US3218644A - Frequency independent slot antenna - Google Patents

Description

D. G. BERRY FREQUENCY INDEPENDENT SLOT ANTENNA Nov. 16, 19 65 3 Sheets-Sheet 1 Filed June 19, 1965 INVENTOR.

DAVID G. BERRY ATTORNEYS D. G. BERRY FREQUENCY INDEPENDENT SLOT ANTENNA Nov. 16, 1965 5 Sheets-Sheet 2 Filed June 19, 1963 INVENTOR.

DAVID G. BERRY W 4/ Man lll llln I.

ATTORNEYS D. G. BERRY FREQUENCY INDEPENDENT SLOT ANTENNA Nov. 16, 1965 3 Sheets-Sheet 3 Filed June 19, 1963 v wt & mm Q INVENTOR DAV/D G. BERRY BY j? r I 1 ATTORNEYS United States Patent Iowa Filed June 19, 1963, Ser. No. 288,961 8 Claims. (Cl. 343770) This invention relates generally to logarithmic periodic antennas and, more specifically, to an antenna comprising a large wave guide structure with slots arranged therein in accordance with log periodic principles, which principles are discussed in detail in US. Patent 2,989,749, issued June 20, 1961, to R. H. Du Hamel and D. G. Berry, entitled Unidirectional Frequency Independent Coplanar Antenna, and incorporated herein by reference.

In certain applications, it is desirable to have antenna mountings that are flush with a particular surface. For example, it is desirable to mount an antenna flush with the surface of an aircraft for obvious aerodynamic reasons. On other occasions for reasons of simplified and inexpensive construction, as well as to avoid easy detection from airborne observers, it is sometimes desirable to construct antennas flush with the surface of the ground, or with the surface of some object built upon the ground, such as the side or top of a building.

The prior art discloses antennas which can be flushmounted with a surface. Some of these prior art antennas are log periodic antennas which employ rods formed into dipoles. It is necessary, however, to place some type of insulation between the log periodic antenna and the surface on which it is mounted, if the surface is conductive. For example, a log periodic antenna could not be laid flat upon the ground since the earth itself is an electrical boundary. Similarly, a log periodic antenna could not be mounted directly upon the metallic surface of an airplane. A base of some type of insulated material must be supplied as a mount for the antenna. Other types of prior art flush-mounted antennas require cavities behind them in order to radiate efficiently. Such antennas have been comprised of wave guides, with slots therein, from which radiation occurs. The slotted type antennas, which will be generally referred to herein as slot antennas, fall into two general categories. The first category includes those antennas formed by making slots in wave guides and in which the frequency bandwidth is limited by three factors: the guide bandwidth, the impedance property of the slot itself, and the change in electrical slot spacing with frequency.

The second type of slot antenna is'the type which is fed by a TEM quasi coaxial circuit. The latter type of antenna is limited in frequency bandwidth by changes in electrical spacing between the slots with frequency, change in slot impedance with frequency, and higher order modes that are generated in the transmission system by the slots. In addition to the disadvantageous characteristics listed above, the cavities required with the aforementioned prior art antennas have been of the order of one-quarter wavelength deep which, although not too important at microwave frequencies, at UHF frequencies and lower frequencies become prohibitive for aircraft mounting.

It is an object of the present invention to provide a slot antenna which is substantially frequency independent over a wide bandwidth and which does not require any appreciable depth of cavity.

A second object of the invention is a slot antenna in which the slots are arranged in accordance with the log periodic principle, which principle will be discussed in detail hereinafter.

A third object of the invention is a log periodic antenna element employing slots as a radiating element which F CC may be formed directly upon a metallic surface, such as a metallic skin of an aircraft.

A fourth object is the improvement of frequency independent slot antennas, generally.

In accordance with the invention there is provided a parallel conductive plate in which slots have been cut on a log periodic basis in one or both. The antenna is fed from the vertex of the triangle formed by the slots by a coaxial cable into a wave guide structure consisting of the two parallel plates and a center conductor positioned symmetrically between the two plates and passing adjacent the midpoints of the slots. Such a wave guide has a field configuration similar to a coaxial transmission line and operates very much like a coaxial transmission line. In the center conductor of this line are series stubs comprised of short cylindrically shaped sections arranged consecutively around the center line to form what might generally be described as a segmented cylinder positioned around the center conductor. The gaps between the cylindrically shaped segments are positioned such that the stub impedances presented to the center line by the cylindrical seg ments are located at the geometric means between the slots, with reference to the apex of the triangle formed by the slots. Consequently, the transmission line is presented with an alternate arrangement of series impedances (slots) and admittances (stubs). When a wave is launched on the transmission system at the feedpoint, it progresses toward the longer slots until it reaches the slot that has a length approximately one-half the wavelength of the supplied signal, at which point the supplied signal will have been substantially totally reflected and radiated by the slots so that no current flows beyond the one-half wavelength slot. Thus, there is no appreciable end effect.

It is a feature of the invention that the antenna is substantially frequently independent over a wide frequency bandwidth and, further, that the depth required between the two plates forming the wave guide is not a function of the wavelength and can be, in fact, a very small fraction of a Wavelength.

The above-mentioned and other objects and features of the invention will be more clearly understood from the following detailed description thereof when read in conjunction with the drawings in which:

FIG. 1 is a perspective view of the invention;

FIG. 2 is a blown-up illustration of a portion of the center conductor and the cylindrically shaped stub elements;

FIG. 3 is a cut-away perspective view of the structure of FIG. 1, showing in more detail the relationship between the slots and the gaps formed by the cylindrical segments around the center conductor; and

FIG. 4 is a cut-away perspective view of an embodiment of the invention wherein the slots exist only in the upper plate; the bottom plate completing the waveguide and having no slots therein.

Referring now to FIG. 1, there is shown a pair of parallel conductive plates 10 and 11 with slots cut therein. More specifically, in the upper plate 10 there are cut slots 13 through 23, inclusively, which increase in length to form a generally triangular configuration with an imaginary apex at the point 24. The distance between these slots is expressed by the following relationship:

n+2 n+l n wherein 1- is a constant. Thus, the distance of any slot from the apex 24 bears a ratio 7 to the radial distance of the adjacent slot next farthest removed from said apex 24.

A similarly arranged group of slots 25 through 34, in-

clusively, are cut in a lower plate 11 and lie directly below the corresponding slots in the upper plate 10, as can be seen from the structure of FIG. 3. The signal is supplied to the antenna structure by means of a coaxial cable 38, having an inner conductor 40 and an outer conductor 41. The outer conductor 41 is electrically connected to metallic spacer 36, which functions to supply the one polarity of the input signal to the plates and 11, as well as to perform the function of spacing the plates 10 and 11 properly. The inner conductor 40 of the coaxial input lead 33 is extended between the plates 10 and 11 and past the midpoints of the slots in said plates 10 and 11.

The center conductor 40 is actually a part of a segmented coaxial type of structure shown in more detail in FIG. 2. In FIG. 2 there is shown two of the cylindrically shaped segments 43' and 44 which are positioned concentrically around the center conductor 40. Each of these cylindrical segments is shorted at one end thereof to the center conductor by a shorting member as shorting member 49. The other end of each cylindrical segment, such as segment 43', is spaced from the center conductor by an insulating spacer, such as ring spacer 47. Each of the gaps between the cylindrical segments, such as gaps 50, 51, and 53 are positioned at the geometric means between the two adjacent slots with respect to the apex of the antenna triangle. More specifically, the slot 50' of FIG. 3 is located at a distance r from the imaginary vertex 24 of the antenna where r bears the following relationship to R R," T It might also be noted that the radial distances of each of the gaps 50, 51 and 53' of FIG. 3 have a radial distance with respect to each other in accordance with the following expression:

Returning again to FIG. 2, each of the cylindrical sections, such as cylindrical section 43, together with its shorting disc 49, presents a shunt parallel resonant LC impedance to the center conductor 40; said impedance being presented as an admittance to the center conductor at positions substantially coincident with the gaps. Since the cylindrical sections increase in length, in accordance with the log periodic principle, as the distance from the vertex 24 increases, the wavelength of the resonant frequency of the transmission line stub increases as the wavelength of the resonant frequency of the slots increases.

Therefore, the resonant frequency of the transmission line presented to the center conductor between the slots 13 and 14 is larger than the resonant frequency of the transmission line stub presented to the center conductor between the slots 22 and 23. More specifically, the resonant frequency of any of the transmission line stubs presented to the center conductor is approximately the geometric means of the frequencies whose wavelengths are equal to twice the physical length of the two adjacent slots.

In its operation, the antenna (FIG. 3) is fed by means of a coaxial cable input 38'. The two parallel plates 10 and 11 form a TEM wave guide with a symmetrically placed center conductor 40". The resultant wave guide has a field configuration and operates quite similarly to a coaxial transmission line. When a wave is launched down the center conductor 40', it will progress to the left in the drawing of FIG. 3 toward the longer slots until it reaches that particular slot whose length is approximately one-half the wavelength of a supply signal. When the wave reaches such particular slot it will have been totally reflected and/or radiated. Consequently, no end effect producing current will exist behind the slot. Thus, if the slot 21 in FIG. 3 (only one-half of slot 21 is shown in FIG. 3) is equal to one-half the wavelength of the applied signal, substantially all the supplied signal will be radiated through the slots 13 through 21 with the greatest percentage of the energy being radiated through the slot 21. The energies radiated through the slots 13 through 21 bear phase relationships to each other such as to produce a radiation pattern which is polarized transversely to the plane of the wave guide; that is, transversely to the plane of the plates 10 and 11. It is to be noted that two radiation lobes will be created; one of said lobes being created by radiation through the plate 10 and the other lobe being created by radiation through the slot plate 11. The resultant overall radiation pattern thus consists of two separate lobes.

Referring now to FIG. 4, there is shown another embodiment of the invention in which slots are present in only one of the plates. More specifically, in FIG. 4 the lower plate has no slots therein, only the upper plate 10" having slots formed therein. In FIG. 4, however, the nonsymmetrical discontinuity introduced by the slots in the upper plate It) excites the parallel plate transmission mode between the upper plate 10" and the lower plate 60, which propagates in all directions and causes end effect, thus destroying the frequency independence characteristic of the antenna. Such end effect is eliminated by properly placing rows of shorting pins or continuous metal plates, such as plates 62, 63, 64, 65, 66,

67, 68, 69, 70, and 71 between the slots. Although no pins are shown in FIG. 4, it is to be understood that a row of pins could replace each of the plates. The plates 62 through 71 serve to prevent the parallel plate mode from propagating, but do not interfere with the main pseudo coaxial mode of the system. With the lack of slots in the lower plate 60, however, radiation will only occur through the slots present in the upper plate 10 so that the over-all radiation pattern will be a single lobe from the upper plate, polarized transversely to the plane of said upper plate.

It is to be noted that the forms of the invention herein shown and described are but preferred embodiments thereof and that various changes may be made in configurations and proportioning without departing from the spirit or scope thereof.

I claim:

1. A frequency independent antenna comprising:

first and second parallel positioned conductive plates with at least one of said conductive plates having a first plurality of parallel positioned slots cut therein, said slots being arranged with their ends lying along a generally triangular configuration, the radial distance of any slot measured from the vertex of said triangle bearing a ratio 7' to the radial distance of the adjacent slot next farthest removed from the said vertex of said triangle,

center conductor means extending between said conductive plates and adjacent the midpoints of said slots,

segmented cylindrical means separated by gaps and positioned concentrically around said center conductor, the lengths of the cylindrical segments being constructed to position a single gap between adjacent slots to cause the radial distance of each gap from the vertex of said triangle to bear a ratio /1- to the radial distance of the adjacent slot next farthest removed from said vertex,

shorting means for shorting one end of each cylindrical segment to said center conductor,

and means for supplying an input signal across said parallel plates and said center conductor.

2. A frequency independent antenna in accordance with claim 1 in which the other of said conductive plate has a second plurality of slots formed therein, said second plurality of slots being arranged substantially identically with said first plurality of slots and directly opposite said first plurality of slots.

3. A frequency independent antenna in accordance with claim 1. comprising:

a plurality of conductive spacers positioned substantially perpendicular to and between said conductive plates and parallel with said slots at the gaps between adjacent cylindrical segments, each of said conductive spacers having an aperture therein through which said center conductor and the segmented cylindrical sections pass.

4. A frequency independent antenna in accordance with claim 3 in which each of said conductive spacers comprises a row of conductive pins which extend normal to and between said pair of conductive plates.

5. A frequency independent antenna comprising:

a pair of parallel positioned conductor plates with at least one of said conductor plates having a first plurality of parallel positioned slots formed therein, said slots being straight and arranged with their ends lying along a line formed by a generally isosceles triangular configuration, the radial distance of any slot measured from the apex of said triangular configuration bearing a ratio r to the radial distance of the adjacent slot next farthest removed from the apex of said triangular configuration,

center conductor means constructed to extend between said conductor plates and past the midpoints of said slot,

cylindrical means positioned around said center conductor, said cylindrical means being separated into cylindrical segments with gaps which are short, relative to the length of the segments, existing between adjacent segments, the lengths of said cylindrical segments being constructed to position the gaps therebetween at the geometric midpoint of said slots with respect to the apex of said triangular configuration, the length of said cylindrical segments being constructed to cause one gap to occur between each pair of adjacent slots,

shorting means for shorting one end of each cylindrical segment to said center conductor,

and means for supplying an input signal across said parallel plates and said center conductor.

6. A frequency independent antenna in accordance with claim 5 in which the other of said conductive plate has a second plurality of slots formed therein, said second plurality of slots being arranged substantially identically with said first plurality of slots and directly opposite said first plurality of slots.

7. A frequency independent antenna in accordance with claim 5 comprising:

a plurality of conductive spacers positioned substantially perpendicular to and between said conductive plates and parallel with said slots at the gaps between adjacent cylindrical segments, each of said conductive spacers having an aperture therein through which said center conductor and the segmented cylindrical sections pass.

8. A frequency independent antenna in accordance with claim 7 in which each of said spacers comprises a row of conductive pins which extend normal to and between said pair of conductive plates.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner.

Claims (1)

1. A FREQUENCY INDEPENDENT ANTENNA COMPRISING: FIRST AND SECOND PARALLEL POSITIONED CONDUCTIVE PLATES WITH AT LEAST ONE OF SAID CONDUCTIVE PLATES HAVING A FIRST PLURALITY OF PARALLEL POSITIONED SLOTS CUT THEREIN, SAID SLOTS BEING ARRANGED WITH THEIR ENDS LYING ALONG A GENERALLY TRIANGULAR CONFIGURATION, THE RADIAL DISTANCE OF ANY SLOT MEASURED FROM THE VERTEX OF SAID TRIANGLE BEARING A RATIO $ TO THE RADIAL DISTANCE OF THE ADJACENT SLOT NEXT FARTHEST REMOVED FROM THE SAID VERTEX OF SAID TRIANGLE, CENTER CONDUCTOR MEANS EXTENDING BETWEEN SAID CONDUCTIVE PLATES AND ADJACENT THE MIDPOINTS OF SAID SLOTS, SEGMENTED CYLINDRICAL MEANS SEPARATED BY GAPS AND POSITIONED CONCENTRICALLY AROUND SAID CENTER CONDUCTOR, THE LENGTHS OF THE CYLINDRICAL SEGMENTS BEING CONSTRUCTED TO POSITION A SINGLE GAP BETWEEN ADJACENT SLOTS TO CAUSE THE RADIAL DISTANCE OF EACH GAP FROM THE VERTEX OF SAID TRIANGLE TO BEAR A RATIO $$ TO THE RADIAL DISTANCE OF THE ADJACENT SLOT NEXT FARTHEST REMOVED FROM SAID VERTEX, SHORTING MEANS FOR SHORTING ONE END OF EACH CYLINDRICAL SEGMENT TO SAID CENTER CONDUCTOR, AND MEANS FOR SUPPLYING AN INPUT SIGNAL ACROSS SAID PARALLEL PLATES AND SAID CENTER CONDUCTOR.

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