US2983919A - Tuning means for slot radiator - Google Patents

Description

United States Patent TUNING MEANS FOR SLOT RADIATOR Matti S. O. Siukola, Woodlynne, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 16, 1957, Ser. No. 690,471

2 Claims. (Cl. 343-467) The invention relates to a tuning device for a radiating slot in a metallic surface, and has for an object to provide an improved tuning device that permits the resonant frequency of the slot to be easily varied.

The tuning device is primarily intended to be used with -a radiating slot or slots in a transmission line or antenna. However the device can be used to vary the resonant frequency of any slot. Generally, each of the slots is relatively narrow and has sides that are substantially longer than its width as measured across the respective ends. The tuning device for each slot comprises at least one conductive adjustable plate that is positioned near one of the ends of the slot. A portion of the plate is arranged so that it projects over a portion of the slot near the one end thereof, thus eifectively changing the electrical characteristics of the slot mainly by restricting the magnetic field. If desired, a second similar adjustable plate may be positioned near the other end of the slot. Since the plates are adjustable, the electrical characteristics, and hence the resonant frequency, of the slot can be easily and quickly varied.

The invention is explained in detail in connection with the accompanying drawing, in which:

Figure 1 shows an elevational view of a slot antenna having radiating slots with which the tuning devices of the invention may be used;

Figure 2 shows an enlarged plan view of one of the slots shown in Figure 1, and also shows one embodiment of the adjustable plates of the tuning device;

Figure 3 shows an enlarged plan view of a slot having another embodiment of the adjustable plates of the tuning device, and also shows a protective cover for the slot; and

Figure 4 shows a cross-sectional view taken along the lines 4-4 of Figure 3.

Referring to the drawing, Figure 1 shows a traveling wave antenna which includes a hollow, cylindrical outer conductor 12, and a cylindrical inner conductor 14 concentrically positioned within the outer conductor 12. The traveling wave antenna 10 is designed to radiate radio frequency signals in the television band, and is preferably mounted in a vertical position on some hollow structure such as a building or a tower. In Figure 1, the antenna 10 is shown mounted in or supported by a suitable pedestal 20 which may house or contain suitable means for applying radio frequency energy to the antenna 10. The outer conductor 12 of the antenna 10 extends above the pedestal 20, and is provided with a plurality of elongated slots 16, each positioned with its long dimension (about one-third wavelength at the operating frequency) substantially parallel to the longitudinal axis of the antenna 10. The slots 16 are designed to have a relatively high Q, the exact value depending upon the gain and bandwidth to be provided :by the antenna 10. It has been found that a slot having an enlarged width at the ends provides a higher Q than a slot having a uniform width throughout. The configuration of such a slot can be seen in Figure l, and is similar to a dumbell. The slots 16 are arranged in a spiral about the outer conductor 12 of the traveling wave antenna 10. There is one pair of slots 16 positioned at the same horizontal level every quarter wavelength at the operating frequency up the vertical height of the antenna 10, the slots 16 at each level being diametrically opposite each other. This has not been shown in detail in Figure 1 in order not to detract from the clarity of the illustration. Each successive or higher pair of slots 16 lies in a vertical diametrical plane that forms an angle of substantially degrees with the plane in which the adjacent lower pair of slots 16 lies.

Radio frequency energy may be applied to the antenna 10 by any suitable means, such as a coaxial transmission line whose outer conductor is connected to the outer conductor 12 of the antenna, and whose inner conductor is connected to the inner conductor 14, of the antenna 10. It is preferred that this radio frequency energy, when excited in the antenna 10, be in the TEM mode. This energy is applied at the base or pedestal 20 of the antenna 10 and travels upward through the antenna 10 toward the top. Equal amounts of this radio frequency energy are coupled to the slots 16 by any suitable means. In order that all of the energy in the antenna 10 be radiated, terminating slots 18 are provided at the top of the antenna 10. The terminating slots 18 are positioned in the same manner as the radiating slots 16, but have a substantially rectangular configuration and uniform width throughout so as to provide a relatively low Q slot. A complete termination for the antenna 10 is provided by some suitable means, such as a plate 22, which shortcircuits the inner and outer conductors 14, 12 at the proper point at the top of the antenna 10.

Antennas such as shown in Figure 1 have been built in about 30 foot sections from lengths of 60 feet to feet overall with a diameter of approximately 10 to 20 inches. Because of such large physical dimensions, the mechanical tolerances must be quite lenient. Thus in cutting the slots 16, 18 in the outer conductor 12 of the antenna 10, certain tolerances must be permitted. These tolerances, while necessary, result in slots which are not all exactly resonant to the desired frequency of operation. Thus some means for tuning the slots must be provided so that they resonate at the desired operating frequency. Such an arrangement is shown in Figure 2.

Figure 2 shows an enlarged plan view of one of the slots 16 in the outer conductor 12 of the antenna 10. The slot 16 has enlarged portions 24 at its ends, these enlarged portions 24 serving to provide the slot 16 with a higher Q than a conventional rectangularly shaped slot. As mentioned, the necessary manufacturing tolerances usually result in the slots 16 having different resonant frequencies, the differences being appreciable in some instances where large antennas are constructed. In order that the slot 16 may be tuned to the desired resonant frequency, rectangularly shaped, conductive plates 26 are provided at the ends of the slot 16. These plates 26 are attached or fastened to the outer conductor 12 near the enlarged portions 24 of the slot 16 so that a portion of each of the plates 26 projects over or covers the enlarged portions 24 of the slot 16. The plates 26 may be fastened by any suitable means, such as bolts which pass through elongated slots in the plates 26. However, it is not necessary that the plates 26 be directly connected to the outer conductor 12. As long as the plates 26 be in a position to vary the magnetic field of the slot 16, they may be mounted in any convenient fashion. This is one advantage provided by the invention. These elongated slots in the plates 26 permit the plates 26 to be adjusted back and forth until the slot 16 has the desired resonant frequency. A change in the position of the plates 26 also changes the magnetic field of the slot 16, thus changing the resonant frequency of the slot 16. Although the plates 26 are shown as being rectangular in shape, other shapes can be used as well. For example, the long sides of the plates 26 may be tapered. The terminating slots 18 in Figure 1 may also have such tuning plates.

While the tuning plates 26 perform satisfactorily to change the resonant frequency of the slot 16, the problem of making the slot 16 weatherproof is also present. An arrangement for providing tuning plates 26 as well as for making the slots 16 weatherproof is shown in the plan view inFigure 3. A longitudinal cross-sectional view, taken along the lines 44 in Figure 3, is shown in Figure 4. A conductive bracket 30 is fastened to the outer conductor 12 near each of the enlarged portions 24. The brackets 30 include a tuning plate 26' and a retaining element 28 which is positioned above, but spaced from the tuning plate 26. The space between the plate 26 and the retaining element 28 serves to receive and hold a suitable covering 32 for the slot 16. In effect, this space provides a groove positioned with respect to the slot 16 to hold the member 32 in slot covering position. The covering 32 for the slot 16 may be made of any suitable dielectric material, such as polyethylene or polystyrene. The brackets 30 may be attached to the outer conductor 12 by suitable means such as bolts which pass through elongated slots in a rear projection 34. The bracket 30 is mounted at each end of slot 16 so as to hold the covering 32. After the bracket 30 is adjusted so that the tuning plate 26' provides the desired resonant frequency for the slot 16, it is then fastened into position so that it holds the covering 32 tightly at its ends. The bracket 30 provides an improved way of holding the covering 32, as well as providing tuning means for the slot 16, because it projects above the surface of the outer conductor 12 a minimum amount. Such a feature is highly desirable in reducing the wind loadings required for the antenna structure.

In addition to using the plates for changing the resonant frequency of the slots, a magnetic material, such as a low loss powdered iron core, may be placed in the slots to change their resonant frequency. The plates may be considered to restrict the magnetic field of the slots, while the magnetic material may be considered to increase the magnetic field of the slots, both changes serving to change the resonant frequency of the slots.

The tuning device described can be produced at relatively low cost, since it can be made of thin metallic plates. Further, these plates protrude only slightly above the surface of the antenna, thus maintaining wind loads at a minimum. In addition, the plates present no electrical contact problems such as presented with capacitor plates.

4 And finally, insulation problems are reduced because the plates are at the ends of the slot whichis a low voltage point.

The invention claimed is:

l. A conductive member presenting an exposed surface and having a slot therein terminating in said exposed surface for radiating radio frequency energy, said slot having sides that are substantially longer than its ends and being enlarged at said ends to form a substantially dumbbell-shaped slot so as to transfer radio frequency energy therethrough, means for changing the resonant frequency of said slot by varying the magnetic field of said slot, said last named means comprising a pair of conductive members each positioned with respect to said slot to project longitudinally along said slot from an end thereof, each of said conductive members being narrower than the width of said slot whereby to cover a longitudinally extending portion of said slot, each of said conductive members being provided with a groove extending laterally with respect to said slot, and a dielectric covering member being positioned in said grooves to cover said slot.

2. A conductive member presenting an exposed surface and having a slot therein terminating in said exposed surface for radiating radio frequency energy, saidslot having sides that are substantially longer than its ends and being enlarged at said ends to form a substantially dumbbell-shaped slot so as to transfer radiofrequency energy therethrough, means for changing the resonant frequency of said slot by varying the magnetic field of said slot, said last named means comprising a pair of conductive members each positioned with respect to said slot to project longitudinally along said slot from an end thereof, each of said conductive membersbeing narrower than the widthof said slot whereby to cover a longi' tudinally extending portionof said slot, each of said conductive members being provided with a retainingrelement which is positioned above, but spaced from said conductive members and a dielectric covering member extending over said slot and being positioned between said spaced retaining elements and said conductive members.

References Cited in the file of this patent UNITED STATES PATENTS 2,507,528 Kandoian May 16, 1950 2,512,468 Percival June 20, 1950 2,629,051 Lindenblad Feb. 17, 1953 2,658,143 Fiet et al. Nov. 3, 1953 2,763,783 Lorcnzen Sept. .18, 1956 FOREIGN PATENTS 600,433 Great Britain Apr. 8, 1948

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