US3425005A - Dielectric variable attenuator - Google Patents

Description

Jan. 28, 1969 A. T. HAYANY DIELECTRIC VARIABLE ATTENUATOR Sheet Filed Feb. 23. 1967 Jan. 28, 1969 v NY 3,425,005

DIELECTRIC VARIABLE ATTENUATOR Filed Feb. 23, 1967 Sheet 2 of 2 wms/f/a a anew 14/767; a4 tar- QW -Q Ze /a/W e (MW? k f/fafr/m/ 164 7792 0/ United States Patent 3,425,005 DIELECTRIC VARIABLE ATTENUATOR Adnan T. Hayany, Kansas City, Mo., assignor to Western Electric Company, Incorporated, New York, N.Y., a

corporation of New York Filed Feb. 23, 1967, Ser. No. 617,984

US. Cl. 333-81 Int. Cl. H01p 1/22 11 Claims ABSTRACT OF THE DISCLOSURE Background of invention It has been determined theoretically that under proper circumstances, the use of solid dielectric :waveguide in long distance high frequency transmission systems in place of more conventional waveguide can result in looser tolerances, greater power handling capacity, increased bandwidth, and relatively low loss per unit length.

Since long distance single-conductor transmission systems are presently formed from hollow conductively bounded tubing and since in any event existing microwave generators employ metallic cavities and/or output coupling sections, it was recognized that the substitution of such dielectric elements for corresponding hollow sections would have to be on a selective replacement basis compatible with the remaining metallic units in the system.

Such replacements, however, have been discouraged in the past because of mismatch and spurious radiation inherently introduced at an interface between a solid dielectric run or component and the adjacent metallic waveguide apparatus. Fortunately, this problem has been alleviated to a large degree by the development of an efiicient hollow metallic-to-solid dielectric transition, as described and claimed in applicants copending application, Ser. No. 608,149, filed Jan. 9, 1967, and assigned to the assignee of the instant invention.

Thus, it has now become feasible to think in terms of standard components (such as attenuators, directional couplers, and filters) constructed entirely in solid dielectric waveguide for use either in future dielectric transmission systems or, for more immediate applications, as discrete dielectric elements to be introduced into existing hollow metallic systems.

In furtherance of this end, the particular problem treated by the instant case is to provide a solid dielectric attenuator suitable for introducing a controllable amount of loss into a single-conductor wave transmission system.

Summary of the invention One solution to this problem is the provision of an attenuator constructed in accordance with the instant invention. In a preferred embodiment, a thin elongated vane is disposed adjacent, and mounted for rotation in a plane parallel to, a 'wide wall of a section of solid dielectric waveguide having a rectangular cross-section. The vane is constructed of material having a dielectric constant substantially higher than that of the solid dielectric guide and, in particular, may be formed from conductive material. A precisely controllable variation of attenuation is obtainable by rotating the vane from a minimum attenuation position perpendicular to the axis of the section to a Patented Jan. 28, 1969 maximum attenuation position parallel to the axis. An indication of the angle of rotation of the vane with respect to the axis may be obtained from an electrically transparent, graduated dial plate interposed between the vane and the wide wall.

The advantages of the resulting attenuator are significant, as shown by a comparison with a metallic waveguide rotary-vane attenuator to which it is roughly analogous. Unlike the metallic design, for example, the solid dielectric unit manifests a low VSWR, and a negligible variation of VSWR and electrical phase shift with rotation without the necessity of providing separate amplitude and phase compensating sections. Also, the dielectric unit can be built entirely in waveguide of non-symmetrical crosssection, with no auxiliary circular guide section necessary to house the vane. In addition, because of the extremely broad-band nature of solid dielectric waveguide, any undesired non-linear distortion of the attenuation vs. vane angle characteristic due to metallic waveguide cutoff phenomena is avoided to a large degree.

Brief description of the drawing The nature of the present invention and its various advantages will appear more fully from the following detailed description when taken in connection with the appended drawing, in which:

FIG. 1 is a sectional side view of a solid dielectric transmission line employing one form of attenuating element constructed in accordance with the invention;

FIG. 2 is a cross-sectional view of the transmission line of FIG. 1, with the attenuating element omitted;

FIG. 3 is a plan view of the attenuating element of FIG. 1;

FIG. 4 is a curve showing a typical attenuation vs. vane angle characteristic for the attenuating element of FIGS. 1-3;

(FIG. 5 is a sectional side view of another form of attenuating element constructed in accordance with the invention;

FIGS. 6A and 6B are curves respectively showing typical variations of VSWR and WOW with frequency for the attenuating element of FIGS. 1-3 or FIG. 5;

FIG. 7 is a set of curves showing typical variations of electrical length vs. vane angle at several frequencies for the attenuating element of FIGS. 1-3 or FIG. 5; and

FIG. 8 is a sectional side view of a hollow waveguide transmission system adapted to receive the attenuating element of FIGS. 1-3.

Detailed description Referring now in more detail to the drawing, FIG. 1 depicts a single-condution transmission line formed from a continuous run 12 of a low-loss solid dielectric material (such as polystyrene) and along which a suitable electromagnetic wave from a source (not show) is adapted to propagate. As shown in FIG. 2, the run 12 has a rectangular transverse cross-section defined by a pair of opposed wide walls 13-13 joined by a pair of opposed narrow walls 1414. The dimensions A and B are chosen to support the TE wave mode over a wide frequency range, which is assumed to be centralized in the 3.6-4.3K mc. band. For this purpose, a run cross-section having A=2.2" and B=1.0 has been found satisfactory.

Since the run 11 has no conductive boundaries, an electromagnetic wave guided therein will have finite field components extending beyond the walls 13-13 and 14-14 into the surrounding air. This serves two functions: (1) to increase the elfective, cross-section of the run and thus its potential bandwidth, and (2) to couple the run to elements external thereto without the use of specially constructed holes, slots and the like in the adjacent Wall of the run.

In accordance with the invention, the run 12 includes an attenuating portion 16 for effectively varying the impedance presented to the flow of energy in the run 12 (FIG. 1) and, consequently, for varying the amount of energy propagating through the run 12. The attenuating portion 16 is coupled to a thin elongated vane 17 constructed of conductive material such as stainless steel. The vane 17 is disposed adjacent an upper wide wall 13 of the run 12. The vane 17 has a length C, a width D, and a thickness which can be assumed to be negligible. The plane of the vane 17 is parallel to the wide wall 13. As an aid in matching, a pair of opposite ends 18-18 (FIG. 3) of the vane 17 may be tapered.

The vane .17 is mounted for rotation in its own plane so that a longitudinal axis 19 thereof defines a selectable angle with respect to a transverse reference line 20 disposed perpendicular to a longitudinal axis 21 of the run 12. For this purpose, the vane is provided with a central aperture 22 having a diameter E, the peripheral wall of which is tightly engaged by a thin rod 23 (FIG. 1) that passes perpendicularly therethrough. One end 24 of the rod 23 extends through an apertured dial 25 and is received within a recess 26 in the top wall 13. The dial 25, which is assumed to have a negligible thickness, is disposed essentially flush with the top wall 13. The rod 23 and the dial 25 are formed from a low-loss material of low dielectric constant, such as polystyrene or polyethylene. The vane 17 is mounted in engagement with a surface 27 of the dial.

As shown best in FIG. 3, the dial 25 is provided with a plurality of equally spaced radial graduations 28-28 around the periphery thereof to set the angle 0 during the operation of the attenuator. In order to view this indicated angle from above the vane 17, the latter is provided with a second aperture 29 in register with the graduated region of the dial 25. The aperture 29 may be filled with a low-loss, low dielectric constant material, as above, and a radial hairline 31 of one graduation width or less is provided in the center of the aperture 29 to indicate the angle 0. The angle 0 may be manually adjustable with the aid of a phenol fiber knob 32 (FIG. 1) mounted at the top end of the rod 23. The bottom surface of the knob 32 is disposed a distance F from the top surface of the vane 17.

The attenuation introduced by the vane 17 as a function of the angle 0 varies in generally logarithmic manner from a minimum when 0:0 (axis .19 perpendicular to axis 21) to a maximum when 0:90 degrees (axis 19 parallel to axis 21). A typical variation of attenuation as a function of vane angle is given in FIG. 4 for a frequency of 3900 me. in the assumed band. The characteristic shown was obtained with the vane 17 (FIG. 1) having dimensions C=8.0" and D=%", and the rod 23 (formed from polyethylene) having a diameter E of /2.

Because of the essentially zero backlash of the arrangement depicted, the logarithmic change of attenuation with vane angle is precisely controllable and repeatable. In the 0-2 db range of attenuation, for example, a resolution in the order of 0.01 db was obtained. In order to minimize the insertion loss of the attenuator in the 0:0 position, it was found that the distance F from the vane 17 to the bottom of the knob 32 should be approximately an odd number of quarter wavelengths in the rod 23. For example, the 0.3 db insertion loss shown in FIG. 4 was obtained when F=2%".

The attenuation provided by the vane is also a function of its length; in particular, it was noted that in the maximum "attenuation (0:90") position, the total attenuation obtainable was about db per guide wavelength in the run 12. In order to decrease the total vane length required for a given maximum attenuation, the attenuator may include a second vane ganged with the vane 17 and symmetrically disposed with respect thereto on the other side of the run axis 21.

One illustrative gauging arrangement of this type is shown in FIG. 5, in which elements corresponding to FIG. 1 have been given corresponding reference numerals. An auxiliary vane 33 extends parallel to the lower wall .13 of the portion 16 in such a manner that the axis (not shown) of the vane 33 is aligned with the axis 19 (FIG. 3) of the vane 17. The vane 33 (FIG. 5) is supported with respect to the lower wall 13 by means of a downwardly projecting extension 34 of the rod 23. The so-extended rod is received in a bore 36 that passes through the portion 16, and is secured at a lower end 37 thereof to the vane 33 by any convenient means. In this case, the extended rod is preferably formed from polystyrene to prevent minimum interference with waves propagating in the polystyrene waveguide of the run 12. It will be apparent that with the arrangement shown in FIG. 5, the aligned and ganged vanes 17 and 33 can be rotated together through a desired angle 0 when the knob 32 is turned.

While the vane 17 has been assumed to be formed of a conductive material, it has been found that a dielectric vane may also be employed with like results provided that the dielectric constant of the vane is significantly higher than the dielectric constant of the material forming the run 12. The VSWR of the solid state attenuator described above is inherently low, and no auxiliary matching section is required as in metallic rotary-vane attenuators. A typical variation of VSWR with frequency for a dielectric unit having the dimensions described in connection with FIG. 4 is shown in FIG. 6A. The vane also has a low WOW, i.e., the VSWR is relatively insensitive to rotation of the vane. This is shown in FIG. 6B, which depicts the increase in VSWR when rotating from the minimum to the maximum attenuation positions.

A further feature of the above-described arrangement should be noted. This is the negligible inherent variation of effective electrical length of the attenuation as a function of vane angle. A typical phase shift variation as a function of vane angle for three separate frequencies in the assumed band is shown in FIG. 7. It will be recognized that this desirable characteristic can only be obtained in rotary-vane metallic attenuators when built-in phase compensating units are employed.

While the invention has been described in connection with a continuous run 12 (FIG. 1) of solid dielectric waveguide, the desirable attributes of this attenuator (illustratively in the form shown in FIGS. 1-3) can also be employed by substituting it directly for an attenuator (such as the rotary-vane type) presently employed in hollow waveguide transmission systems. This is shown in FIG. 8, wherein elements corresponding to FIGS. 1-3 have been given corresponding reference numerals. The portion 16 is coupled to a pair of runs 38 and 39 of hollow metallic waveguide at its input and output through a pair of identical back-to-back transitions 41 and 42, respectively. The latter transitions are of the type described in applicants above-mentioned copending application. In particular, the transitions 41 and 42 individually include solid dielectric sections 43 and 44 aligned with and having cross-sections identical to the attenuating portion 16. The transitions 41 and 42 are also provided with a pair of longitudinally extending reflectors 46 and 47. In the arrangement shown, the reflectors 46 and 47 are transversely spaced from the associated sections 43 and 44 and are antisymmetrically disposed on opposite sides of the common longitudinal axis 21. The spacing of the reflector 46 from the adjacent surface of the section 43 decreases monotonically from a miximum at the input of the section 43. Similarly, the spacing of the reflector 47 from the adjacent surface of the section 44 decreases monotonically in the reverse direction from a maximum at the output of the section 44.

The interfaces between the attenuating portion 16 and the sections 43 and 44 are defined by a pair of complementary tongue and groove joints 48 and 49. However, it will be apparent that these interfaces may be eliminated and a continuous length of polystyrene employed between the input and output metallic runs 38 and 39 with the same results. It will also be apparent that the attenuator of FIG. 8 will function in a manner identical to that of FIGS. 1-3.

It is to be understood that the above-described arrangements of elements are simply illustrative of the principles of the invention, and many other modifications may be made without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for introducing attenuation into a solid dielectric waveguide, which comprises:

an elongated member constructed of material that will present a relatively high impedance to a flow of energy through the waveguide when positioned parallel to the longitudinal axis of the waveguide and a relatively low impedance to the flow when positioned transverse to the longitudinal axis of the waveguide; and

electromagnetically transparent means supporting the member externally of an adjacent the waveguide for rotation between the position of relatively low impedance and the position of relatively high impedance.

2. A variable attenuator for electromagnetic wave energy, which comprises:

a section of solid dielectric waveguide for receiving the wave energy;

an elongated vane electromagnetically coupled to a surface of the section, the vane being formed from material having a dielectric constant different from that of the section; and

means for selectively rotating the vane in a plane parallel to the surface to vary the attenuation of the section.

3. An attenuator as defined in claim 2, in which the vane is formed from conductive material.

4. In a transmission system employing a run of solid dielectric waveguide for guiding wave energy, apparatus for varying the amount of energy transmitted through the run, which comprises:

a thin elongated vane formed from material of substantially different dielectric constant than said run; and

means for rotatably mounting the vane adjacent one surface of the run with the plane of the vane extending parallel to the plane of the surface.

5. An attenuator as defined in claim 4, in which the run has a rectangular cross-section, and the mounting means positions the vane for rotation adjacent a generally central location on a wider wall of the run.

6. A variable attenuator for electromagnetic wave energy, which comprises:

a section of solid dielectric waveguide for receiving the wave energy;

first and second elongated vanes individually coupled to opposite surfaces of the section; and

selectively operated ganging means individually supporting the first and second vanes with their axes aligned for rotation in planes parallel to the associated surfaces.

7. A variable attenuator for electromagnetic wave energy, which comprises:

an elongated section of solid dielectric waveguide for receiving the wave energy, one surface of the section having a recess therein;

an elongated vane having a central aperture, the vane being formed from material having a dielectric constant higher than that of the section;

a rod extending perpendicularly through the aperture into the recess for supporting the vane in a position parallel to and adjacent the surface; and

means for selectively rotating the rod within the recess to vary the angle between the axis of the vane and the axis of the section.

8. A variable attenuator as defined in claim 7, in which a peripherally graduated dial is disposed on the surface of the section for indicating the last-mentioned angle, and the vane further includes an additional aperture in alignment with the periphery of the dial for viewing the indicated angle.

9. A solid dielectric attenuator adapted to be interposed between first and second spaced sections of conductively bounded hollow waveguide, which comprises:

a third section of solid dielectric waveguide interconnecting the first and second hollow sections;

first and second substantially identical, longitudinally extending reflectors respectively spaced from and antisymmetrically disposed with respect to the third section, the first reflector extending from the input of the third section to a first intermediate cross-section thereof, the second reflector extending from the output of the third section to a second intermediate cross-section thereof, the first and second cross-sections being longitudinally spaced to define a reflectorfree region of the third section; and

an elongated vane rotatably mounted adjacent one surface of the third section in the reflector-free region, the plane of the vane extending parallel to the lastmentioned surface.

10. Apparatus as defined in claim -9, in which the vane is formed from material having a dielectric constant higher than the dielectric constant of the third section.

11. Apparatus as defined in claim 9, in which the spacings of the first and second reflectors from the third section decrease monotonically with longitudinal distance from maxima at the input and output, respectively, of the third section.

References Cited Circuit Components in Dielectric Image Lines," King, IRE Transactions on Microwave Theory and Techniques, volume MTT-3, No. 6, December 1955, TK 7800 I23, pages 35-39.

HERMAN KARL SAALBACH, Primary Examiner.

MARVIN NUSSBAUM, Assistant Examiner.

U.S. Cl. X.R.

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