US 3831176 A
A partial-radial-line antenna comprises a sector of a radial waveguide with the outer edges of its two parallel spaced plates terminating in identical circular arcs which define the antenna aperture. The radial sector is connected at its center or apex to a rectangular-waveguide input transmission line such that the wave excited in the radial sector propagates with its electric or E-field lines extending along circular arcs between the diverging side walls of the same radial waveguide sector. For circularly polarizing the radiated energy, the circularly-shaped antenna aperture is covered by a polarizer, and a plurality of arcuately displaced radial absorption vanes are mounted between and normal to the plates of the radial waveguide sector. The antenna is excited with energy in the TE10 radial line mode such that the E-field lines are circular and emanate from the centered feed point. The E-field is thus perpendicular to all the absorption vanes and to the diverging side walls. With an appropriately designed polarizer, antennas of this type are designed to radiate elliptically polarized waves characterized by low or near zero db axial ratio (i.e., nearly circular polarization) over entire angular sectors ranging from 60 DEG to 300 DEG for a broad frequency band extending over at least an octave.
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Description (OCR text may contain errors)
343- 75oe 5R United States Patent [191 [111 3,831,176
Epis et al. [451 Aug. 20, 1974 PARTlAL-RADIAL-LINE ANTENNA radial waveguide with the outer edges of its two paral-  Inventors: James Epis Sunnyvale; Ewest lel spaced plates terminating in identical circular arcs Robles, Mountain View both of which define the antenna aperture. The radial sector 18 Ca I connected at its center or apex to a rectangularwaveguide input transmission line such that the wave A g GTE y a a n rpor excited in the radial sector propagates with its electric Mountain View, Calif or E-field lines extending along circular arcs between  Filed, June 4 1973 the diverging side walls of the same radial waveguide sector. For circularly polarizing the radiated energy,
[21 Appl. No.: 366,558 the circularly-shaped antenna aperture is covered by a polarizer, and a plurality of arcuately displaced radial absorption vanes are mounted between and normal to [58 Field of 's' r'ci. 343/756, 780, 786, 753, excted energy TEN 343/754 755 such that the E-field lines are circular and emanate from the centered feed point. The E-field is thus per- [561 62:33:51,221;as,azz ziigtgaizaitgntr:3:
UNITED STATES PATENTS larizer, antennas of this type are designed to radiate 2,721,263 9/1955 Spencer 343/780 lli i ll polarized waves characterized y low or 2,933,731 4/1960 Foster et al 343/756 near zero db axial ratio (i.e., nearly circular polarization) over entire angular sectors ranging from 60 to 300 for a broad frequency band extending over at least an octave.
2,978,702 4/1961 Pakan 343/773 Primary ExaminerEli Lieberman Attorney, Agent, or Firm-John F. Lawler; Norman J. OMalley; Elmer J. Nealon 57] ABSTRACT 6 Claims, 10 Drawing Figures A partial-radial-line antenna comprises a sector of a PATENTEmummm sum 20: s
MEI aor 5 FIG. 8
PAIENIEMZ H 3.831.176
sum sor 6 FIG. IO
BACKGROUND OF THE INVENTION This invention relates to antennas and more particularly to such antennas which have contoured fanshaped radiation patterns.
Certain electronic systems are required to transmit and/or receive circularly polarized waves within relatively broad azimuth sectors of angular width ranging between 60 and 300, and within relatively narrow elevation angular sectors, i.e., in planes perpendicular to the principal azimuth plane. In practice, it is highly desirable that these antennas be capable of providing energy with nearly true circular polarization throughout the entire angular region described above and over a broad operating frequency range, preferably greater than an octave. Furthermore, the radiation in the azimuth plane should have a substantially constant beamwidth at all frequencies of operation of the system. Such performance characteristics are necessary in order to ensure uniformity of signal strength in the azimuth sector of interest as the frequency is swept over the octave range. There is no known prior art antenna capable of meeting all of these performance criteria.
OBJECTS AND SUMMARY OF THE INVENTION An object of this invention is the provision of a sectoral or partial-radial-line antenna which provides elliptically polarized radiation of small axial ratio (or ideally, circular polarization) over a broad sector of substantially constant angular width in one principal plane, called the azimuth plane, and over a relatively narrow angular sector in planes normal to the azimuth plane, called vertical or elevation planes.
A further object is the provision of such an antenna that is operative over a broad frequency range, i.e., at least an octave.
Still another object is the provision of a circularly polarized antenna in which the polarization axial ratios as measured at any given signal frequency are substantially the same in all directions within the broad angular sector of the azimuth plane and over the region confined by the -l db points on the radiation pattern in any elevation plane.
A further object is the provision of an antenna of the type described which may be constructed with different broad-pattern beamwidths in the principal or azimuth plane without materially affecting the beamwidth of the more directive pattern in elevation planes normal to the azimuth plane.
Still another object is the provision of an antenna having a radiation polar diagram displaying a nearlyconstant field intensity over a broad angular sector in the azimuth plane and a directive pattern in the elevation planes, and achieving-these radiation pattern characteristics over broad frequency bands with the beam width and other characteristics of the broad pattern displaying substantial constancy as the signal frequency is changed.
These and other objectsof the invention are achieved by constructing a partial radial waveguide sector terminating in similar circular arcs, which form the antenna aperture, and by feeding that sector precisely at the center or apex of those arcs so that the electric fieldlines of the electromagnetic waves in the partial radial waveguide, or radial line, are both circular and concentric with the aperture. This permits the use of radially extending absorption vanes in the radial line sector in planes perpendicular to the E-field to ensure optimum performance of the polarizer in producing radiation with near-circular polarization without adversely affecting other antenna performance characteristics. Reduction in the angle of azimuth coverage is achieved without deterioration of performance characteristics by an increase in the radius of the radial line; beamwidth in the elevation plane decreases almost linearly with increase in spacing of the radial line plates.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view partially in section of an antenna embodying the invention;
FIG. 2 is a front view of the antenna taken on line 2-2 of FIG. 1;
FIG. 3 is a side elevation of the antenna taken on line 33 of FIG. 1;
FIG. 4 is a section taken on line 4-4 of FIG. 1;
FIG. 5 is a greatly enlarged section taken on line 55 of FIG. 1 showing a radial absorption vane;
FIG. 6 is an enlarged section of part of the absorption vane taken on line 6-6 of FIG. 5;
FIGS. 7, 8 and 9 are plots of radiation patterns with superimposed polarization axial ratios in the principal or azimuth plane of an antenna embodying the invention and operating at frequencies of 8 GHz, 12 Gl-Iz and 16 GI-Iz, respectively; and
FIG. 10 is a plot of the radiation pattern with superimposed polarization axial ratios in the elevation plane of such antenna operating at a frequency of 12 GHz.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, FIGS. '1, 2 and 3 illustrate an antenna 10 embodying the invention comprising a rectangular waveguide feed section 11 having broad walls 11a and 1 lb and narrow walls 1 1c and 11d, a radial waveguide sector 12 defining at its front end a circularly curved antenna aperture 13 having a radius R. For circularly polarized operation, sector 12 has a circularly curved polarizer 14 on aperture 13. Polarizer 14, by way of example, may comprise a meanderline array assembly of the type described in application Ser. No. 268,479 of J. J. Epis entitled BROADBAND AN- TENNA POLARIZER, now US. Pat. No. 3,754,271.
Feed section 11 preferably comprises a double ridged waveguide having interior ridges l6 and 17, see FIG. 4, projecting inwardly from broad walls 11a and 11b, respectively, with the ridges tapering to zero height within section 11 as shown in FIG. 1. This provides the desired bandwidth in the feed section as required for broadband operation of the antenna. An unridged transition waveguide portion 19 between ridged waveguide section 1 1 and radial sector 12 uniformly and gradually changes the interior dimensions of the ridged waveguide'to those of the rectangular feed port 20 of the radial sector 12, see FIG. 2; the height a of ridged waveguide 11 tapers to the smaller height a of port 20 and waveguide width b diverges to the larger width b of port 20. Thus walls 19a and 19b of transition portion 19 converge from waveguide walls 11a and 11b, respectively, to port 20 and walls 190 and 19d diverge from waveguide walls and 11d, respectively, to the port.
Radial sector 12 comprises identical parallel spaced plates 22 and 23 with circular front edges 22a and 23a, and side walls 25 and 26 which diverge radially outwardly from plate rear edges defining feed port 20 to front edges 22a and 23a and perpendicular to plates 22 and 23. The angle 20 between walls 25 and 26 is about ten percent larger than the desired sector of radiation of the antenna and may vary between approximately 65 and 330, the embodiment shown in FlG. 1 being about 160. Polarizer 14 is disposed across front edges 22a and 23a of plates 22 and 23 and extends from wall 25 to wall 26 so as to cover antenna aperture 13. Polarizer 14 thus has the shape of a circular are or cylindrical cover with a center along the midpoint of port 20. Antenna sector 12 and polarizer 14 therefore have a true radial configuration concentric with the center of port 20.
In order to preserve the desired substantial circularity of the polarization of radiated waves, i.e., to minimize the axial ratio, absorption vanes 30 are disposed between and perpendicular to plates 22 and 23 in arcuately spaced planes which extend radially from the center of feed port 20. Each vane 30 has a length less than the radius R of sector 12 and may comprise resistance card made of a thin fiberglass sheet 31 coated on one or both sides with carbon 32, see FIGS. and 6. Alternatively, the vanes may be made of metallized mica. These vanes absorb the small component of energy reflected by polarizer 14 that is not normal to the vanes, which component would otherwise have a magnified deleterious effect on the polarization axial ratio.
An inherent feature of the design of the antenna embodying this invention is that the angular sector of the radial line is always about ten percent larger than the desired sector of coverage of the broad radiation pattern in the principal or azimuth plane. A second inherent feature of the design is that the radius of the partial radial line must be made larger and larger as that azimuth coverage is decreased. For example, over the frequency range from 7 to 17 GHz, desirable performance is obtained from an antenna with 240 azimuth coverage using a partial radial line of radius equal to 4.7 inches; achieving the same quality of performance for an azimuth coverage of 90 requires use of a partial radial line of radius close to 20 inches.
The radiation pattern beamwidth in all elevation planes is nearly independent of the radius of the radial line. This beam-width decreases almost linearly with the spacing between the similar parallel metallic plates of the partial radial line.
An antenna embodying the invention has been constructed and tested successfully and has the following characteristics and dimensions:
Radius R of sector 12 Angle (20) of sector 12 Width b of port 20 (spacing between plates of radial line) 4.7 inches 260 degrees inches Polarizer 5-layer meanderline array The axial ratio patterns in the azimuth plane of the foregoing antenna at 8 GHz, 12 GHz and 16 GHz are shown in FlGS. 7, 8 and 9, respectively, while FIG. 10 shows such a pattern in the elevation plane at 12 GHz. These patterns were obtained as follows: The antenna was connected to conventional receiving apparatus with the signal output of the latter connected to a standard radiation pattern recorder. The transmitting system consisted of a linearly polarized pyramidal horn antenna spaced 40 feet from and aimed at the receiving antenna and connected through a rotary joint in a feed transmission line to a tunable microwave oscillator; the transmitting antenna was continuously rotated as a function of time. The data was taken by rotating the partial-radial-line receiving antenna about an axis perpendicular to its azimuth plane at a very slow angular velocity relative to the speed of rotation of the transmitting antenna, i.e., approximately times slower. Under these conditions, the measured data plotted by the recorder provided an accurate representation of the polarization axial ratio value of the partial-radialline antenna as a continuous function of angular direction on the radiation pattern; the axial ratio value specifically is represented by the amplitude Z of the sinusoidal ripple on the pattern. For example, in FIG. 7, the axial ratio Z is 1.5 db in the azimuth direction of d) 30.
FIG. 10 illustrates the measured axial ratio pattern of the foregoing antenna in the central elevation plane, i.e., in the vertical plane at zero azimuth angle. This plot is representative of the pattern at other elevation planes throughout the sector angle 20 and also at frequencies over the range of 7 to 17 Gl-lz with the exception of absolute axial ratio values and the fact that the elevation patterns have broader beamwidths for frequencies below 12 GHz and narrower beamwidths for frequencies above 12 GHz.
What is claimed is:
1. An antenna system comprising a partial-radial-line antenna comprising a pair of substantially identical spaced coextensive parallel plates, each plate having a circularly shaped front edge and arcuately spaced side edges extending radially inwardly from said front edge and a rear edge interconnecting said side edges opposite from said front edge, said rear edge containing the center of formation of said circular front edge,
a pair of side walls, each side wall extending perpendicular to said plates at adjacent side edges thereof, said side walls defining with said plate front edges the antenna aperture and with said plate rear edges a rectangular feed port,
a rectangular waveguide having broad walls and narrow walls and adapted to propagate linearly polarized electromagnetic waves having an electric (E) field normal to said broad walls, and
a waveguide transition section interconnecting said waveguide with the feed port of said antenna, said transition section having first and second walls connecting said broad waveguide walls, respectively, to said side walls of the antenna and having third and fourth walls connecting said narrow waveguide walls, respectively, to said antenna plates whereby the E-field of the wave mode in the antenna is circular and normal to said antenna side walls.
2. The antenna system according to claim 1 with a polarizer disposed on said antenna aperture, said polarizer being shaped as a portion of a cylinder having an axis passing through said centers of formation of the plate front edges, and a plurality of arcuately spaced radially extending radio frequency absorption vanes disposed between and perpendicular to said antenna plates.
3. The antenna according to claim 1 in which said waveguide feed section comprises doubly ridged waveguide.
4. An antenna system comprising a partial-radial-line antenna having parallel plane plates and side walls defining with said plates an aperture at one end and a rectangular feed port at the opposite end,
said aperture having a circular shape centered at the mid-point of said feed port, and
a rectangular waveguide feed section coupled to said port for exciting said antenna with linearly polarized electromagnetic waves having an electric field extending in a direction parallel to said plates and perpendicular to said side walls.
5. The system according to claim 4 in which the broad dimension of said feed section extends in a direction parallel to the broad dimension of said port, and a transition waveguide section connecting said feed section to said port of the antenna.
6. An antenna system comprising a partial-radial-line antenna having parallel plane plates and side walls defining with said plates an aperture at one end and a rectangular feed port at the opposite end,
said aperture having a circular shape centered at the mid-point of said feed port,
a polarizer disposed on said antenna aperture,
said polarizer being circularly shaped corresponding to the aperture shape,
a plurality of arcuately spaced radially extending energy absorption vanes between said plates of the antenna,
a rectangular waveguide feed'section coupled to said port for exciting said antenna with linearly polarized electromagnetic waves having an electric field extending in a direction parallel to said plates and perpendicular to said side walls, the broad dimension of said feed section extending in a direction parallel to the broad dimension of said port, and
a transition waveguide section connecting said feed section to said port of the antenna.