker. The resonating magnetic field strengths H.sub.ci and H.sub.cj of two absorption lines of the six absorption lines arising from Mn.sup.2+ are calculated from the applied microwave frequency, the nuclear spin quantum numbers m.sub.Ii and m.sub.Ij producing the two absorption lines, a predetermined g value g.sub.o intrinsic to Mn.sup.2+, and a predetermined isotropic hyperfine coupling constant A. The magnetic field strength H.sub.cx at a position of interest in the ESR spectrum is determined from the distance between the absorption lines, the distance between the position and one of the two absorption lines, and the magnetic field strengths H.sub.ci, H.sub.cj.

Abstract: An electron paramagnetic resonance (EPR) spectrometer employing a reference arm microwave bridge provides efficient, simultaneous generation of orthogonal I and Q signals. The simultaneous generation is enabled through the use of a field effect transistor (FET) microwave amplifier in the signal arm feeding a balanced quadrature mixer. The I and Q signals are then combined to produce a signal suitable for use as a discriminator in an automatic frequency control (AFC) circuit. An important characteristic of the discriminator signal is that it is independent of the relative phase angle .alpha. between the reference arm and signal arm of the bridge. In a further embodiment, an apparatus is disclosed which computes .alpha. from the I and Q signals, and uses the computed value for .alpha. to extract pure absorption and pure dispersion information from the I and Q signals regardless of the setting of .alpha. in the bridge.

Abstract: An electron spin resonance spectrometer is disclosed, in which a sample made of a semiconductor material or the like is illuminated with light while being applied with a magnetic field and a microwave, to detect the electron spin resonance due to a defect in a light-illuminated portion of the sample, on the basis of a photo-induced voltage appearing across the sample or a reduction in microwave intensity caused by the microwave absorption of the sample.

Abstract: The high frequency magnetic field generating portions are arranged at spatially different positions, respectively, and high frequency currents having phase difference corresponding to a spatial deviation between the high frequency magnetic field generating portions are supplied to the high frequency magnetic field generating portions, resultant high frequency magnetic fields being combined to produce a single rotating magnetic field.

Abstract: The invention relates to a method and apparatus to detect the superconductive state by measuring the change in the resistance of a sample. A dc magnetic field is imposed on the sample while the temperature is swept. The strength of the magnetic field is held below the critical value and is frequency modulated by the application of an ac field in such a way that total magnetic field is always positive. The resistance of the sample is measured incrementally as the temperature is swept and is phase detected at the modulation frequency. According to this embodiment, only magnetic field dependent changes demonstrating a precipitous drop in resistance identify a composition as superconductive.

Abstract: This invention involves resonance enhancement of signals obtained from magnetic specimen undergoing changes in magnetic state. This is accomplished by placing the sample, magnetized by a quasi-static magnetic field, in a high frequency resonant cavity. By adjusting the resonance frequency of the sample-containing cavity so that it is resonant at a d.c. field corresponding to domain wall motion or domain rotation, clear signals can be obtained from the sample.

Abstract: An electron spin resonance spectrometer comprises a resonator containing a sample and arranged in a magnetic field of constant strength and high homogeneity. A microwave bridge can be supplied with microwave energy in the form of an intermittent signal. Measuring signals emitted by the resonator are supplied to a detector and a signal evaluation stage. A line provided between a microwave source and the microwave bridge is subdivided into parallel pulse-shaping channels, one of them containing a phase shifter, an attenuator and a switch for the signal passing through the pulse-shaping channels. In order to be able to set, if possible, an unlimited plurality of pulse sequences for experiments of all kinds, the pulse-shaping channels are supplied in equal proportions from the line by means of a divider. All pulse-shaping channels are provided with a phase shifter and an attenuator. The pulse-shaping channels are re-united by means of a combiner arranged before the input of a common microwave power amplifier.

Abstract: An electron spin resonance spectrometer is disclosed which includes a sample and a reference arm (16, 18). The sample arm has a circulator (30) for directing radiation to a resonator (22) in which a sample for analysis is deposited and for receiving radiation from the resonator and directing it to a detector (32). The detector detects the microwave radiation from the sample and reference arms (16, 18) to determine a property or properties of said sample. The sample arm (16) and said reference arm (18) are formed as an integrated microwave bridge which includes a divider (14) having a first output portion coupled to an attenuator (20), said attenuator being coupled to said circulator (30) by a microwave transmission line (110). The circulator (30) is coupled to at least one output by a second microwave transmission line (124).

Abstract: A resonance system for ESR spectroscopy has a resonance cavity (23) which connected by axial slots (25) with chambers (24) coaxial to the resonance cavity (23). The resonance cavity (23) and the chambers (24) are located within a metallic screen (21). The wall sections (26) of the resonance cavity (23), separated by the axial slots (25), form the inductances of the resonance circuit, whereas the wall sections of the axial slots (25), located opposite one another, form the capacitances. The entire resonance system, including the screen, is made of metal and is, therefore, suitable for use at extremely low temperatures.

Abstract: An electron spin resonance spectrometer comprises a resonator arranged in a magnetic field of constant strength and high homogeneity and containing a sample. The resonator can be supplied, via a microwave bridge, with high-power microwave energy in the form of an intermittent signal. Measuring signals emitted by the resonator can be supplied to detector arrangements and signal evaluation arrangements. In order to be able to cut a pulse-shaped output signal of the microwave power amplifier off sharply at its trailing edge, the attenuator is designed as a switching stage comprising a circulator whose input is connected to the output of the microwave power amplifier. A subsequent circulator connection is wired up to a series connection comprising a microwave switching diode that can be switched via a control input and a waveguide termination, and another subsequent circulator connection is connected to the microwave bridge (FIG. 10).

Abstract: An electron spin resonance spectrometer comprises a resonator arranged in a magnetic field of constant strength and high homogeneity and containing a sample. The resonator can be supplied, via a microwave bridge, with microwave energy in the form of a continuous-wave signal (CW) or in the form of an intermittent signal (P). Measuring signals emitted by the resonator can be supplied to a detector arrangement and a signal evaluation arrangement. In order to be able to carry out individual experiments with continuous-wave signals or intermittent signals of low power or with intermittent signals of high power in arbitrary combination and with an arbitrary succession of microwave pulses, the spectrometer comprises a first channel for supplying a continuous-wave signal or an intermittent signal of small power in the mW range and a second channel for supplying an intermittent signal of high power in the W range. The channels are connected by their inputs to a common microwave source and united at their outputs.

Abstract: In the control of a refinery process, vanadium content of the hydrocarbon streams is determined by measuring the intensity of the 5 isotropic and 6 anisotropic ESR spectral peaks. The ln of the sum of the intensities of these two peaks has a linear relationship to vanadium content.

Abstract: An ESR spectrometer is equipped with a split-ring resonator comprising an electrically conductive cylinder or loop which may be excited by a microwave antenna. The conductive cylinder or loop is secured to a surface of an insulating sheet whereby the other surface may be brought to bear upon the specimen being examined. Preferredly, the insulating sheet comprises two insulating sheets with a Faraday shield sandwiched therebetween whereby the magnetic field generated by the microwave energy applied to the loop leaks to the surface of the specimen but the electric field is inhibited.

Abstract: A local probe for use in nuclear magnetic resonance imaging includes a pair of loop-gap resonators which are connected together and positioned adjacent a region of interest. FID signals produced in the region of interest induce signals in coupling loops mounted near each loop-gap resonator, and these connect to a transmission line leading to the receiver. Three embodiments are disclosed, one best suited for imaging appendages, a second best suited for imaging the head and neck, and a third best suited for the spine.

Abstract: A method and apparatus for imaging the concentration of paramagnetic species inside a sample is disclosed wherein a sample is placed inside an NMR imager and an additional microwave field is introduced. A microwave field is chosen in such a way that there is a simultaneous resonance of nuclear spins (protons) and electrons in the same main magnetic field. The microwave signal is modulated which provides a modulation transfer to the intensity of the NMR signal. The modulation is extracted from the NMR signal to produce an image representative of the local electron spin resonance (ESR) and thereby the concentration of paramagnetic species in the sample. In addition, electrical activity of the brain can be detected and measured by measuring the broadening of the width of NMR spectral lines. A discharging neuron in the brain introduces an inhomogeneity in the magnetic field which reveals itself as a broadening of resonance lines which can be measured to determine the neuron discharge current flux.

Abstract: The prior art split-ring resonator for use in an electron spin resonance (ESR) spectrometer comprises a circular cylinder made from an electrically conductive material and housed in a metallic tube. The cylinder is provided with a split extending axially of the cylinder. A sample to be investigated is inserted into the cylinder in the axial direction. In accordance with the invention, a shielding member which is wider than said slit and made from an electrically conductive material is disposed so as to shield the slit from the inside of the cylinder, whereby the resonance frequency of the resonator is not significantly affected by the presence or absence of the sample or by the amount of the inserted sample.

Abstract: Magnetic resonance images of the distribution of a substance within a sample are obtained by splaying a pair of magnetic field generating coils relative to each other to generate a magnetic field gradient along an axis of the sample. In other aspects, electron spin resonance data is derived from animal tissue, or images are derived from a sample that includes dissipative material, using a radio frequency signal of sufficiently low frequency.

Abstract: A fast low angle shot ("FLASH") NMR method is described, which uses radio frequency excitation pulses having a flip angle substantially less than 90 degrees, and subsequent reading gradient reversal for producing a gradient echo signal. The small flip angle pulses create an equilibrium state between longitudinal and transverse magnetization, which allows to apply the radio frequency pulses repeatedly with high repetition rate to produce gradient echo signals of essentially equal strength, without any intervening delay for magnetization restoration or recovery.

Abstract: This disclosure relates to a microwave amplifier (22) having an active X-Band microwave cavity (33) for use in electron paramagnetic resonance (EPR) measurements on large, lossy, irradiated, samples such as a human finger. The amplifier comprises an input section (31), a drift section (32) and an output section (33) which is the aforementioned active microwave cavity. An electron beam is used to input RF energy into the cavity. The input and output section have small helical couplers (34) for coupling RF energy onto and off of the electron beam. The RF wave is essentially "trapped" in the cavity because of the non-reciprocal nature of the electron beam medium and because the guide (i.e., drift section) at the input to the cavity has dimensions such that the frequency of the RF microwave energy is below the guides lower cut-off frequency. Also, a frequency "locking" effect occurs in the output cavity. The input frequency can be varied from 1-2% yet the output frequency does not change.

Abstract: An antenna device for exciting a magnetic high-frequency field and/or for receiving corresponding high-frequency signals in a nuclear spin tomography apparatus contains a tubular antenna part which is designed to largely pass low frequencies, for magnetic gradient fields, consists of electrically highly conductive nonmagnetic material and has means for connecting to an external energy feeding or receiving device. In order to permit a simple design for this antenna device, only the tubular antenna part is provided as a circular waveguide for exciting or receiving the high frequency field. The circular waveguide is provided at least in the region of the cross-sectional plane extending through the center of its axial dimension with a system of coupling elements for coupling in or out the corresponding high-frequency power. In addition, further coupling systems may be provided in at least two cross-sectional planes symmetrical to the central cross-sectional plane.

Abstract: A cavity resonator serves to generate magnetic dipole transitions in a sample, for instance for carrying out electron spin resonance measurements. The natural frequency of the cavity resonator (10) is influenced by a reduction in length of the E flux lines. To this end, conductive areas (20) are arranged at the points where the E flux lines are encountered. This results in an increase of the space factor, related to an unchanged specimen volume, and thus in an improvement of the measuring sensitivity.

Abstract: The thickness of a web is measured by passing the web through the gaps of a loop-gap resonator. Changes in web thickness alter the resonant frequency of the resonator which is detected by a control circuit. Three embodiments of the invention are described, and in one embodiment, the moisture content of the web is also measured.

Abstract: An exploration method is disclosed for mapping subsurface migration of crude petroleum by measuring electron spin resonance signals from materials within samples of the subsurface formations and identifying the samples that contain petroleum crude by detecting enhancement and/or suppression of the electron resonance signals due to the addition of a chemical material. Electron spin resonance signals from crude oil are enhanced by the presence of iodine and suppressed by the presence of ferric chloride.

Abstract: A well logging method is disclosed for identifying the presence of crude petroleum within subsurface earth formations by detecting electron spin resonance signals along a well bore and identifying crude petroleum as the source of those signals by enhancing and/or suppressing the signal by the presence of a chemical. Electron spin resonance signals from crude oil are enhanced by the presence of iodine and suppressed by the presence of ferric chloride.

Abstract: The ends of a loop-gap resonator are extended along a closed path to define an enclosed chamber with a gap extending along its length. A lumped circuit resonator is formed and the magnetic field is confined to the chamber. Openings may be formed in the chamber wall to direct magnetic flux through sample materials outside the chamber, and the cross sectional area of the chamber may be altered to control the energy density distribution of the magnetic field.

Abstract: A capacitively shortened coaxial resonator for making nuclear magnetic resonance measurements includes inner and outer concentrically arranged metal cylinders. The cylinders define therebetween a sample region and are shorted electrically by a conductive cap which closes one end of the inner and outer cylinders. A capacitor is connected across the cylinders at the opposite open end and functions to resonate with the inductance of a coaxial conductor formed by the short circuited cylinders. Samples placed in the sample region are mounted on an air bearing and rotated by means of air holes formed in the inner and outer cylinders.

Abstract: A magnetic resonance apparatus uses a first harmonic signal produced by magnetic field modulation as a correction signal and the second harmonic signal to provide a property of a sample.

Abstract: A structure for supporting a loop-gap resonator includes a cylindrical housing which defines a space into which microwave energy is injected. The resonator is mounted to a support element which is inserted into the space through an opening at one end of the housing. The support element and attached resonator form an interchangeable module.

Abstract: A microwave resonator is formed by a cylindrical conductive loop and one or more gaps which extend along its length. The loop forms an inductive element and the gaps form capacitive elements which determine the resonant frequency. The loop is surrounded by a conductive shield which improves the quality factor (Q) of the resonator and high frequency radio signals are coupled to the resonator by an inductive loop positioned at one of its ends.

Abstract: A specimen head for electron spin resonance and paramagnetic electron resonance measurements, having an annularly cylindrical resonator, with a waveguide being coupled to one of its end walls in such a manner as to excite an H.sub.011 wave. The other end wall is formed by a piston capable of being shifted for tuning, the piston being attached at the end of a hollow piston rod (6) mounted in a bearing block (4), which also contains the resonator (16), in such a manner that it is concentric relative to the axis of the resonator and longitudinally shiftable. The waveguide (13) is attached to the bearing block (4) on that side of the resonator (16) which is opposite the piston (7) and extends along the outside of the bearing block parallel to the piston rod (6) after having been deflected 180.degree..

Abstract: A microwave resonator is formed by a cylindrical loop and one or more gaps which extend along its length. The loop is formed from a machineable insulating material and a layer of electrically conductive material is deposited over its surfaces.

Abstract: Method for enhancing electron spin resonance signals from crude petroleum by contacting the crude petroleum with a chemical which effects the population of free radical asphaltenes in the crude petroleum. The enhancement is in a positive sense when the chemical is a halogen and particularly iodine. The enhancement is in a negative sense (suppression) when the chemical is ferric chloride.

Abstract: A test head for electron spin resonance and paramagnetic electron resonance measurements comprises a resonator of circular cylindrical shape adapted to accommodate a coaxially arranged test tube, and a waveguide arrangement for exciting the H.sub.011 wave in the resonator, said waveguide arrangement comprising one section having its one end arranged adjacent one face of the resonator and communicating with the resonator via an opening provided in the face in eccentrical arrangement relative to the resonator. In order to enable the coupling coefficient to be varied and, thus, the resonator to be adapted to the waveguide section, the waveguide section carrying a H.sub.1n wave communicates with the resonator via a coupling slot the angular position (.phi.) of which can be varied relative to the radial plane of the resonator passing through its center. Preferably, the waveguide section exhibits a circular cross-section and can be rotated about its axis. It guides the H.sub.