US 2533900 A
Description (OCR text may contain errors)
Dec. 12, 1950 J, P, SHANKLIN 2,533,900
ANTENNA Filed 59W 9, 1949 FIG. I.
Patented Dec. 12, 1950 ANTENNA J ohn P. Shanklin, Cedar Rapids, Iowa, assigner' to Collins Radio Company, Cedar Rapids, Iowa,
a corporation of Iowa Application September 9, 1949, Serial No. 114,781
This invention relates to antennas, and more particularly to antennas of the broadband omni- A directional kind.
A principal object of the invention is to provide an improved broadband omni-directional antenna employing a series of folded dipoles.
Another object is to provide an antenna comprising an array of folded dipoles assembled and united in such a way as to provide an omni-directional antenna having broadband and vertically polarized characteristics.
A feature of the invention relates to an array of folded dipoles assembled in squirrel-cage or basket-like formation and supported symmetrically around a central supporting mast.
Another feature relates to a novel antenna having a single central support mast and a stacked series of folded dipoles, each series comprising a plurality of folded dipole sections assembled in substantial squirrel-cage or basket-like formation.
Another feature relates to an improved design of squirrel-cage or basket-like antenna unit which can be mounted on a single central oontinuous metallic mast without exciting any substantial currents in the mast, While at the same time providing the antenna as a whole with broadband omni-directional and vertically-polarized characteristics.
Another feature relates to a squirrelcage or basket-like antenna constituted of a series of folded dipole units with novel feed or transmission line connections thereto.
A still further feature relates to the novel organization, arrangement and relative location 4and interconnection of parts which cooperate to provide a simplified and rugged omni-directional broadband antenna employing folded dipole units.
Other features and advantages not specifically enumerated, will be apparent after a consideration of the following detailed descriptions and the appended claims.
In the drawing:
Fig. 1 is a schematic representation of a conventional folded dipole antenna.
Fig. 2 is a perspective view of an antenna according to the invention.
Fig. 3 is a modification of Fig. 2.
In the conventional folded half-wave dipole antenna, such as shown in Fig. 1, the input impedance Z is determined by the ratio of the currents I1 and I2 in the fed and unfed legs of the dipole. Thus where feed impedance of a single legged or commonbidipole;
Z=feed impedance of a folded dipole; and
Id =current in a single legged common dipole.
If the two legs of the folded dipole are identical and not spaced too widely, I1 willA equal I2. In this case the impedance step-up Z d will equal 4.
` While it has been proposed heretofore to employ an array of folded dipoles, certain difficulties have usually arisen where the folded dipoles are to be supported from a common central conduction mast. Unless special precautions are taken, excessive currents may be excited in the mast which would interfere with the desired symmetry and polarization of the radiation or field pattern. This is particularly true where the antenna is to have a broadband frequencycharacteristic as well as an omni-directional and vertically-polarized pattern. I have found that by the novel arrangement such as shown in Fig. 2, these` disadvantages are overcome.
Referring to Fig. 2, the antenna comprises a pair of horizontal metal rings I, 2, which are bridged by four straight rods 3 5. Integrally attached to the center of each rod at its respective voltage nodal point, for example by brazing, soldering, etc., are respective radial struts 'l-Ill whose inner ends are likewise integrally fastened to a central metal support mast Il.
Integrally attached to ring I are four down,- wardly extending rods I2-I5. Likewise integrally attached to ring 2 are four upwardly extending rods Iii-I9 which are in vertical alignment with corresponding rods I 2-I5, but with gaps 20-23 between aligned pairs of rods. The conductor 24 of a short length of twin conductor transmission line 25, is connected between the free ends of rods I2 and I5. The other conductor 25 of this transmission line section is connected between the free ends of rods IS and I9. A similar short section of transmission line 21 connects the ends of rods I3, Ill, and the ends of rods I'I,` I8. A main feed line 28 is connected to the electrical midpoints of the line conductors 24, 26, and likewise to the electrical midpoints of the conductors of line 2l, so that the points of connections to the respective lines 25 and 21 are diametrically opposite. AWith this manner of feeding, it is found possible .to generate a radiation pattern of small beam width in the vertical plane, but omni-directional and circular in the horizontal plane.
In one particular antenna structure that was found to produce the desired results, the mast II had a radius of 0.04K; the rings I and 2 each had a radius of 018k; and the vertical spacing of the rings I and 2 was 033k, where A is the geometric mean of the wavelength range over which the antenna is to operate. The rings I and 2, and the Various rods, are preferably, although not necessarily, made of metal tubing. The diameter of this tubing is not critical, and in the particular antenna .having a center band frequency of 470 megacycles, quarter-inch tubing was employed.
In order to maintain the desired horizontal circular pattern, it was found necessary to feed the antenna at the diametrically opposite points above indicated, and in order to arrive at a desired feed impedance, for example of 100 ohms at each feed point, it was found necessary to feed the legs as indicated in the drawing, wherein the lines 25 and 21 were 200 ohm lines. Since this resulted in matched impedances between the line 28 and the antenna, the length of the lines 25, 21, was substantially immaterial. With the particular dimensions above mentioned, the antenna had a very broadband characteristic, and the standing wave ratio on the main feed line 28 remained less than two-to-one over a 32% band width.
It will be seen that in effect the antenna may be considered as made up of four folded dipoles, one dipole consisting of the legs I2, I6 and 3, and the intervening arcuate portions of the rings I, 2; the second folded dipole consisting of the legs I3, I1 and 4, and the corresponding arcuate section of the rings I, 2; likewise the third folded dipole consisting of the legs I4, I8 and 5, and the intervening arcuate sectionsof rings I, 2; and finally the fourth folded dipole consisting of the legs I5, I9 and 6, and the corresponding intervening arcuate sections of the rings I, 2. v
While the antenna has been described as con# sisting of eight legs constituting four folded dipoles, it will be understood that the invention is not limited to this particular number. For example, a similar antenna was constructed employing twelve legs consisting of six continuous rods similar to rods 3-6, and six sets of gapped rods to which the transmission line was connected. In other Words, the antenna had twelve vertical legs, and the radius of the rings I and 2 was approximately 0.24).; the radius of the mast II was approximately (1,053).; and the vertical lpacing ofthe rings I and 2 was approximately In any case, it was found that by increasing the radius of the rings I and 2, the band coverage could be increased, while increasing the radius of the mast I I decreases the band coverage, probably because the unbroken conductive mast shortcircuits the electric field. Preferably where the radius of the rings I and 2 is increased above 025;., it may be necessary to employ more than two feed points in order to maintain a circular pattern. Preferably the vertical spacing of the rings I and 2 should be considerably less than because of the capacity loading effect of these rings and the effective large diameter of the dipole assembly. One of the additional advantages of this construction of antenna is that it is possible to mount a series of such units vertically spaced along the mast II a full wavelength apart along the mast as schematically illustrated in Fig 3.
While certain particular embodiments and dimensions have been described herein, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.
What is claimed is:
1. An antenna, comprising a pair of end rings, a first antenna leg bridged across said rings, a pair of other antenna legs extending towards each other from respective rings and having a gap between their adjacent ends, said first antenna leg forming with said other antenna legs and with the intervening portions of said rings aV folded dipole, a second antenna leg bridged across said rings, an additional pair of other antenna legs extending towards each other from respective rings and having a gap between their adjacent ends, and transmission line means connected to said adjacent ends of both said :pairs of other antenna legs.
2. An antenna, comprising a pair of end rings, a first pair of rods bridged between said rings, a second pair of rods attached to one end ring between said first pair of rods, a third pair of rods attached to the other ring between said first pair of rods, the rods of said second and third pairs having their free ends spaced apart, transmission line means having one conductor bridged across the free ends of the second pair of rods, and another conductor bridged acrossV the free ends of the third pair of rods.
3. An antenna according to claim 2, in which all said rods are located in substantially the same cylindrical boundary and are supported from a common central mast extending parallel to said rods.
4. A squirrel-cage antenna of the type Vdescribed, comprising upper and lower end rings, a first pair of rods downwardly depending4 from the upper ring, a second pair of rods upwardly extending from the lower ring with the adjacent ends of the rst and second pairs of rods spaced apart, transmission linetmeans interconnecting thesaid spaced ends, a third pair ofrods downwardly extending from said upper ring substantially diametrically opposite from the first pair of rods, a fourth pair of rods upwardly extending from the lower ring and having the ends thereof spaced from the ends of the third pair of rods, transmission line means connecting the said spaced ends of the third and fourth pairs of rods, and continuous rods extending between said rings said continuous rods forming with said pairs of rods corresponding respective folded dipole units. p Y
5. An antenna according to claim 4, in which said continuous rods have. conductive struts attached thereto and radially extending towards and attached to a single central support mast for the antenna. Y
- JOHN P.Y SHANKLIN.
REFERENCES CITED The following references are of record in the le of this patent:
UNITED STATES PATENTS Number Name Date 2,175,252 Carter .'.v...' Oct. 10, 1939 2,237,778 carter Apr. 8, 1,941 2,253,914? Carter May 26, 194:2