US2772400A - Microwave polarization changer - Google Patents

Description

1956 A.. J. SIMMONS 2,772,400

MICROWAVE POLARIZATION CHANGER Filed Jan. 8, i954 3 Sheets-Shes; 1

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D INVENTOR A LA'N J. SIMMONS BY i /WI? ATTOR NEYJ Nov. 27, 1956 A. J. SIMMONS ,77

MICROWAVE POLARIZATION CHANGER Filed Jan. 8, 1954 3 Sheets-Sheen 2 QT MM I W MH UTE ALAN J. SIMMONS flw/lgfig ATTORNEY5 NOV? 1956 A. J. SIMMONS MICROWAVE POLARIZATION CHANGER 3 Sheets-Sheerv 3 Filed Jan. 8, 1954 United States Patent 015cc 2,772,400 Patented Nov. 27, 1956 MICROWAVE POLARIZATION CHANGER Alan J. Simmons, Hyattsville, Md., assignor to the United States of America as represented by the Secretary of the Navy Application January 8, 1954, Serial No. 403,081

6 Claims. (Cl. 333-21) (Granted under Title 35, U. S. Code (1952), sec. 266) This invention relates in general to phase shifting devices and more particularly to rectangular waveguide devices for changing the phase between orthogonally polarized waves in a linear polarization to produce a circular polarization.

It is commonly known that circular polarization may be set up from linear polarization when two orthogonally polarized waves of equal amplitude in a transverse mode are shifted in relative phase one quarter wavelength. In the prior art several devices have been developed to produce the necessary phase shift between the orthogonally polarized waves. The phase velocity of either one wave or the other has been retarded to produce the one quarter wavelength phase delay. One familiar device loads the waveguide uniformly by the insertion of a longitudinal metal fin which alters the phase velocity of the wave parallel to the fin. Another device loads the waveguide periodically by the insertion of small screws at intervals within the guide to alter the phase velocity of only one wave. Still another device in the prior art has a dielectric plate longitudinally positioned within the waveguide to retard the phase velocity in both polarizations. In this device the phase delay is dependent on the dielectric material used and on a ratio of the amount of dielectric in the path of each wave. It is readily seen by those familiar in the art that said dielectric plate will present a serious matching problem unless properly designed. Unfortunately a variation of the plate configuration will usually introduce a difiicult phase delay determination problem which may tend to offset any matching advantage obtained.

While waveguide transmission lines are relatively broadband polarization changing devices for use with these lines, with the exception of the above described dielectric plate device, are generally narrowband. Whereas a broadband polarization device of simple design is particularly desirable in many applications:

It is an object of this invention to provide a unique waveguide phase shifting device with a wide band pass characteristic for translating linear polarization to circular polarization.

It is also an object of this invention to provide a means for producing a phase shift between orthogonally polarized waves of a linear polarization whereby one Wave is advanced and the other wave is delayed to produce a relative phase shift.

It is an additional object of this invention to provide a broadband waveguide phase shifting device which may be simply and sturdily constructed to produce any amount of phase shift desired.

It is still another object of this invention to provide a means for producing a phase shift to be used in the translation of linear polarization to circular polarization within a rectangular waveguide in which matching means are incorporated as an integral part thereof.

Other objects of the invention will become apparent from a better understanding of the invention for which reference is had to the accompanying detailed description and drawings of the invention.

In the drawings:

Figure 1 is a side view of one embodiment of a phase shifting device according to this invention;

Figure 2 is an end view of the embodiment of Figure 1;

Figure 3 is an elevational view, cut away and in perspective to show detail and assembly of the embodiment as shown in Figure 1;

Figures 4a, 4b, and 4c are a group of graphs showing the phase shift (no) between irises for various dimensional parameters in the described embodiment.

Figures 5a, 5b, and 5c are a group of graphs showing the voltage standing wave ratio (V S W R) for various dimensional parameters in the described embodiment.

In the examination of the wave energies within the waveguide device of this invention only the transverse electric (TE) field, in which the electric field lies across the waveguide and no E lines point in a longitudinal direction within the guide shall be considered. The relative phase difference between modes discussed herein shall refer to the phase difference between the orthogonally polarized TEn,1 and the TE1,0 modes. It is understood that the discussion which follows might also be applicable to a transverse magnetic (TM) field analysis.

Briefly, the invention comprises a periodic loading of a rectangular waveguide by means of a series of windows, hereafter called irises, transversely disposed within the waveguide section. Each pair of irises capacitively affects (delays) one mode and inductively affects (advances) the other mode to produce its proportionate phase shift, the total phase shift being the sum of the individual phase shifts within the waveguide section. With a change in frequency this capacitive-inductive efiect self-compensates to give the invention a characteristic constant phase shift over a wide band of frequencies. In the device of this invention the amount of phase shift is dependent on the dimensions of the waveguide, the size and separation of the incorporated irises, the number of irises and the free space wavelength of a chosen center frequency.

Referring now to the drawings in detail:

in Figure 1 a rectangular waveguide section is shown according to this invention in which thin metal fins have been transversely placed as obstacles at determined intervals within the section to form a series of irises. In the embodiment as shown there are 10 metal fins forming 5 irises within the section. All the transverse fins are shown equally spaced along the tube and each is of similar thickness and height with the exception of the 4 end fins which are reduced in height to provide a proper matching impedance for the end section, the fins being so spaced and dimensioned for reasons which shall appear more fully hereafter.

The invention may incorporate any number of irises, dependent upon the amount of phase shift desired. In a particular waveguide section the spacing between irises may be reduced provided the non-progating field produced by one iris do not interact with the corresponding fields of the preceding or following iris. It is of great importance in the basic analysis of the invention that the susceptance of each iris may be determined independent of the next and it is obvious that any interaction between the adjacent irises would complicate such a simplified analysis. It is understood that the invention itself is not to be so operatively limited in the spacing of the irises. Attention is called merely to show the point at which a simplified analysis of the device may become more difficult.

In Figure l the fins are shown penetrating the walls of the wave guide in accordance with a simplified construction procedure wlrereny the fins are inserted in narrow slots and then soldered in place before the usual silvering From a complex matrix formulation it may be shown of the unit. It is understood that it is not essential to the invention that the fins penetrate the walls of the wavel cos [COS Bl Bl] (4) guide and that this innovation is shown merely to display T above formulation has eonsldeled botb B a11d S a preferred and simplified means by which s invention. 5 as lndependent variables. In the broadbandmg analysis, may be constructed however, it is essential that the dependence of these two As shown in the drawing of Figure 1, the distance be- Variables on frequency 011 free Space Wavelength tween the irises is represented by l and the distance across be considered Where M is the Wavelength tbe guide the inside of the waveguide section is represented by b. for One mode, M is the Wavelength in the guide for the Likewise, the distance between opposite fins of each of (Fiber mode and b the heigb'E 0f guide it Can be the center irises is represented by d and the corresponding SbeWIl that distance in the case of the larger end irises by e. The 21rl 21rl b separate end view of the waveguide, shown in Figure 2, BC "1': T'X C (5) illustrates the transverse positioning of the fins within and the section. The width of the waveguide section is repre- 2 l 2 l b sented by a as shown in this end view. B Z= (6) Figure 3 shows a portion of the embodiment of Figure 1 in a prgjected end iew. This utaway iew more Where a iS th Width of the guide it can be SllOWll that clearly portrays the transverse metal fins in their correb 1 b i 2a 2 sponding' relation to one another. 1 (7) Figures 4a4c and Sa-Sc will aid in the under- 2 standing of the operation of this invention which is deand scribed hereafter in more detail. b 1 2a b 2 t:In alrlialyzingfthf'i operation of thlis ingentilon thehetifect T ')(E) o eac pair o a 'acent irises wit in t e piase s iting device should be ccinsidered individually. In this analysis Substituting Equanon 4 m Equatlon 2 the eifect of each intermediate iris is considered twice, A0c=cos [cos ficl-Sc sin ficl]l3cl (9) once with "each of its adjacent irises. For the purpose when;v substituting Equation 7 in Equation of simple analysis an infinitesimal fin thickness is assumed and the basic theory of inductive and capacitive irises of Z 2a 2 Zero thickness (as described in Waveguide Handbook- 561:7- E (10) Marcuvitz M. I. T. Rad. Lab. Series volume 10, pages 218430) is applied to determine the phase advance and 1 phase delay contributing to the proportionate phase shift B=C0S [cos flLl-SL sin ml] /3Ll (11) for each pair of irises. A short summary of the theory where substituting Equation 8 in Equation 6:

involved in this unique application of said basic theory I of admittance and susceptance is outlined below. For a l=L\/ 1 (12) more comprehensive understanding of the properties of b a thin irises reference is had to the Marcuvitz text.

Similarly, substituting Equation 4 in Equation 3 Thus it can be seen that an expression for the capaci- Typically, a loaded waveguide with admittance Yo, tive phase delay A00 for a pair of irises may be obtained used for calculating reflecting properties, and propagation in terms of waveguide height, width, admittance and susconstant [2, used for calculating phase shift, may be receptance from a simple substitution of Equation 10 in placed by an equivalent transmission line of new char- Equation 9. acteristic admittance Y0 and new propagation constant Similarly it can be seen that an expression for the [3. For the purpose of this description 2S represents inductive. phase advance ABL. may be obtained in the same either the positive or negative susceptance of each iris. terms from a substitution of Equation 1'2 in Equation 11. In the discussion 28 also represents the susceptance of The susceptance of standard waveguide elements, such the phase shift section (between irises) but it should be as capacitive and inductive irises, has been measured and understood that this is considered to be the summation recorded in. such readily available sources as the reference of one half the susceptance of the preceding and of the text (pages 220223). It is convenient, therefore, to following iris. Accordingly, it follows that the end irises consult such graphs to determine the values of the capacishould be considered to have a susceptance S. tive susceptance So in Equation 9 and the inductive sus- As previously referred to, a compensating capacitiveceptance SL in Equation 11 and to observe the dependence inductive efiect is inherent in the operation of this invenof Se and St. on the iris aperture and on the waveguide tion. Each of the thin irises acts as a shunt capacitance height, width and free space wavelength. for one mode to introduce a phase delay and as a shunt By substituting the expressions thus obtained for A inductance for the other mode to introduce a phase adand A01. in Equation 1', the total relative phase shift for vance. The proportionate phase shift, A t, for each pair each pair becomes at t a s srsteer01-weei of irises is the summation of the capacitive delay A00 5 The total phase. shift for the device is the sum of the and the inductive advance A01. as individual; phase shifts for each adjacent pairs of irises. v For identical; intermediate. iris, dimensions as shown in A(I IMCl+|A0Ll (1) the illustrated embodiment, the calculation of the phase Considering the equivalent transmission line, the 0&- shift of one pair will hold for each of the others except Paeltive delay y be represented y the pairs, including the larger end irises. Thus it will be A0C:Bc,l 8cl (2)v seenv that using only two different sized fins to form the irises: makes determination of the phase shift much easier.

In constructing the phase shifting device of this inven- =I -B (3): tion it is convenient, knowing the propagation constant wherel is the. actual separation between irises. B for the medium, to first select a value for fil in the and the inductive advance by region of rr/Z and then, using the formulae disclosed herein; to determine the total number of phase sections between irises necessary .to give a 90 phase shift between orthogonal modes for the total line.

Using this convenient procedure for determining the phase shift the distance between irises over the height of the guide, Z/b, will be arbitrary until the free space wave length of the chosen center frequency 7m and either I or b have been selected.

Considering bandwidth only, the proper size iris may be determined by use of calculated graphs similar to those shown by Figures 4a-4c. On these graphs a bandwidth ratio for a 15% variation in phase shift is indicated. (Bandwidth ratio is the ratio of maximum to minimum value of Za/Ao). In Figure 4a, Ad has been plotted for values of the parameter d/b for the center irises. Obviously a similar plotting might be made for values of the parameter e/ b for the end irises. In Fig ure 4a it is seen that as the aperture of the iris is made larger the bandwidth increases. In Figure 4b, no has been plotted for values of the parameter l/b. It is seen that as the ratio 1/ b is increased, the bandwidth gradually decreases. Figure 4c shows that a variation of the ratio b/a from the value 1.0 decreases the bandwidth and also shifts the band of operation.

A further consideration in the design of this device is the maximum voltage standing wave ratio (V S W R). Maximum V S W R refers to the value of (Yd/Yo) or (Yo/Y's) (whichever is larger) for an equivalent transmission line having an electrical length which is an odd multiple of 1r/2. Y0'/Y0 is the ratio of equivalent line admittance to actual line admittance of one phase section and is essential to the calculation (using standard transmission line formulae) of the input admittance Yin/Y0 of the total waveguide section.

It may be shown:

In Figures 561-50, the maximum values of V S W R for the dimensional variation in Figures 4a4c are shown. A solid line shows the maximum possible V S W R for the capacitively loaded mode and a dotted line shows the maximum V S W R for the inductively loaded mode. Specifically, in Figure So it is seen that the V S W R is lower for a larger ratio d/b. While d/b should be chosen as large as feasible, it should be noted that as d/b is increased the number of sections necessary for a given amount of phase shift-also increases and the cumulative eliect of waveguide machining tolerances may tend to cancel the more apparent advantages of a larger d/b.

In a typical instance of the disclosed embodiment of this invention, in which a 90il0% phase shift having a bandwidth as large as possible and centered at 8300 me. u)=3.6 cm.) is desired, the following values are determined. Selecting l/b=0.40 for maximum bandwidth, with a minimum phase shift for the total four phase sections of 80 or for each phase section a minimum phase shift of 20 and with both a and b=2.96 cm., the center of the band will be at 2a/)\o=1.65 and the iris spacing 1 will be 1.19 cm. Choosing d/b=0.7 the center iris aperture d will be 2.07 cm. and the end iris aperture 6 will be 2.30 cm.

This invention as shown and described provides phase shifting device having a wide bandpass characteristic which may be simply and sturdily constructed :to produce a readily determined relative phase shift between orthogonally polarized waves in a waveguide transmission system. it is understood that this invention is adaptable to any square or near square waveguide section and :that it is not to be restricted to the disclosed square waveguide structure. Further, it is understood that while all the fins are shown equally spaced and the intermediate ones are shown of equal height :these dimensions, being illustrative of only one embodiment, are not to be considered defin- 6 ing or limiting features of the invention and that this invention is to be limited by the scope of the appended claims alone.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the pay ment of any royalties thereon or therefor.

What is claimed is:

l. A waveguide polarization converter for shifiting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a waveguide sec tion, a plurality of conducting fins transversely mounted and longitudinally spaced at intervals therein, said fins being dimensioned and relatively spaced to delay one of said modes and to advance its orthogonal mode, the number of fins in said plurality being suificient to provide a total relative phase shift between said orthogonal modes of approximately degrees, and means for introducing to the section an incident wave having orthogonal trans verse modes of equal magnitude.

2. A waveguide polarization converter for shifting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a substantially square waveguide section, a plurality of thin conducting fins transversely mounted and longitudinally spaced at regular intervals therein, said fins being dimensioned and relatively spaced to delay one of said modes, and to advance its orthogonal mode, the number of fins in said plurality being sufficient to provide a total relative phase shift between said orthogonal modes of approximately 90 degrees, and means for introducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

3. A waveguide polarization converter for shifting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a waveguide section, a plurality of pairs of thin conducting fins longitudinally spaced within said waveguide section, each of said pairs being transversely disposed to form an aperture therein, said fins being dimensioned and relatively spaced to delay one of said modes and to advance its orthogonal mode, the number of fins in said plurality being sufficient to provide a total relative phase shift between said orthogonal modes of approximately 90 degrees, and means for introducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

4. A waveguide polarization converter for shifting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a waveguide section, a plurality of pairs of thin conducting fins longitudinally spaced within said waveguide section, each of said pairs being transversely disposed to form an aperture therein, each of the terminal pairs of said plurality having a greater aperture than an intermediate pair of fins in said plurality, said fins being dimensioned and relatively spaced to delay one of said modes and to advance its orthogonal mode, the number of fins in said plurality being sufficient to provide a total relative phase shift between said orthogonal modes of approximately 90 degrees, and means for introducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

5. A waveguide polarization converter for shifting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a waveguide section, five pairs of thin conducting fins longitudinally spaced at regular intervals within said waveguide section, each of said pairs being transversely disposed to form an aperture therein, said fins being dimensioned and relatively spaced to delay one of said modes and to advance its orthogonal mode, the number of. fins in said plurality being sufficient to provide a total relative phase shift between said orthogonal modes of approximately 90 degrees, and means for introducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

6. A waveguide polarization converter for shifting the relative phase of the orthogonal transverse modes of the wave to be converted which comprises: a square waveguide section, five pairs of thin conducting fins transversely disposed to form apertures and longitudinally spaced at regular intervals within said waveguide section, the interval between pairs of fins being approximately 0.4 the transverse width of the waveguide section, said fins being dimensioned to delay one of said modes and to advance its orthogonal mode, the number of fins in said plurality being sufficient to provide a total relative phase shift between said orthogonal modes of approximately 90 degrees, and means for introducing to the section an incident wave having orthogonal transverse modes of equal magnitude.

References Cited in the file of this patent Publication I, Montgomery: Microwave Transmission Circuits, vol. 9, M. I. T. Radiation Lab. Series, published 1948 McGraw-Hill, pp. 369474. (Copy in Patent Office Library.)

Publication II, King, et al.: Transmission Lines, Antennas and Wave Guides, published 1949 McGraw-Hill, pg. 275.

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