US2966679A - Unloaded helical antenna - Google Patents

Description

Dec. 27, 1960 E. F. HARRIS UNLoADz-:n HELIcAL ANTENNA Filed Nov. 13. 1957 a All. .um qm .93 M9555 j @lmllrll Nrw/ United States Patent UNLOADED HELICAL ANTENNA Edward F. Harris, 4729 Farwell, Lincolnwood, lll.

Filed Nov. 13, 1957, Ser. No. 696,226

2 Claims. (Cl. 343-895) M-y invention relates to an improved helical antenna having small length in relation to the wave length of the radiated energy and characterized by a highly favorable current distribution and radiation resistance Without the use of massive loading elements on the antenna structure.

In vehicles, boats, and other applications it is desirable to provide an efficient radiator or'antenna of as small length in relation to the wavelength of the radiated signals as is possible. Moreover, for maximum utility such antennas should be free of loading coils, loading capacitors or other massive physical elements that must be supported during vehicle movement and under conditions of wind, icing, and the like. Also aesthetic considerations favor an antenna of the so-called whip type consisting of a single upstanding rod having its sleek appearance unmarred by other elements.

Efficient electrical performance in a short antenna demands operation in resonant fashion and with a maximum radiation resistance value. Resonant action at a shortened axial length can be achieved by the use of a helical construction, the antenna conductor being in the form of a Wire Wound helically about a straight or slightly tapered nonconducting core or rod at uniform pitch. Such arrangement is shown in my copending patent application, Whip Antenna and Process of Fabricating Same, Serial No. 687,286, filed September 30, 1957, now Patent No. 2,938,210. However, when such antenna is made with a very small axial length in relation to the wavelength, the radiation resistance falls to a low value. For example, su-ch an antenna of one tenth Wavelength axial physical length theoretically would have a radiation resistance of about six ohms. This resistance is small as compared to the inevitable contact resistance, conductor resistance, and the resistance equivalent of other energy loss producing elements, so that the eiciency of such antenna is relatively low. When the antenna is made even shorter the radiation resistance falls to smaller values. Additionally, the low radiation resistance imposes a troublesome impedance matching problem with respect to the common transmission line impedance values, which are of the order of 50 ohms and more, In addition, the generally sinusoidal current distribution on an antenna or radiator of this type gives rise to low radiating eliciency.

In accordance with the present invention a helical antenna is provided with a varying pitch providing a nearly uniform current amplitude from the base to a point near the top. The current amplitude then rapidly drops to Vzero at the tip. It has been found that by so constructing the antenna the radiation resistance value is considerably more favorable than that of a simple helical antenna, and the efficiency of radiation is greatly enhanced over that associated with an equivalent helix of constant pitch. Moreover, this antenna-because of its short length-can be placed on a vehicle, such as a car,

in a more favorable radiating positionv thana conventional l 2,966,679 Patented Dee. 27, 1950 whip antenna (even though loaded), achieving a maximum efficiency of operation.

It is therefore a general object of the present invention to provide an improved, short, unloaded whip type helical antenna.

A more specific object of the present invention is to provide an antenna of the above type which is characterized by a relatively large radiation resistance in relation to other unloaded antennas of equivalent length.

Still another object of the present invention is to provide a short helical antenna of the whip type having improved electrical characteristics.

Another object of the present invention is to provide an improved helical antenna wherein variations in pitch serve to provide improved radiation resistance and other performance despite the short axial length in relation -to the resonant wavelength.

Yet another object of the present invention is to provide a helical antenna in which variation in pitch serves to provide improved electrical characteristics.

Still another object of the present invention is to provide an improved unloaded helical antenna in which the current sharply increases in value from the free end to reach a substantially uniform value that is effective for the major proportion of the length.

It is another object of the present invention to provide an improved short helical whip antenna or antenna element having features of construction, combination, and arrangement rendering it easily and inexpensively manufactured, readily installed on a vehicle for effective operation, ecient and reliable in operation even under adverse conditions, readily adjusted to accommodate varying frequencies, of attractive appearance, and of maximum commercial utility.

It is yet another object of the present invention to provide a resonant helical antenna element suitable for use as an individual element, as part of a dipole, as part of a directional array, or for other purposes in which variations of pitch serve to provide improved radiation resistance and other performance despite the short axial length in relation to the resonant wavelength.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention, itself, together with further objects and advantages thereof, will best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

Figure l is an elevational view of a helical whip antenna constructed in accordance with the present invention;

Figure 2 is an enlarged, fragmentary, elevational crosssectional view of the antenna of Figure 1, with portions broken away to shorten the overall length;

Figure?) is a graph showing the current distribution along the length of various types of antennas; and

Figure 4 is a View thro-ugh section 4 4 of Figure l;

Figure 5 is a somewhat schematic view of a multielement array incorporating the radiator of the present invention.

In the antenna of Figures l and 2, the conducting Wire 13 is wound about flexible core Ztl. A ferrule or base member 14 is connected to the base end of the core, as described hereafter. A protective insulating covering 16 envelopes both the conducting wire 1S and the core 2t), and a cap 12 is telescoped over the free end of the core, as shown.

The base end of the wire l extends radially inwardly through the core 20 and downwardly through the axial hole 201 of the core and axial hole 14C of the ferrule, as shown in Figures 2 and-4. Connection is made to a source of radio frequency oscillations indicated at 34. The latter source is also connected as shown to the conducting support socket 36. This socket is insulatingly supported on the vehicle (or other support) by insulating washer 37 or by other suitable means which is in turn supported by the body 38 of the Vehicle or by other appropriate support. The source of oscillations 34 is connected as shown to the conducting socket 36 and to the body 38, the latter acting to define a ground plane with respect to which the antenna radiates.

The other end of wire 18 is received in slot 22 as the free end of core 20. The conducting wire 15 and ilexible core 20 are preferably fabricated with a protective covering in the same manner as that described in my co-pending application Whip Antenna and Process of Fabricating Same, Serial No. 687,286, f led September 30, 1957.

Core 20 is made of resilient material capable of holding an erect shape when supported at one end and which is light weight and dielectric. Any resilient core with these characteristics may be used, but preferably the core is of a plastic impregnated glass composition, such as is commonly used in lishing rods, and may, for example, be made of molded glass fibers mechanically bound with a polyester resin. The diameter of core 20 progressively decreases from the base end (received in ferrule 14) to the free end topped with cap 12, although this is not necessary to the practice of the present invention.

Ferrule or base member 14 is tightly received on the base end of core 20. The exposed outer face of the ferrule is` of hexagonal conformation. The ferru-le has a cylindrical bore 14a in one end into which core 2t) is cemented. The ferrule has an extending threaded stub 1412 at its base end to be threadedly received in socket 36 and establish a conducting connection therewith. The socket 36 may, for example, be the top or engine hood of a car. Because the structure of the present invention permits the efiicient use of a relatively short antenna, some latitude in the choice of the mounting location is permitted and the antenna may be mounted on a flat conducting surface such as the engine hood or the body which denes an effective ground plane from which radiation takes place. Passage 14e extends through stud part 14b of ferrule 14 and connects to a hollowed center portion 14d of the ferrule. Conducting wire 18 extends radially upwardly, passes through the end of core 20, and is received in the ferrule to extend into the passage 14c where it is soldered to ferrule 14.

Wire 18 is wound helically along the core 2t). The wire is wound in a series of segments 20a, 20h, Zic, 20d and 20e, each having turns of like pitch or spacing. The top segment 20c has the turns wound against each other to provide minimum pitch. As shown, the turns in segments 20d, 20c, Ztlb, and 2tlg each have greater pitch than those of the preceding segment. The purpose and effect of the variation in pitch is described in detail hereafter.

The antenna specifically shown in Figures l and 2 is intended for use in the 15 meter (20 megacycle) range. In this antenna the conductor 18 is of #20 A.W.G. enameled copper wire. The antenna totals 47.5 inches in axial core length. With the form shown, the 'total length of the conductor 18 is about 398 inches, and the respective conductor lengths in the portions 2th: to 20e, inclusive, are 9, l0, 13, 14 and 353 inches. The effect of this distribution is to provide a current distribution along the axial length of the antenna of approximately the form shown at curve 24, Figure 3, where axis 3th represents distance from the base and axis 32 represents current amplitude. 1t will be noted that the current increases from zero value at the antenna tip to almost maximum value at the lower end of section 20e. Below this point, the current has a substantially constant value.

The current distribution ofcurve 24, Figure 3,'should be compared with that `of curve 26, Figure 3, for a uniformly wound helix, and curve 28, Figure 3, for a base loaded antenna. Curve 26 is in the form of a sine wave, with continuously decreasing value as the top of the antenna is approached. The curve 28 is essentially a straight line decreasing current curve due to the near straight line conformation of the rst portions of a sine curve and the fact that the base loading coil accounts for the remainder of the standing current wave.

It has been found that the nearly rectangular current distribution curve of curve 24, Figure 3, provides highly desirable antenna characteristics. In the first place, essentially the entire length of the antenna operates at high current density and hence a relatively high radiated energy. This same high radiated energy is reflected in an increased radiation resistance, which means that the efciency and the amount of the radiation is increased. Alternatively, f a predetermined radiation elciency is desired, an antenna such as that of Figures 1 and 2 and having the current distribution of curve 24 may be made shorter than an antenna having the current distribution of curve 26 or 28.

Computations indicate that for a given axial antenna length that is short in relation to a quarter wavelength, the antenna of Figures l and 2 has about twice the radiation resistance of a helix of like length and a uniform pitch (curve 26), and more than three times the radiation resistance of a short base loaded antenna of like length (curve 29).

In use, the antenna should be trimmed to a length providing resonant action at the driven frequency. This can be readily accomplished by removing cap 12, and cutting the length of the antenna until the reactive impedance cancels out at that frequency. The use of a matching network (not shown) between the source of oscillations (or antenna energizing cable) and the antenna provides some adjustment for error in actual length and for matching the actual complex antenna impedance to the cable or source termination.

One of the advantageous features of the antenna lies in the short axial length that may be used. One of the problems in mounting vertical antennas on automobiles is that of avoiding excessive vertical height. With such conventional antennas it is often necessary to mount them on the vehicle bumper. This is highly undesirable because no efficient and uniform ground plane is defined, the vehicle body often parallels the lower part of the antenna, and such eld as is developed is highly distorted. The antenna of the present invention-because of the short length possible-may more readily be mounted on the metallic roof or engine hood where a relatively good ground plane is established, radiation is directionally uniform, and the vehicle body does not parallel the antenna in any substantial respect.

Another advantage of the antenna of the present invention is that it requires no elevated loading coil or capacitor. This is highly important because such elements are necessarily massive, are unslightly, and impose troublesome tuning and support problems.

=In a practical antenna constructed in accordance with the present invention for use on an automobile in the 15 meter amateur band, the following dimensions were used:

Core-plastic impregnated rod of glass ber, 48" long CVS diameter at base tapering to 0.275" at tip.

Conductor-#20 A.W.G. enameled copper wire.

Segment 2in-11.5 inches long, 26 turns per inch, 299 turns, total wire length 353 inches, wires wound for minimum pitch.

Segment 26d- 11.5 inches long, l/s inch pitch, total wire length 13.7 inches.

Segment 29c--l'0.5inches long, 1A inch pitch, total wire length 12.7 inches.

Segment 20b-8 inches long, 1/2 inch pitch, total wire length 10.2 inches Segment 20a- 6 inches long, 1 inch pitch, total wire length 9.11 inches. Total axial length of winding on core-47.5 inches. Jacket 16--vinyl tubing dilated, to slip over antenna and dilator thereafter permitted to evaporate. Ferrule or base 1li-approximately 2 inches long.

In the design of antennas pursuant to the present invention, the current value should be made to increase as rapidly as possible from the tip end down, and below the top segment, the conductor should be wound to provide a practically constant current value. This means that the pitch at the top of the antenna should be as small as the diameter of the conductor and insulation) permits, and that this portion should continue until at least half of the total wire, and preferably substantially more than half of the wire, is used along a comparatively short axial length, preferably less than a quarter of the antenna length. Below this closely wound section, the pitch should increase progressively and substantially, so that a configuration similar to that of Figures 1 and 2 is obtained. In theory a uniformly increasing pitch from turn to turn is desirable. For production reasons, however, it is desirable to wind the conductor in a series of adjacent segments, each having a constant pitch. Preferably these should be of progressively shorter length as the base end of the antenna is approached and the pitch of each such segment should be about twice that of the preceding segment as the base is approached.

While the above description relates to a unit Where the helical element radiates in relation to a ground plane, the element may also be applied to various kinds of multielement antennas. It may, for example, be used in a dipole construction, such as is shown in Figure 5. Here, an insulating support 40 has a pair of conducting end members 40a and 4Gb, each of which is threaded to receive stud 14b, Figures l and 2, to define a dipole with two elements of the type shown in Figures l and 2. The dipole is energized from source 42 through the transmission line 44. This construction is especially useful for a horizontal antenna structure, particularly in a directional unit mounted on a rotatable structure.

The antenna element of the present invention may be used for reception as well as transmission, in which case a receiver is connected in the same manner as the oscillation sources 34 and 42. The term radiant energy apparatus is used in the appended claims and denotes both transmitting and receiving apparatus.

While I have shown and described specific embodiments of the present invention, it will, of course, be understood that other modifications and alternative construction may be used Without departing from the true spirit and scope of this invention. I therefore intended by the appended claims to cover all such modifications and alternative constructions as fall within their true spirit and scope.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A normal mode resonant antenna characterized by relatively high radiation resistance in relation to axial length and resonant frequency and suitable for radiating action in relation to a plane of uniform potential, said antenna comprising in combination: a lengthy insulating whip-like support means extending in direction normal to said plane with one end substantially in said plane and the other end remote therefrom, the support means extending in its lengthy direction substantially less than a quarter Wavelength at said frequency and having its largest cross-sectional dimension less than one-ftieth wavelength at said frequency and a uniform low resistance conductor wound helically about said support means in a series of at least four axially spaced segments each having a uniform pitch, the turns of the segment adjacent said other end of the support means incorporating at least a major proportion of the total length of the conductor in not more than the quarter of the length of the support means adjacent said other end, the remaining segments having progressively shorter axial lengths as the end of the supporting means adjacent said plane is approached, the pitch of the segment adjacent said first mentioned segment being substantially greater than the pitch of said rst mentioned segment, and each successive segment as said first end of the supporting means is approached having substantially greater pitch than the preceding segment, the total number of turns on the helix being such as to provide resonant quarter Wave action in normal mode at said frequency, and the total length of the conductor being approximately one-fourth wave at said frequency.

2. A normal mode resonant antenna characterized by relatively high radiation resistance in relation to axial length and resonant frequency and suitable for radiating action in relation to a plane of uniform potential; said antenna comprising in combination: a lengthy insulating Whip-like support means extending in direction normal to said plane with one end substantially in said plane and the other end remote therefrom, the support means extending in its lengthy direction substantially less than a quarter wavelength at said frequency and having its largest cross-sectional dimension less than one-ftieth wavelength at said frequency; and a uniform low constant resistance conductor wound helically about said support means with progressively decreased pitch as said other end of the support means is approached, the length of the conductor wound about the quarter of the support means adjacent said other end constituting at least percent of the total length of the conductor, said conductor being resonant in normal mode at said frequency and of total length approximately one-fourth Wave at said frequency.

References Cited in the le of this patent UNITED STATES PATENTS y2,681,412 Webster June 15, 1954 2,682,608 Johnson June 29, 1954 2,712,602 Hallen July 5, 1955 2,850,732 Kandoian et al Sept. 2, 1958 2,875,443 Kandoian Feb. 24, 1959 FOREIGN PATENTS 810,325 France Dec. 28, 1936 OTHER REFERENCES Antennas, by J. D. Krause, copyright 1950, pp. 213 to 216.

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