US 3680138 A
A circular aperture waveguide emitting element of cross-section comparable with the emitted wavelengths is made to have a linear polarization output by a cross-mode reflector which is a resonant bar mounted on a window located in the aperture.
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Description (OCR text may contain errors)
UR 3968Oa138 United States Patent Wheeler  July 25, 1972 CROSS-MODE REFLECTOR FOR THE Refer n Cited FRONT ELEMENT OF AN ARRAY UNITED STATES PATENTS AN TENNA 3,26l,0l s 7/1966 Mast ..343/7s9 nvcntor; Harold A. wheeler Smithtown 3,541,560 11/1970 Lyon et al.... ...,.343/756 [731 Ass'gneei The Unmd America is 3,553,707 1/1971 Yang et al ..343/786 represented by the Secretary of the Army 22 Filed; Sept 2 1970 Primary Examiner-Eli Lieberman Attorney-Charles K. Wright, Jr., William G. Gapcynski,  APPL 73,822 Lawrence A. Neureither, Leonard Flank, Jack W. Voigt and James T. Deaton  U.S. Cl ..343/756, 343/778, 343/784,
333 9 M  ABSTRACT  Int. Cl. ..H0lq 19/00 A circular a perture waveguide emitting element of cross-sec-  Field of Search ..343/756, 768, 770, 786, 789, ion comparable with the emitted wavelengths is made to have a linear polarization output by a cross-mode reflector which is a resonant bar mounted on a window located in the aperture.
5 Claims, 2 Drawing Figures PATENTEBJlILzs I972 3.680.138
IN ENTOR Horbld A.Wheeler,
CROSS-MODE REFLECTOR FOR TIE FRONT ELEIVENT OF AN ARRAY ANTENNA BACKGROUND OF THE INVENTION This invention is directed to the field of radar elements, particularly to the emitting front element of a phase-array radar. It is necessary to have a circular aperture for the front element so as to obtain good broadband performance. This is also necessary to obtain a better scan performance. The circular design is also the easiest shape for achieving the hard, flush 1 design made necessary in a nuclear environment. This environment also makes necessary the use of a ceramic window in the circular aperture. When an uncompensated circular front element array is scanned in the skew planes, the cross TE-ll mode can be excited in the elements by mutual coupling. The power in the cross-mode will be reflected inside the elements and reradiated. Since the normal and the cross TE-ll modes will in general differ in phase, elliptical polarization will result. The best way to solve this cross-mode problem would be to make the aperture of the element a slot with the narrow dimension below cutoff to the cross-mode. Since the design and tactical conditions will not allow this, there is a need for the present invention which uses circular aperture front elements with a dimension below cutoff but modified so that they look like a slot in terms of polarization performance.
SUMMARY OF THE INVENTION This invention is directed towards the modification of the front element of a phase-array radar so that the beam radiated from the array remains essentially linearly polarized even through the front elements have circular apertures. The radiating front elements each consist of a full-wave loop and a ceramic window enclosed in a circular waveguide shell. The window is located in the aperture plane of the array. The input power to the element is provided by a coaxial connector. One side of the full-wave radiating loop is fastened to the coaxial connector and the other side is connected to the base (floor) of the element. The ceramic disc is mounted in the waveguide shell at the front. A resonant cross-mode reflector is fastened to the window. Ground-plane contacts are provided to give electrical continuity between the elements. A mounting flange is used to bolt the element to the array structure. In order to obtain peak-power capacity, the cross-mode reflector has all its corners rounded and its ends are spherical and bent away from the dielectric window. The cross-mode reflector acts as a short circuit to any cross-polarized radiation, while having little effect on the normal mode impedance. Radiated polarization from the array of the circular elements having the crossmode reflectors present will be similar to the radiation of a magnetic dipole, or a narrow slot with its narrow dimension below cutoff at the frequency of operation.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 schematically illustrates the circular-hole array element, partially cut away, in relationship with other elements in the array; and
FIG. 2 illustrates the circular-hole array element, partially cut away, in greater detail.
DESCRIPTION OF THE PREFERRED EMBODIMENT plane 5 such that the apertures of the elements are all located in a single plane. Each element is provided with a cross-mode reflector 7 shown as a resonant straight bar of conducting material in FIG. 1. The cross-mode reflector 7 is brazed to the dielectric window 9. FIG. 2 shows the structure of the elements in greater detail. The full-wave exciter loop 3 and the beryllia ceramic dielectric window 9 are enclosed in a circular waveguide shell 12. The full-wave loop excites a TE-ll mode in the element. The waveguide shell 12 has two sections; each 0 having a different diameter. This is done for impedance matching. A 50-ohm coaxial line, not shown, is connected to the coaxial connector 14 to provide input power for the exciter loop 3. One side of the loop is connected to the coaxial connector and the other side to the base or floor of the element. The window is brazed to the waveguide shell at the front. A cross-mode reflector 16 of a different shape than that shown in FIG. I is brazed to the rear of the window. Groundplane contacts 18 serve to support the element and provide electrical continuity by contact between the elements when mounted in the array structure (not shown) by mounting flange 20 and bolts on the array structure (not shown).
The cross mode reflector 16 is incorporated in the elements so that the beam radiated from the array of the radar remains essentially linearly polarized, independent of scanning. This is accomplished even though the dimensions of the circular waveguide shell 12 are comparable with the operating wavelength. The waveguide shell is made large so to have broadband performance. The resonant cross-mode reflector presents a short circuit at the aperture to the cross-mode radiation. The reflector is positioned at a right angle to the full wave loop 3 so as to have little effect on the normal mode radiation. All of the comers of the reflector are rounded, and the ends of bar 16 are spherical and bent away from the dielectric window 9. This shape provides the best peak-power capacity of the array. A possible specific configuration of the cross mode reflector would be a bar with its middle third brazed to the window and each of its end thirds bent at a 15 angle from the window but still in the plane at right angles to the full wave loop.
l. A radiation waveguide emitting element having a circular waveguide cavity and circular aperture with dimensions in all directions comparable with the wavelength of emitted radiation; and a reflecting bar located in the aperture so as to short circuit any cross-mode radiation emissions.
2. A radiation emitting element as set forth in claim 1, wherein a dielectric window is secured in said aperture and said reflecting bar is elongated and secured to one surface of said dielectric window.
3. An element as set forth in claim 2, wherein said reflecting bar has its corners rounded, and its ends are spherical and bent away from the aperture.
4. An element as set forth in claim 3, wherein the circular waveguide is a shell of two sections having different diameters; a dielectric window brazed to the shell so as to be located in the aperture; a full-wave radiating loop; a coaxial connector connected through said shell to the full-wave radiating loop; and said reflecting bar being a cross-mode reflector which is brazed to the dielectric window inside the shell and at a right angle to the full-wave radiating loop.
5. An element as set forth in claim 2 wherein a plurality of such elements make up front elements of a phased-array radar; and the radiation emitted therefrom is linear polarized.