Circularly Polarized Antenna Using Slotted Cylinder and Conductive Rods

This invention relates to circularly polarized antennas and, more particularly, to such antennas using slotted conductive cylinders.

Although television has been broadcast in horizontal polarization in the U.S.A., it appears from recent test results that circularly polarized broadcasting might well greatly improve television reception in large metropolitan areas. Further, broadcast antennas must often broadcast an omnidirection pattern about the tower such that when this tower is erected in the center of a city, for example, substantially equal coverage is provided about the city.

Slot antennas like that described by Bazan in U.S. Pat. No. 2,981,947 are presently being used for providing horizontally polarized television broadcasting. This type of antenna provides a good omnidirectional pattern. Since broadcasters have expended considerable cost for this slot type antenna, it would be desirable to provide some means of converting this type of antenna to provide a circularly polarized broadcasting antenna. This is especially true for UHF stations where the operating budgets are smaller and there is a wide use of the slot antenna.

Briefly, a circularly polarized antenna system is provided by longitudinally extending slots in a conductive cylindrical mast and a pair of conductive elements associated with each of said slots. Each of said slots are at least one-half wavelength long at the operating frequency of the antenna. Each of the slots are fed to excite horizontally polarized waves. Each of the pair of conductive elements excite vertically polarized waves. The conductive elements are each approximately one wavelength long with one conductive element fixed near the mid point of one elongated side of the slot and having a free end portion extending in one longitudinal direction and the other conductive element being fixed at one end to a feed point near the mid point of the opposite side of the slot and having a free end portion extending in an opposite longitudinal direction.

FIG. 1 is an elevation view of an antenna system according to the present invention.

FIG. 2 is an elevation view of a subsystem of FIG. 1 according to one embodiment of the present invention.

FIG. 3 is a cross-sectional view of the antenna subsystem taken along line 3 - 3 in FIG. 2.

Referring to FIG. 1, an omnidirectional antenna system 10 is shown comprising four identical stacked subsystems 51, 52, 53 and 54. Each subsystem comprises four longitudinal slots and linear conductors. The omnidirectional antenna system includes a coaxial transmission line having an inner conductor 20 and an outer conductive mast 19. Signals at the operating frequency of the antenna are coupled at the bottom end of the mast 19 between the center conductor and the outer conductor or mast 19. The mast 19 has a terminating short at the remote end 19a to excite a standing wave. The longitudinal center of the first plurality of slots of subsystem 51 is located, for example, three-quarters of a wavelength at an operating frequency of the antenna from the terminated end 19a. The term "wavelength" when used herein refers to a wavelength at an operating frequency of the antenna system. The center of the second plurality of slots of subsystem 52 is spaced a full wavelength from the longitudinal mid point of the slots of subsystem 51. Similarly, the center of the third plurality of slots of subsystem 53 are spaced a full wavelength from the mid point of the slots of subsystem 52. Similarly, the center of the slots of subsystem 54 are spaced a full wavelength from the mid point of the slots of subsystem 53. These slots are thereby centered at the high impedance points of the standing wave. Associated with each slot of the plurality of slots is a pair of approximately one wavelength long linear conductors 18.

Referring to FIGS. 2 and 3, the subsystem 52 including the slots is illustrated in more detail. The four slots 11, 13, 15 and 17 of subsystem 52 extend along the longitudinal axis of the mast a length of about one-half wavelength (.lambda./2) at an operating frequency of the antenna. The slots 11, 13, 15 and 17 were made 0.05 wavelengths or slightly greater than one-half wavelength at the operating frequency. The width of each of the slots is on the order of 0.08 wavelength (5/8 inch -- for an operating frequency of 1500 MHz for example). The slots 11, 13, 15 and 17 are located 90.degree. of arc and about one-half wavelength from each other about the mast. The slots 11, 13, 15 and 17 are excited for example by elongated coupling probes 31, 33, 35 and 37. The probe 31 extends about one-half wavelength long along the inside longitudinal edge 11b of slot 11. Similarly, probes 33, 35 and 37 are about one-half wavelength long and extend along the inside longitudinal edges 13b, 15b and 17b of the respective slots 13, 15 and 17. The probes 31, 33, 35 and 37 are similar to the probes in Bazan, U.S. Pat. No. 2,981,947. Similarly, the slots of subsystem 51, 53 and 54 are excited. In the arrangement shown in FIGS. 1 thru 3 with only the slots horizontally polarized waves in an omnidirectional pattern is excited. This antenna with the slots alone would function in a manner similar to that described in the above cited patent.

Circular polarization is achieved herein using the above described well known type of antenna by exciting vertically polarized waves in phase quadrature to the horizontally polarized waves radiated from the slots using the conductive rods 18 fed on each side of the slots. These conductive rods 18 in FIG. 1 are each approximately one wavelength long elements with one end fixed near the mid point of an elongated side of a slot with a one-half wavelength portion at the free end of the rods extending in a vertical direction. Referring for example to FIG. 2 and 3, vertically polarized waves are excited in phase quadrature using the rods 21, 22, 23, 24, 25, 26, 27 and 28 spaced about slots 11, 13, 15 and 17. The rods 21 and 22 extend from points 11c and 11d respectively located near opposite longitudinal sides 11a and 11b of slot 11. The points 11c and 11d are midway between the longitudinal ends of slot 11. Similarly, rods 23 and 24 extend from points respectively on the opposite longitudinal sides 13a and 13b of slot 13. Similarly, rods 25 and 26 extend from feed points on the opposite longitudinal sides 15a and 15b of slot 15 and rods 27 and 28 extend from points on opposite elongated sides 17a and 17b of slot 17. The rod 21 extends generally vertically, and away from the slot over a first portion 21a. The rod 21 extends horizontally away from the mast and away from the slot over the second portion 21b. The length of the rod 21 over the first and second portions is about one-half wavelength at the operating frequency of the antenna. The rod 21 extends vertically downward over a third free end portion 21c. The third portion 21c is slightly less than one-half wavelength long at the operating frequency of the antenna. The third portion 21c is about one tenth of a wavelength less than one-half wavelength or about 0.4 wavelength long. The total length of rod 21 is approximately a full wavelength at the operating frequency. The rod 21 may therefore be considered an end fed full wave radiator. The rod 22 extending from the opposite side of the aperture 11 extends generally downward and away from the slot 11 in an opposite direction over the first portion 22a. The rod 22 over the second portion 22b extends horizontally away from the slot and the mast 19. The length of the rod 22 over the first and second portions is one-half wavelength at the operating frequency of the antenna. The rod 22 extends vertically upward over a third free end portion 22c. The length of the rod 22 over the third portion 22c is slightly less than (.lambda./10 less) one-half wavelength long at the operating frequency of the antenna (about 0.4 wavelength long). The total length of rod 22 is approximately a full wavelength. The rod 22 may therefore be considered an end fed full wave radiator. The rods 21 and 22 are oriented such that the third portions 21c and 22c are centered with respect to the slot 11 as shown in FIG. 2. The rods 23, 25 and 27 are identical to rod 21 and the rods 24, 26 and 28 are identical to rod 22. The third or free end portion of the rods 23, 24, 25, 26, 27 and 28 are slightly less than (one tenth of a wavelength less than) one-half wavelength long and are centered with respect to their respective slots. The first and second portions of rods 21, 22, 23, 24, 25, 26, 27 and 28 extend from the mast 19 such that the third portions are spaced every 45.degree. of arc around the mast 19 as shown in FIG. 2. Since length of the combined portions 21a and 21b of rod 21 is about one-half wavelength from the feed point 11c at the dipole slots to vertical portion 21c and the combined length of the first two portions of the rod 22 to the vertical portion 22c are each about one-half wavelength at an operating frequency of the antenna, quadrature phasing is provided between the horizontally polarized waves radiated from the slot 11 and the vertically polarized waves radiated from the third portion of rods 21 and 22. Similarly, there is quadrature phasing between signals radiated from the slot 13 and the rods 23 and 24, from the slot 15 and the rods 25 and 26 from the slot 17 and the rods 27 and 28. The rod 22 is fed 180.degree. out of phase with respect to rod 21 by the coupling probe 31 on the inside of mast 19. Similarly, the pair of rods 23 and 24, 25 and 26 and 27 and 28 associated with each slot are fed 180.degree. out of phase with each other. The portion 21c of rod 21 is spaced approximately one-half wavelength at an operating frequency of the antenna from portion 22c of rod 22. Since these end radiating portions are one-half wavelength apart, the radiation in the common plane of these end radiating portions cancel. Similarly, the end radiating portions of the two rods 23 and 24, 25 and 26 and 27 and 28 associated with each slot are spaced about a half-wavelength apart to achieve this cancellation. This cancellation of radiation prevents signals radiated in the directions in the plane of the two rods from interfering with the radiation from the other radiators to form nulls or cancellation in these directions and reinforcement in other directions and thereby not achieve the desired omnidirectional circularly polarized pattern.