US3784938A - Microwave spectroscopy - Google Patents

Microwave spectroscopy Download PDF

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Publication number
US3784938A
US3784938A US00216207A US3784938DA US3784938A US 3784938 A US3784938 A US 3784938A US 00216207 A US00216207 A US 00216207A US 3784938D A US3784938D A US 3784938DA US 3784938 A US3784938 A US 3784938A
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Prior art keywords
cell
set forth
cell set
section
channel
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US00216207A
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English (en)
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J Cuthbert
J Stow
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Cambridge Scientific Instruments Ltd
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Cambridge Scientific Instruments Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N22/00—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more
    • G01N22/005—Investigating or analysing materials by the use of microwaves or radio waves, i.e. electromagnetic waves with a wavelength of one millimetre or more and using Stark effect modulation

Definitions

  • ABSTRACT In a Lide type of absorption cell for use in Stark modulation microwave spectroscopy the cell is in two mu tually insulated halves which are secured together by longitudinally spaced releasable clamps allowing the cell to be taken apart readily for cleaning but holding the halves accurately in their correct relative positions when assembled.
  • the clamps can be of C-shape with disengageable leaf springs fitting in their open sides.
  • SHEET 2 OF 2 VIJIIIIIIIIII MICROWAVE SPECTROSCOPY This invention relates to the construction of the absorption cell used in the microwave spectroscopy of gases.
  • Such a cell takes the form of a waveguide of rectangular cross-section, usually a metre or up to several metres in length, which contains the gas under examination and a microwave source feeds in power at one end while a detector at the other end examines the resulting absorption spectra.
  • This electrode must be located with great precision since, if it is slightly nearer to one face than the other at any point it will give a greater electric field strength on that side, and any non-uniformity of the field will result in a broadening of the shifted line, and the non-uniformity of the resulting electrostatic forces on the electrode may cause it to oscillate mechanically, the result of which may be an apparent microwave ab sorption signal at the appropriate frequency and phase. Yet at the same time the electrode must be electrically insulated from the guide. Also, the presence of this Stark electrode introduces appreciable attenuation and tends to produce unwanted reflections despite care in its shaping and location.
  • the aim of the present invention is to provide an absorption cell of this type, which can be readily taken apart for cleaning purposes but which at the same time is rigid when assembled, and keeps the two halves acc urately in their correct relative positions, as well as being substantially free from distortion with changes in temperature.
  • an absorption cell for use in Stark modulation microwave spectroscopy comprising a rectangular-section waveguide split in a central plane midway between and parallel to the planes of the longer sides of the crosssection to form two mutually insulated halves, the two halves being releasably held spaced apart by a series of releasable clamps spaced apart along their length.
  • Each clamp is preferably in the form of a C-shapcd member surrounding three sides of the waveguide, with a spring (which may be a leaf spring) spanning the fourth side and bearing on one of the channel-section members to urge it towards the other.
  • a spring which may be a leaf spring
  • FIG. 1 is a simplified block circuit diagram showing a Stark modulation microwave spectrometer
  • FIG. 2 is a perspective view of the cell according to the invention.
  • FIG. 3 is a cross-section through the cell according to the invention, illustrating the construction of one of the clamps.
  • a microwave spectrometer operating on the principle of Stark modulation comprises basically an oscillator 0, an absorption cell C and a detector D.
  • the oscillator may for example be a Klystron oscillator but is preferably a backward wave oscillator since the latter has a greater ability to be varied in frequency over a range of as much as 1.6 to l purely electrically, without requiring any mechanical movement such as a change of cavity size.
  • the oscillator operates over the 0 band (called the R band in the USA.) which covers 26,500 to 40,000 Megaherz.
  • the signal from this oscillator is passed through the cell C which may be a metre long, and into which the substance under examination is introduced.
  • the substance is a gas it can be fed in at normal temperature, but at a low pressure of the order of 10" Torr.
  • it is a liquid at normal temperatures it will normally vaporise anyway at this pressure, but it may be introduced at an elevated tempera ture, and solids may likewise be heated to vaporise them.
  • the frequency of the oscillator is varied slowly over the whole band, or over a part of the band, by a sweep-frequency oscillator S giving a saw-tooth signal of which the sweep period can be varied between 10 seconds and minutes, and as the frequency of the oscillator 0 passes through the frequency of one of the resonant modes of a molecule present in the substance under examination (which may be a mixture) the energy is absorbed to some degree and the signal in the detector D (which is a crystal diode) falls.
  • the absorption is, in absolute terms, very small (only perhaps one millionth of the total energy passing through the cell, it is impossible to detect directly the fall in signal as the oscillator sweeps through a resonant frequency.
  • the modulation signal is, as far as possible, a square wave signal with as short a rise time as possible, to shift the resonant frequency back and forth between two distinct values.
  • the output of the diode D is fed to an amplifier A tuned to the modulation frequency (which may be 40 kiloherz) and used to control the Y deflection of a cathode ray oscilloscope CR or an XY recorder, the X deflection or time-base of which is provided by the saw tooth signal from the sweep oscillator 0.
  • the modulation frequency which may be 40 kiloherz
  • Additional signals will appear in the diode D as the osciallator 0 passes through frequencies differing by the amount of the modulation frequency from the Stark frequency, and these will appear as so-called Stark lobes on each side of the main Stark signal on the CRO screen or recorder chart.
  • a phase-sensitive detector we make these lobes of opposite sign to the main signal, to produce a spectrum (for one particular resonant frequency) of the kind indicated on the CRO screen in FIG. 1.
  • the absorption cell C according to the invention is shown in FIGS. 2 and 3.
  • a waveguide for the Q band in U.S.A. the R band
  • To apply the Stark electrostatic field by splitting a cell of such dimensions in a central longitudinal plane midway between these two shorter walls and applying the signal between the resulting two mutually insulated halves would, as indicated earlier, produce unstatisfactory results, since the electrostatic field produced would be non-uniform.
  • the degree of Stark shift of the resonant frequency is proportional to the field strength, which needs to be of the order of two or three thousand volts per centimetre for a worthwhile shift of one Megaherz. If this field strength is not uniform the shift will be spread over a range and result in a loss of resolution.
  • the spacing between them is 0.702 cm., which is the normal dimension in this direction for a waveguide in the Q band, and there is a gently tapering transition section T at each end of the cell to change over the internal dimension in the E-plane (but not the H-plane) from the standard waveguide cross-section to that of the cell.
  • the two rectangular channel sections F that face each other to form the cell are machined by milling or electro-forming from drawn metal channel sections. They are held in their correct relationship by a series of C-clamps at intervals along their length, two of the clamps being visible in FIG. 2.
  • Each clamp comprises a flat rigid C-shaped member G extending around three sides of the cell and a leaf spring L of flattened V-shape that engages notches in the inner faces of the two ends of the member G.
  • Opposed pairs of B of PTFE of Teeshaped cross-section fit between the flanges of the two channels F to keep them at the correct spacing apart (0.15 mm. in the example shown) and have their outer faces grooved to engage and be located by the inner edges of the member G.
  • the outside face of the web of one of the channels F (the right-hand one on FIGS. 2
  • clamps and 3) can have shallow transverse grooves machined in it at longitudinally spaced intervals as shown at I in FIG. 3 to receive the inner edges of the webs of the members G of the clamps and thereby locate the clamps axially.
  • the clamps are thus at the electric potential of this channel, normally the grounded channel, and the clamp is insulated from the other channel, by a pad P of PTFE.
  • the ends of the leaf springs L are themselves notched to locate them in the members G. It will be appreciated that the resulting clamp can be readily assembled and dismantled by hand and that, when assembled, it urges the two halves of the cell tightly and resiliently together.
  • the simple shape of the parts, including the channels F enables their dimensions to be closely controlled during manufacture, so that the two halves of the channel are accurately and rigidly located in the correct relative positions and minimum spacing (consistent with the voltage to be applied between them) to provide a smooth waveguide offering a minimum of reflections and unwanted modes.
  • the simple manual way of releasing the clamps ensures that the user can quickly take them apart to dismantle the cell for cleaning purposes. This is particularly useful in a cell intended for use in routine chemical analysis, where unexpected condensation or decomposition, within the cell, of (possibly unknown) constituents may occur.
  • the rigid construction resulting from the channels F and the clamps G,L allows the cell to be heated or cooled without significant distortion.
  • the cell will normally be mounted in a close fitting round cylindrical housing to which the gas or vapour under examination is admitted, and the members G, as well as forming parts of the clamps, also form spaced supports for the cell within the housing, which is indicated in broken lines at H in FIG. 3.
  • at least the lower limb of each member can, as shown, be of part-circular profile, or at least have as an outer periphery containing points lying on a circle centered on the axis of symmetry of the cell, so that the cell is then located centrally in the housing.
  • An absorption cell for use in Stark modulation microwave spectroscopy comprising a rectangular section wave guide of cross-section having longer sides and shorter sides and split in a central plane midway between and parallel to the planes of said longer sides to form two channel-section members, opposed blocks of electrically insulating material extending along the length of said wave-guide and having portions of predetermined width projecting between the mutually opposed flanges of said channel section members to mutually space said members the correct distance apart while mutually insulating one from the other, and a series of longitudinally spaced apart releasable clamps each engaging a portion of each of the opposed blocks and releasably holding said blocks and said members in assembled condition.
  • said clamp comprises a C-shaped member having a web and limbs surrounding three sides of said waveguide, said limbs engaging the portions of said blocks, and a spring spanning the fourth side, said spring bearing on one of said channel-section members to urge it towards the other said channel-section member.
  • each said clamp has a periphery containing points lying on a cirblocks are arranged in longitudinally spaced apart, op-

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
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US00216207A 1971-01-12 1972-01-07 Microwave spectroscopy Expired - Lifetime US3784938A (en)

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Application Number Priority Date Filing Date Title
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20020021197A1 (en) * 1999-10-29 2002-02-21 Berg Technology, Inc. Waveguides and backplane systems
US20100052822A1 (en) * 2008-08-26 2010-03-04 Mitsubishi Electric Corporation Waveguide, antenna and vehicular radar apparatus
US20120086527A1 (en) * 2010-09-30 2012-04-12 Nealis Edwin Systems and methods of waveguide assembly
US20140368376A1 (en) * 2013-05-23 2014-12-18 Texas Instruments Incorporated Calibration scheme for gas absorption spectra detection

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5544904B2 (2) * 1973-09-05 1980-11-14
US4220915A (en) * 1978-06-28 1980-09-02 Rca Corporation Resistivity measurement system

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2155508A (en) * 1936-10-31 1939-04-25 Bell Telephone Labor Inc Wave guide impedance element and network
US2381367A (en) * 1941-07-10 1945-08-07 British Insulated Cables Ltd Guide for the transmission of electric waves
US2433368A (en) * 1942-03-31 1947-12-30 Sperry Gyroscope Co Inc Wave guide construction
DE883460C (de) * 1944-09-21 1953-07-16 Siemens Ag Kupplung fuer Hochfrequenzleitungen
US3315186A (en) * 1964-07-18 1967-04-18 Philips Corp Wave guide joint having non-conductive gap between sections
DE1515958A1 (de) * 1965-09-23 1970-01-02 Krone Gmbh Hohlleiter

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2155508A (en) * 1936-10-31 1939-04-25 Bell Telephone Labor Inc Wave guide impedance element and network
US2381367A (en) * 1941-07-10 1945-08-07 British Insulated Cables Ltd Guide for the transmission of electric waves
US2433368A (en) * 1942-03-31 1947-12-30 Sperry Gyroscope Co Inc Wave guide construction
DE883460C (de) * 1944-09-21 1953-07-16 Siemens Ag Kupplung fuer Hochfrequenzleitungen
US3315186A (en) * 1964-07-18 1967-04-18 Philips Corp Wave guide joint having non-conductive gap between sections
DE1515958A1 (de) * 1965-09-23 1970-01-02 Krone Gmbh Hohlleiter

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6590477B1 (en) 1999-10-29 2003-07-08 Fci Americas Technology, Inc. Waveguides and backplane systems with at least one mode suppression gap
US6724281B2 (en) 1999-10-29 2004-04-20 Fci Americas Technology, Inc. Waveguides and backplane systems
US20040160294A1 (en) * 1999-10-29 2004-08-19 Berg Technology, Inc. Waveguide and backplane systems
US6960970B2 (en) 1999-10-29 2005-11-01 Fci Americas Technology, Inc. Waveguide and backplane systems with at least one mode suppression gap
US20020021197A1 (en) * 1999-10-29 2002-02-21 Berg Technology, Inc. Waveguides and backplane systems
US8446233B2 (en) * 2008-08-26 2013-05-21 Mitsubishi Electric Corporation Waveguide, antenna and vehicular radar apparatus
US20100052822A1 (en) * 2008-08-26 2010-03-04 Mitsubishi Electric Corporation Waveguide, antenna and vehicular radar apparatus
US20120086527A1 (en) * 2010-09-30 2012-04-12 Nealis Edwin Systems and methods of waveguide assembly
US8816799B2 (en) * 2010-09-30 2014-08-26 Aviat U.S., Inc. Systems and methods of waveguide assembly using longitudinal features
US9502743B2 (en) 2010-09-30 2016-11-22 Aviat U.S., Inc. Systems and methods of waveguide assembly using longitudinal features
US10090570B2 (en) 2010-09-30 2018-10-02 Aviat U.S., Inc. Waveguide bend assembly having waveguide flanges with cavity portions therein for attaching the waveguide bend to straight waveguides
US20140368376A1 (en) * 2013-05-23 2014-12-18 Texas Instruments Incorporated Calibration scheme for gas absorption spectra detection
US9128023B2 (en) * 2013-05-23 2015-09-08 Texas Instruments Incorporated Calibration scheme for gas absorption spectra detection

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Publication number Publication date
FR2122172A5 (2) 1972-08-25
GB1320673A (en) 1973-06-20

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