US3495191A - Microwave phase shifter - Google Patents

Description

Feb. 1o, 1970 F. w. zlLKosKl MICROWAVE PHASE SHIFTER Filed Aug. 1, 1966 |Ll|| Il l .I I I lwlluuhdllrlldcl'lmlllllullllll'll FIGI I9 L 3 F|G.4 l 25 g I L- FIG.3

INVENTO I FRANK W. ZIL KI, BY c?. M @Lx HIS ATTORNEY.

United States Patent O 3,495,191 MICROWAVE PHASE SHIFTER Frank W. Zilkoski, Liverpool, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 1, 1966, Ser. No. 569,295 Int. Cl. H03h 7/36 U.S. Cl. 333-31 5 `Claims ABSTRACT F THE DISCLOSURE This invention is directed to a microwave phase shifter construction in which the magnitude of phase shift in a waveguide is controlled by the depth of insertion of a dielectric card member into the guide. The card is inserted through a slot formed in the waveguide broadwall at the closed end of a reverse bend in the guide, so as to extend through the bend area and into the straight runs of waveguide adjacent thereto. Manual or automatic adjustment of the card insertion depth then controls the magnitude of phaseslhift in the guide.

This invention relates to microwave phase shifters and more specifically to phase shifters of the type providing control of the magnitude of phase shift by mechanical adjustment of the phase shifter parts.

The phase shifters of this invention afford a level of performance significantly improved over that of the conventionally constructed phase Shifters, and at the same time afford a significant simplification of structure with consequent improvements in economy and reliability. Among the more troublesome of the shortcomings of conventional phase shifter designs is their usual requirement for matching devices, couplers or chokes which in addition to undesirable complexity and cost are characteristically frequency dependent in their operation and which ras a result tend to limit operation to undesirably narrow bandwidth. These and other such common problems as poor linearity or unduly narrow range of phase shift adjustment, impedance mismatch and variation with phase shift adjustment, are substantially alleviated if not wholly eliminated in the phase Shifters of this invention,

It is accordingly an object of the invention to provide a mechanically adjustable phase shifter construction characterized by wideband frequency capability, Wide range and good linearity of adjustment of phase shift, and simplicity and economy of construction. Another object of the invention is the provision of a phase shifter affording inherent matching with conventional waveguide structures thus avoiding need for frequency critical matching or coupling devices either within the phase shifter or to enable its connection to other waveguide. It is also an object to provide an adjustable phase shifter which presents substantially constant impedance irrespective of phase shift adjustment, which does not change wave polarization, and which permits little leakage even without shielding.

Briefly stated, in one presently preferred embodiment the phase shifter of this invention comprises a length of rectangular section waveguide in the form of two straight runs disposed parallel to each other and connected by an 180 E-plane bend, i.e., a reverse bend formed about a line parallel to the waveguide broadwalls. These broadwalls each have a slot therethrough disposed medially of the broadwall and extending longitudinally through the area of the bend, with at least the inner vbroadwall slot extending beyond the bend and into the straight runs of waveguide on either side of the bend. A dielectric card, preferably having its leading edge shaped for optimized reflective matching to the waveguide, is inserted through the slots in the waveguide broadwalls at the closed end of the bend so as to extend through the bend ICC area and into the straight runs of waveguide. Manual or automatic adjustment of the depth of insertion of the card then controls the magnitude of phase shift of waves in the guide, the relationship between phase shift and card inserted depth being linear through at least part of the available range of adjustment.

The invention will be further understood and its various objects, features and advantages more fully appreciated by reference to the appended claims and the following detailed description when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a view partially in elevation and partially broken away of a phase shifter in accordance with the invention;

FIGURES 2 and 3 are sectional views taken along the lines 2 2 and 33, respectively, in FIGURE 1; and

FIGURE 4 is a fragmentary elevation of an alternative form of dielectric card usable in the phase shifters of the invention.

With continued reference to the drawings, wherein like reference numerals have been used throughout to designate like elements, FIGURE l illustrates the invention in one presently preferred embodiment. The phase shifter aS shown comprises a metallic body member 11 providing wall means 13 defining a length of rectangular-section waveguide 15 opening at its opposite ends 17 and 19 through an end face of the body member, Waveguide 15 includes a reverse bend 21 and two straight runs of guide 23 and 25 interconnected by the bend, these straight runs being disposed parallel to each other as best illustrated in FIGURES 1 and 3.

The bend 21 in waveguide 15 is formed about a line parallel to the guide broadwalls and thus constitutes a E-plane bend with waveguide operation in the normal dominant TELO mode. This bend need not be of the semicircular form illustrated but may be of any of the various other curved or stepped configurations known to afford good VSWR and spurious mode characteristics.

The outer and inner broadwalls of waveguide 15 are slotted as at 27 and 29, respectively, with the slots being disposed medially of each broadwall and extending longit-udinally along the waveguide through the entire area of the bend 21. The slot 29 through the inner broadwall further extends beyond the bend a substantial distance into the two straight runs of waveguide 23 and 25, thus interconnecting them through the slot. It is to be noted that because these longitudinally extending slots through the broadwalls are medially disposed and relatively narrow, and because as well known in the art broadwall slots of this configuration do not cause significant loss or other adverse affect on wave energy transmission in the TEU, mode due to the E and H field patterns characteristic of this mode, the slots 27 and 29 do not themselves noticeably affect waveguide performance.

A dielectric card 31 of quartz or other low-loss dielectric material is inserted through the slot 27 in the outer broadwall at the closed end of the bend 21, then through the slot 29 in the inner broadwall to a point such that the forward edge of the card is within the straight runs of the guide as shown. Card 31 is generally rectangular in shape and of width just equal to the spacing between the outer broadwalls (the clearance shown being exaggerated for clarity of illustration), so that the lateral edges of the card project through the waveguide 15 and into sliding contact with the outer broadwalls. In and out adjustment of the card then will result in controlled shift of phase of waves within the guide 15, due to the changing effective dielectric constant within the waveguide resulting from changes in card length therein.

So long as the card is inserted at least sufiiciently far into the waveguide that its forward edges lie wholly within the straight portions 23-25 of the waveguide, the mag- 3 nitude of phase shift will be linear with the depth of card insertion. This has been shown experimentally and also may be derived from the elementary relations characteristic of waveguides containing dielectric. In such waveguide the guide wavelength is:

Ag: L Pfff where:

A=free space wavelength Ac--cut-off wavelength of guide (air-filled) e=effective value of dielectric constant in the region of guide under consideration. In air-filled regions 6:1; in regions containing the dielectric card the effective value of e depends on the thickness of the dielectric card relative to the width of the broadwall.

Since the guide wavelength with the dielectric insert is shorter than that of air-filled guide the effective electrical length Le will be increased with increasing depth of insertion of the dielectric card into the guide, and phase shift is proportional to the variation in effective length of the waveguide resulting from insertion of the card. Waveguide effective length is given by the expression:

Substituting for AE1 and XE2 there results where as previously noted ec will lie somewhere between the dielectric constant of air and that of the card material, at a value depending on card width and the dimensional relationship between card thickness and broadwall width.

It will be noted that in this expression the Values of all parameters except L1 are constant in any given phase shifter design and at given frequency, so that the waveguide effective length and thu-s the phase shift within the guide become linear with L1. Phase shift thus is linear with depth of card insertion.

The range of adjustment of phase shift may also be seen from this expression to be a function of a number of design parameters perhaps the two most flexible of which are the depth of insertion (L1) which is subject to no limit except to the extent that dimensional limitations may be encountered in the particular installation under consideration. The effective dielectric constant ec also may be varied quite widely, by use of different dielectric cornpositions for the card and by change of the dimensional relationship between card thickness and waveguide broadwall width.

Card thickness should be kept reasonably small, however, because if the card and the slots in which it slides are of excessive dimension undesirably high losses may occur. This results from the fact there is effectively an upper limit on the product of card thickness and card dielectric constant. This product determines the cutoff frequency of the -slots in which the card slides, and if such cutoff frequency were made to fall below the operating frequency a dominant mode or modes then could be ver established in the card and would propagate through the slots with resultant excessive leakage an radiation. Card thickness of about 10 percent of broadwall width have in practice been found quite suitable with conventional dielectric materials, but proportions differing from this value by a factor of two or more in either direction may be found desirable in certain applications.

For optimizing the reflective matching between airfilled and card-containing portions of the waveguide, card 31 preferably has its forward edge tapered as lshown at 33 in FIGURE l or stepped as shown at 37 in FIGURE 4. Such edge treatment is alone adequate to hold VSWR and spurious mode generation to acceptably low levels without need for other matching or coupling devices, thus obviating the problem of frequency sensitivity which the use of such other devices normally involves.

For further reduction of VSWR and spurious mode control two additional features of construction have been found helpful. As illustrated in FIGURES l and 2, matching at the ends of the inner broadwall slot 29 may be improved by provision of a taper as at 39, providing more gradual transition between the unslotted and slotted portions of the waveguide. Also, to reduce undesired transmission of wave energy from one of the straight runs of guide into the other by propagation through the portion of the dielectric card 31 lying between these waveguide runs, the wall member defining the inner broadwall slot 29 may be provided with a coating or inset 41 of load material such as a resistive epoxy or lossy film. This load material should cover approximately the area shown in FIGURE 1, and serves to dissipate most if not all of any wave energy which might otherwise propagate in spurious` modes through the dielectric. Similar resistive load means may be provided if necessary adjacent the outer broadwall slot 29 for dissipating any spurious mode which might otherwise propagate through the card and out the slot. Such controls are not intended to be effective against any dominant mode transmission through the dielectric card and slot, of course; this is prevented by selection of card thickness and dielectric constant as explained above so that the cutoff frequency within the slot always is above the operating frequency.

It'will be noted that since the outer broadwall slot 27 is the only external opening required in the phase shifter body member for insertion of the dielectric card, a staticr or moving seal can readily be accomplished in conventional manner where necessary to enable pressurization of the guide. Little if any microwave radiation will occur through this opening even if left completely Unshielded, again provided the operating frequency is kept below the cutoff frequency of the opening. It will also be noted that in addition to these and the various other advantages previously mentioned, the phase Shifters of this invention offer the further advantage that they do not affect wave polarization and that dielectric card movement for phase adjustment does not affect impedance.

While in thi-s description of the invention only certain presently preferred embodiments have been illustrated and described by way of example, many modifications will occur to those skilled in the art and it therefore should be understood that the appended claims are intended to cover all such modifications as fall within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A microwave phase shifter comprising:

wall means defining a rectangular section waveguide including a E-plane bend connecting two straight runs of guide, said wall means further defining a longitudinallly extending slot medially disposed in each of the waveguide broadwalls in the area of said bend with at least the slot in the inner broadwall extending into said straight runs of guide; and a generally rectangular dielectric card member, of width substantially equal to the spacing between said outer broadwalls, inserted through said slots in the inner and outer broadwalls and extending into said straight runs of guide.

2. A microwave phase shifter as defined in claim 1 the area of the bend, said slot in the outer broadwall being of length at least equal to the length of the bend, and said slot in the inner broadwall being of length suchthat it extends through and beyond said bend and into said straight runs of guide and inter- 5 connects them; and

a generally rectangular dielectric card member projectwherein the forward corners of said dielectric card member are of like shape optimized for reflective matching in the guide.

ing through said slots in said inner and outer broadwalls and extending into said straight runs of guide, said card being of width at least equal to the spacing between said outer broadwalls so as to project through the guide in both said straight runs thereof for control of .phase shift therein.

5. A microwave phase shifter as defined in claim 4 r wherein said wall means defining said slot in the inner broadwall includes resistive load means disposed adjacent the dielectric ca rd between said straight runs of guide for dissipating energy `which might otherwise propagate through the card.

walls with both slots being medially disposed in their respective 'broadwalls and extending longitudinally through the area of the bend and with at least the inner broadwall slot extending a substantial distance beyond the bend and into said straight runs of guide connecting thereto; and 20 a generally rectangular dielectric card member inserted through said slots in said inner and outer broad- References Cited UNITED STATES PATENTS walls and extending into said straight runs of guide, 2,762,973 9/1956 Kallman 324-58 said card being of width at least equal to the spacing 3,192,492 6./ 1965 Linder 333-31 between said outer broadwalls so as to project through 25 3,005,168 10/ 1961 Fye 333-31 the guide in both said straight runs thereof and being 2,454,530 11/ 1948 Tiley 333--31 movable in said slots so as to enable adjustment of 2,630,492 3/1953 Muchmore 333-31 phase shift by adjustment of card insertion depth. 3,316,509 4/ 1967 Ayer et al. 333-31 4. A microwave phase shifter comprising:

wall means defining a rectangular section waveguide in the form of a 180 E-plane bend connecting two straight runs of guide, said wall means further defining a longitudinally extending slot medially disposed in each of the waveguide broadwalls through 30 HERMAN KARL SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner j U.S. Cl. X.R. S33-34, 84

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