US 3521289 A
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United States Patent 3,521,289 HELICAL DIPQLE ANTENNA ELEMENT Paul E. Mayes, Champaign, IlL, and David T. Stephenson,
Ames, Iowa, assignors to University of Illinois Foundation, Urbana, Ill. Continuation-in-part of application Ser. No. 488,402, Sept. 20, 1965. This application Nov. 13, 1967, Ser. No. 687,414
Int. Cl. HOlg 1/36, 9/16 US. Cl. 343---806 7 Claims ABSTRACT OF THE DISCLOSURE A helical dipole or monopole antenna element having conductors substantially in the form of helices with the element lengths determined by one of the odd harmonic resonance frequencies of the fundamental one-half guide wavelength resonance frequency to provide an increased directivity (superdirective) radiation pattern at the selected odd harmonic frequency. A superdirective helical dipole or monopole antenna element with helical shunt feeding to provide increased radiation resistance and bandwidth.
This application is a continuation-in-part of our copending application U.S. Ser. No. 488,402, now abandoned, filed on Sept. 20, 1965.
This invention relates to antenna elements and more particularly to dipole elements in which the directivity of such an element operating at a desired frequency has been increased by the use of helical dipole elements.
Various antenna systems employing dipole elements are well known. In such systems, the dipoles are excited by applying an alternating voltage across a small gap symmetrically located with respect to ends of the dipole halves. The overall length of both dipole halves is most frequently chosen to be approximately one-half the wavelength corresponding to the desired operating frequency. At low frequencies, since the necessary dipole lengths can become quite large and perhaps excessive because of economic, mechanical, or space limitations, there has been proposed the use of helical dipole elements in the antenna array. By suitable choice of the design parameters of the helix, the dipole element can be made to resonate at a frequency much lower than the frequency for which the dipole is a half wavelength long in overall dimension, thus reducing the overall size of the dipole antenna array.
The present invention has application in antenna systems where high directivity is desired. In such antenna systems, high directivity can be used to advantage since it affords the only means of discriminating against atmospheric disturbances which propagate around the earth in the same manner as radio signals. While there is no theoretical upper limit on the directivity which can be achieved from an antenna of a given size, there are practical difficulties which are encountered when attempting to design an antenna to achieve directivity in excess of that obtained with a uniform current. Antennas which achieve this are called superdirective and are subject to low efficiency, due to their low radiation resistance, very narrow bandwidth, and extreme sensitivity to construction tolerances. However, in many applications efficiency is not of prime importance; and instead, high directivity from a minimum of antenna space is desired.
In order to increase the power transfer, matching networks can be provided external to the dipole or the antenna can be constructed as a conventional folded element type. The use of matching networks, while practical, is not desirable since additional equipment would be required, thus increasing the cost of the antenna system ice in terms of initial construction and maintenance of the installation. The efficiency of small network elements is not as high as that for a distributed network having lower stored energy per unit volume. The use of a folded element type of antenna structure increases the antenna input resistance, but offers no improvement in bandwidth. In fact, in order to increase the bandwidth with conventional folded dipole techniques, it has been found necessary to use values of the order of one ohm for the characteristic impedance of the transmission line formed by the two dipole wires. This is far too low a value to be realized in practice.
In accordance with one aspect of the present invention, by suitable choice of the design parameters of a helical dipole antenna element, a superdirective helical dipole antenna can be provided with a directivity above that of a linear antenna element of the same size.
In a preferred embodiment of the invention such a superdirective or directivity increased helical dipole antenna can be constructed with both a significantly higher input resistance and bandwidthwithout resorting to matching networks external to the dipole. It has been found, for instance, that a directivity increased helical dipole antenna, when fed through a helical shunt feed, not only retains the desired superdirective radiation pattern, but has twice the input resistance and almost eight times the bandwidth, thus reducing the problem of match ing the impedance of the external circuits.
The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic plan view of a helical dipole constructed in accordance with one aspect of the invention to provide a superdirective radiation pattern;
FIG. 2 is a graph illustrating the beam width of various increased directivity helical dipoles in accordance with the present invention;
FIG. 3 is a graphical illustration of the side lobe level of the increased directivity helical dipoles corresponding to FIG. 2;
FIG. 4 is a graphical illustration showing the directive gain of a preferred increased directivity helical dipole embodiment in accordance with the invention;
FIG. 5 is a schematic view of a preferred embodiment of the directivity increased helical antenna of this invention illustrating helical shunt feeding to provide a significant increase in input resistance and bandwidth;
FIG. 6 illustrates an alternative embodiment of a superdirective helical dipole with helical shunt feeding constructed in accordance with the principles of the invention as shown for example in FIG. 5 and FIG. 7 illustrates a directivity increased helical monopole according to the invention.
As shown in FIG. 1, a helical dipole element 10 is composed of two halves 12 and 14, each of the dipole halves comprising a conducting element such as a wire wrapped in the form of a helix 16 around a support 18 made of a suitably nonconducting material such as polystyrene. The feeder system 20, comprising a balanced pair of conductors, feeds the dipole element 10 at its midpoint. The helical dipole element 10 as shown in FIG. 1 is wound such that the helix 16 has a length to diameter ratio which is very large. The guide wavelength x and the free space Wavelength x0 are related by xg=S \0 where the factor s is a shortening factor, defined as the length of a loaded dipole relative to the length of a linear dipole resonant at the same frequency. Thus, the factor s is equal to 1 for a linear dipole and becomes less than 1 as the dipole is helically wound and shortened.
The length or height h of one of the dipole element halves 12 is shown in FIG. 1 as corresponding to 3/4Ag,
or in other words the dipole element may be characterized as a helical 3/2)\g dipole. It is known, of course, that a monopole of height it over a perfectly conducting ground screen behaves, in the half space above the screen, exactly as'does a dipole of length 211 in free space. Because of this known relation between monopoles and dipoles, to avoid confusion, in the following description the term first resonance applies to a resonant dipole or monopole for which h=1/4)\g; similarly second resonance applies to h=3/4 \g; third resonance applies to l1=5/4 \g; etc. Thus, the helical dipole in FIG. 1 is shown as operating at the second resonance frequency, as indicated by the illustrated current distributionor which frequency might be described as the third harmonic of the fundamental frequency corresponding to l/4Ag.
Radiation patterns corresponding to helical dipoles operating at frequencies near the second, third and fourth resonances were calculated, and the resulting E-plane beam width and side lobe level for each of these higher order resonances as a function of the shortening factor s is shown in the diagrams of FIGS. 2 and 3.
Referring now to FIG. 2, it will be noted that a 3/ 2kg dipole has a narrower half-power beam width than a 1/2)\0 linear dipole (at the point labeled 11:1/ 4X0 corresponding to 2/1=l/ 2A0) for all values of s greater than about 0.15. It may be further noted that for a shortening factor s:0.3 the 2h=3/2)\g dipole is 0.45M in length, and has a beam width of approximately 53. In comparison, note that the 2h=l/2)\0 dipole exhibits a beam width of 79. Thus, when operated near the second resonant frequency, a helical dipole is more directive than a slightly longer linear half-wave dipole. By referring to FIG. 3 it may be further seen that the 3/2 \g dipole when constructed with a shortening factor of s=0.3, theoretically has no side lobes. Another interesting observation from these figures is that at a shortening factor of s=0.4, the 3/2)\g dipole is 0.6 free space wavelength long, just 0.1 wavelength longer than the l/ZAO linear dipole, but its beam width is only 44 compared with 79 for the linear dipole, and the side lobes are down 22 db.
FIG. 3 illustrates that the side lobe levels for the third resonance (5/4)\g) and the fourth resonance (7/4Xg) are much higher than for the second resonance (3/4 \g). FIG. 3 also shows that although a given beam width might be achieved by any of the three modes, the overall dipole lengths will be about the same in each case. Since superdirectivity is often characterized by many changes in sign of the current distribution along the antenna element, and since a highly superdirective element is not only sensitive to small changes in current distribution but also features large 1 R losses, the best choice for a given beam width appears to be the second resonance.
Accordingly, in the diagram of FIG. 4 there is shown for the 3/2 \g dipole the values of directive gain broadside to the dipole or in other words at an angle of 0 equal 90, as a function of the shortening factor, s. The directive gain has been calculated with respect to an isotropic radiator. As can be seen from this figure, a maximum directive gain broadside to the dipole of nearly three db relative to a linear half wave dipole is obtained for a value of s between 0.5 and 0.6; at this value, the overall dipole length is just over three-fourths of a free space Wavelength. Also, it may be noted that directive gains of a helical dipole in second resonance can be obtained which are somewhat greater than that possible for a linear half wave dipole and at shortening factors which allow the helical dipole to be slightly smaller in size than the linear half wave dipole.
In order to demonstrate the performance obtainable with antennas of the present invention, a direct comparison was made between a directivity increased helical monopole and a linear 1/4 \0 monopole. The helical mono- 4. pole was constructed with a shortening factor s of 0.5565 at a second resonant frequency of 464.5 me. A comparison method was used, in which the signal received by the helical monopole, when tuned carefully into a 50-ohm line and a matched detector, was compared to the signal received by the linear 1/4 \0 monopole, also carefully tuned line to the same line and detector. A calibrated variable attenuator in the transmitting circuit was used to set the received signal to the same level for each antenna, and gain readings were then taken from the attenuator. The helical dipole exhibited a power gain of approximately 2.7 db relative to the 1/4 \0 monopole. As expected, the radiation efficiency of the helical dipole was only approximately 74%. However, in some applications where efliciency is of secondary importance, as in the design of receiving antennas for use at frequencies below approximately 30 me. where atmospheric noise rather than receiver noise determines the signal to noise ratio, the increased directivity obtained according to this aspect of the present invention can be used to reject this ever present atmospheric noise. Thus, where rejection of a noise signal is more important than a high efliciency, a helical dipole according to the present invention offers a large range of side lobe angles near 6 equals 0 and through which its sensitivity to signals is much lower than that of a linear element of the same length. A range of shortening factor s between 0.3 and 0.55 appears to offer the most useful increase in directivity without reaching extreme size reduction and superdirectivity, while in practice it has been found that the advantages of this invention can be provided with a factor s between 0.3 and 0.7.
A specific application of the present invention can be made at low frequencies, for instance, less than 20 me. At these frequencies where small antenna size is desired for rotability, and where narrow beam width and a high front to back ratio of radiation pattern is required, a 1/2 wavelength dipole is not suitable. However, a two element hybrid network array utilizing two helical dipole elements operating at the second resonance frequency according to the present invention and coupled into a hybrid network can perform at these frequencies as required.
Also, where a monopole element is desired, such an element operating at the second resonance frequency and constructed in accordance with the teachings of this invention provides a narrower beam width than a 1/4 free-space wavelength monopole.
To increase the value of radiation resistance of the helical dipole, instead of feeding the element at the gap between the dipole halves as shown in FIG. 1, the gap instead may be short circuited and the feed line tapped in at an appropriate position along the Wound helix. This technique has been suggested for use with normalmode helical antennas in first resonance (h=l/4 \g); however, in attempts to use this type of shunt feed in a helical dipole at second resonance it was found that the desired superdirective radiation pattern could not be obtained.
It has been found that a directivity increased helix constructed as described hereinabove, when fed through a helical shunt feed, retains the superdirective radiation pattern and yet has significantly increased input resistance and bandwidth. Thus, shunt feeding of the directivity increased helix by means of a helical feed has proven successful in controlling the impedance properties of the directivity increased helix without destroying the radiation pattern.
Referring to FIG. 5 there is illustrated a preferred embodiment of the invention, comprising a helical shuntfed directivity increased helical dipole 30 fed through a helical feed 32. The wires 34 and 36 comprise essentially a bifilar helix with the illustrated pitch p, diameter D, and length 2h. The wires are wound around a rod 38 made of a suitably nonconducting material such as polystyrene and otherwise similar in construction to the support 18 illustrated in FIG. 1.
The two wires 34 and 36 may be regarded as a parallel wire transmission line with characteristic impedance Z The wires are connected together at the taps 40 and 42 with each tap located at a point on the wires 34, 36 defined by the ratio b/ h equal approximately to 0.3. The input terminals 44 and 46 are formed approximately at the midpoint of the antenna element by breaking the wire 36. It may be noted that the remaining wire 34 continues unbroken adjacent the input terminals across the center of the dipole. Between the tap points 40, 42 and the ends 48, 50 of the dipole 30, the two wires 34 and 36 behave as one wire, and may in fact be replaced by a single wire extending from the respective tap points.
Such a configuration can be seen by referring to FIG. 6, wherein a single wire 52, 54 extends from the respective tap points 56, 58 to the ends 60, 62 of the directivity increased helical dipole 64. A similar helical feed 66 is utilized for the antenna 64 of FIG. 6, and this antenna is similar to the antenna 30 shown in FIG. 5, except for the illustrated single wires 52 and 54 extending from the tap points to the respective dipole end.
The appearance of the antennas 30 and 64 are very similar to that of a folded helical dipole. In both cases, the increase in radiation resistance and bandwidth theoretically results from the individual admittances of the dipoles and the transmission lines, as functions of frequency, adding together. The difference lies in the fact that in the conventional folded helical dipole, the length of the transmission line sections from the input terminals to the tap points, illustrated as the length b, is one-quarter of a wavelength for the transmission line currents in the direction of the wire, whereas in the present invention the length b is approximately one-half wavelength. Therefore, the impedance behavior of the shunt-fed directivity increased helix of the present invention is entirely different from that of the conventional folded helical dipole, and substantial broad banding is obtained with practical values of the required characteristic impedance of the transmission line.
As an example of the improvement in impedance properties provided by the helical shunt feed of the present invention, reference may be made to the impedance of a constructed shunt-fed directivity increased helical dipole for which Z =160 ohms, s=0.47, and b/h=0.272. The input impedance of this dipole exhibited a maximum VSWR of 1.65 with respect to 20.6 ohms over a 3% frequency band. This is to be compared with a conventional center fed directivity increased helical dipole, such as shown in FIG. 1, of the same pitch, diameter, length, resonant frequency, and Wire size, for which the same VSWR, namely 1.65, encompassed a frequency band of only 0.39% and this VSWR was with respect to only 10.5 ohms. Thus, in using the helical shunt-feed aspect of the present invention to obtain the significant improve ment in radiation resistance and bandwidth, the desired superdirective radiation pattern of the directivity increased helix such as shown in FIG. 1, was readily obtained.
As alternative embodiments, the helical shunt feeding aspect of this invention is also applicable to the directivity increased helical monopole 70, such as is illustrated in FIG. 7. The directivity increased helical monopole 70 is essentially one-half of the helical dipole illustrated in FIG. 1. The monopole '70 is positioned perpendicular to a large conducting screen 72 which serves as the ground plane. As illustrated in FIG. 7, one wire 74 is connected to the ground plane 72 and to the outer conductor of a coaxial connector 80 at a point 76, and the other wire 78 is fed through the center conductor of thecoaxial connector 80 which forms the antenna input terminal. The ratio of b/h illustrated in FIG. 3 should be about 0.3, similar to that for the dipole hereinbefore described. As in the dipole embodiment, the two wires 74 and 78 can be interconnected, twisted together or replaced by a single wire between the tap point 82 and the monopole end 84.
The enhanced input impedance behavior of the shuntfed dipole or monopole helix as previously described could also be obtained with values of b/ h in the neighborhood of 0.6 and 0.9, due to the periodic nature of the behavior of a length of shorted two wire transmission line as the shunt is moved along the length of line.
Therefore, the foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.
What is claimed is:
1. A helical dipole antenna element for operating at a selected odd harmonic resonance frequency of the fundamental one-half guide wavelength resonance frequency, said helical dipole element comprising:
an insulating support member; and
a pair of bare conductor wires each extending from the midpoint of said antenna element towards respective ends of said antenna element substantially in the form of a helix around said insulating support member and forming dipole halves thereon;
said helical dipoles each having a shortening factor, s, of between 0.3 and 0.7, where s is defined as the length of a loaded dipole relative to the length of a linear dipole resonant at the same frequency, and wherein the dipole halves have lengths substantially corresponding to said selected odd harmonic resonance frequency to provide an increased, directivity antenna element radiation pattern and power gain significantly above that of said linear dipole.
2. A helical dipole antenna element as claimed in claim 1, wherein said dipole halves have lengths substantially corresponding to three-quarter guide wavelengths at the second resonance frequency or the first odd harmonic of the fundamental resonance frequency.
3. A helical dipole antenna element for operating at a selected odd harmonic resonance frequency of the fundamental one-half guide wavelength resonance frequency, said helical dipole element including two halves each comprising conductor means substantially in the form of a helix having a shortening factor, s, of between 0.3 and 0.7, where s is defined as the length of a loaded dipole relative to the length of a linear dipole resonant at the same frequency, and wherein the dipole halves have lengths substantially corresponding to said selected odd harmonic resonance frequency to provide an increased directivity antenna element radiation pattern;
said conductor means comprising a pair of spaced parallel wires extending from the midpoint between the ends of said antenna element towards respective ends of said antenna element, the length between said midpoint and each of said antenna element ends defined as length It;
said parallel wires being interconnected at symmetrical tap points b on each side of said midpoint and with respect thereto, where b is defined as an integral multiple of approximately 0.3 h;
one of said wires being noncontinuous at said midpoint to provide input terminals for feeding said antenna element,
whereby said helical dipole antenna element provides an increased radiation resistance and operating frequency bandwidth.
4. A helical dipole antenna element as claimed in claim 3 wherein said parallel wires are terminated at said tap points, and including an extension wire extending from each of said tap points towards said respective antenna element ends.
5. A helical antenna element for operating at a selected odd harmonic resonance frequency of the fundamental one-half guide wavelength resonance frequency, said helical element comprising:
a conductor means substantially in the form of a helix having a shortening factor, s, of between 0.3 and 0.7, where s is defined as the length of a loaded element relative to the length of a similar linear element resonant at the same frequency, and wherein the element length substantially corresponds to said selected odd harmonic resonance frequency;
a ground plane below said helical element; and
said conductor means further comprising a pair of parallel Wires helically extending from said ground plane, said antenna element being fed between one of said wires connected to said ground plane and the other of said wires to provide a helical monopole antenna element having an increased directivity radiation pattern.
6. The method of operating a helical dipole antenna element having an insulating support member, and a pair of bare conductor wires each extending from the midpoint of said antenna element towards respective ends of said antenna element substantially in the form of a helix around said insulating support member and forming dipole halves thereon, said helical dipoles each having a shortening factor, s, between 0.3 and 0.7, where s is defined as the length of a loaded dipole relative to the length of a linear dipole resonant at the same frequency, including feeding an operating signal to the dipole halves at a fre- 8 quency substantially corresponding to an odd harmonic resonance frequency of the fundamental one-quarter guide wavelength dipole halves resonance frequency to provide an increased directivity antenna element radiation pattern and power gain significantly above that of said linear dipole.
7. The method as claimed in claim 6, wherein said operating signal corresponds to substantially the first odd harmonic or three-quarter guide wavelength second resonance frequency.
References Cited UNITED STATES PATENTS 6/1935 Round 343-895 6/1954 Johnson 343802