Multielement Radio-frequency Antenna Structure Having Linear And Helical Conductive Elements

Abstract

A multi-element antenna structure is provided which may be fabricated with an improved characteristic impedance for impedance matching purposes. The elements in an antenna include both linear and helical electrically conductive elements that are structurally supported and protectively encased in a dielectric material comprising a fiber-glass-reinforced synthetic resin matrix.

Parent Case Text

This application is a continuation of Ser. No. 53,062, filed July 8, 1970, now abandoned.

Description

BACKGROUND OF THE INVENTION

The antenna structure of this invention is primarily adapted to mobile installations for both transmitting and receiving functions such as citizens-band operations in connection with automotive vehicles although the antenna structure is adaptable to other frequency band allocations. In installations of this type, the antennas of prior art constructions comprises a single electrically conductive element effective as both a receiver and radiator of electromagnetic wave energy in the radio-frequency spectrum and is of a physical construction to accomodate the mechanical forces that may be applied as a consequence of vehicular movement. Antennas of prior art construction for mobile installations are most commonly an electrical quarter wave in length and metallic ranging in length from about 9 feet for 27 megahertz to about 6 inches for 470 megahertz. These antennas are usually vertically mounted and supported only at the bottom and are end-fed. Vertical quarter wave antennas of a length in the range of 9 feet are physically unwieldly; however, an antenna in the 1 -500 megahertz range may be physically shortened by adding inductance in series. Conversely, the physical length, commonly called aperture, may be increased by adding capacitance in series.

Quarter-wave antennas are desirable because, when end-fed, they approach resonance. Resonance is the state where inductance and capacitive reactances are equal, but, since they are of opposite sign, the resultant total impedance of the antenna is its resistance which includes both radiation resistance and loss resistance.

In two-way radio communications, the transceiver and antenna are connected with coaxial cable, and the most commonly used coaxial cable has a characteristic impedance of 52 ohms. The output stage of the transceiver is adjustable to 52 ohms. However, the terminal impedance of an end-fed quarter wave is well below 52 ohms, perhaps as low as 15 ohms. Maximum power transmission results when the terminal impedance of the end-fed quarter wave antenna matches the impedance of the coaxial transmission line. With the single element antennas of prior art the impedance mis-match is great enough to seriously impede the efficiency of power transferral. Some installations are operated inefficiently with this mis-match, while other installations rely on complex impedance matching networks to correct this mis-match.

BRIEF DESCRIPTION OF THE INVENTION

The antenna structure provided by this invention comprises a multiplicity of elements including both linear elements and helical elements that are electrically interconnected to form a composite structure having the desired impedance at the design operating frequency of a particular antenna structure. This multiple element design enables construction of an antenna having a more advantageous impedance match with that of a connecting coaxial transmission cable. A desired nominal operating frequency within a frequency band throughout the 1 -500 megahertz spectrum is readily obtained for a particular antenna structure through appropriate selection of the component elements and physical configuration of each element while providing a wide band-width and maintaining a relatively high response and radiation characteristic for the entire band. A basic embodiment of the invention antenna comprises as the electrically conductive elements thereof, a combination of linear elements and helical elements with there being a plurality of either the linear elements or the helical elements. Other embodiments may comprise pluralities of both linear elements and helical elements in various combinations. A structurally supporting body is formed for the selected elements from a dielectric material such as a synthetic resin reinforced with strands of fiber glass with the completed structure capable of accomodating the structural or mechanical forces encountered in a mobile vehicular installation. These and other objects and advantages of this invention will be readily apparent from the following detailed description of embodiments thereof and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary elevational view partly in section, of an antenna structure embodying this invention.

FIG. 2 is a transverse sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a transverse sectional view taken along line 3--3 of FIG. 1.

FIG. 4 is a diagrammatic illustration of the electrical equivalent circuit of the antenna structure of FIG. 1.

FIGS. 5 and 6 are graphic representations of the frequency response characteristics of the elements forming the antenna structure.

FIGS. 7-11 are diagrammatic illustrations of the electrical equivalent circuits of modified antenna structures embodying this invention.

DETAILED DESCRIPTION OF THE INVENTION

Having reference to FIGS. 1-3 of the drawings, an antenna structure embodying this invention is illustrated in detail. This antenna structure comprises a multiplicity of electrically conductive elements, indicated generally at 10, encased in a structurally supporting body 11 that includes a central core 12 and an outer coaxially formed sheath 13 although the core 12 and sheath 13 are preferably integrally formed in fabrication of a composite antenna. In this embodiment, the four elements 15, 16, 17 and 18 are arranged in pairs thus forming two members which are effective, at the design radio frequencies, for radiation or reception of electromagnetic wave energy or two elements 15 and 16 are helically coiled to define an elongated cylinder and are each serially connected with a respective linear element 17 and 18.

The conductive elements 15, 16, 17 and 18 are preferably formed from small diameter copper wire, such as No. 24 A. W. G., to obtain the desired electrical characteristics and, consequently, the elements will not be structurally self-supporting. An antenna structure of this invention is particularly adapted to utilization with mobile vehicular installations and these installations are normally operated in the 1 -500 megahertz frequency spectrum where a quater-wave antenna will have a substantial length. In the case of equipment operating in the citizens-band frequency spectrum at the nominal operating frequency of 27 megahertz, the physical length for an electrical quarter-wave length will be of the order of 3 to 8 feet and it will be readily seen that this length precludes reliance on the structural strength of the conductive elements for structural integrity of the antenna structure. Accordingly, a structurally supporting body 11 is provided to adequately maintain the several conductive elements in a specific configuration and permit vertical mounting of the antenna on a vehicle. This body 11 is formed from a dielectric material which is a synthetic resin matrix having the necessary mechanical characteristics as to flexural strength and modulus for the specific design application to withstand the static and dynamic loads that may be encountered in vehicular installations and maintain the specific element configuration. Preferably, the synthetic resin matrix which may comprise a thermosetting polyester or epoxy, also includes strands of fiber glass 19 distributed throughout the body to enhance the mechanical properties of the antenna structure. These strands of fiber glass 19 are preferably oriented longitudinally of the antenna structure and are included in both the core 12 about which the helical elements 15 and 16 are wound and the outer sheath 13.

Interconnection of the antenna with radio installation, as well as mechanical support or mounting of the antenna, is accomplished by means of a mounting ferrule 20. The ferrule 20 is formed from an electrically conductive metal with a central socket 21 in which an end of the antenna structure is inserted and secured as by a suitable adhesive or bonding material. The terminal ends of the conductive elements 15 and 16 extend into an aperture 22 formed in the ferrule and are electrically connected to the ferrule as by a solder connection 23. The ferrule 20 is also provided with a threaded portion 24 for mechanical interengagement with a mounting socket (not shown).

The two helical elements 15 and 16 may be formed conveniently by simultaneously winding both elements on the core 12 which is performed in a preliminary step in the antenna fabrication process. Each turn of the helix is longitudinally spaced from an adjacent turn, a distance of the order or 1/16 inch in the 27 megahertz embodiment with the core diameter or internal diameter of the helical elements being of the order of 1/8 inch. The diameter of the outer sheath 13 may be of the order of 3/8 inch and will become integral with the core 12 during the thermosetting step thereby providing a unitary structure. At least one of the helical elements, 15 or 16, may be provided with a dielectric sheath 25 to assure electrical insulation of the two elements throughout their length except for the terminal ends electrically interconnected by the solder 23. This dielectric sheath 25 in the present embodiment comprises a suitable varnish; however, other well-known materials that do not provide electromagnetic shielding may be utilized. If desired, the dielectric sheath 25 may be omitted if the element spacing is otherwise maintained or both elements may be provided with a similar dielectric sheath.

Connected to each helical element 15, 16 at the end opposite the solder connection 23 is a respective one of the two linear elements 17, 18 which extend longitudinally in axial alignment to the helical elements. The linear elements 17, 18 are also electrically insulated from each other as by a dielectric sheath 26 which may also be varnish applied to one of the elements. These linear elements 17, 18 are disposed in spaced parallel relationship but are not electrically connected at their free ends thus preventing electric current circulation within the pairs of conductive elements 15, 17 and 16, 18.

Through selection of conductive elements 15, 16, 17 and 18 of appropriate cross-sectional area and length and through proper spacing of the elements, an antenna structure may be constructed having a predetermined value of antenna input impedance. In the usual mobile installation, the desired antenna input impedance for proper matching is 52 .OMEGA. as this is the impedance of the most commonly used commercially available coaxial transmission cable.

The antenna structure of this invention will preferably be a quarter-wave length for the specific design frequency band. One of the parameters controlling the physical length of an end-fed electrical quarter-wave antenna is the diameter of the conductor. For a given electrical quarter-wave, the physical length of the conductor decreases as the conductor diameter increases, but not as a straight line function. FIG. 5 illustrates this condition. Curve M shows the response of a conductor of a given diameter, while curve N is the response of a conductor of another diameter. FIG. 5 shows that their resonant frequencies are at different frequency values in the spectrum and illustrates how a multiplicity of conductors of dissimilar diameters broadens the effective band-width of an antenna.

FIG. 6 illustrates how conductors of different physical lengths have their maximum response at dissimilar frequencies when end-fed as quarter-wave antennas. Curves P and S represent conductors of different physical lengths, and their resonant frequencies may be widely displaced.

A slight difference in length of the pairs of conductive elements 15, 17 and 16, 18 is obtained in the illustrated embodiment where the two helical elements 15 and 16 are seen to be of dissimilar diameters. This results from the two elements being wound with the same pitch with the same internal diameter and this causes the pitch diameter which determines the lineal dimension to be different. The larger diameter element of 15 and 16 will thus be longer. A further change in length can be obtained through adjustment in relative length of the two linear elements 17 and 18.

Utilizing a combination of helical and linear elements permits fabrication of an antenna having improved impedance match through appropriate relative dimensioning of the helical and linear elements and also having a better resonance characteristic.

The central core 12 may also comprise a ferrite material having a magnetic permeability greater than unity to further enhance the electrical properties of the antenna structure. Ferrites such as iron carbonyls and magnetic iron oxide in particulate form may be embedded in and distributed throughout the resin matrix forming the core 12.

Other antenna structures embodying this invention may be constructed with various combinations of helical and linear elements. Several combinations are diagrammatically illustrated in FIGS. 7-11 which may be readily fabricated utilizing the previously set forth detailed construction principles relative to the form shown in FIG. 4 to obtain specific antenna characteristics. FIG. 7 illustrates a configuration opposite or inverted to that previously described with the linear elements being electrically interconnected and forming the end fed terminal connection. The configurations shown in FIGS. 8 and 9 are further variations of the embodiment described in detail in that the linear or helical elements may be formed in separate sections having the other elements interposed at an intermediate point. FIGS. 10 and 11 illustrate embodiments that include either a single helical element or single linear element while having a plurality of the other elements. In these embodiments, the single element is connected to an antenna terminal connection with the multiple elements being electrically interconnected at one end and to the single element. It will be apparent that combinations other than those illustrated may be fabricated such as having more than two conductive pairs or having multiples of the configurations shown in FIGS. 10 and 11.

It will be readily apparent that a novel antenna structure is provided which may be readily constructed with the desired impedance at the connector terminal for optimum matching with the impedance of an interconnecting cable. Utilizing conductive elements of dissimilar diameters and lengths widens the frequency response and a desired antenna characteristic is obtained through appropriate combination of linear and helical elements.

Claims

I claim:

1. A radio-frequency antenna structure comprising a linear element formed from an elongated, longitudinally extending electrical conductor and a plurality of helical elements axially disposed relative to said linear element, said helical elements being elongated electrical conductors formed into cylindrical helixes of the same pitch and internal diameter disposed in coaxial, side-by-side relationship with each element electrically insulated from the other throughout their length but all being electrically connected together at one end, said linear element series connected electrically at one end with said helical elements at their interconnected ends with the other end of said linear element forming a connecting terminal of the antenna, said helical and linear element conductors being of selected cross-sectional area to provide a desired antenna impedance, and a body structure formed from a dielectric material in which said linear and helical elements are embedded for support thereof in relatively fixed relationship said dielectric material having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristic to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure.

2. An r-f antenna structure according to claim 1 wherein said dielectric material is a snythetic resin matrix reinforced with strands of fiber glass extending longitudinally throughout the length of the antenna structure.

3. An r-f antenna structure according to claim 1 having a central core around which said helical elements are wound with said core formed from a material having a magnetic permeability greater than unity.

4. An r-f antenna structure according to claim 1 wherein said helical elements are of dissimilar cross-sectional area to provide a relatively greater effective band width at the nominal operating frequency.

5. An r-f antenna structure according to claim 1 wherein said helical elements are of dissimilar lengths to provide a relatively greater effective bandwidth at the nominal operating frequency.

6. A radio-frequency antenna structure comprising a plurality of linear elements of elongated, longitudinally extending electrical conductors disposed in spaced parallel relationship electrically insulated one from another throughout their length, a plurality of helical elements of elongated electrical conductors formed into cylindrical helixes of the same pitch and internal diameter disposed in coaxial, side-by-side relationship electrically insulated one from another throughout their length, said linear and helical elements relatively axially disposed with each of said linear elements series connected electrically with a respective one of said helical elements and having all of the series connected elements electrically interconnected at an end point forming an antenna terminal, said linear and helical element conductors being of selected cross-sectional area to provide a desired antenna impedance, and a body structure formed from a dielectric material in which said linear and helical elements are embedded for support thereof in relatively fixed relationship, said dielectric material having a relatively low loss characteristic as to electromagnetic wave energy and physical strength characteristic to maintain the physical configuration of the conductive elements and assure structural integrity of the antenna structure.

7. An r-f antenna structure according to claim 6 wherein said dielectric material is synthetic resin matrix reinforced with strands of fiber glass extending longitudinally throughout the length of the antenna structure.

8. An r-f antenna structure according to claim 6 having a central core around which said helical elements are wound with said core formed from a material having a magnetic permeability greater than unity.

9. An r-f antenna structure according to claim 6 wherein said linear elements are electrically interconnected together at one end to form said antenna terminal.

10. An r-f antenna structure according to claim 9 having a helical element intermedially interposed in each linear element.

11. An r-f antenna structure according to claim 6 wherein said helical elements are electrically interconnected together at one end to form said antenna terminal.

12. An r-f antenna structure according to claim 11 having a linear element intermedially interposed in each helical element.

13. An r-f antenna structure according to claim 6 wherein said linear and helical elements are of dissimilar cross-sectional area to provide a relatively greater effective band width at the nominal operating frequency.

14. An r-f antenna structure according to claim 6 wherein said series connected linear and helical elements are of dissimilar lengths to provide a relatively greater effective band width at the nominal operating frequency.