US3014216A - Wideband antenna - Google Patents

Description

Dec. 19, 1961 N. YARU WIDEBAND ANTENNA Filed June 2, 1958 BY M AQENI,

United States Patent O 3,014,216 WIDEBAND ANTENNA Nicholas Yarn, Santa Monica, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed June 2, 1958, Ser. No. 739,119 2 Claims. (Cl. 343-801) The present invention relates to wideband antennas and more particularly to an antenna of the dipole type having a substantially constant field pattern over a wide frequency band.

It has long been a problem to obtain antennas suitable for use over a wide band of frequencies. Most antennas, when operated over a wide range of frequencies, exhibit a radically varying eld pattern as the frequency is varied. For example, a conventional dipole antenna, when operated at the frequency at which it is half an electrical wavelength long, has a two-lobed iigure-of-eight field pattern. As the frequency is increased the iield pattern deteriorates until at the frequency for which the dipole antenna is one electrical wavelength long, the eld pattern is multi-lobed due to phase reversals of the current distributed along the antenna.

The problem of obtaining a simple inexpensive broadband antenna has become extremely acute since the advent of television whereby stations radiate signals over a 4 to 1 frequency band. Many antenna systems have been developed to overcome this problem. However, many such antennas are still unsatisfactory in some respect. Some broadband antenna systems comprise a plurality of narrow band antennas. Attempts have been made to cor* rect for current phase reversal in a dipole antenna by means of reactive elements. Most prior art broadband antennas are unsatisfactory because the antenna eld patterns often tend to deteriorate in some portion of the frequency range while the input impedance is highly reactive,

vespecially for those modified dipole antennas which utilize transmission line tuning stubs. Other antenna systems having a more satisfactory field pattern and impedance characteristics are complex in construction. Such antenna systems are large, unwieldly and expensive.

A more desirable approach would appear to be the modification of the dipole antenna current by providing more than one current distribution along the antenna in such a manner that the currents would combine to produce a resultant current having a predetermined desirable distribution. A particularly desirable current distribution would ibe that in which the phase of the resultant current would beconstant along the antenna but the amplitude would be greater in the middle and less near the ends than is normal for a half-wave dipole antenna. An antenna having such a current distribution would have low side lobes and broader main beam.

. Accordingly, it is an object of the present invention to providean antenna having a substantially constant field pattern over a 4 to l frequency band.

Another object of the invention is to provide a simple, light-weight and inexpensive broad band antenna.

A further object of the present invention is the provision of a modified dipole antenna having an in-phase current distribution over a 4 to l frequency range. i In accordance with the invention there is provided an antenna comprising three elongated elements disposed collinearly. The overall length of the antenna is one-half electrical wavelength at the low-frequency end of. the band. The antenna is excited bya transmission line connected at two pairs of points, these being the two pairs of adjacent ends of the three antenna elements. By transposing the transmission line connected to one of the pairs of connecting points, the two current components along 3,014,216 Patented Dec. 19, 1 961 the center element are in phase at the high frequency end of the band. In the outer elements, a small amplitude, out-ofphase current is cancelled by a large amplitude, inphase current. Thus the combined current has an inphase distribution along the entire antenna and greater amplitude along the center element than along the outer elements at the high end of the frequency band. At the low frequency end of the band, the usual half wave dipole current distribution is developed.

In accordance with another feature of the invention, there may be provided a fourth element as long as the overall length of the antenna and disposed parallel to the three collinear elements. The outer ends of the fourth element are coupled to the outer ends of the outer two of the three collinear elements to improve the mid-band impedance, thus providing a modified folded dipole antenna. A somewhat detrimental impedance effect at the high end of the band due to the fourth element may be Vcompensated for vby providing two inductive transmission line stubs individually connected in series with the two pairs of transmission line connecting points. The stubs cause no deleterious effects at lower frequencies because they become a short circuit as the frequency is lowered.

For a better understanding of the invention together with other and further objects thereof, reference is made vto the following description taken in connection with the ,accompanying drawing in which embodiments of the invention are illustrated by way of exampleonly, like reference characters designate like parts throughout the figures thereof, and wherein:

FIG. 1 is a diagram of an antenna and its connections in accordance with the invention;

FIGS. 2a and 2b are explanatory diagrams of the current'components present in the antenna of FIG. 1 at the high frequency-end of the band; v

FIGS. 3a, 3b and -3c are diagrams of the field patterns of the antenna yof FIG. 1 at the low frequency end `of the band, at mid band, and at the high frequency end of the band, respectively;

FIG. 4 is a diagram of an embodiment of the antenna of FIG. l; and

FIG. 5 is a diagram of `a modification of the antenna of FIG. l in accordance with the invention.v

Referring now to FiG. 1 of the drawing there is provided an antenna 10 comprising three identical elongated elements or rods 11, 12 and 13. The elements 1'1, 12 and 13 are disposed collinearly, .that is, in line with one an- -other and end to end, By being spaced a short distance apart, the'elements 11, 12 and 13 are electrically insulated one from the other. The adjacentspaced-apart ends of the elements 1:1, 12 and 13 define two pairs of coupling points 14, 15 and 16,- 17. The right end of the left hand element 11 andthe left end of the central element l2 define one pair ofv couplingpoints 14, 15 and the right cnd of the central element 112.V and the left endof the righthand element 13 define the other pair of coupling points 16, k17. y; l v v A coupling transmission line 18, 20 interconnects the two pairs of couplingzpoints 14,:15 and 16, 17. The coupling line 18, 26' is transposed once betweenthe two pairs of coupling points 14, 15 and 16, 17. That is, the point 14 of element 1-1 is connected by conductor 18 of the coupling line .18, 20 to the point 16 of the centrally disposed element 1-2. Similarly,.the point 17 of element 13 is connected by conductor 20-of the coupling line 18, 20 to the point 15 of the centrally disposed element 12.`

A feed-transmission line 21, 2.2 couples theA antenna 10 to a utilization device which may be either a transmitter or receiver, as it will be understood that the antenna 10 may be used either to radiate or intercept electromagnetic waves. The feed line 21, 22 is connected to the center points 23, 24 of the coupling line 18, 20. The center point 23 of conductor 18 of the coupling line 18, 20 is connected to the conductor 211 of the feed line 21, 22. Similarly, the center point 24 of conductor 20 of the coupling line 1S, 20 is connected to conductor 22 of the feed line 21, 22.

Each of the elements 11, 12 and 13 is selected or adjusted to be one-half electrical wavelength at the high frequency end of the frequency band over vwhich the antenna is to operate. A length of thirty inches has been found satisfactory for the length of each element for an antenna to roperate in the present-day television band of 54 to 216 megacycles. The coupling line 1S, 20 should not be longer than is necessary to couple the elements 11, 12 and 13, and with an element length of thirty inches may be, for example, thirty-six inches long.

In operation as a transmitting antenna, current is supplied to the antenna 10 via the feed line 21, 22 and the coupling line `18, 20. The current distribution in the antenna 10 at the low frequency end ofthe band will be the usual half-wave dipole current distribution indicated by the line 25 of FIG. l. That is, the current will be in phase throughout the antenna 1G and will have a maximum amplitude at the center and a minimum amplitude at the two ends. This is because the overall length of the collinear elements 11, 1-2 and 13 is one-half electrical wave length at the low frequency end of the band.

ln FIGS. 2a and 2b there are illustrated the individual current components from the two pairs of coupling points 14, 15 and 16, 17 at the high frequency end of the band. Considering first FIG. 2a', in which is illustrated the distribution of the current applied solely at the right hand pair of coupling points 16, 17, the end of the coupling line 18, at the other pair of coupling points 14, 15 will appear as an impedance Z as shown. The value of the impedance Z will be the impedance presented by that end ofthe coupling line 18, 20. The currents along elements 12 and 13 supplied from coupling points 16 and 17 will be in phase with each other and will be of equal amplitude as shown by distribution curves 26 and 27. The current from coupling points 16 and 17 along element lil, however, will be out of phase with the current along the other two elements 12 and 13 and will be of a lesser amplitude due to Vthe series impedance Z as illustrated by distribution curve 28.

Considering now FIG. 2b in which solely the current applied at the left hand pair of coupling points 14, 15 is considered, the end of the coupling line 18, 20 will again present the aforementioned impedance Z at the right hand pair of coupling points 16, 17. Due to the transposition of the coupling line 18, 20 between the coupling points 14, 15 and 16, 17, the current components along the centrall element 1-2 lfrom both pairs of coupling points 14, 15 and 16, 17 will be in phase. The current from coupling points 14 and 15 along element lil will be in phase with the current along the central element 12 and the currents along these two elements 11, 12 will be of equal amplitude. This is indicated by distribution curves 30 and 31. The current alongy element 13, indicated by distribution curve 32, however, will be of the opposite phase and will have a lesser amplitude due to the series impedance Z.

Itfwill be obvious that the individual current components of FIGS. 2a and 2b will combine to develop the resultant current distribution indicated in FIG. 4. The

lout-of-phase current components 28 and 3G (FIGS. 2a

-the antenna 10 have been eliminated. In FIGS. 3a, 3b, 3c.there is illustrated the radiated field pattern, in one plane, of the antenna 10 of FIG. 1 at the low end of the band, `mid band and at the high end of the band, respectively. The diagrams in the figure are polar plots of equal values of radiated energy versus radial angular distance from the center of the antenna 10, measured in the horizontal plane of the antenna 10. FIG. 3a indicates the field pattern at 55 megacycles for an antenna in accordance with the invention having three elements, each being 30 inches long and an overall length of 91 inches. FIG. 3b illustrates the field pattern of the same antenna at 10() megacycles and FIG. 3c indicates the pattern at 215 megacycles. It will be observed that an antenna in accordance with the invention exhibits a substantially constant two-lobed pattern over a frequency range having approximately a 4 to l ratio.

Although the operation of the antenna 10 has been described `with reference to its use as a transmitting antenna, it will be understood by those skilled in the art that as pointed out before, by the reciprocity theorem, the antenna 10 exhibits the same broad band directional characteristics when operated as a receiving antenna.

FIG. 4 illustrates one practical embodiment of the antenna 10 of the presentinvention. The elements 11, 12 and 13 may be metallic tubes, such as sections of aluminum tubing. The elements 11, 12 and 13 may be secured in a collinear relationship in any convenient electrically insulated manner. The coupling transmission line 18, 20 may be two-wire line of the type known as twin lead ribbon and may have a characteristic impedance of 30() ohms. The ends of the coupling line 18, 20 are electrically connected to the elements 11, 12 and 13 at the coupling points 14, 15 and 16, 17. In order to secure the proper current phasing, the coupling line 18, 2i) should be transposed, as by giving the two-wire line a half twist, before securing both ends to the elements 11, 12 and 13. That is, conductor 18 is connected between points 14 and 16 and conductor 20 is connected between points 15 and 17. At the center 23, 24 of the coupling line 18, 2t), the feed transmission line 21, 22 is connected, as by soldering. Conductor 21 of the feed line 21 22 is connected to the center point 23 of conductor 18 and conductor 22 of the feed line 21, 22 is connected to the center point 24 of conductor 20. The feed line 21, 22 may also be 300 ohrn two-wire ribbon line and extends to a utilization device, such as a television receiver.

FIG. 5 illustrates a modification of the antenna 10 of FIG. l. In order to improve the mid-band impedance properties of the antenna 10 of FIG. 1, a fourth element 36 is provided. The `fourth element 36 is equal in length to the overall length of the three collinear elements 11, 12 and 13. The fourth element 36 is disposed parallel to the collineary elements 11, `12 and 13 and is connected thereto. More particularly, the left end 37 of element 36 is connected to the left end 38 of element 11 and the `right end 40 of element 36 is connected to thepright end inductive transmission line impedance matching stubs 42,

43 may be provided. kMatching stub 42 is connected in series with conductor 18 and point 14 While matching `stub 43 is connected in series with conductor 20 and point 17. The stubs 42 and 43 are selected or adjusted `to be inductive, that is, to have a length less than onequarter wavelength at the high frequency end of the band. The stubs k42' and 43 will not vaffect the impedance at mid-band frequencies or low frequencies because they become much shorter than one-quarter of a wavelength as the frequency decreases and thus effectively become short circuits.

Thus there has been described an antenna having a substantially constant field pattern over a frequency band having a 4 to 1 ratio. In accordance with the invention, the antenna operates as a half Wave dipole antenna at the low frequency end of the band and at the high end of the band exhibits no phase reversals of the current distribution along the antenna.

What is claimed is:

1. An antenna having a substantially constant bidirectional iield pattern over substantially a four-to-one frequency band comprising three elongated substantially straight elements of equal length disposed collinearly and electrically insulated one from the other, a fourth element equal in length to the combined length of said three collinearly disposed elements, said fourth element extending parallel with said three elements, the two ends of said fourth element being individually electrically connected to the outer ends of said three collinear elements, a first conductor connecting one end of the centrally disposed one of said three elements to the inner end of the one of said three elements adjacent the other end of said centrally disposed element, a second conductor connecting said other end of said centrally disposed element to the inner end of the other one of said three elements, and afeed point established at the center of said irst and second conductors, said iirst and second conductors each forming a stub having an inductive reactance at the upper end of said frequency band.

2. A wideband antenna of the folded dipole type having a substantially constant two-lobe eld pattern over a four-to-one frequency range comprising three elongated members of equal length disposed in collinear fashion, a fourth elongated member disposed parallel to said three collinear members and having a length equal to the come bined length of said three collinear members, the ends of said fourth member being individually coupled to the outer ends of the outer two of said three collinear members, and transmission line feed means individually interconnecting the inner ends of the outer two of said three collinear members with the opposed ends of the centrally located one of said three collinear members, for providing in response to energization of said feed means a plurality of current components along said members which combine to develop resultant in-phase currents along each of said members over said frequency range.

References Cited in the file of this patent UNITED STATES PATENTS 2,217,321 Runge Oct. 8, 1940 2,418,124 Kandoian Apr. 1, 1947 2,567,235 Rabuteau Sept. 11, 1951 2,632,108 Woodward Mar. 17 1953 2,655,599 Finneburgh Oct. 13, 1955 FOREIGN PATENTS 782,901 France July 5, 1935

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