US2956247A

US2956247A - Broad band microwave phase shifter

Info

Publication number: US2956247A
Application number: US561401A
Authority: US
Inventors: Peter J Sferrazza
Original assignee: Sperry Rand Corp
Current assignee: Sperry Corp
Priority date: 1956-01-26
Filing date: 1956-01-26
Publication date: 1960-10-11
Anticipated expiration: 1977-10-11

Description

Oct. 11, 1960 2,956,247

P. J. SFERRAZZA BROAD BAND MICROWAVE PHASE SHIFTER Filed Jan.' 2e. 195e United States Patent 1 Eice Patented Oct. 1l, 1960 BROAD BAND MICROWAVE PHASE SHIFIER Peter I. Sferram, Wantagh, N.Y., :signor to Sperry Rand Corporation, a corporation nl Delaware Filed 1.11.26, 195s, sa. No. 561,401

s Chim. (cl. asa-s1) This invention relates generally to microwave phase shifting devices, and more particularly, is concerned with a structure for introducing a substantially fixed predetermined phase shift along a wave transmission line over a broad frequency band.

It is frequently desirable in microwave transmission systems to introduce a fixed amount of phase shift into one transmission path, for example, to change the phase relative to a signal transmitted by second transmission path. One rather simple and straightforward way of accomplishing this Iresult is by making one transmission path longer than the other by a -action of a wavelength .corresponding to the desired phase shift. Such a scheme,

however, is highly frequency sensitive since any change in wavelength changes the relative phase of energy transmitted by the two paths.

Other arrangements for introducing a fixed phase shift include the introduction in one wave guide transmission path of a dielectric insert which effectively changes the wavelength of the energy transmitted in the region of the dielectric. It can be shown that by proper tapering vor stepping of the dielectric insert, the frequency over which a given phase shift is achieved can be extended. However, the insertion of dielectric within the wave guide has the objection that it substantially reduces the power handling capacity of the transmission line in that the dielectric -absorbs energy to a degree, which in the presence of high incident energy, results in overheating of the dielectn'c.

Yet anotherarrangement heretofore proposed for introducing phase shift into a wave guide transmission line has been to use filter sections incorporating reactive elements such as irises and probes. The 4reactive elements of the filter sections introduced into the wave guide transmission iline greatly reduce the power handling capacity of the transmission line by rincreasing the probability of voltage breakdown. v

It is the general object of this invention to avoid and overcome the foregoing and other diiculties in and objections to the prior art practices by the provision of a microwave phase shifter which provides a substantially constant phase shift over an extended frequency band yet does not materially reduce the power handling capacity of the associated wave guide transmission line.

Another object of this invention is the provision of a phase shifter that introduces negligible reflection without special matching connection means.

These objects and other objects are achieved according to the present invention by providing a guided path for microwave energy, the path being bonded by conducting surfaces into one of which are inserted branch microwave transmission devices. The branch devices are dimensioned and terminated so as to produce a coeicient of coupling with respect to the guided path whereby a corresponding predetermined phase shift is introduced into the microwave energy yflowing in` the guided path. In a typical embodiment of the present invention, there is provided a plurality of rectangular wave guide shorted stub sectionseoupled toa broad wall of a main rectangular wave guide section, theA stub sections being series connected to the lmain wave guide section. lThe stub sections are an eighth wavelength long at the design frequency as measured from the broad wall of the associated main wave guide to the shorted end of the stub section. Adjacent stub sections are spaced from each other along the wave guide la center-to-center distance of a quarter wavelength at the design frequency.

For a better understanding of the invention, reference should be had to the accompanying drawings, wherein:

Fig. 1 is a perspective view of one embodiment of the present invention;

Fig. 2 is a schematic showing of a branched-guide coupler useful in explaining the operation of the present invention;

Fig. 2a is a vector diagram of the voltages in Fig. 2;

Fig. 3 is a schematic diagram of the embodiment of the present invention shown in-Fig. l;

Fig. 3a is a vector diagram of the voltages in Fig. 3;

Fig. 4 is a graph useful in explaining the design characteristics of the phase shifter; and

Fig. 5 is a schematic showing of a modification of the phase shifter of Fig. l.

In the embodiment of the invention as shown in Fig. 1, the numeral 10 indicates generally a section of rectangular wave guide for coupling energy, for example, from a microwave source 12 to a load 14. A xed phase shift is introduced along the length of rectangular wave guide 10 by a pair of shorted wave guide stub sections 16 and 18. These shorted stub sections are joined at one end to a broad wall of the rectangular wave guide section 10, forming E-plane or series type T junctions with the wave guide 10. The center spacing between adjacent stubs 16 and 18 is electrically a quarter wavelength at the design frequency of the phase shifter, while the length of the shorted stub sections is electrically an eighth wavelength from the shorted end to the end joined to the broad wall of the wave guide section 10. The narrow dimension of the rectangular wave guide stub sections 16 and 18 (the dimension extending parallel to the longitudinal axis of the wave guide section 10) is established according to the degree of phase shift desired, as will hereinafter be explained.

Operation of the phase shifter of Fig. 1 can best be understood by reference to the operation of the wellv known branched-guide directional coupler, as shown schematically in Fig. 2. The theory of operation of the branched-guide coupler has been extensively analyzed heretofore. See, for example, vol. ll, Radiation Laboratory Series, McGraw-Hill Book Company, page 866. The branched-guide coupler, -as shown in Fig. 2, includes a pair of rectangular

wave guide sections

20 and 22, forming

arms

1, 2, 3, and 4 of the coupler, and two'coupling stub sections 24 and 26, the stub sections being a quarter wavelength long and spaced apart -a quarter wavelength. lf a voltage E1 at an assigned zero phase angle is fed into arm 1 of the directional coupler, part of the energy will be coupled down the same wave guide section 20 to arm 2 and part of the energy will be coupled in one direction in the wave guide 22 to arm 3. Due to the directivity characteristic of the coupler no energy is coupled to arm 4. Such a directional coupler has an input impedance, coupling, and directivity that are substantially independent of frequency to a first order approximation, as analyzed in the above identified publication.

As is characteristic of directional couplers of high directivity, the energy coupled into the wave guide section 22 and out arm 3 is shifted by substantially 90 from the energy coupled directly down the wave guide section 20 and out arm 2. 'Ihe relative magnitude of the two output voltages from arm 2 and arm 3 of the relative phase angle of -90.

directional coupler Adepends upon the 'amount of coupling between the two

wave guide sections

20 and 22. Thus the energy out of arm 3 in the wave guide section 22 hasa magnitude CE1 at a relative phase angle of 90 as compared to the input wave (neglecting phase change due to propagation path length) where c is the coupling coeicient determined by the degree of coupling provided by the stub sections 24 and 26. The coupling coefficient varies between zero and unity and can be controlled by changing the narrow dimension of the stub sections 24 and 26. The energy out of arm 2 of the coupler accordingly has a magnitude of \/lc2E1 at a relative phase angle of as compared to the input wave.

Now consider a second input voltage E2, having an assigned zero phase angle so as to be in phase with the input voltage E1 fed into arm 4. Part of the energy will be coupled directly down the wave guide section 22 with a magnitude of \/1c2E2 while the other portion 4will be coupled out of arm 2 with a magnitude of cli.n at a The resultant output from arm 2 of the directional coupler therefore is the sum of the two voltages \/l-c2E1 and CE, having a phase quadrature relationship, as shown by the vector diagram of Fig. 2a. It will be readily apparent from Fig. 2a that the phase angle 0 of the resultant voltage depends on the relative magnitude of the two voltages E1 and E, and the value of the coupling coeflcient c.

If it is assumed that E1 and E, are equal, a voltage null exists in the stub sections 24 and 26 at a point halfway between the ends of the stub sections. This can be seen qualitatively by the vector arrows representing the voltages El and E, in Fig. 2. At the center of the stub sections 24 and 26, the two voltages produced by E1 and E2 are equal in magnitude and 180 out of phase so that complete cancellation takes place. Thus, a short circuit may be introduced in the st-ub sections at their midpoint, as indicated by the broken line 28, without any apparent change in the

output ofvarms

2 and 3. The resulting structure with the top portion removed is shown in Fig. 3, which it will be seen is a schematic showing of the phase shifter of the present invention, described in connection with Fig. l.

Itis evident by comparing lthe structure of Fig. 3 with Fig. 2 as described above that an output voltage is provided by the phase shifter of Fig. l having two components whose relative amplitudes are proportional to the input voltage where the respective proportionality factors are \/1c2 and c. These two components have a 90 phase relationship. The vector diagram of Fig. 3a shows that the resultant output voltage is equal in magnitude to the input voltage but varies in relative phase through an angle 0 to 90 depending upon the value of the coupling coelicient c, which in turn is determined by the narrow dimensions of the wave guide stubs, as described above.

The narrow dimensions of the branch wave guides in the structure of Fig. 1 can be determined by the following equations:

In the above equations, 0 is the phase shift introduced by the phase shifter, c is the coupling coetlcient of the equivalent branch guide coupler, C is the coupling in db provided by the T junctions, Z is the impedance of the branch guide or stub normalized to the impedance of the main wave guide transmission line, and b and b' are the narrow cross-sectional dimensions of the main and branch wave guides respectively, as indicated in Fig. 3. Equation 3 is plotted in Fig. 4. When using Equation 4, the value of nz (a correction factor for frequency) is determined from the frequency correction curves given in vol. l0, Radiation Laboratory Series, McGraw-Hill Book Company, page 346. At cut-off n2 is equal to one.

For example, assume that the main wave, guide is 1 x 2" operating a't a design wavelength of 2.5 inches and a phase [shift of 45 is desired. Bry Equation 1, c would be equal to .707. From Equation 2, C would then be -3 db. From the curve of Fig. 4, Z would then be .75, and therefore b', uncorrected for frequency would be 0.75". Corrected for frequency, the value of b would be about 0.9".

It should be noted that if b' exceeds b, the coupling, according to the curve of Fig. 4, begins to drop olf. As apractical matter therefore the stub wave guides are always smaller than the main wave guide, and if larger phase shifts are desired than can be obtained by two stubs with limited coupling, additional pairs of stub wave guides are provided.

While the phase shift introduced by the double stub device of Fig. 1 has the same broad band frequency response characteristics of the branched-guide coupler, the frequency band can be even further extended by providing additional stub. sections, as shown in Fig. 5. Again the stub sections are an eighth wavelength long at the design frequency from the short circuit end to the end joined to the broad wall of the main wave guide section, and the stub sections are spaced at center-tocenter distance of a quarter wavelength. However, to improve the frequency characteristics of the phase shifter, the narrow dimensions of the wave guide stubs are varied according to Iwell known broad-banding techniques. Ihe narrow dimensions of the stub wave guides can be proportioned, for example, so that the amplitude of coupling bythe respective stub wave guides varies according to the coecients of a binomial expansion or the well known Tchebyschetf polynomial expansion, in the same manner as has been heretofore taught in connection with multiple-hole broad band directional couplers.

'Ihe coefficients of a binomial expansion are given in the following table:

Number of stubs N- and for the Tchebyschel polynomial are given in the following ltable for p=2, where p is the ratio of the guide wavelengths at the edges of the operating frequency band.

Relative amplitude a 'Ihe relation between the coupling coecient c1 for the smallest pair of stubs having equal coupling and the coupling coeflicient c of all the stubs may be expressed by the equation If the quantity in the brackets of Equation 5 is small, Equation 5 can be rewritten as The coupling of any other pair is then found from the expression C,= C, +20 10g (s) The size of the narrow dimension 6' of the stub wave guides can then be determined as before from the graph of Fig. 4. If there are an odd number of stubs, calculations are made in the same way except the center stub is considered as one element of a pair.

From the above description it will be seen that the various objects of the invention have been achieved in one embodiment by providing means for introducing a predetermined phase shift iuto a rectangular wave guide transmission line. 'Ihe phase shifter, utilizing stubs that are an eighth wavelength long and spaced a quarter wavelength apart, has the same broad band properties as the phase shifter comprising a main rectangular wave guide transmission line, a plurality of rectangular wave guide shorted stub sections each joined at one end to the broad wall of the main waveguide in a series T junction, the stub sections being substantially an eighth wavelength long at the design frequency as measured from the junction end to the shorted end, and adjacent stub sections being spaced from each other along the main wave guide a center-tocenter distance of substantially a quarter wavelength at the design frequency, the stub sections being dimensioned so as to produce a total coupling coefficient c related to the desired phase shift 0 according to the rclationship c=sin 0 where c is the ratio of the amplitude of the total energy coupled out of the transmission line at the junctions and the amplitude of the energy coupled into the transmission lline at the input thereof.

3. A phase shifter as defined in claim 2 wherein the rectangular stub sections have the same broad dimensions as the main wave guide and the narrow dimensions vary such that amplitudes of the energy coupled out at the respective T junctions vary according to the coelicients of a binomial expansion.

4. A phase shifter as defined in claim 2 wherein the rectangular stub sections have the same broad dimen- Well known branched-guide hollow Wave guide direc- Y tional coupler, and can be even further improved in frequency response by providing a multiplicity of stubs in a suitable array, e.g. one in which the coupling is varied according to the coetlicients of a binomial or Tchbyscheif polynomial expansion. 'I'he phase shifter does not materially affect the power handling capacity of the wave guide line, since no reactive elements or dielectric material is introduced into the main wave guide transmission line section itself. Any amount of phase shift can be achieved by using several double-stub phase shifters in tandem if desired. v l

It should be noted that where the dimensions are given in terms of wavelengths, the wavelengths are the electrical distances not actual distances, and a correction for fringing field effects at the junctions must be considered, as taught in vol. 10, Radiation Laboratory Series, chapter 6, to get physical distances corresponding to the desired electrical distances specified.

Since various changes can be made in the above con struction and many apparently Widely different embodiments of this invention could be made without departingfrom the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not n a limiting sense.

What is claimed is:

1. A wave guide phase shifter operative over a broad band of frequencies encompassing a design frequency, said phase shifter comprising a main rectangular wave guide transmission line, a pair of rectangular wave guide shorted stub sections each joined at one end to the broad wall of the main wave guide in a series T junction, the stub sections being substantially an eighth wavelength long at the design frequency as measured from the junction end to the shorted end, and the stub sections being spaced from each other along the main wave guide a center-to-center distance of substantially a quarter wavelength at the design frequency, the stub sections being dimensioned so as to produce a total coupling coeilicient c related to the desired phase shift -0 according to the relationship c=sin 0 where cis the ratio of the amplitude of the total energy coupled out of the transmission line at the junctions and the amplitude of the energy coupled into the transmission line at the input thereof.

2. A wave guide phase shifter operative over a broad band of frequencies encompassing a design frequency, said sions as the main wave guide and the narrow dimensions vary such that amplitudes of the energy coupled out at the respective T junctions vary according -to the coeicients of a Tchebyschet polynominal expansion.

5. A microwave phase shifter operative over a broad band of frequencies encompassing a design frequency, said phase shifter comprising a transmission line having input and output terminals for the transfer of microwave energy from an energy source to a load, said input terminals being directly connected to said source and said output terminals being directly connected to said load, a pair of stub sections each joined at one end to said transmission line, the stub sections being substantially an eighth wavelength long at the design frequency as measured from the junction end to the other end thereof, and the stub sections being spaced from each other along the transmission line a center-to-center distance of substantially a quarter wavelength at the design frequency, the stub sections being dimensioned and terminated so as to produce a total coupling coefficient c related to the desired phase shift 0 according to the relation c=sin 0 where c is the ratio of the amplitude of the total energy coupled out of the transmission line at the junctions and the amplitude of the energy coupled into the transmission line at the'input thereof.

6. A microwave phase shifter operative over a broad band of frequencies encompassing a design frequency,

` said phase shifter comprising a transmission line having input and output terminals for the transfer of microwave energy from an energy source to a load, said input terminals being directly connected to said source and said out put terminals being directly connected to said load, a plurality of stub sections each joined at one end to said transmission lne, the stub sections being substantially an eighth wavelength long at the design frequency as measured from the junction end to the other end thereof, and the stub sections being spaced from each other along the transmisison line a center-to-center distance of substantially a quarter wavelength at the design frequency, the stub sections being dimensioned and terminated so as to produce a total coupling coeiiicient c related to the desired phase shift 9 according to the relation c=sin 0 where c is the ratio of the amplitude of the total energy coupled out of the transmission line at the junctions and the amplitude of the energy coupled into the transmission line at the input thereof.

7. A phase shifter as defined in claim 6 wherein the stub sections are dimensioned and terminated such that the amplitudes of the energy coupled out of the transmission line at the respective junctions vary according to the coefficients of a binomial expansion.

7 8. A phase shifter as defined in claim 6 wherein the stub sections are dimensioned and terminated such that the amplitudes of the energy coupled out of the transmission line at the respective junctions vary according to the cocicients of a Tchebyschcf polynomial expansion.

References Cited in the file of this patent UNITED STATES PATENTS 2,147,807 Alford Feb. 21, 1939 2,155,508 Schelkuno `Apr. 25, 1939 2,159,648 Alford May 23, 1939 2,540,488 Mumford Feb. 6, 1951 2,561,130 McClellan July 17, 1951 8 White Sept. 11, 1951 Kock Oct. 23, 1951 Fox Mar. 4, 1952 Lovcridgc Dec. 23, 1952 Reed Feb. 17, 1953 Lines June 9, 1953 Cutler Nov. 17, 1953 Warnecke et al Aug. 31, 1954 Pierce May 10, 1955 Krutter et al Iune 21, 1955 King ..5 June 19, 1956 FOREIGN PATENTS Great Britain .1 Apr. 7, 1954