US3462713A - Waveguide-stripline transducer - Google Patents

Description

Aug.19,1969 RHMNERR 3,462,713

4 WAVEGUIDESTRIPLINE TRNSDUCER I Filed July 19, 1967 5 Sheets-Sheet 1 A TTORNEY Aug. 19, 1969 R. H. KNERR wAvEGUIDE-STRIPLINEA TRANSDUCER Filed July 19, 1967 5 Sheets-Sheet 2 FIG. 2B

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Aug. 19, 1969 R.H. KNERR 3,462,713

WVEGUIDE-STRPLINE TRANSDUCER l Filed July 19. 1967 r 5 sheets-sheet s United States Patent O1 ice 3,462,713 WAVEGUIDE-STRIPLINE TRANSDUCER Reinhard H. Knerr, Bethlehem, Pa., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill, NJ., a corporation of New York Filed July 19, 1967, Ser. No. 654,480 Int. Cl. H0111 1/16 U.S. Cl. 333-21 5 Claims ABSTRACT F THE DISCLOSURE A broadband waveguide to stripline transducer in which the end of the stripline center conductor is directly inserted into the waveguide and is provided with lumped reactive elements which form a plurality of stagger tuned resonant circuits with distributed reactances of the inserted portion.

BACKGROUND OF THE INVENTION This invention relates to radio frequency transducers and more particularly to a transducer for coupling together a radio frequency waveguide of the hollow conductor type and one of the parallel strip, multiple conductor type, such as the stripline or Microstrip.

Mixed microwave circuits, in which part of the circuit is in conductively bounded waveguide and part is in the form of parallel strip conductors, are becoming increasingly popular with the development of integrated circuit techniques. In these circuits it is generally necessary to transfer energy one or more times between guides of these different types. One form of transducer now being used in the art for this purpose employs a short sectional coaxial line matched at one end to the stripline and having the center conductor thereof at the other end forming a probe which extends into and is matched to the waveguide. Examples of transitions between stripline and coaxial conductors may be found, for example, in United States Patent 2,794,174 granted May 28, 1957 to M. Arditi et al. or 3,201,721 granted Apr. 17, 1965 to R. W. Voelcker. Transitions between coaxial lines and waveguides are shown in publications by S. B. Cohn in the September 1947 Proceedings of the I.R.E. at p. 920 or by W. W. Mumford in the February 1953 Proceedings of the I.R.E. at p. 256. This combined transducer is, however, cumbersome and expensive.

If the center conductor of the stripline itself could be directly inserted into the waveguide in a manner analogous to the coaxial probe, the transition could be physically simplified. This approach has not heretofore proved satisfactory, except for very narrow bandwidths, because the stripline center conductor so inserted forms with the waveguide a sharply resonant circuit at some frequency. Unless this frequency falls within the band 0f interest there will be insufficient coupling, but if the frequency can be adjusted to fall within the band, its sharp resonance will severely limit the band of coupled energy.

SUMMARY In accordance with the invention, the portion of the center conductor directly inserted into the waveguide is provided with a pattern of substantially lumped reactive elements, at least one of which is within the guide, the others, if any, being close to the guide. When these reactive elements are tuned with the distributed reactance of the stripline center conductor to a plurality of spaced individual frequencies within the band, the resulting series of stagger tuned circuits produces a broadband match. Each reactive element may be either in the form of a capacitive crosspiece or stub with respect to the center conductor, or altematvely, an inductive notch in the 3,462,713 Patented Aug. 19, 1969 center conductor. Both stubs and notches have been used in the art as reactances to modify the characteristic impedance of the strip for impedance matching purposes and it is known that they can be substituted one for the other if accompanied by physical relocation of one-quarter wavelength along the length of a stripline.

BRIEF DESCRIPTION OF DRAWING FIG. l is a cutaway perspective view of a waveguidestripline transducer in accordance with the invention;

FIG. 2a is a cross sectional view taken through FIG. 1 as indicated to show the stripline pattern in accordance with the invention;

FIG. 2b is the approximate equivalent circuit of the pattern of FIG. 2a;

FIG. 3 is a typical coupling versus frequency characteristic for the embodiment of FIGS. l and 2a; and

FIGS. 4a through 6a are alternative stripline patterns together with their approximate equivalent circuits in FIGS. 4b through 6b respectively.

DETAILED DESCRIPTION Referring more particularly to FIG. 1, an illustrative embodiment of a transducer is shown which provides coupling between a rectangular conductively bounded waveguide 10 and a parallel conductor stripline 11. For illustration, line 11 is specifically shown as the symmetrical type having a thin center conductor or strip 12 interposed between a pair of ground planes which are formed in the particular embodiment illustrated by the wider inside surfaces of a channel cut in body 13. Strip conductor 12 is typically supported in this channel by being formed of conductive material plated or printed upon a supporting substrate 14 of high dielectric material. lIt should lbe understood, however, that the invention may be applied to lines of self-supporting center conductor or to the unsymmetrical type of line, sometimes referred to as the Microstrip, in which a first thin conductor is related to only one ground plane. Furthermore, the ground planes may be printed or plated surfaces in the familiar sandwich construction.

The coupling between guide 10 and stripline 11 is formed by extending center conductor 12 through an elongated aperture 15 in the wide wall of guide 10` which aperture preferably has the same cross sectional dimensions as and is aligned with the channel in body 13. Thus the extended center conductor forms a strip shaped probe 16 within the guide parallel to the undisturbed electric field polarization in guide 10. The position of aperture 15 is displaced away form the longitudinal center line of guide 10 in accordance with known impedance matching considerations and the end of guide 10 is closed by a short circuit, piston or conductive transverse wall 19 spaced from the position of the probe 16. Details concerning the location of both the probe and the conductive short are disclosed, for example, in terms of coaxial probes in the above-mentioned article The Optimum Piston Position for Wide-Band Coaxial-to- Waveguide Transducer by W. W. Mumford, appearing in the February 1953 Proceedings of the I.R.E. at p. 256. It has been determined that it is immaterial whether the plane of strip shaped probe 16 is parallel to a narrow wall of guide 10 or parallel to wall 19.

It should now be noted that the stripline ground planes which as to the upper portion of strip 12 comprised the surfaces of the channel in body 13, are now effectively replaced as to probe portion 16 by the walls of guide 10.

In accordance with the invention it has been found thatV condition in the band of interest. An equivalent circuit will be described hereinafter in connection with FIG. 2 which attempts to identify the components of distributed reactance which porduce the observed condition of resonance. Unless this resonant frequency falls within the band of interest there is limited coupling and if the resonance does fall within the band, the coupled band is very narrow.

It is known in the art that stripline stubs extending on one or both sides of the center conductor constitute capacitive reactances, as Shown for example, in the above-mentioned Arditi patent. In accordance with the present invention the coupled bandwidth is broadened by stubs 17 and 18 of this type placed, however, upon the probe 16 portion of conductor 12 within guide 10. In particular stubs 17 and 18 have such dimensions and locations on probe 16 that they form with the distributed reactances of probe 16 within the guide, a second resonance at a frequency within the band of interest but adjacent to and different from the resonant frequency produced as a result of the distributed reactances of probe 16. The interaction of these two resonant circuits has the same effect as stagger tuned circuits used in filters or equalizers, for example, and broadens the coupled bandwidth as will be shown in connection with FIG. 3.

FIG. 2a is -a cross sectional view taken through guide in front of dielectric support 14 and shows the stripline center conductor 12 forming probe 16 as it extends within guide 10. An equivalent circuit is shown in FIG. 2b having inductances L and certain capacitances C and Ca representing the distributed reactances of that portion of strip 12 within guide 10. Outside of guide 10 reactances corresponding only to L and C are present, and customary design usually results in a net inductive reactance and nonresonant condition at the frequency of interest. Within the guide, however, capacitance Ca represents that portion of the total distribtued capacity that is in series with the center conductor due to the electric field component in guide 10 parallel to probe 16 of strip 12 which depends in turn upon the distance a of FIG. 2a. It is this capacity which produces the band limiting effect to which the present invention is directed. The capacity C then represents the remaining small distributed capacity due to the electric eld perpendicular to the strip. Stubs 17 and 18 produce a substantially lumped capacity represented on the equivalent circuit of FIG. 2b by Cb, large compared to C, and dependent upon the dimension b of FIG. 2a as well as upon the width of stubs 17 and 18. When the circuit loops invloving Ca and Cb, as indicated in FIG. 2b, are tuned to different frequencies f1 and f2 within the band, improved coupling in accordance with the invention is achieved as is shown in FIG. 3 by the familiar double humped coupling versus frequency characteristic of stagger tuned circuits. The spacing Af between the respective hump center freqeuncies f1 and f2 is readily adjusted by control of the spacing of stubs 17 and 18 from the end of probe 16. This spacing determines the amount of distributed inductance L along with a smaller amount of distributed capacitance C included within each circuit loop. Obviously, in a practical embodiment the parameters are not as discrete as the equivalent circuit seems to indicate. Furthermore, since other variables include 'the position of the short, the length of the probe and its position, proper proportions of the center conductor pattern are best determined on an empirical basis after an approximate choice of the other parameters on the basis of known coaxial probe relationships.

One or more additional capacitive reactances, further producing one or more separate resonant circuits, may be added to further increase the bandwidth. Thus in FIG. 4a stubs 21 and 22 are added which have dimensions different from those of stubs 17 and 18 and are spaced therefrom along center conductor strip 12. The dimension c of stubs 21 and 22 determines a capacity Cc as shown on FIG. 4b and the spacing of stubs 21 and 22 from stubs 4 17 and 18 determines the value of L and C as required to produce resonance at a new frequency f3.

Notches such as 23 and 24 of FIG. 5a reduce the width of center conductor 12 and introduce an inductance in series with the line as indicated by inductance Ld of FIG. 5b. In general a notch is equivalent to a stub located one-quarter Wavelength further along the line from the position of the stub which is replaces. Thus a plurality of notches 25 as shown in FIG. 6a and having the equivalent circuit of FIG. 6b can similarly be employed.

It should be understood that the added reactances can only produce the improvement in accordance with the invention if they tune with the distributed reactance of probe 16 within the guide. Otherwise added reactance only serves to modify the characteristic impedance of the line in accordance with prior art teachings, and only incidentally, if at all, to effect bandwidth of any coupling means at the end of the strip. It has been found, however, that if at least one of the added reactances is within the guide, additional improvement may be obtained from others not physically within the guide but which are relatively close thereto. It is believed that improvement in accordance with the invention can be obtained so long as the added reactance is in the transition region where field patterns are changing from those of the waveguide mode to those of the stripline mode. It is further believed that a reactance more than one-quarter wavelength away from the guide can have little direct effect upon the coupled bandwidth.

While in the particular form described, the plane of the stripline was shown inserted parallel to the narrow side of the guide, it should be noted that any given pattern may be turned ninety degrees in the guide so that its plane is perpendicular to the narrow side without effecting the coupled bandwidth. This versatility is one of the primary advantages of the coupling in accordance with the invention. In certain experimental models it was, however, found necessary to make slight readjustment of the shorting piston position when the pattern is rotated.

In all cases it is to be understood that the abovedescribed arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A combination in which a hollow conductively bounded waveguide is coupled for frequencies of electromagnetic wave energy within a given wide band to a second waveguide of the type having a strip conductor disposed in parallel spaced relationship to at least one conductive ground plane, said coupling being produced by extending a portion of said strip conductor into said hollow waveguide, means for introducing a first substantially lumped reactance to a first portion of said strip within said guide, characterized in that said lumped reactance is tuned with a portion of the distributed reactances of said portion to resonance at a first frequency within said given band and further characterized in that means are provided for introducing a second substantially lumped reactance to a second portion of said strip displaced along the length thereof from said first portion but in coupling relationship to fields of wave energy in said hollow waveguide, said second reactance being tuned to resonance within said band at a frequency slightly different from said first frequency to broaden the transmission band of electromagnetic energy coupled between said waveguides.

2. The combination according to claim 1 wherein one of said lumped reactances comprises a conductive crosspiece upon said strip within said hollow guide and the other comprises a notch in said strip.

3. The combination according to claim 1 wherein one of said lumped reactances comprises at least one conductive stub upon said strip within said guide.

5 6 `4. The combination -according to claim 1 wherein one OTHER REFERENCES of said lumped reactances comprises at least one notch Parallel-Plate Transmission Systems for Mirowave Frequencies-A. F. Harvey--The Proceedings of the Institution of Electrical Engineers, London (Part B No.

5 26) March 1959 (vol. 106) pp. 129-133.

in said strip within said guide.

5. The combination according to claim 1 wherein said lumped reactances comprises conductive crosspieces of diierent size pon said strip within said guide.

References Cited HERMAN KARL SAALBACH, Primary Examiner UNITED STATES PATENTS MARVIN NUSSBAUM, Assistant Examiner 2,829,348 4/1958 Kosrrtza et al. 333-84 XR 10 U5 C1. XR. 2,877,426 3/1959 KostllZa et al. 333-84 XR 2,884,601 4/1959 KostriZa et al. 333-84 XR 3331-84, 98 2,979,676 4/1961 Rueger 333--34 3,265,995 8/1966 Hamasaki 333-21

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