US 2946055 A
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Description (OCR text may contain errors)
" bit? 1 July 19; 1960 C. E. FAFLICK PARASITIC DIPOLE SLOT ANTENNA Filed Dec. '29, 1958 INVENTOR. CARL E. FAFLICK ATTORNEY.
United States Patent signments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Dec. 29, 1958, Ser. No. 783,560
'4 Claims. (Cl. 343-727) This invention relates to antennas and more particularly to antennas employing a slot radiator and especially adapted for use on aircraft, such as high speed planes and missiles.
Due to the high speed of modem aircraft and missiles it is important that the size of protuberances from the surfaces of the craft be kept small, or if possible, eliminated. Since most such craft have communication equipment of one kind or another mounted thereon, with the attendant requirement for antennas, considerable work has been done toward reduction of the size of such antennas and their location within the body of the aircraft or mounting flush with the surface of the craft. Antennas employing a slot radiator are particularly useful in the latter connection, the aperture being located flush with the skin of the craft and preferably backed up by a resonant cavity for exciting the slot. A slot radiator, however, produces a linearly polarized radiation pattern which is unsatisfactory in certain applications. That is, some systems require an antenna having the characteristics of a slot radiator (electrical as well as mechanical), but having circularly rather than linearly polarized patterns.
It is a principal object of the present invention to provide an antenna capable of producing circularly polarized slot radiator patterns; that is, patterns rela tively broad in one principal plane and relatively narrow in the other principal plane.
Another object of the invention is to provide an aircraft antenna having the above electrical characteristics and yet be capable of being entirely contained within the skin of the craft and consuming a minimum of space within the craft.
These and other objects of the invention are realized by means of a slot or open waveguide radiator in combination with a parasitic dipole antenna mounted in the aperture. The dipole is tilted at a small angle with respect to the aperture thereby producing a component of electric field in the direction of the dipole which excites the dipole parasitically. The dipole is terminated by a phase shifter which may consist, for example, of a section of short-circuited transmission line approximately one-quarter wave length long, or other lumped phase shifter. The transmission line is preferably placed in the neutral plane of the waveguide so that the antenna is not excited by the phase shifter. The phase shifter controls the phase of the current on the parasitic dipole relative to the field in the waveguide aperture. The resultant fields of the dipole radiator and the waveguide slot radiator may be adjusted to produce circular polarization.
While intended primarily for use on aircraft, it will be evident from the following description that the antenna is equally applicable for use on other vehicles and in fixed installations such as in the wall of a building or tower.
For a better understanding of the invention and other objects and features thereof, reference is had to the following description of a preferred embodiment taken in connection with the accompanying drawings in which:
Fig. 1 is a perspective view of the antenna, partially cutaway;
Fig. 2 is a front view of the antenna, illustrating an alternative construction of the dipole; and
Fig. 3 is a cross-sectional view taken along line 3-3 of Fig. 2.
Referring now to Fig. 1, there is shown a conductive plate 10, which may be the skin of the aircraft, through which the waveguide slot antenna projects. It is well known that a rectangular waveguide, such as waveguide 12, terminated in a conductive flange; that is, with the aperture of the waveguide flush with the conductive plate 10, constitutes a slot radiator. In accordance with the present invention, a dipole antenna 16 is positioned in the aperture of the waveguide with the radiating elements of the dipole tilted slightly with respect to the long axis of the waveguide aperture. An angle of tilt of approximately 20 has been found satisfactory. Dipole 16 is positioned in the neutral plane of the waveguide, that is, equidistant from the broad walls of the guide, and may be supported by a dielectric support '17 as shown, or by conductive supports, provided they are located in the neutral plane of the waveguide. A thin dielectric window (not shown) is preferably secured across the waveguide aperture to prevent entry of moisture and to minimize drag.
In the illustrated embodiment, the radiating elements are supported on a short-circuited section of transmission line 18 positioned in the neutral plane of the waveguide. The transmission line 18, approximately onequarter wavelength long at the operating frequency, provides proper phasing of the two dipole elements. Slight adjustment of the length of the line may be necessary to achieve circular polarization, the exact length depending upon the size of the dipole, the length of the radiating elements, the thickness of the elements, and the angle of the dipole with respect to the long axis of the aperture. Dimensions of the antenna thus far described which have been successful are indicated in the drawing and are tabulated as follows: Long dimension of waveguideapproximately W wavelength. Short dimension of waveguideapproximately /a wavelength. Angle of tilt-approximately 20. These dimensions should be construed as illustrative, however, as other dimensions may be selected which will support a radiating mode. For each aperture dimension there will exist an optimum angle of the dipole radiator and an optimum phase shift.
The antenna is energized by launching electromagnetic energy down the waveguide 12, and would, if dipole 14 were not present, function as a slot radiator. However, the dipole positioned as described above, is parasitically excited by the energy propagated in the guide, and produces a field, which when combined with the field produced by the aperture alone, results in circular polarization. The indicated tilt of the dipole produces a component of dipole field in a direction perpendicular to the long axis of the aperture, this excitation field being approximately in phase quadrature with the current on the dipole over a distance comparable to the short dimension of the aperture, which is small in terms of wavelengths; namely, of the order of one-third wavelength. The phase and amplitude of the resultant field at the aperture of the waveguide relative to the current on the dipole can be adjusted by changing the length of the phasing section 18. The horizontally polarized radiation produced by the dipole element is exactly out of phase with the vertically polarized radiation field of the waveguide, provided the depth or length of line 18 is of the proper dimension. A nominal line length of (free-space A) provides the proper phase relationship, but this is subject to some minor adjustment with variations in the angle of tilt of the dipole. The angle of tilt mainly affects the amplitude of the two fields, so long as the tilt angle is small, and by proper adjustment, the amplitudes of the resultant vertically polarized field and the horizontally polarized field can be made substantially equal, this condition coupled with the 90 phase relationship between the horizontally and vertically polarized fields providing circular polarization. It has been found that almost perfect circular polarization can be achieved at any given frequency.
The radiation pattern of the antenna corresponds approximately to that which would be produced by a linearly polarized slot having the same dimensions, an average beam width of approximately 120 in the direction of the narrow dimension of the aperture, and a beam width of approximately 60 in the direction of the long dimension of the aperture. It has been found that the polarization ellipse does not change with aspect angle, the patterns being substantially identical for all components in the plane of the aperture and diifering only slightly in the plane perpendicular to the aperture where the patterns are very broad and affected more by the finite ground plane, that is, plate 10, in which the aperture is positioned.
An alternative construction of the parasitic dipole is illustrated in Figs. 2 and 3. Instead of the wire or rod radiating elements, the dipole radiators 16 may comprise conductive strips 16' which are printed, or etched, or otherwise afiixed to the interior surface of a dielectric window 22, which is sealed across the aperture of waveguide 12. As in the arrangement of Fig. 1, the elements 16' are tilted relative to the long central axis of the aperture, and thus are adapted to be parasitically excited by energy launched down the waveguide. In order to obtain proper phasing, a quarter wavelength short-circuited transmission line 18', positioned in the neutral plane of the waveguide, is supported at one end on the window 22 and is electrically connected to the radiating elements 16'. The inner end of line 18' may be supported by a dielectric post in the manner shown in Fig. 1. Electrically, the antenna of Figs. 2 and 3 is substantially identical to the structure of Fig. l, and operates in the same manner to provide circular polarization. I
From the foregoing it is seen that applicant has pro-, vided an antenna in which the same element, namely, the waveguide, is utilized to excite a linearly polarized dipole and also to radiate directly. The result is a circularly polarized antenna capable of producing pattern shapes heretofore possible only with a linearly polarized antenna.
What is claimed is:
1. A circularly polarized antenna system including a conductive plate having a rectangular aperture therein, a rectangular waveguide connected to said plate behind said aperture and open at said aperture, a dipole disposed in the plane of said aperture and tilted relative to the longer axis of said aperture, and a short-circuited quarter-wavelength transmission line connected to said dipole and extending into said waveguide.
2. A circularly polarized antenna system including a conductive plate having a rectangular aperture with length and width dimensions formed therein, a hollow rectangular waveguide connected to said plate behind said aperture and open at said aperture, a dipole arranged with its radiating elements disposed in the plane of said aperture centrally thereof with the axis through the radiating elements tilted at a small angle relative to the length dimension of said aperture, and a short-circuited transmission line connected to the radiating elements of said dipole and disposed within said waveguide,
the length of said transmission line being substantially one-quarter wavelength at the frequency of operation.
3. A high frequency circularly polarized antenna system comprisinga conductive plate having a rectangular aperture therein, a hollow rectangular waveguide having cross-sectional dimensions corresponding to the dimensions of said aperture connected to said plate behind said aperture and open at said aperture, said aperture being adapted for excitation by high frequency electromagnetic energy propagated in said rectangular waveguide, and a dipole arranged with its radiating elements in the plane of said aperture and supported on a short-circuited transmission line extending into said waveguide and lying along the central axis thereof, the length of said transmission line being substantially one-quarter wavelength at the frequency of operation and the axis of said radiating elements being tilted approximately twenty degrees with respect to the long axis of said aperture.
4. A high frequency circularly polarized antenna system comprising a conductive plate having a rectangular aperture therein, said aperture having length and width dimensions of substantially nine-tenths and one-third of a wavelength at the frequency of operation, a hollow reetangular waveguide having cross-sectional dimensions corresponding to the dimensions of said aperture connected to said plate behind said aperture and open at said aperture, said aperture being adapted for excitation by high frequency electromagnetic energy propagated in said waveguide, and a dipole arranged with its radiating elements in the plane of said aperture and supported on a short-circuited transmission line extending into said waveguide and lying along the central axis thereof, the length of said transmission line being substantially one-quarter wavelength at the frequency of operation, the axis of said radiating elements being tilted approximately twenty degrees relative to the long axis of said aperture whereby said dipole is adapted to be parasitically excited by energy propagated in said waveguide.
No references cited.