Bifilar wound quarter-wave helical antenna having broadside radiation

3,083,364

March 26, 1963

BIFILAR

M. W. SCHELDORF

WOUND QUARTER-WAVE HELICAL ANTENNA HAVING BROADSIDE RADIATION

Filed July 23, 1958

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3,083,364

Patented Mar. 26, 1963

United States Patent Office

3,083,364 ^B®^{LAR wo}^ND quarter-wave helical antenna HAVING BROADSIDE RADIATION

Marvel W. Scheldorf, Palos Heights, Ill., assignor to Andrew Corporation, Chicago, III., a corporation of Illinois

Filed July 23, 1958, Ser. No. 750,417 16 Claims. (Cl. 343—843)

This invention relates to antennas, and more specifically to antenna structures having resonant radiating elements which are relatively short considering the wavelength of the transmissions in connection with which they are employed.

Resonant antennas of conventional construction, employing straight conductors, normally have radiating elements of a minimum length of approximately one-quarter wavelength. (It will be understood that the term “wavelength as herein used refers, as is conventional, to approximately free-spaced wavelength, except where the context indicates otherwise.)

Such antennas, when employed for mobile use, as on automobiles, etc., are of prohibitive height when used at frequencies other than extremely high frequencies of the order of hundreds of megacycles. Accordingly, until recently, it was customary to sacrifice efficiency by using an antenna of substantially shorter than resonant length and “turning out” the capacitive component of the impedance of the non-resonant antenna thus presented by means of a lumped inductance element. This commonly used expedient, however, results in poor radiation efficiency, as is well known.

Another approach to the problem of reducing the length of antenna elements is the employment of helical, rather than straight, elements. Resonance may be produced in a helical element, of substantially shorter overall length than the corresponding length of straight wire, the resonant frequency of a helical radiating element lying somewhere between the resonant frequency of a straight wire of the same length as the axial length of the ⁴⁰helix and of a straight wire of the same over-all or “developed” length as the wire employed in the helix, the relationship between the length of the helix and the resonant frequency being a function of numerous factors including the pitch and diameter of the helix. \ „„„„„ versions of such helical antennas or elements have been heretofore proposed or used, those most commonly employed for mobile use being, in essence, adaptations of the single wire quarter-wave vertical antenna, employing, in general, the metal top or other portion of the 50 vehicle upon which the antenna is used as the ground plane. Such antennas, however, are subject to the serious objection that they possess an extremely low radiation resistance, thus producing either a serious mis-match with the 50. ohm coaxial cables which are in standard 55 use for feeding antennas, or wasting power by introducing loss by the use, for example, of small wires, thus producing a proper match, but only at the expense of the production of dissipated, rather than radiated, power in a substantial portion of the resistive impedance of the 60 antenna.

In work by. Li and Beam, reported in the Proceedings of the National Electronic Conference, 1957, there are described helical antennas in which there is employed a folded structure wherein the fed conductor is accompanied by a corresponding grounded conductor in a manner analogous to the use of a grounded conductor in folded quarter-wave straight-conductor antennas. However, as is shown by Li and Beam, in the case of the Meal e,™,. _{which tottIconnecte} 70 the fed and grounded elements is not desirably placed at ’ the extreme outer ends of the conductors, as in the case of the corresponding straight-conductor antenna. The input impedance of a folded antenna may be analyzed on the basis of two separate components acting in parallel, these components being the impedance due to currents in the conductors which are in phase, called the radiation component, and the impedance due to currents in the conductors which are in phase opposition, called the transmission line component, i.e., the impedance it would present if no radiation were occurring. Although it is impossible to measure these two quantities or components of input impedance separately, or even to make highly accurate theoretical calculations of the magnitudes of the components, the knowledge of their existence leads to a qualitative understanding of the action of such antennas which makes possible the performance of experiments leading to radical improvement in size and performance characteristics of such antennas.

In folded quarter-wave antennas employing straight conductors, a short between the conductors at their outer end is required in order to prevent the transmission line component of the input impedance from constituting a short at the input, since in the absence of such a short, the input impedance of the antenna would constitute essentially that of an open quarter-wave line, or a virtual short circuit in parallel with the radiation resistance. The short at the outer end, however, gives the transmission line component of the input impedance such a high value that the transmission line component is negligible in determining the input impedance of the antenna. ° As applied to helical antennas, the use of such a short at the end is extremely undesirable commercially, because of the attendant difficulty of tuning the antenna to a desired frequency. In general, it is highly desirable that adjustment of the length of the antenna to correspond to the exact resonant frequency desired must be capable of being accomplished by the user, rather than the manufacturer, of the antenna. In the case of antennas employing linear conductors, such adjustments may readily be made by provisions such as telescoping of the conductors, sliding connections, and other adjustments. In the case of helical conductors, however, length adjustments may not be so simply effected where the presence ? ΤηΓ'ΤηΤζ £ 1 ---·**ν*._ν mv J&gt;lVJvllVV νΞ 45 ?^{f &amp; shorting} connection is required. In the single con_v ι,θΣ ductor helical antennas previously mentioned, tuning to frequency is accomplished by cutting off the antenna to the desired length. However, where a folded antenna is to be employed, if it is so designed that it must employ a shorting element across the end similar to that employed with straight conductors, there is introduced the necessity of the user, after cutting to length, exposing the end portions of the conductors (which are normally covered with an insulating covering, in addition to possessing an over-all jacket or protective covering for the entire antenna structure), and adding the short circuit required to preserve high impedance at the input. The impracticality of such a structure for commercial purposes is further multiplied by the finding of Li and Beam that in the case of the folded helical antenna, the short is best placed at some point inward of the end of the antenna, because of the fact that the velocity of propagation of the transmission line mode or component differs from the velocity of propagation of the radiation mode or component, so that a quarter-wave for the transmission line mode is substantially shorter than a quarterwave for the radiation mode. Thus, complicated as it would be for a user to add a short at the outer end after cutting of the antenna to desired length, this complication becomes completely prohibitive when the short must end.

3,083,364 •3 o .

It is the principal object of the present invention to provide a folded helical antenna structure which achieves high impedance characteristics without introducing complexities in the operation of preparing the antenna for any desired exact frequency such as to render commercial use and manufacture extremely difficult. This principal object has, in the course of the research and development which has culminated in the present invention, led to a number of subsidiary objects which are accomplished by the present invention. :

A first improvement over prior structures flows from the finding by the present inventor that the importance of the presence and the placement of the shorting conductor can be greatly reduced by raising the characteristic impedance of the structure when regarded as a transmission line. Since the input impedance to a transmission line at any frequency is, for any given length of line, proportional to its characteristic impedance, the raising of the characteristic impedance diminishes the importance of the presence and exact location of the shorting bar in achieving the ultimate object of making the impedance of the transmission line component so high as to make its shunting effect negligible as compared with the radiation resistance. In accordance with the present invention, a number of structural features have been found advantageous for this purpose.

The present invention uses, as did the prior devices, a pair of conductors helically wound about a common axis, the conductors being parallel throughout their length. However, in the prior art there was employed a structure wherein the two conductors are closely adjacent throughout their length, as in a bifilar winding, all corresponding portions of the two· conductors being closely adjacent in position as regards angular relation on the circular cross-section of the helix. In the present invention, all corresponding longitudinal portions of the members of the pair of conductors are displaced by an arc of at least 90° with respect to the axis of the helix, the arc of displacement preferably being 180° so that all corresponding portions lie diametrically opposite each other at any point. The lower ends of the two conductors are displaced by a substantial arc, preferably at least 90°, and this arc of separation is preserved throughout the length of both conductors (and the axial length of the helix) by the parallel relation of the conductors. The characteristic impedance of the transmission line component is further increased by appropriate selection of the pitch angle of the helix. It is found that the optimum pitch angle varies somewhat with the angular spacing between the conductors. Where the conductors are displaced by 180°, the optimum pitch angle is approximately 45°, thus creating a condition where the corresponding longitudinal portions of the conductors are essentially perpendicular to each other at any place along the axis of the helix. However, both because of improvements mentioned above, and others to be scribed below, it is found unnecessary to employ optimum pitch angle to obtain highly satisfactory suits; it will of course be understood that the objective of shortening the length of the antenna element required to produce resonance makes it desirable to employ the minimum pitch angle which can be employed without bringing the over-all input impedance value down to- the point where there is excessive mis-match with the exciting transmission line. It is found that displacement of the corresponding portions of the conductors by an angle of at least 90° produces satisfactory results with helical windings of pitch between 5° and 60°. It will of course be understood that the helix diameter is important in determining the characteristic impedance of tlie transmission line mode with any given pitch and angular spacing of the conductors, since conductor spacing will vary with helix diameter under these conditions.

But it will readily be seen that the diameter of the helix 75 obtained.

and the spacing of the conductors must remain small¹compared to the wavelength of the highest desired frequency of operation.

As previously indicated, in the helical antenna experiments heretofore performed, it was found that the difference in the velocity of propagation for the two modes was sufficiently high to make the desired location of the shorting conductor used to produce high impedance of the transmission line mode substantially further from the outer end of the element than is the case where straight conductors are used for the folded antenna.. This property has been utilized in the present invention. 10 produce a folded helical antenna in which the shorting •bar may be eliminated altogether, by proper selection of the variables to produce a condition wherein the axial velocity of propagation for the transmission line mode is apuroximately one-half the axial velocity of propagation for the radiation mode, so that the length of the structure at resonance is effectively a half-wavelength for the transmission line mode when it is effectively a quarter-wavelength for the radiation mode, thus producing a condition wherein the frequency of resonance for the radiation mode, i.e., the frequency at which the element is effectively one-quarter wavelength long, corresponds to&gt; 25 a condition wherein the transmission line mode appears; at the input as a half-wave open line, thus making the· presence of a shorting conductor at the quarter-wave; point of the transmission line mode unnecessary.

With the structure as thus described, there may readily' 30 be constructed in a simple fashion folded helical antennas which can be tuned to desired frequency over a wide^:range by the mere operation of cutting the antenna to length. Since the only requirement for satisfactory performance is primarily merely to make the transmission 35 line impedance component high compared to the radiation resistance, exact achievement of complete optimum conditions is not necessary. The combination of raising of the characteristic impedance of the transmission line mode with the selection of parameters giving a ratio of ⁴⁰ velocities of propagation in the neighborhood of 2. to 1 makes the transmission line mode impedance sufficiently high so that variation of either or both of these factors considerably away from optimum will still produce a transmission line input impedance which does not shunt the radiation mode impedance beyond acceptable limits.

From the discussion above of the manner in which the objects of the invention have been attained, persons skilled in the art will readily recognize that acceptable performance in accordance with the teachings of the in50 vention may be obtained with a wide variety of selection of the variables involved, the general manner of selection of such variables as diameter, pitch, and arcuate spacing being roughly suggested by the theory of the present man-_r ner of achievement of the objects generally described above, when coupled with the skill of the art regarding the general theory and characteristics of two-conductor transmission lines and radiating elements, and the effect of the variables involved upon characteristics such as the· 60 characteristic impedance and axial velocity of propagation. Although the theory has not, in the present state of the -art, become sufficiently advanced so that it would be possible to compute accurately the optimum selection of variables, the theory of operation is adequately decs scribed by the general principles discussed above to permit the design of many antennas by simple experimentation. Accordingly, the teachings of the invention may readily be applied by those skilled in the art from what has already been stated herein. However, in accordance with the requirements of the patent laws, there is illustrated in the annexed drawing, and described below, an embodiment of the invention, together with certain of its performance characteristics and results experimentally the dethe re70

3,083,364

In the drawing:

FIGURE 1 is a view in side elevation of an antenna structure embodying the invention, with a more or less schematic representation of the manner in which the antenna element is mounted on a ground plane such as an automobile top, shown in section;

FIGURE 2 is a top plan view of the antenna of FIGURE 1;

FIGURE 3 is a sectional view taken along the lines 3—3 of FIGURE 1 in the direction indicated by arrows; . FIGURE 4 is a fragmentary perspective view of a portion of the structure of FIGURE 1; and

FIGURE 5 is a fragmentary view illustrating a modified form of the invention.

From the description of the invention contained above, persons, skilled in the art will readily recognize that the device illustrated in FIGURE 1 is a simple and elementary form of the antenna of the invention. The device is mounted upon a metallic ground plane 1®, such as the metallic top of a vehicle. The metallic ground plane 1® is provided with a conventional coaxial receptacle 12 into which is threaded a coaxial plug 14 upon which the antenna structure, is mounted. The plug 14 has a flanged shell 16, constituting the grounded portion of the connector, a center feed conductor 18, and an insulator 2® supporting the center conductor in the shell 16 in conventional fashion. The antenna structure is supported by insulating support posts 22 at the corners of the flanged shell 16. It will be understood that the support and electrical connection means illustrated are selected for ease and convenience of illustration and description, since the support and connection means in themselves constitute no portion of the present invention, a large variety of support and connection means being employable with the antenna element of the invention.

The antenna conductors are wound on a cylindrical tubular core 24 of a suitable insulating material such as resin-impregnated fiberglass. The conductors of the folded antenna comprise a fed conductor 26 having its lower end connected to the central feed conductor 18 and ^a ground conductor 28 having its lower end grounded to the shell 16. These two conducting wires extend through diametrically opposed apertures 30 and 32 in the lower ®nd of the core 24 and are wound upon the outer surface of the core 24 in the manner generally described above 45 and now to be described in greater detail with regard to the particular embodiment selected for illustration and detailed description.

As earlier stated, because of the factors mentioned, satisfactory operation can be obtained over a fairly wide ⁵⁰range of values of the variables involved when the teachings. of the present invention are employed. In the embodiment described, there was used a tubing of one-inch inside diameter and of a wall thickness resulting in a helix diameter of approximately 1.102 inches. The fed conductor was of 25.mil diameter and the grounded conductor of 50 mil diameter. The employment of a grounded conductor of larger diameter than the fed conductor is, of course, employed to increase the radiation component of the impedance, and has only a second-order effect on the transmission line component. The pitch angle, of the helix was approximately 16°, thus producing Vfi^{0 de}y^el°P^ed conductor length to axial length of slightly less than 4 to 1. This structure was tested at 3 frequencies, 106 megacycles, 150 megacycles, and 212 megacycles, experimentation' with length being required to obtain resonance at each of the desired frequencies, since, as pointed out below, it is found that the resonant frequency is not exactly inversely proportional to the length. In. addition, data was taken on the impedance characteristics demonstrated by the antenna over a range on each side of these resonant frequencies. Since the antenna was designed to match a 50 ohm feed cable, the factor of interest was the closeness of the match to the 50 ohm impedance of the cable, and the data below regarding the impedance at the input terminals is expressed in terms of the ratio of the measured impedances to the 50 ohm impedance of the cable.

.Resonance at approximately 106 megacycles was obtained with an axial length of 12% inches and a conductor length of 45½ inches. (Actual resonant frequency in this case appears from Smith Chart plotting of the data to be given to be a few hundredths of a megacycle below 106). The impedance data for frequencies in the neighborhood of 106 megacycles with the above lengtn is as follows (negative and positive reactance values representing capacitive and inductive reactances, as is conventional):

<td> Frequency</td><td> R/Z„</td><td> X/Zo</td>

<td> 100______________</td><td> 1 AR</td><td></td>

<td> 102_________</td><td> 1. ίο QR</td><td> 2.90</td>

<td> 104________</td><td> . no</td><td> 1.82</td>

<td> 106___________</td><td> • σύ OR</td><td> . 87</td>

<td> 108_________</td><td> . yo 1 13</td><td> +. Uo</td>

<td> 110_____________</td><td> 1 97</td><td> +. 80</td>

<td> 112____________</td><td> 1. z/ 1</td><td> +1.63</td>

<td></td><td> 1. oo</td><td> +2.65</td>

The structure was then cut to an over-all length of 8¼ inches, being a conductor length of 29¾ inches. It was found that the antenna was resonant a few tenths of a megacycle below 150 megacycles, the data obtained being as follows:

Frequency

B/Zo .93 .86 .87 .894 .96

1.10

1.31

X/Zo

144.

146.

148.

150.

152.

154.

156.

-1.45

-.04

-.44

+.086

+. 68 +1.26 +2.0

The same structure was then cut to an axial length of 5¼ inches, corresponding to a conductor length of 19½ inches, producing resonance at a frequency of almost exactly 212 megacycles. The data obtained with this cutting of the antenna is as follows:

<td> Frequency</td><td> R/Z_o</td><td> X/Zo</td>

<td> 206___________</td><td rowspan="8"> .98 .84 .80 .80 .84 .9 .97</td><td rowspan="8"> -1.66 -1.01 -.58 +. 006 +.38 +. 78 +1.06</td>

<td> 208__________</td>

<td> 210_____</td>

<td> 212________</td>

<td> 214______</td>

<td> 216_________</td>

<td> 218________</td>

<td></td>

It will be seen that the antenna illustrated and described produces a highly satisfactory impedance match to the 53 50 ohm line over an extremely wide range of frequencies by mere cutting of the antenna to length, and further, that at any given length of the antenna, the frequency band of proper operation and impedance characteristics is sufficiently broad for all communications purposes normally 60 required. The physical heights (axial lengths) are respectively only .111, .103 and .094 wavelength.

Further embodiments of the invention, constructed to achieve a less perfect impedance match, but one still commercially acceptable, resulted in a finding that considering 65 a 2 to 1 standing wave ratio as commercially acceptable, antennas with equal conductor lengths may be built in accordance with the present teachings down to a physical height, or axial, length, of approximately .06 wavelength. Such an embodiment was constructed using a helix of the 70 same diameter as that previously described with a Vi inch pitch, the fed conductor being 32 mils and the grounded conductor 103 mils. With this construction, resonance was obtained at 106 megacycles with an axial length of 7 inches, corresponding to a developed conductor length of 75 49 inches; resonance was obtained at 150 megacycles

3,083,364 with an axial length of 4¾ inches and a developed conductor length of 31 inches, and resonance was obtained at 212 megacycles with an axial length of 2.85 inches and a developed conductor length of 20 inches. Thus, with the present construction, an antenna having at resonance an impedance of approximately 25 ohms can be constructed with a physical length of only about .06 wavelength, retaining this impedance and satisfactory radiating characteristics over a frequency range of 2 to 1.

A variant on the folded antenna structure illustrated, employed for the purpose of still further raising of the input impedance, is the cutting of the fed conductor to a longer length than the grounded conductor. Such a construction is shown in FIGURE 5, wherein the fed conductor 26α is longer than the grounded conductor 28α. Data on this type of structure indicate that the resonance frequency of such a construction lies between the resonance frequencies of structures with equal-length conductors of the two respective lengths.

Measurements of the latter type were made on the characteristics of such antennas. 50 mil wire was used for both conductors. The pitch employed was such as to produce 1 inch of axial length for each turn of the helix. With 9 turns of fed conductor and 8 turns of grounded conductor, resonance was obtained at slightly more than 145 megacycles, the impedance at resonance being 85 ohms. When the fed conductor was reduced to 8% turns, leaving the grounded conductor unchanged, resonance was obtained at approximately 151 megacycles with an impedance at resonance of 65 ohms. When the fed conductor was further reduced to 8 turns, thus producing an antenna substantially the same as those originally described, the resonant frequency was slightly above 155 megacycles, the impedance being 45 ohms. With equal conductors, fed and grounded, resonance at 150 megacycles was reached with 8.3 turns on each, the impedance being very slightly above 50 ohms. This structure was then altered to produce the same resonant frequency of 150 megacycles with a grounded conductor of 7.2 turns and a fed conductor of 8.8 turns. The impedance of this structure at resonance was approximately 135 ohms.

As previously seen,- a highly satisfactory impedance match can be obtained in accordance with the present invention, when matching a 50 ohm line, by using equal conductor lengths. For certain purposes, where higher impedance is desired, or where the antenna impedance would otherwise be too- low, it may be raised by slightly elongating the fed conductor, as indicated above. Where high impedance is desired in an antenna structure cut off for tuning by the user, such relative elongation may readily be produced by making the cut-off along a line angularly related to the axis, rather than perpendicular to the axis. Best results are achieved with the longer conductor between 5 percent and 20 percent longer than the shorter conductor.

The antenna structure of the invention has been discussed herein throughout in connection with the type of antenna which is effectively of one-quarter wavelength, employing a ground plane. But the use of the folded helical structure described herein as an element in other ^b0types of antennas, such as dipoles, etc., will be obvious to those skilled in the art. Also, the dimensions and other parameters herein given as examples are suited for the particular frequencies stated, and adaptation of the an- _6gtennas presently disclosed to other frequencies will readily be made by such persons. Likewise, although the invention has been discussed in connection with transmitters, its application to receiving antennas will be obvious. Accordingly, the scope of the invention should not be γθ deemed to be limited by the particular embodiments herein described in detail, but shall be determined only from the appended claims.