US2681412A - Whip antenna structure - Google Patents

Description

`lune 15, 1954 M. WEBSTER WHIP ANTENNA STRUCTURE Filed Jan.k 29, 1951 ATTORNEYS Patented June 15, 1954 UNITED STATES PATENT OFFICE 5 Claims.

This invention relates to radio antennas of the whip type, such as are used in connection with vehicular radio equipment of many types, but particularly transmitting equipment.

There are numerous radio applications where whip antennas are the only type of receptor or radiator of radio signals which can be viewed practically. This is the case not only with automobile radio equipment, such as police radio, but also with some types of radio equipment for small craft, submarines, and even aircraft radio for some wavelengths.

Whip antennas in these services are subject to rapid depreciation. Not only are they likely to strike obstacles, especially in the case of automobile radio equipment, but they are subject to violent vibration which fatigues the metal of which they are formed and renders them liable to breakage under stresses which, when new, they could easily bear. The duty upon these structures is so severe that some police departments have reported that the normal life of a transmitting whip antennaJ is in the neighborhood of only about two months.

The purpose of this invention is to provide a whip antenna which will substantially obviate the difficulties mentioned. Pursuant to this broad purpose, among the objects of the invention are to provide a whip antenna structure which is sufciently stiff to remain Substantially erect during normal motion of the vehicle to which it is attached but is resilient and flexible enough to withstand exure through 90 or more; to provide an antenna structure which will not take a permanent set following flexures of the magnitude mentioned; to provide an antenna structure which is substantially immune to fatigue due to mechanical vibration; to provide a structure which can be rigidly mounted to a vehicle, without the necessity for shock mountings of any kind; to provide an insulated antenna structure which will prevent high frequency burns in case of accidental contact therewith when it is excited but which does not, at the same time,

occasion measurable losses in transmitting efciency; to provide an antenna which is not subject to warpage under eXtreme temperature changes and to provide an antenna which will not rattle or chatter underextreme mechanical vibration.

The antenna of this invention comprises an outer tubular sheath, preferably tapered, formed of glass ber cloth impregnated with thermosetting plastic. Longitudinally within this sheath is a central `conductor of materially smaller diameter than at least a major portion of the bore of in the internal diameter of the sheath at the eX- treme tip without harm. The conductor is preferably of resilient material-semi-sprng brass is excellent for the purpose-and longer than the sheath, and is bent so that it contacts opposite walls of the sheath at intervals along the length thereof. Within the sheath and embedding the conductor is a lling of plastic, which is also preferably of the thermosetting variety, but which has a considerable degree of pliability. Various plastics are suitable for the purpose, as will be set forth more specifically hereinafter. The butt end of the antenna is preferably connected, as, for example, by hard soldering or brazing, to a metal fitting including a collar within which the sheath is cemented and which is provided with exterior screw threads or other means for mounting to the vehicle and connection with the transmitter or receiver with which the antenna is to be used. The tip end of the conductor may either be left flush with the end of the sheath, slightly embedded Within the plastic at the end of the sheath, or, preferably, may be provided with a small ball or knob for distribution of electrical stresses and prevention of the formation of corona which might occur at any sharp edge when the antenna is excited by a relatively high powered transmitter.

In the drawings, Fig. 1 is a longitudinal crosssectional view of a preferred form of the device of this invention;

Fig. 2 is a similar longitudinal sectional view of a modication thereof; and

Fig. 3 is a section through Fig. 1 on the line 3 3.

Considering rst Fig. 1, a sheath I, which provides the mechanical strength for the device, is formed of a number of layers of glass fibre cloth impregnated with a thermosetting resin and consolidated under heat and pressure. The material and the mode of manufacture are similar to those of the so-called glass fishing rods which are obtainable from various sources. Sheets or strips of the glass fibre cloth are wound tightly upon a tapered mandrel and are subjected to the treatment suited to the particular resin chosen for impregnation. The resins used for this purpose may be of the well-known phenolic type, but other thermosetting resins are even preferable for the purpose, such as Paraplex P-43, which is a proprietary product the composition of which has not been publicized. The composition, however, is sold by Rohm &

e Haas Co., of Philadelphia, Pennsylvania, under the indicated trade name.

The dimensions of the sheath will, of course, vary with overall size of the antenna to be used.

A typical whip, eight feet in length, such as is' illustrated in Fig. l, for instance, may have a butt diameter or" l" outside and a tip diameter' of 9g". The walls may be of the uniform thickness of about 116 or they, too, may be slightly tapered in thickness from butt to tip. Extending longitudinally within the sheath is the conductor 3. For the size antenna described this may be aas" in diameter, so that it nearly iills the bore within the sheath at the tip that is materially smaller than the internal diameter of the base. For best results the rod should be longer than the sheath so that it may be bent to contact opposite sides of the sheath alternately under intervals. The preferable material for the conductor is of semi-spring character. A preferable mode of bending is in the zig-Zag form shown in Fig. l. In forming the conductor, before it is inserted Within the sheath, it may be bent in this form, the angles, however, being greater than those shown so that it will be stressed and press firmly against the inner sides or walls of the sheath after it is inserted. It Will thus be held by its own resiliency so that on the average it is centered Within the sheath.

The reason for forming the central conductor as thus described lies primarily in the very great difference ybetween the coeiiicients of thermal expansion of the sheath and the central conductor. The coefiicient of the glass and resin sheath is relatively small in comparison to that oi the metal conductor. It is practically impossible to hold a straight conductor, of the same nominal length as the sheath, in a strictly axial position in the course of the iilling operation next to be described. If it is attempted to do so there `will almost certainly be some lack or symmetry and diierential expansion will either cause large stresses to be set up within the structure or will cause it to warp or bend with changes in temperature. Where such changes are moderate this may not be noticeable, but where they are extreme, as is frequently the case in vehicular equipment of this character, the exures caused are at best unsightly and at worst may so change the position of the antenna with respect to its surroundings as to cause changes in its electrical characteristics.

In the curing process, which is done at about 90 F. and which is a necessary phase in the operation to initiate the chain reaction in the resin curing, the internal conductor 3 expands at such a different rate than the sheath that kinks occur in the absence of the Zig-zag (or the spiral or helical, later to be described) formation of the central conductor. The shaping is adequate to provide the spring action necessary to maintain the desired structural formation.

Filling the sheath and embedding the conductor is a mass of pliable thermosetting plastic 5. There is a considerable choice available for the plastics used. A mixture of polyethlene and polystyrene in the proportions of about 3 to l will give the necessary degree of pliability; such a mixture has been used in solid dielectric coi axial cables for radar use and such cables may be ratio of 3 to l being satisfactory. Other resins having the same or approximately the same mechanical and electrical characteristics would be suitable. The necessary feature is that they should not be brittle; under the duties to which an antenna of this character is subjected a hard resin such as the unmixed styrenes or the unmixed P-43 resin mentioned above will very quickly be reduced to a granular form. The mixtures mentioned, however, will permit the relative motions of sheath and interior conductor without damage. While they are firm to the touch they will compress and permit elastic flow oi Athe iilling even ii the composite structure is bent into semi-circular form. Both the small diameter and the Zig-zag structure of the conductor permit it too to withstand such a iiexure without taking a permanent set, while if a solid conductor which iilled the entire bore of the sheath were used such a ilexure would destroy its usefulness. The iilling also prevents vibration o the conductor independent of that of the sheath. Such independent vibration can become very noisy as well as leading to eventual breakage, particularly adjacent the ends of the structure.

Preferably, although not necessarily, the end of the conductor is finished with a terminal knob or ball not only to give a finished appearance but 'to prevent the formation of corona at the sharp tip of the conductor. The ball may be threaded or soldered in place.

The butt end oi the structure is provided with a fitting 9, preferably of brass although other materials may be used. Such a iitting Will comprise a deep collar i! having an inside diameter such as to accommodate the butt of the sheath which is cemented therein with an adhesive suitable for use with the plastic used for impregnation of the glass nbre cloth, e. g., Bakelite cement if the phenolic is used for impregnation of the cloth or the same material which is used for iilling the sheath if one of the Paraplex resins is employed for the purpose. The 4fitting is preferably provided with a flange i3 for bearing against a suitable mounting bracket and terminates in a threaded stub I5. The conductor 5 is electrically connected to the fitting by soldering or brazing in a central hole in the stub end of the fitting.

The modication of the invention illustrated in Fig. 2 is substantially identical with the preferred form shown in Fig. l except as to the formation of the conductor 3', of a tapered helix. The reference characters used for the other parts of the structure are therefore the same as those employed in Fig. l. The form oi the device shown in Fig. 2 possesses no material advantage over that shown in Fig. l and is somewhat more diicult to construct. The showing is included, however, as indicating that the zig-zag form of the central conductor is not the only one which will maintain the average central position of the conductor which is desirable and would permit flexure of the antenna without permanent deformation.

While the plastics mentioned above havebeen referred to as thermosetting" it is to be understood that this term includes those which will, with the aid of catalyzers such as the peroxides, set at room temperatures. The use of such catalytic curing processes is, in fact, frequently desirable. The important feature about the plastics used for the construction oi the device is that they should not either liquefy under elevated temperatures or become unduly brittle at the low temperatures to which the device may be subjected as would be the case were any of the thermal-plastics known to me to be used. Antenna structures of the type here set forth may, under extreme conditions of direct sunlight and a hot climate, reach temperatures of between 125 and 150 F. or, on the other hand, be subjected to temperatures of 40 F. or more below 0 F, As constructed in accordance with the description here given the antenna of this invention will withstand such extremes.

An eight-foot antenna of the dimensions given will withstand bending into semi-circular form and will return to its original shape without taking a set. It is impervious to moisture and it will withstand extreme degrees of vibration, including vibration of its own mechanical resonant frequency, Without harm.

1 claim:

1. A whip antenna structure comprising a tubular outer sheath of glass fiber cloth impreg.. hated with a thermosetting plastic, a conductor of relatively small diameter with respect to at least a vmajor portion of that of theinside diameter of sheath extending generally longitudinally within said sheath for the entire length thereof, and a lling of pliable plastic within said sheath embedding said conductor.

2. A whip antenna structure comprising a tubular outer sheath of glass fiber cloth impregnated with a thermosetting plastic, a conductor of relatively small diameter with lrespect to at least a major portion of that of the inside diameter of sheath extending generally longitudinally within said sheath for the entire length thereof, said conductor being formed of resilient material and bent to contact opposite sides of said sheath at intervals therealong, and a filling of pliable plastic within said sheath embedding said conductor.

3. An antenna structure in accordance with claim 1 wherein said sheath is tapered from an inside diameter approaching the outer diameter of said conductor to a diameter materially greater.

4. An antenna in accordance with claim 1 wherein said inner conductor is of resilient material and is of zig-zag conformation so as to bear alternately on opposite sides of the interior of said sheath.

5. An antenna in accordance with claim 1 wherein said inner conductor is of resilient material and is of helical conformation so as to bear on all sides of said sheath.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,373,660 Clossom Apr. 1'?, 1945 2,456,330 Scott Dec. 14, 1948 FOREIGN PATENTS Number Country Date 490,414 Great Britain Aug. 9, 1938' 583,409 Great Britain Dec. 17, 1946

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