US2741744A

US2741744A - Microwave apparatus for circular polarization

Info

Publication number: US2741744A
Application number: US225211A
Authority: US
Inventors: Driscoll Clare
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-05-08
Filing date: 1951-05-08
Publication date: 1956-04-10
Anticipated expiration: 1973-04-10

Description

C. DRISCOLL April 10, 1956 MICROWAVE APPARATUS FOR CIRCULAR POLARIZATION Filed May 8 1951 FIG? FIG.5

FIG.6

INVENTOR.

CLARE DRISCOLL FIG.I

FIG. 3

United States Patent MICROWAVE APPARATUS FOR CIRCULAR POLARIZATION Clare Driscoll, Washington, D. C., assignor m the United States of America as represented by the Secretary of the Army Application Ma a, 1951, Serial No. 225,211 4 Claims. c1. 333-21 This invention relates in general to microwave waveguides and is particularly directed to a broad band transformer for converting plane polarized waves to waves of circular polarization.

An electrical wave is said to be circularly polarized when the field thereof can be resolved into two equal components 90 apart in space and 90 apart in time. If the two vectors are not of equal amplitude, or if the time difference between them is other than 90, the wave is said to be elliptically polarized; in the special case in which the time phase angle reduces to zero, the wave is linearly polarized. Thus, both linear polarization and circular polarization can be looked upon as special cases of elliptical polarization.

The equation of this ellipse may be expressed as follows:

tude, and 6 is the time phase angle of the two components.

=sin 5 and A=B so that the components are 90 apart in time phase and are of equal amplitude the equation of the ellipse reduces to E: +Ey =A which is the equation of a circle. The wave is then circularly polarized. If 6:0 or 1r (components in phase or 180 out of phase), the equation becomes that of a straight line the wave is linearly polarized.

Various methods may be used to produce a field having components of substantially equal amplitude 90 apart in space in a square or rectangular wave guide. For example the two components may be separately introduced into the waveguide or they may be generated by applying a linearly polarized wave to the waveguide having an electric vector rotated approximately 45 with respect to either of the two pairs of sides of the square or substantially square waveguide, so that the electric vector will have substantially equal components perpendicular to each pair of sides.

It is necessary, in order to produce a circularly polarized wave, to create a phase shift in time of 90 in the two components of the wave in space quadrature. This has been previously accomplished by utilizing a rectangular waveguide of asymmetric proportions or, in a square waveguide, by the introduction of a longitudinal dielectric slab inserted parallel to one pair of sides of the waveguide.

The dimensions of the waveguide or of the dielectric slab are chosen so as to produce the desired time phase shift at the center of the desired frequency band. At frequencies above and below the center frequency, however, the time phase-shift will vary from the desired 90 time phase-shift, causing the polarization to deviate from circular polarization at these frequencies. The frequency 2,741,744 Patented Apr. 10, 1956 'ice band in devices of this nature is, therefore, restricted by the limits of ellipticity which are considered satisfactory.

It is consequently an object of this invention to provide apparatus which overcomes the limits of the prior art by reducing the deviation from circular polarization over a given frequency range.

It is a specific object of this invention to provide improved apparatus for producing circular polarization within desired limits of ellipticity over a broad frequency range.

.In accordance with this invention two wave guide transformers are connected in series to produce a total time phase shift between the transformers which is subject to less variation with change in frequency than would occur through the use of a single waveguide transformer. One transformer is designed to produce a time phase shift which is greater than the desired phase shift. The other transformer is designed to produce a correcting phase shift which reduces the total phase shift to approximately the desired value over a broad frequency range.

The foregoing and other objects of the invention will be best understood from the following description of specific embodiments thereof, reference being had to the accompanying drawings, wherein:

Fig. l is an isometric view of one embodiment of the invention;

Fig. 2 is an isometric view of another embodiment of the invention;

Figs. 3, 4 and 5 are cross sectional views of the arrangement of Fig. 1; and

Figs. 6 and 7 are cross sectional views of the arrangement of Fig. 2.

- Referring now to Fig. 1, a view of a device for converting a linearly polarized wave propagated in space quadrature into a substantially circularly polarized wave is shown, said device comprising a rectangular waveguide section 11 of constant cross section, as shown in Fig. 3, having a pair of long sides and a pair of short sides. A second section 13 of rectangular wave guide of constant cross section, as shown in Fig. 5, is provided having a length and a dimension ratio of long side to short side which difier from the length and dimension ratio of sides of section 11. The dimensions of the narrow walls of the

sections

11 and 13 are, of course, large enough to support propagation in which the electric field voltage vectors are perpendicular to the narrow walls for otherwise a circularly polarized wave would not be produced by the waveguide.

The long sides of section 11 are joined to the short sides of section 13 and the short sides of section 11 are joined to the long sides of section 13 by tapered waveguide section 15. Waveguide section 15 changes in cross sectional dimension along its length from the dimensions of section 11 shown in Fig. 3 through a square cross section shown in Fig. 4 to the dimensions of section 13 shown in Fig. 5.

A wave propagated into the open end of waveguide section 11 in space quadrature may be derived from any desired source (not shown) in a conventional manner. This wave has a composite electric vector lying in the cross sectional plane of the waveguide and inclined at an angle of approximately 45 to both pairs of sides of the rectangular waveguide. This electric vector can be resolved into two components Ex and B each perpendicular to a separate pair of sides of the waveguide.

At the open end of waveguide section 11 Ex and By are still in time phase and the resultant is a linearlypolarized wave. In order to satisfy the condition for a circularly polarized wave the two components must also have a time phase of or 270.

The waveguide section 11 as seen by the component B:

is different in width from that seen by B Therefore their velocities of propagation are somewhat different, and in thetpassage of the components through section 11 the phase angle between -Ex and B progressively departs from zero' and the wave becomes elliptically polarized.

This progressive increase in time phase angle continues with the passage of the wave into the tapered waveguide section 15 up to the 'point in section 15 where the 'dimem sions of the two pairs of sides become 'equal. At this point, indicated by the cross sectional View of Fig. -4, the components Ba and By see equal dimensions, and past thispoint in section 15 and in waveguide "section--13, it is apparent that components Ex and By see a reversal in pro Frequency: Phase shift, degrees To 9 fo-l-Af 75 fo-Af 110 In order to narrow the time phase shift over the desired frequency range the length of this Waveguide section may be doubled so as to produce a time phaseshift of 180 at its center frequency. This wave guide 'will have the following characteristics:

Frequency: Phase "shift, degrees f0 180 ftH-Af 150 f0-A), 220

connected waveguide section corresponding to section 13 of Fig. 1 having different cross sectional dimensions and therefore a difierent ratio of phase shift to frequency variation. Tfiiis section can be designed to have the following characteristics:

Frequency: Phase shift, degrees f minus 90 tori-n? minus 55 fuhjf minus 125 The total phase shift of these 'two sections connected in series would be:

Frequenc f0 r w :2 "yo-A7 95 r This is a substantial improvement over the phase shift variation over the-same frequency range obtainable from a single waveguide section.

lt-should be noted that'in theapar'ticular construction disclosed in Fig. 1 the tapered section willalso produce a-shift in the-time phase angle. 7 This must be taken into consideration in the design of'seetions 1'1 and 13. This phase shift will be first-in one direction and then in the other, reversing at the plane in which the cross section ofthe'taper is symmetn'ca1. Section 15 canbe-cdnsidered as two zguide sections, each sectionof whichcanbe considered'as'havinga taper to asymmetrical end which-is mated with the facing end-of the other sections. Carrying this point of view -to:its limit, :none of thewa'veguide sections 'wouldbe uniform throughoutdts complete length and the entire ipola'rization process could' becarited out in a properly dimensioned taper with no straight se'ction. Fig. 'Z' ShOWS a viewsot a second :embodime'nt'of "the Phase shift, "degrees ing the dimensions and the dielectric-constant of the slabs th'ewtivghide. I I

by placing a second longitudinal dielectric slab having different dimensions in a succeeding section 21 of the waveguide parallel to'the sec'ondpairofsid'es. 13y selecta variation of phase shift over thetdesired frequency range may 'be obtained which is similar to that previously described for the "structure illustrated in Fig. 1. While there have been described "what are presently considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention. a

What is claimed is: -1. A device :for introducing in a band of frequencies asub'st'a'ntial'ly 90 "time phase'displacement'betweenthe space quadrature com onents of the plane-polarized Waves'of said band of -frequencies comprising first hollow waveguide means for increasing the relative time phase angle between the space quadrature components of the center frequency of said band of frequencies by a given amount greater than 90 and shifting the relative'time 'phase angle between the space quadrature components quency variation which differs from that of said first waveguide means for reducing the shift in relative time phase angle between the space quadrature components of the output waves of said first waveguide means or all frequencies in'said band to substantially 90.

2. A device as claimed in claim 1 wherein said 'first means comprises a waveguide section having asymmetrical dimensionsand'said second means comprises a waveguide section of asymmetrical but dilferent dimensions than said first 'waveguidemeans. l

3. A device as cla'imed in claim 1 whereinsaid'first means comprises a waveguide rectangular in cross secti'o'n and havingrretativelygreaterspacing between its side walls than :"s'pacing :b'etweeniits top and bottom surfaces, said ise'c'ond me'an's comprises "a waveguide rectangular in cross section V and having relatively greater 'spacing betw'een i'ts top and bo'ttom :surltaces' than the spacing between its si'd'e walls, and :further including a waveguide component formed to join the opp'os'i'ng 'short "walls-of each rectangularwaveguidesection with'the opposing long walls of the other rectangular waveguide section.

4. A device as claimed in claim 1 wherein said first andsecon'd waveguides are '-'square in 'cr'o's's section and further including :a first dielectric :s'lab mounted within said fir st waveguide parall'ellto the 'walls "thereof anda