US 3050730 A
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Aug. 21, 1962 B. J. LAMBERTY BROADBAND PLATE ANTENNA Filed July 9, 1959 2 Sheets-Sheet l HVVENTUR.
BERNARDJ.LAMBERTY ATTORNEY g- 1962 B. J. LAMBERTY 3,050,730
BROADBAND PLATE ANTENNA Filed July 9, 1959 2 Sheets-Sheet 2 FIE-17 INVENTOR.
BERNARD J. LAMBERT-Y A AM I ATTORNEY id weenie Patented Aug. 21, 1962 3,050,730 BROADBAND PLATE ANTENNA Bernard J. Lamberty, Santa Clara, Calif., assignor to Sylvania Electric Products Inc, a corporation of Dela- Ware Filed July 9, 1959, Ser. No. 825,996 3 Claims. (CL 343-843) This invention relates to antennas, and more particularly to a class of physically small, broadband, mediumgain antennas.
A difficulty commonly experienced in antenna design is achieving broadband operation while holding the physical size to a minimum. For a better understanding of terms, broadband operation as used herein means the antenna requires no tuning to match its impedance to some relative impedance having a voltage standing wave ratio (VSWR) of less than 4.0 to 1 over a frequency band at least three times the lowest operating frequency. 'Efiorts in the past to build broadband small size antennas resulted in anternia configurations which are a function of angle only and not of length, that is, the antenna is impliedly infinite in extent. Spirals and modifications of spirals are examples of such antenna configurations. The difiiculty with this type of antenna is that the size of the antenna must be at least a wavelength at the lowest operating frequency. Furthermore, these antennas do not have a large bandwidth when operated over a ground plane and usually are complex and difiicult to construct. Other types of antennas in this class are conical dipoles which must be at least a quarter wavelengh high and have rather limited bandwidth.
The class of antennas to which this invention relates operates over an extremely broad frequency band, in the order of 20 to 1, has a medium-gain characteristic, and is small in physical dimension-in the order of a sixth of a wavelength or less at the lowest operating frequency. The fundamental antenna design on which this class of antennas is based is a folded loaded unipole antenna characterized by several folds between the input end and the opposite end, the latter end being shorted to the ground plane over which the antenna is operated. The fingers comprising the folds are spaced apart and are made of conducting material, the entire antenna generally resembling an end-fed glove. A second embodiment comprises a similar structure without a shorted end, and with a center feed. A third embodiment comprises a center-fed rectangular plate. Modifications of the second and third embodiments comprise a balanced centerfed glove structure with and without a reflector, crossed rectangular plates, and a periodic structure having a plurality of rectangular center-fed plates of diminishing area and interplate spacing. The latter modifications of the basic structures exemplify the variety of different kinds of antennas which result from optimization of the desirable features of the basic structures. The advantages of small physical size relative to a wavelength, and uniform patterns and impedance response are achievable in varying degrees With all these antennas.
An object of the invention is to provide a broadband antenna having a small physical size with respect to a wavelength. Another object is the provision of a broadband antenna having medium-gain characteristics and relatively uniform radiation patterns, and whose largest physical dimension is in the order of a sixth of a wavelength or less at the lowest operating design frequency. A further object is the provision of a physically small antenna having relatively constant radiation pattern and impedance characteristics over frequency bands in the order of 20 to 1 or greater. Still another object is the provision of a broadband plate-type antenna which can operate above a ground plane. A more specific object is the provision of an antenna having a radiating element made of a solid or continuous rectangular conducting plate. A further object is the provision of a periodic antenna structure having rectangular plates for radiating elements. Still another object is the provision of a balanced fed single-plate antenna capable of operating without a ground plane.
These and other objects of my invention will become apparent from the following description of the various embodiments thereof, reference being had to the accompanying drawings in which:
FIGURE 1 is a side elevation of an end-fed rectangular digital plate antenna embodying my invention.
FIGURE 2 is a section taken on line 2-2 of FIG- URE 1.
FIGURE 3 is a side elevation of a center-fed digital plate antenna.
FIGURE 4 is a section taken on line 44 of FIG- URE 3.
FIGURE 5 is a side elevation of a center-fed rectangular continuous plate antenna.
FIGURE '6 is a section taken on line 66 of Fl"- URE 5.
FIGURES 7 and 8 are side elevation and end views, respectively, of the antenna of FIGURE 5, showing schematically the current and field distribution for low frequency operation.
FIGURES 9 and 10 are similar to FIGURES 7 and 8 except that the current and field distribution is for high frequency operation.
FIGURE 11 is a perspective view of an antenna structure comprising two center-fed crossed rectangular plates.
FIGURES 12, 13 and 14 are perspective, front, and side views, respectively, of a periodic center-fed antenna structure comprising continuous rectangular plates.
FIGURE 15 is a side elevation of an antenna structure comprising a combination of two antennas of the type shown in FIGURE 1, and fed by a balanced line.
FIGURE 16 is a section taken on line ire-16 of FIG- URE 15.
FIGURE 17 is a perspective view of the antenna of FIGURE 15 in a combination with a reflector element.
Referring now to the drawings, FIGURES 1 and 2 show a rectangular plate antenna 8 comprising a plurality of fingers 9, four being shown in this embodiment, which lie in a common plane and which extend from a base portion 10 in laterally spaced relation. This rectangular digital antenna has an outline configuration resembling a glove. The antenna extends over a ground plane 12 and is fed by a coaxial input line 13, the center conductor of which is connected to the lower corner A of the structure and the outer conductor of which is connected to the ground plane 12. The opposite corner 14 of the structure is shorted to the ground plane. The lower edge 15 of the antenna is tapered to improve the VSWR over the operating frequency range.
The digital plate antenna 8 operates generally on the principle or" the folded unipole (wire-type) antenna which has a multiple-fold configuration, the folds serving to substantially reduce the physical size of the structure. Antenna 8 is shorted to the ground plane at one point only, however, rather than at each fold and resonance occurs approximately at the frequency for which one-half wavelength is equal to the outer distance around the structure from the feed end A to the shorted end 14. The folded wire unipole is a narrow band antenna. By filling the space between each fold with conducting material so as to load it, and by extending each loaded fold closely to the ground plane, I have greatly increased the bandwidth of the antenna while maintaining a VSWR of less 3 than 4.0 to 1. At the lower frequencies, currents are more concentrated at the feed end and in the folds, whereas at the higher frequencies, radiation occurs from currents in the vicinity of the lower edge 15. Tests conducted on the antenna of FIGURE 1 in which the height h and the width w were each 5 inches 6 at design frequency) and the lower edge was tapered, indicated a VSWR of less than 3.0 to 1 from about 350 megacycles to 7000 megacycles-a frequency range of to 1.
'Lack of uniformity of the radiation patterns, however,
imposes a limitation on the use of the digital plate structure over extremely wide frequency bands.
The low frequency cutoff, f of the antenna of FIG- URE 1 is a function of the distance around the outer edge of the antenna from feed point A to shorted end 14. The value of J can be reduced approximately 15% by increasing this distance. This is accomplished with the digital plate antenna 18 (FIGURE 3) which is the same in construction as antenna 8 except that the input feed line 13' is connected to the lower edge 19 at point B midway between the side edges of the structure, and the short to the ground plane is removed. This center-fed antenna is matched closer to a 50 ohm coaxial input line than is the end-fed structure 8 of FIGURE 1, up to about 4 At Sf the VSWR increases from less than 2.0 to 1 to a ratio of 3.5 to 1, and gradually decreases and remains small at higher frequencies. Overall improvement of VSWR results from rounding of the lower corners 20 and 21 of the plate.
In addition to improved bandwidth, the center-fed digital plate antenna 18 has an improved radiation pattern compared to the end-fed structure 3 of FIGURE 1. The H-plane patterns are omnidirectional from f to B have broadside directivity from 3f to Sf clover leaf from Sf to 7f and tend to have end-fire directivity from 7.5f to mm; or greater. There is some improvements also in the E-plane patterns, particularly as to symmetry, although to a lesser extent than those in the H-plane. A further and important advantage of the center-fed structure is that it is physically smaller in terms of wavelength than the end-fed structure. The former is approximately one-eighth of a wave-length square in outside dimension at the lowest operating frequency.
A further modification of the rectangular plate antenna structures of FIGURES l and 2 is the continuous rectangular plate design which may be visualized as a digital antenna 18 with the spaces between the fingers filled with conducting material. This structure obviously is considerably easier and less expensive to fabricate than the digital structures. A center-fed continuous plate antenna 24 is shown in FIGURE 3 with coaxial feed line 13" being connected centrally at B on the lower edge of the plate 19 and to ground plane 12".
For a clearer understanding of terms, the three antennas shown in FIGURES 1, 3 and 5 are described in this specification and in the claims as being rectangular plates, since each lies in a single plane and has a generally rectangular outline. Digital is used to describe the spaced finger construction of the antennas of FIG- URES 1 and 3 in contradistinction to the word continuous, which means the non-digital, solid or homogeneous structure of FIGURE 5.
VSWR of the center-fed continuous plate antenna 24 is slightly higher on the average than the center-fed digital antenna lit-about 2.3 to 1 for the former, as opposed to 1.8 to 1 over a frequency range of 20 to l. VSWR of antenna 24 also is a function of the distance .9 between the lower edge 26 of the plate and the ground plane 12". For example, a square plate 24 measuring 0.125). on a side has a minimum average VSWR of less than 3.0 to 1 for a 20 to 1 frequency band when the distance sis equal to about 0.006
Antenna 2.4 has an E-field vector in the vertical direction as indicated by the arrow in FIGURE 6, and has somewhat more radiation and cross-polarization overhead than does digital antenna 18. It has been determined that currents I and electric field E at low frequency operation are distributed over the rectangular plate antenna as shown in FIGURES 7 and 8, while at high frequencies, the distribution is as indicated in FIGURES 9 and 10. Referring now to FIGURES 9 and 10, consider a horizontal strip of Width x across the plate 24. This strip may be considered as a source of uniform amplitude distribution with phase proportional to length. The E-fiel in the H-plane from all such strips tends to be broadside, that is, normal to the plane of the antenna with directivity increasing with frequency for each given strip length L. At f L is only an eighth of a wavelength and therefore radiation is nearly omnidirectional. Elongation of the pattern occurs in the broadside direction as the frequency increases. are present until the width of the rectangular plate is slightly less than a half wavelength (3f At frequencies, above 3] some broadside radiation takes place. At higher frequencies, currents are concentrated within the space between edge 26 and the ground plane 12" as shown in FIGURES 9 and 10. The radiation patterns, therefore, are similar to those from a transmission line radiator with little of the low frequency broadside eifects present; that is, at high frequencies the continuous plate antenna has an end-fire radiation.
The foregoing antenna structures achieve many of the objectives of the invention regarding size and performance, and yet there are practical limitations on their respective frequency ranges because of non-uniformity of radiation patterns. Further modifications of these antenna structures as discussed below have alleviated these limitations and have permitted more effective pattern control and, have produced Igreater pattern uniformity.
As mentioned above, the homogeneous or continuous rectangular plate antenna 24 has a pattern which tends to change direction from broadside to end-fire as the frequency increases, VSWR being low throughout this frequency hand. These characteristics were combined in antenna 30 (see FIGURE 11), which comprises two center-fed rectangular plates 31 and 32 crossed at right angles to each other along the feed point and over ground plane 33. The center conductor 34 of the coaxial feed line is joined to the lower edges of the plates at their points of intersection. The effect of this configuration is that the radiation pattern minima of one plate are largely filled by the radiation pattern maxima of the other, since the plates are orthogonal to each other, as are the radiation pattern maxima and minima of the single plate structure. Orthogonal placement of two such plates also results in elimination of the undesirable lobes.
The VSWR of the cross-plate antenna 30 averages at about 3.0 to 1 over a frequency range of 20 to 1. H-plane patterns are omnidirectional to within 3-db variation over the 20 to 1 frequency band. E-plane patterns are essentially the same as those of a single center-fed continuous rectangular plate antenna 24. The height of the crossed plate antenna 30 is approximately one-eighth of a wavelength at the lowest operating frequency Another modified form of the invention is a periodic structure shown in FIGURES 12, 13 and 14. This antenna, indicated generally at 36, consists of a succession of center-fed rectangular continuous plates 37, 38 and 39, the area and spacing of which vary in progressive increments of a predetermined ratio. These plates are parallel and are oriented in space so that a plane containing the axis of symmetry of each plate is perpendicular to the plates, the spacing of these plates being along this plane. Plates 37, 38 and 39 are fed in series from a coaxial feedline 49 having a center conductor 41 connected to a common conductor 42 which joins to the mid-points of the lower edges of the several plates. The composite structure 36 operates over a ground plane 43.
Antenna 36 illustrated in the drawings has plates with a size and spacing which vary progressively by a factor of;
Essentially omnidirectional patterns three; that is, plate 38 has linear dimensions three times greater than those of plate 39 and one-third of those of plate 37, and is spaced three times farther from plate 37 than from plate 39. During operation, only large plate 37 radiates in the range from f to 3f plates 38 and 39 being below cutoff. The patterns are similar to those of a single rectangular plate; that is, omnidirectional to slightly broadside. In this frequency range, plates 38 and 39 act as large capacitive reactances in parallel with the feed line and so have little effect on the impedance match. At frequencies from three to nine times f plates 37 and 38 radiate. Since energy arrives at plate 38 prior to plate 37, the former tends to radiate more of the power. Since a single rectangular plate tends to radiate end-fire at frequencies from about 7f plate 37 has this characteristic in the 3f to 9 frequency range. Plate 37 also reflects radiation from plate 38. Therefore, the characteristic dip in the pattern of the large plate 37 is filled in the direction toward the feed end because of radiation from the smaller plate 38. A null, is however, present in the reverse direction because of the reflecting action of the large plate and due to a dip in the pattern of this large plate in that direction. In general, unidirectional patterns are obtainable at frequencies from three to nine times f and excellent directivity is present above Sf At frequencies above 9f radiation is possible from all three plates. However, because of the sequence in feeding, little power reaches large plate 37 and the smaller plates 38 and 39 radiate in the frequency band above 9f in much the same manner that plates 37 and 38 radiate at lower frequencies. As a result, a fairly uniform, medium-gain pattern exists over a large frequency band.
In actual tests conducted on the three-plate antenna structure of FIGURE 12, VSWR characteristics were measured at less than 4 to 1 for all frequencies from f to ZOf except for one peak of approximately 5 to 1 occurring at lOf More than three radiating plates can be used in the construction of the periodic antenna, so that the frequency band over which uniform patterns are attainable may be extended practically indefinitely. Periodic ratios other than that illustrated and described herein may be used to achieve more uniform radiation patterns and improved impedance matching.
The above described antennas were adapted for use with an unbalanced line and a ground plane. In order to provide for operation without a ground plane, two symmetrically arranged end-fed digital antennas of the type shown in FIGURE 1 were combined to form antenna 45, see FIGURE 15, and the composite structure is fed by a balanced line comprising conductors 46 and 47. These lines connect to opposite halves 45a and 45b of the structure, and conductor 48 at the end of the structure completes the feed circuit. The E-field of antenna 45 extends generally in the plane of the antenna normal to the inner edges 49 and 50 thereof, and the E-plane radiation patterns are essentially in the shape of a figure 8. The H-plane radiation pattern for this balanced structure shows a maximum gain toward the feed end. In order to achieve a unidirectional pattern, a reflector 52 may be provided with the balanced structure 45 as shown in FIG- URE 17. The effect of the reflector 52, in addition to providing directivity, is to cause a slight decrease in bandwidth due to spacing in terms of wavelength between the antenna element and the reflector.
In general, balanced structures of the type shown in FIGURES l5 and 16 provide medium gain over a relatively large bandwidth. An important feature of these antennas is that each can be used independently of a ground plane and for vertical or horizontal polarizations. Because of the minimum overall size and the simplicity and economy of construction, these antennas are ideally suited for television receiving systems.
From the above description, it will be seen I have provided a class of broadband antennas having relatively uniform patterns, small physical size, and reasonable impedance characterisitcs. With certain modifications, the basic antenna structures are readily adapted to provide medium gain, to have omnidirectional patterns, and to operate with balanced feeds.
Modifications, changes and improvements in the above described embodiments of my invention may be made by those skilled in the art without departing from the precepts of my invention. The scope of the invention, therefore, is defined in the appended claims.
1. A periodic high frequency untuned antenna comprising a plurality of generally rectangular continuous plates of conducting material in separate planes, said plates being spaced from a ground plane with a plane containing the axis of symmetry of each plate being perpendicular to planes of the plates, the linear dimensions and spacing of successive plates varying in progressive increments of a predetermined ratio, and a transmission line connected to said plates for feeding them in series.
2. A periodic high frequency untuned antenna comprising a plurality of generally rectangular continuous parallel laterally spaced conducting plates disposed in separate planes and mounted over a ground plane, a transmission line for feeding said plates in series comprising a conductor connected to the midpoints of the edges of said plates proximate to said ground plane, the linear dimensions and spacings of successive ones of said plates varying in progressive increments at a predetermined ratio.
3. A high frequency untuned antenna comprising a pair of generally rectangular continuous plates of conducting material arranged over a ground plane with a plane containing the axis of symmetry of each plate perpendicular to the plates, each of said plates having a maximum length along any side thereof equal to oneeighth of a Wavelength at the lowest design operating frequency for said plate, said plates being spaced closely to and normal to a ground plane, the linear dimensions and spacing of successive plates varying in progressive increments of a predetermined ratio, and a transmission line for feeding said plates comprising a coaxial line having an inner conductor connected to the edges of said plates closest to the ground plane and an outer conductor connected to the ground plane.
References Cited in the file of this patent UNITED STATES PATENTS 2,505,751 Bolljahn May 2, 1950 2,688,080 Van Atta Aug. 31, 1954 2,701,307 Cary Feb. 1, 1955 2,826,756 Cary Mar. 11, 1958 2,845,624 Burberry July 29, 1958 2,949,606 Dorne Aug. 16, 1960