US2763003A - Helical antenna construction - Google Patents

Description

m- 1956 E. F. HARRIS 2,763,003

HELICAL ANTENNA CONSTRUCTION Filed July 1, 1953 United States Patent HELICAL ANTENNA CONSTRUCTION Edward F. Harris, Lincolnwood, lll. Application July 1, 1953, Serial No. 365,355

3 Claims. (Cl. 343-895) This invention relates generally to high frequency antennas and more particularly is concerned with a construction of a helical antenna.

The helical antenna, as it is known, is an unusual type of antenna due to its characteristics which render it distinct from the other type of high frequency antenna such as yagis, paraboloids, and corner reflectors. The primary consideration is that all other antennas are linearly polarized, usually vertically, while the helical antenna is circularly polarized. For unidirectional service, therefore, it is apparent that the most desirable of antennas is the helical, because it will provide a beam which is not only circularly polarized, but polarized either right handed or left handed, depending upon the manner in which the helix winds. Its transmitted wave can be received only by an antenna which is likewise polarized circularly and in the same sense.

Along the propagation path of the usual linearly polarized antenna, interference between the directly received wave and the reflected wave results in fading, diurnal effects, and the like. Since the reflected wave from a circularly polarized antenna rotates substantially in the opposite direction, the receiving antenna will only receive the primary wave. It will be substantially free from the fading. and atmospherica-l-ly caused phenomena attendant upon the use of linearly polarized antennas. Tests have shown that helices of opposite rotational sense can discriminate side by side, i. e., where one is properly arranged to receive the transmitted wave and the other is not, by a signal difference of the order of 20 db. This ability to discriminate also makes possible efficient adjacent and back-to-back systems.

Accordingly, it is a principal object of the invention to provide a uni-directional antenna which is constructed to afford all of the above mentioned advantages of the helical beam.

Other considerations of the helical beam antenna are in the matter of gain, cost, and critical dimensions. At 450 megacycles, for which this invention is especially suitable, the three types of linearly polarized antennas mentioned heretofore afford good gain. The corner being of the order of 8 to 10 db, the yagi being even better, and the paraboloid exceeding even that. The gain through the use of the helical beam antenna is of the same order as that of the paraboloid.

The cost of installation of the corner reflector and the paraboloid, at least of such dimensions as to give performance comparable to that of a given size of helical antenna is considerably greater than the cost of installat-ion of the helical antenna. Even the yagi requires expensive feed lines, and systems which are anything but simple. All of these antennas require feeding by means of dipole or modified linear radiators. Transformation from the unbalanced mode of the almost universally used coaxial cable and the feed balanced dipole is an expense and complication which are also eliminated by the construction of my invention. The helical beamantenna presents substantially a natural termination for the coice '2 axial cable transmission line feeding the same in the construction described hereinafter. The outer conductor of the cable is connected to the ground plane, and the helix itself is a continuation of the central conductor.

The Wave on the helix is a travelling wave and hence attenuates appreciably by the time itv reaches the end of the helix. There is very little reflected wave, therefore, and the coaxial cable line has practically constant terminal impedance.

The provision of such advantages of the helical antenna over others are other objects of the invention.

A most important consideration in using antennas of the kind discussed hereinabove is that the dimensions of such antenna are critical, making them difficult to manufacture, maintain, and mount, but more important the existence of ice built up upon these antenna not only distorts their pattern but substantially lowers their efficiency. As an example, the yagi, which incidentally has less band width than either of the other two linearly polarized antennas and certainly less than the helical antenna, can lose as much as 50% efficiency through build-up of snow and ice. The helical antenna is characterized by a complete lack of critical dimensions and adjustments. It has wide band width, high operating efficiency, high radiation resistance, and its pattern and output are practically unaltered by staggering loads of snow and ice.

The use of helical beam antennas has been heretofore greatly limited because of the difficulties in constructing the same and mounting the same. cantilever structure of the antenna, proper non-metallic support capable of withstanding severe weather conditions has not been devised. It is therefore an important object of the invention to provide a structure which is simple, economical, durable and above all has great strength.

A further object of the invention is to provide a helical beam antenna which is formed of simple and easily obtained parts, to provide an antenna construction through the use of which the antenna is rendered totally weatherproof, and to provide an antenna which will at the same time have all of the advantages enumerated above.

Another important object of the invention is to provide a novel method for manufacturing the antenna of the invention in a speedy and economical manner.

Many other objects will occur to those skilled the art to which the invention appertains. For enabling a full :and clear understanding of the invent-Len, its structure, assembly, use and mode of construction and practice I have illustrated a preferred embodiment and described the same in considerable detail from which it will become apparent that I have made a substantial and important advance in the arts and sciences.

In the drawings:

Fig. l is a perspective view of an installation of -a helical beam antenna constructed in accordance with my invention.

Fig. 2 is a side elevational view of the same with portions broken away to show the interior construction thereof.

Fig. 3 is a sectional view on an exaggerated scale to show the manner in which the conductor is laid upon the core of the antenna.

Fig. 4 is a sectional view taken generally along the line 44 of Fig. 2 and in the direction indicated.

Fig. 5 is a diagrammatic sectional view showing the connection between the coaxial cable and the antenna.

Fig. 6 is a perspective view showing the antenna in Due to the inherent.

The problems attendant upon point to point communications at relatively high frequencies are capable of solution through the use of helically polarized antennas, but until the advent of the invention herein the use of such antennas were impractical. This was true principally because of difficulties of construction, mounting and support. My antenna has been developed principally in connection with use in the so-called citizens frequency band, i. e., from 450 to 470 megacycles, but is applicable to other frequencies as well. As a matter of fact, tests which I have conducted on antennas specifically designed for 450 megacycles show that the side lobes and non-uni-directional effects (i. e. spurious responses) are substantially negligible between 390 megacycles and 600 megacycles.

I have provided a novel method of producing an antenna of unique construction which will render practical the use of helical beam antennas. As for the article of manufacture, it consists of a cylindrical core member of cellular plastic having its axis coincident with the line of propagation, having the conductor of flexible braid enwrapped about the surface thereof in helical formation, and the whole being sealed in a cylindrical structure of layers of fiberglass impregnated with resin. The whole is mounted upon a base plate and provided with suitable supports, the transmission line being connected to the back of the base plate with its outer conductor electrically connected to the base plate and its inner conductor connected directly to the antenna element. As for the novel method of manufacture, it consists of mounting the cylindrical core upon the plate and placing the assembly upon a lathe-like rotating mandrel, cementing one end of an elongate sheet of glass fibre cloth upon the surface of the cylinder with bonding resin, and then slowly rotating the mandrel and hence the cylinder while enwrapping the sheet about the same, the operator in the meantime applying the resin coating with a brush, much like glue. After this, the assembly is cured in a mold. A cap of resin or resin impregnated fiberglass may be applied to the end either before or after the sheet is enwrapped upon the cylinder, and is cured in the mold also.

Referring now to the figures, in Fig. 1 there is illustrated an antenna which is mounted for uni-directional service upon a pole or tower 11. The direction of propagation or reception of the antenna 10 is coincidental with the axis of the cylinder comprising the antenna as will shortly be explained. The antenna 10 comprises a central elongate cylindrical core 12 which has its left end as viewed in the figures engaged within the upstanding cylindrical ring 13 secured to an annular base plate 14. The base plate and ring may be integrally formed by moulding or may merely consist of a shaped angle iron or strap. The plate 14 may be riveted or welded to a relatively large rectangular member 22 comprising the antenna ground plane and hereinafter referred to as such. Obviously this ground plane 22 is formed of metal. The preferred arrangement need not have the base plate 14, the ring 13 welded to the ground plane plate 22 being satisfactory.

Wound about the core 12 is a length of conductor 17 which is preferably formed of pliable metal braid of the conventional construction useful for shielding purposes. This conductor provides a flexible easily worked material which has good current carrying capacity, and which will lie flat. I form the conductor 17 into a plurality of equidistant helical convolutions, having a sense which depends upon the system to be used. For example, in the example shown, looking out from the antenna along the line of propagation, or reception (since the identical construction is used in both cases), the conductor 17 is wound to the right or clockwise going out. The antenna is therefore intended to be used with a mate which has its conductor similarly wound.

In the particular embodiment illustrated, the antenna 4 was cut for 450 megacycles, and there were six turns of braid of 14 pitch, so that the overall length of the entire structure, including end cap, to the ground plane was approximately 29 inches. The total diameter of the antenna was about 6 /2 inches, the ground plane being formed of quarter inch aluminum 16 inches square. The braid was of copper, normally inch in diameter.

The core 12 is formed of a cellular or foam plastic, of extremely light weight and easily worked. This material has excellent dielectric properties for use in antenna constructions. This core is approximately 6 inches in diameter for the example described, and preferably is a bit more than that when the outer crust which is molded over the core is in place, the thickness of the crust is about Ms inch. In the figures this thickness is exaggerated in order better to illustrate the construction.

The outer crust 19 comprises a multiplicity of layers of fiberglass fabric or cloth, several of which are illustrated at 20, well-impregnated with a liquid thermosetting plastic resin, preferably of the polyester type, so that the convolutions of the conductor 17 are embedded in the crust and the entire core and ring 13 likewise sealed against the atmosphere. An end cap 21 of plastic, either formed of layers of impregnated fiberglass or of the resin itself is provided. Many methods of moulding the antenna 10 may be utilized, but I prefer a novel process which will hereinafter be described.

The ground plane 22 mounts a coaxial connector 23 which is of the type commercially known as type N. It enables a coaxial transmission line 24 to be led to the antenna and suitably connected thereto. In Fig. 5 there is illustrated diagrammatically the simple connection between the antenna and the coaxial cable. For convenience the connector is not shown. The coaxial cable includes an outer insulating cover 25 and an outer braided conductor 26 just inside the insulating cover. The central conductor 27 is embedded in the central insulating layer 28. The central conductor 27 extends through the ground plane plate 22 and is insulated therefrom as indicated by the insulating member 29 and is electrically connected at 30 to the inner end of the antenna conductor 17. Note that the opposite end of the antenna conductor 17 is not connected to anything but merely poised in free space. There is a bond between the conductor 26 and the ground plane plate 22 as indicated by the connection 31.

The plate 22 has swivel brackets 34 bolted thereto by means of which the entire assembled antenna may be secured to the pole 11, as for example by shackles, tie-rods, bolts, etc. Obvious means of securement will suggest themselves to the artisan. It is only of importance that since the antenna is provided with a simple and rugged ground plane plate 22 which serves as mounting means for the physical antenna, considerable savings in effort and expense is effected in installing the antenna.

In manufacturing the antenna, I have devised a novel and effective method which may be practiced by workmen not possessing a high degree of skill, and yet the resulting product is of excellence which would be achieved through the use of more expensive methods, by diemoulding, for example. First, the ring 13 is welded to the ground plate 22. This ring may or may not have the base plate flange 14. Next, the connector 23 is mounted on the plate 22 and a proper length of braid 17 is measured out and soldered to that portion of the connector 23 which extends through the plate. For example, as shown in Fig. 5, the braid end is soldered as at 30 to the conductor 27. In the 450 megacycle antenna which I have described, the braid will be approximately 9 feet long. Thereafter, the core 12 is forced into the ring 13 and the helix is wound on the core. The size of the ring 13 is preferably chosen so that there is a force fit of the core within the ring.

The helix may be cemented in place or held upon the Surface of the cylindrical core 12 by a pressure sensitive tape, or it may be mounted in grooves formed in the core, the latter expedient not being necessary, but of possible convenience.

As thus far described, the method is fairly obvious from the structure of the antenna, but at this point the thus far described assembly is treated in a novel manner. The plate 22 is bolted to a mandrel 40 mounted on a rotatable head 41. This could be for example the head end of a lathe. By the use of a wheel 42 or power, the mandrel 40 is rotated thereby rotating the core 12. A length of fabric of glass fibres is then started upon the core. The length is illustrated at 43 and it is out so that when completely enwrapped about the core, the desired final diameter will be attained. The leading end 44 is laid down and cemented in place by the use of the plastic resin that is used for bonding and impregnating the fabric. The mandrel 40 is slowly turned while the length of fabric 43 is laid upon the core, enwrapping the same. As this process proceeds, the resin in fluid form is literally painted upon the core and the fabric with a brush, this being illustrated generally at 45. The resin is liberally applied and the fabric tightly wrapped so that the resin will ooze through the interstices of the fabric and completely impregnate the same. When the crust is cured, the result is substantially an integral wall.

Simultaneously with applying the layers 20, the end cap 21 may be applied, or if desired, this can be done in the next step. The antenna is removed'from the mandrel 40, and a mold such as the hinged device 47 is engaged upon the cylindrical portion. The halves of the mold are tightened and the mold permitted to remain in this condition until all of the resin has hardened. The end cap may merely consist of a plug formed by filling the upper end of the mold with fluid resin once the antenna has been placed therein. A flat plate 48 engaged upon the end and pressed upon the cap will provide a smooth end surface.

It will be noted from the figures that the ring 13 is moulded into the interior of the antenna so that any danger of leaking at this point is eliminated. This is done simply by having the fabric layers cover the same so that the crust formed after curing of the resin engages over the ring.

It is believed that the invention has been sufficiently described such as to enable those skilled in the art to 6 which the same appertains to construct and use the same. Many variations are possible without in any way departing from the spirit or scope Otf the invention as defined in the appended claims.

What is claimed by Letters Patent of the United States is:

1. A helical, circularly polarized antenna horizontally arranged and cantilever mounted with no metallic members along its electrical length, said antenna comprising a vertically planar metallic base plate and means for securing said base plate to an upright, a circular flange member having a central cavity secured to said base plate with its axis horizontal, an elongate cylindrical core member of cellular plastic having one end in said cavity and being coaxial with said flange member whereby to protrude perpendicularly from said base plate with its opposite end unsupported, a flexible conductor helically engaged about the core member surface along the length thereof in configuration to provide helically polarized characteristics for said antenna along the axis of said core member, said core member, flexible conductor and flange member having a thin coating of thermosetting resinous plastic adhesively securing and bonding said core member and coating to said flange member and base plate rendering said antenna impervious to moisture While providing substantial cantilever support therefor, and a transmission line having one lead insulatedly extending through said base plate and electrically connected with the inner end of said flexible conductor and having a second lead electrically engaged with said base plate.

2. An antenna as described in claim 1 in which the flexible conductor comprises flattened metal braid.

3. An antenna as described in claim 1 in which said thin coating consists of glass fibres impregnated with a polyester resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,296,781 Farny Sept. 22, 1942 2,552,599 Stout May 15, 1951 2,575,377 Wohl Nov. 20, 1951 2,590,493 Berberich et al. Mar. 25, 1952 2,614,058 Francis Oct. 14, 1952 2,630,530 Adcock et a1 Mar. 3, 1953 2,637,533 Sheldorf May 5, 1953

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