US3500258A - Wave mode converter - Google Patents

Description

March 1-0, 1970l E. R. NACELBERG WAVE MODE CONVERTER Filed Dea. 1e. 1968 ($338930 Nl) ll BCIIIDBAVM NI SHGOW mamas a'loNv asvud '.)NVENTOR E. R. NAGELBERG y ATTORNEY United States Patent O U.S. Cl. 333--21 2 Claims ABSTRACT OF THE DISCLOSURE A straight waveguide structure is disclosed with a portion of the structure having a first inside diameter and the remainder of the structure having a second inside diameter to form a circularly symmetric discontinuity. This structure includes an annularly shaped dielectric member in the larger diameter portion immediately adjacent to the discontinuity. In use, the structure converts energy in the TE11 mode into energy in the TE11TM11 dual mode.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to electromagnetic wave mode converters which convert energy in the TE11 mode into energy in the TE11TM11 dual mode.

Description of the prior art One very simple prior art dual mode converter effects conversion by way of a circularly symmetric discontinuity formed by a waveguide structure having a first portion at one inside diameter and another portion at another inside diameter.

In this converter the diameters are chosen so that the smaller diameter portion supports energy in the TE11 mode only, while the larger diameter portion supports energy in the TE11 and the TMu" modes only. When energy in the TEU mode is fed into the smaller diameter portion, some of this energy is converted into the TMll" mode by the discontinuity so that energy in both modes appears in the larger diameter portion to produce a socalled dual mode. A detailed discussion of this appears in Mode Conversion in Circular Waveguides by E. R. Nagelberg and I. Shefer, Bell System Technical Journal, vol. 44 (1965), pages 1321 through 1338.

Notwithstanding the highly desirable simplicity of the above-described converter, the phase angle between the two modes forming the dual mode varies as a function of frequency to an extent which is undesirable in applications such as feed lines for low noise antennas.

SUMMARY OF THE INVENTION An object of the present invention is to reduce, in the above-described dual mode converter, the variation with frequency of the phase angle between the two modes forming the dual mode.

This and other objects of the invention are achieved through the use of an annularly shaped dielectric member inserted in the above-described dual mode converter. In particular, this member has an outside diameter substantially equal to the inside diameter of the larger of the two inside diameters of the waveguide structure and an inside diameter substantially equal to the smaller inside diameter. Furthermore, the member is located in the larger diameter portion adjacent to the smaller diameter portion; in other words, it abuts the discontinuity formed by the two diameters. As is discussed in greater detail in the following description, this relatively simple and inexpensive addition to this prior art converter results in a substantial reduction in the variation, with frequency, of the phase angle between the two modes forming the dual mode.

3,500,258 Patented Mar. 10, 1970 Other objects and features of the invention will become apparent from a study of the following detailed description of a specific embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows a cut-away view of an embodiment of the invention; and

FIG. 2 is a graph showing results of measurements performed while using the embodiment in FIG. 1.

DISCRIPTION OF THE DISCLOSED EMBODIMENT FIG. 1 shows a cut-away view of an embodiment of the invention comprising a pair of waveguides 10 and 11. Waveguide 10 has an inside diameter which supports electromagnetic wave energy in the TE11 mode only, while waveguide 11 has an inside diameter which supports the TE11 and TM11 modes only. Furthermore, one end of waveguide 10 is electrically joined to one end of waveguide 11 in a conventional coaxial manner. As appreciated by those skilled in the art, this structure may be made out of a single piece of metal. In addition, it may be considered as a single waveguide whose diameter through one portion is greater than that throughout the rest of the waveguide. In either case, the conguration of the structure is identical to that discussed in the above-mentioned Bell System Technical Journal article.

Unlike the structure of the previously mentioned article, the embodiment of FIG. l includes an annularly shaped dielectric member 12. This member has an outside diameter substantially equal to the inside diameter of waveguide 11 and an inside diameter substantially equal to that of waveguide 10. Furthermore, this member is iixed in waveguide 11 so as to abut the junction of the two waveguides.

The two arrows shown to the left and right of the embodiment of FIG. 1 indicate that when electromagnetic wave energy in the TE11 mode is fed into smaller diameter waveguide 10, energy in the TE11TM11 dual mode appears in the larger diameter waveguide 11. The conversion of some of the TEHJ mode energy into TM11 mode energy occurs as a result of the discontinuity at the junction. The effect of the dielectric member on this conversion may be appreciated by considering the following discussion in conjunction with the curves shown in FIG. 2.

FIG. 2 shows a phase angle vs. frequency graph on which appears a pair of lines 13 and 14 derived from measurements made while using the embodiment of FIG. 1. In the embodiment, the inside diameters of waveguides 10 and 11 were 2.1 and 2.8 inches, respectively. Furthermore, member 12 was made of polystyrene and had a thickness of 0.454 inch. A succession of`discrete-frequency waves in the TE11 mode were fed into waveguide 10 with the phase angle between the two modes in waveguide 11 measured for each input wave. Line 13 is a smooth line drawn through the measured phase angles. Identical measurements where made with member 12 removed so that the structure was identical to the prior art structure. Line 14 is a smooth line drawn through the measurements made in the absence of member 12.

From FIG. 2, it is believed readily apparent that the presence of member 12 reduces the variation of the phase angle with frequency. This improvement is attributed to the effect of the dielectric loading on the phase angle at the junction between the waveguides (conversion occurs at this junction) and the time delay introduced by member 12.

Measurements were also made using dielectric members of other thicknesses. The following effects were noted:

(1) The amount of mode conversion and its frequency dependency increases slightly as the thickness of member 12 is increased.

(2) The slope of phase angle line I-Sdecreases and then reverses as the thickness of member 12 is increased.

(3) Phase angle characteristics at the higher frequencies are virtually unaffected by the thickness of member 12..

The actual thickness chosen, therefore, is a matter of design choice.

What is claimed is:

1. A mode converter for converting energy in the TE11 mode to energy in the TE11TM11 dual mode, said converter comprising,

rst and second circular waveguides joined together in coaxial alignment for electrical transmission therethrough, said rst waveguide having an inside diameter to support energy in the TE11 mode only and said second waveguide having an inside diameter to support energy in the TE11 and TM11 modes only, and

an annularly shaped dielectric member having an outside diameter substantially equal to the inside diameter of said second waveguide and an inside diameter substantially equal to the inside diameter of said rst waveguide and, furthermore, located in said second waveguide adjacent to said rst waveguide.

2. A mode converter for converting energy in the TE11 mode to energy in the TE11TM11 dual mode, said converter comprising,

-- I a waveguidestructure having a iirst portion with an inside diameter to support energy in the TE11 mode only and a second portion with an inside diameter to support energy in the TEMo and TM11 modes only, and

5 an annularly shaped dielectric member having an outside diameter substantially equal to the inside diameter of said second portion of said waveguide structure and an inside diameter substantially equal to the lo inside diameter of said first portion of said waveguide structure and, furthermore, located in said second portion of said waveguide structure adjacent to said first portion of said waveguide structure.

15 References Cited UNITED STATES PATENTS 3,305,870 2/1967 Webb 333--21 X 3,413,641 11/1968 Turrin 333-21 X 20 3,413,642 11/1968 Cook 343-781 ELI LIEBERMAN, Primary Examiner MARVIN NUSSBAUM, Assistant Examiner 25 U.S. Cl. X.R.

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