US 3077569 A
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Feb. 12, 1963 K. IKRATH 3,077,569
SURFACE WAVE LAUNCHER Filed Nov. 5, 1959 FIELD FICTiTIOUS MAG. DIPOLES INVENTOR, KURT IKRATH ...Wx/5. a.
United States Patent @dice $77,559 Patented Feb. l2, 1963 3,677,569 SURFACE WAVE LAUNCHER Kurt llirrath, 2*5 l-irhiand Ave., Elheron, Nil., assigner to the United States of America as represented by the Secretary oit the Army Filed Nov. 3, 1959, der. No. @563,742 5 Qlairns. (Cl. 33E- 95) (Granted under Title 35, U.S. Code (1952), sec. Zoon The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
This invention relates to a waveguide and more particularly to an electromagnetic wave launcher which is capable of propagating surface waves along the boundaries of a current carrying wire mesh structure and materials of different electromagnetic properties. n
The electrical and mechanical features of the invention make it highly useful as a nonres-onant coupler of radio frequency power for feeding conventional surface wave transmission lines from conventional cables and as a nonresonant travelling wave antenna element.
Wave launchers comprised of combinations of feeders, couplers, and antennas of various types have been used before in Iapplications t-o which this invention is adopted but have proved to be unsatisfactory in many respects. This invention provides a relatively'simple device to replace the combinations of elements previously used.
It is well known that electromagnetic waves can be propagated along surfaces where discontinuities of the electromagnetic properties of media exist; for example, along the plane or spherical boundaries between ground and air or water and air, along the cylindrical boundary surface of copper wire and air and specially designed surface transmission lines utilizing dielectric coated wire and air as media (G-strings), along dielectric rods, and along cylindrical holes in dielectric material. These surface wave modes of propagation prior to applicants invention were excited by conventional methods with dipoles, probes, and open-ended waveguides such as horns and various combinations thereof.
intimately linked with guided wave phenomena is radiation from the guided surface wave, i.e., the breaking away into space of electromagnetic energy from the guiding structures as a result of discontinuities on the structure ofthe guide. Such a structure then becomes a travelling wave radiator and is known as a travelling wave wire antenna or dielectric antenna. These radiators have the common property that they produce highly directional multi or single lobe radiation patterns such that their directivity is proportional to their length, a distinctive characteristic of this type of antenna. Although these antennas are inherently nonresonant wide band devices, in prac ice bandwidth limitations are caused by the conventional feeder and coupler systems. 1
An object of this invention is to provide la unidirectional travelling wave antenna which physically incorporates feeder, coupler and radiating action into one element.
A second object of this invention is to provide a nonresonant distributed coupling means -to a surface wave mode of electromagnetic energy from a conventional waveguide or transmission line mode of wave propagation.
A further object of this invention is to provide a unidirectional travelling wave antenna whose physical length is less than that of conventional long wire antennas with similar directional characteristics.
A preferred form of the invention consists of a coaxial arrangement of a cylindrical inner conductor electrically insulated from a cylindrical envelope or enclosure which contains an inner layer in the forni or a conductive mesh of closely spaced apertures and an outer layer of high permeability material of suitable thickness. The ends of the inner conductor and the inner layer of the envelope, in actual use of the invention, are connected to the inner and outer conductors, respectively, of a cable connector to permit feeding from a conventional cable and/or connection to the terminals of power generators and loads.
Another embodiment of the invention is a flat, sandwich-shaped variation of the above description.
Further objects and advantages of the invention will be apparent from the following description taken in connection with the drawing, wherein:
FGURES 1 and 2 illustrate in diagrammatic form the basic principles of the invention;
FIGURES 3 and 3a illustrate the invention in coaxial form;
FIGURES 4 and 4a illustrate a planar version of the invention;
FIGURE Str shows the mesh apertures of the outer conductors; and
FIGURE 5b graphically shows a single aperture for purpose of the lmathematical development.
The same reference numbers are used to represent the same or similar elements throughout the iigures Iof the drawing.
Referring to FIG. l of the drawing, a braided or meshwire conductor )itl of the type commonly used in coaxial transmission lines as the outer conducting element `has a primary magnetic field Hp induced in the space beneath it by the wave travelling through the line. Leakage of primary iield Hp through the apertures li. into the originally ield-free space above the apertures il at high frequencies exceeds by far the leakage through the metal part of braid l@ and is represented by tield lines i2. The field thus formed in the space above the apertures can be considered to emanate from a fictitious magnetic and elec tric dipole, indicated lschematically in the apertures 11. The close proximity of apertures li and hence the close spacing of the tictitious dipoles located within the apertures lll results in a strong coupling between the fictitious dipoles to form the magnetic leakage r'icld ft2. The dipole concept explains the experimentally observed shape of the radiation pattern from a line constructed in this manner whose main lobes tend to emanate in the direction of the exciting travelling wave and which have the forward tilt of slow wave type radiation patterns. The length of the rneshwire lo has a direct effect on the sharpness of the lobes, i.e., the longer the cylindrical wire mesh, the sharper is the directivity of the lobes.
ri`he mathematical development of this dipole concept will be described in conjunction with FIGS. 5a and 5b which show portions of the outer meshwire conductor. FIG. 5b serves as an analytical model `for computation of the surface EMF. The magnetic field Hy above lsuch apertures (ie. the tield perpendicular to the plane of the aperture shown in the drawing) is represented by the equation:
Hy=Hy-l-Hy" (l) where Hy is the total held above the mesh, Hy is the eld contributed by the current z" flowing in wires 25', and I y" is the field contributed by the current i" in wires 35. Each individual field coniiguration can be represented in ter-ms of the dimensions of the braid as shown in the following equation for Hy:
where z" is the current flowing in wires Z5 of the braided conductor, n is lthe number of meshes or apertures, a is the distance between the axes of wires 25, and 5 is the aeration distance along the g axis of the oblique X, g, 7;, coordinate system from the origin to wire 25. Carrying out the summation of Equation where i" is the current ilowing in wires 35 and i; is the distance along the axis of coordinate system X, 5, 17, from the origin to wire 35.
Adding Equations 3 and 4 according to Equation 1 the total magnetic field Hy in the plane of the mesh or where g is the distance between the outside surfaces of adjacent parallel conductors in the braid.
The relationship shown in Equation is true for the following validity conditions:
(1) Stationary D.C. like behavior of the wire braid currents is assumed.
(2) Magnetic field distortions at the braid wire crossings are neglected.
(3) The portion of the braid under consideration is considered at and the number of meshes or apertures in this portion is assumed to be large (ne oo).
Choosing the X and Z coordinates to be the diagonals of a diamond shaped aperture as shown in FIG. 5b, the magnitudes of E and v; can be mathematically expressed in terms of X, Z, and a so that X Z -2 cos a-2 sin a (8) and X Z 712 cos et+-2 sin (9) The magnetic iield Hy found by means of Equation l() has been well confirmed experimentally thereby showing the practical Validity of the mathematical approach which led to the development of the practical, operative device to be described in conjunction with FIGURES 2, 3, `and 4. The magnetic iield Hy of Equation l0 is designated as magnetic leakage ield 12 in FIGURES l and 2.
According to Faradays law, a ring voltage is linked to the leakage iiux described above such that an electric eld is produced parallel to the resultant current ilow through the meshwire conductor. The mean electric field strength in the plane of each mesh aperture is equivalent to an per unit length along the surface of the meshwire structure, This EME. per unit length is directly proportional to the primary magnetic field Hp or what is equivalent to the current i in the meshwires, and its phase Velocity equals the phase velecity of the primary field. The EMP. -thus produced excites and supports a surface wave on the surface of the meshwire conductor lo at high frequencies and is linked to the magnetic leakage ux leaking through the apertures il so that the meshwire surface per unit length increases with increased 'frequency of the input wave. This increase ofthe surface EMF. per unit length causes a corresponding increase of the ratio or" the radial to the longitudinal field energy iiow in the vicinity of the meshwire, t-h-at is, a trend from wave guidance to wave radiation of the slow wave type previously discussed.
According to the invention, if the meshwire 1t) is covered with a sheet of material 13, as shown in FIG. 2, having a high permeability constant (low reluctance), negligihle conductivity `and negligible hysteresis loss, the magnetic leakage iiux 12 through the apertures lll will tend to be confined within the highpermeability material 13 and will not stray into the outside space which has a higher reluctance. The density of the magnetic iield l2 is greatly increased in cover sheet 13 as a result. Referring again to the .fictitious dipole conception, it can be said that the high permeability cover sheet 13 acts as a shunt or keeper for the fictitious dipoles in much the same manneras a keeper for a permanent magnet. y
The meshwire conductor l@ and high permeability cover sheet 13 of proper thickness, in effect act as a transformer to couple energy (e.g., a TEM mode) from below the meshwire conductor 1li to the region just above the meshwire conductor it) within cover sheet i3 where the energy is propagated in the form of a surface wave. The high concentration of magnetic energy within cover sheet i3 will increase the per unit length along lthe surface of meshwire conductor l0 to a value considerably higher than that obtained from the device of FIG, 1. The guided surface wave thus formed is in a non-radiating mode tightly coupled to the wave beneath meshwire conductor lit. The degree of this coupling and character of the surface wave is controlled by the size and spacing of apertures il and by the thickness and permeability of cover sheet I3.
Conversion of the non-radiating surface wave to a radiating mode can then be accomplished by feeding the wave into a discontinuity at the end of a surface wave launcher of the type shown in FlG. 2. This will cause a disruption of the intimate coupling between the surface wave and inner wave thereby resulting in the generation of a radiating mode which is reected back and forth between the ends of the launcher in a standing wave pattern being set up by the radiating modes such that there is a net energy flow from the input end to the load or terminal end of the wave launcher. This explains the directivity of the radiation pattern and the proportionality of its sharpness with the length of the launcher.
A preferred form of a surface wave launcher according to the invention is shown in FIGS. 3 and 3a and consists of a coaxial arrangement of a cylindrical inner conductor 14 surrounded by a dielectric material l5' having `a proper dielectric constant which insulates conductor M from an envelope comprised or" an inner meshwire conductor i6 With a thin outer coating l? of a material having high permeability (ferrite for instance). In actual use, a suitable source of high frequency power is connected to one end of conductors 14 and 16 and the other end of the surface wave launcher could be utilized for launching surface waves along a wire (Gf-string) or other suitable loads. `Construction `of this coaxial surface wave launcher in a conical shape will sharpen the radiation pattern considerably. Operation of the embodiment shown in IFIGS. 3 and 3a is otherwise the saine as described in conjunction with FIG. 2.
FIGS. 4 and 4a show a planar or sandwich version of the surface wave launcher described in FIGS. 2, 3 and 3a. In this version a thin planar conductor 18 is sandwiched between two layers of insulating material 19 each of which are in turn covered by a meshwire or braided conductor 20. The braided conductors 20 are then each covered with a sheet 21 of material having a high permeability. This form of the invention is particularly useful for the excitation of surface waves along ice and `air or water and air boundaries.
The high permeability cover sheet may be made of ferrite and could be applied to the coaxial version of the invention by winding ferrite tape around the outer meshwire conductor of the cable, for example.
While the invention has been described with reference to particular embodiments, various other specific embodiments may suggest themselves to those skilled in the art without departing from the spirit and scope of the invention. Therefore, the present embodiments are to be considered illustrative only with reference being had to the appended claims to indicate the true scope of the invention.
What is claimed is:
1. A surface wave launcher comprising a rst conducting means, a second conducting means of relatively open meshwire construction having two surfaces, insulating means separating said first conducting means and one surface of said second conducting means, and a high magnetic permeability cover sheet substantially covering the other surface of said second conducting means.
2. A surface wave launcher according to claim 1 wherein said insulation means has a dielectric constant which matches the phase velocity of the primary eld within said insulation means to 4the surface wave phase velocity.
3. An electromagnetic wave launcher comprising a pair of planar relatively open meshwire conductors arranged in parallel spaced relation with respect to one another, insulation means, a solid plane conductive sheet between said meshwire conductors and separated from each of said meshwire conductors by said insulation means, and a high magnetic permeability cover sheet substantially cov ering each of said meshwire conductors on the side remote from said insulation means.
4. A microwave antenna` comprising inner and outer conducting elements arranged in coaxial relationship and separated by suitable insulating means, said outer con ducting element having a plurality of closely spaced apertures therein, and a. cylindrically shaped enclosure of material having a high magnetic permeability encasing said coaxial conducting elements and arranged in contact with said outer conducting element.
5. The apparatus of claim 4 wherein said inner and outer conducting elements are adapted to be connected -to a suitable source of high frequency power and where said insulating material has a dielectric constant which matches the phase velocity of the primary eld within said insulating means to the phase velocity of a surface wave excited in said high magnetic permeability enclosure.
References Cited in the file of this patent UNITED STATES PATENTS 563,274 Guilleaume July 7, 1896 2,018,353 Gothe Oct. 22, 1935 2,111,651 Wentz Mar. 22, 1938 2,894,226 Wild July 7, 1959 2,913,515 Ebel Nov. 17, 1959 2,915,719 Larsen Dec. l, 1959 2,929,034 Doherty Mar. 15, 1960 2,938,943 Horn May 3l, 1960 2,994,050 Ayer July 25, 1961 FOREIGN PATENTS 633,190 Great Britain Dec. 12, 1949 OTHER REFERENCES Rotman: A Study Guides, published in Proceedings of the IRE., vol 39, Issue 8, pages 952-959, August 1951.
Goubau: Surface Waves Lines, published in Journal of Applied Physics, November 1950, pages 111,94 1128, volv 2 1,No,11,
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,077 ,569 February 12g 1963 Kurt Ikrath It s hereby Certified that ervror appears n the above numbered patent requiring Correction and that the said Letters Patent should read as corrected below.
Column 3, lines 8 and 9 e uation (4) should a ea shown below lnstead of as in'tlnaI patent: pp P as Signed and sealed this 22nd day of October 1963 (SEAL) Attest:
EDWIN L., REYNOLDS ERNEST W. SWIDER ttesting Officer AC ting Commissioner of Patents UNITED STATES PATENT oEETCE CERTIFICATE OF CORRECTION Patent No. 3,077 ,569 February 12, 1963 Kurt Ikrath It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 3, lin-es 8 and 9 e uation y(4l) should a ea shown below instead of as in'tlneI patent: pp r as Signed and sealed this 22nd dey of October 1963.
EDWIN L REYNOLDS ERNEST W. SWIDER Attesting Officer Acting Commissioner of Patents