Abstract:
An antenna comprising a plurality of axially extending, radially spaced, electrically conductive coaxial cylinders and mechanism electrically coupling opposite ends of said cylinders to the adjacent opposite ends of alternate adjacent cylinders.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an antenna, and more particularly to a physically shortened antenna including a conductor having a plurality of folds to permit operation within a limited space. 
     2. Description of the Prior Art 
     Antennas generally comprise a conductor or a system of conductors used either for radiating electromagnetic energy into space or for collecting electromagnetic energy from space. Since it is difficult to radiate large amounts of power efficiently in the lower frequency ranges with shortened antennas, it is especially important that as much of the available signal at the transmitter be converted into radiated energy as possible, and that as much of the radiated energy as possible be picked up at the receiver. 
     A half-wave length conductor is the simplest of the radiating elements. Considerable radiation occurs in this element because of its resonant characteristics and its ability to store large amounts of energy in induction fields. Resonance causes high voltage and high circulating currents and they, in turn, produce strong fields around the antenna. It should be understood that due to resistance of the wire, the movement of waves along an entire wire is somewhat slower than wave movement in space. Wave length on a wire, therefore, is slightly less than that of a wave travelling in space. Physically, an antenna is about 6% shorter than a half-wave travelling in space. This phenomena is sometimes referred to as &#34;end effect&#34;. 
     A quarter-wave grounded antenna is a common type of grounded antenna. This type is called a Marconi antenna as contrasted with a half-wave ungrounded dipole antenna, which is sometimes called a Hertz antenna. The input impedance to the Marconi antenna is approximately 37 ohms when the antenna is fed at its base. In addition, a quarter-wave Marconi is resonant and displays zero reactance as does a half-wave antenna. This assumes that the conducting plane is a perfect conductor. If it is not a perfect conductor, as is the usual case, some of the conditions just discussed would be altered. The conducting plane might be the skin of an aircraft or the frame of an automobile. With ground equipment, the conducting plane is the earth&#39;s surface or an artificial ground called a counterpoise. 
     Energy may be fed to an antenna in a variety of ways. When the exitation energy is introduced to the antenna at the point of high circulating currents, the antenna must be center-fed. When the energy is introduced at the point of maximum voltage, the antenna must be voltage fed. 
     A folded dipole is a full length conductor that is folded to form a half-wave element. It consists of a pair of half-wave elements connected together at the ends. In it, the voltage at the ends of each element, must be the same. In operation, the field from the driven element induces a current in the second element. This current is the same as the current in the driven element. 
     Quarter-wave vertical antennas are usable over a narrow band of frequencies around resonance. When it is necessary to modify the electrical length, the electrically short antenna may be lengthened by adding series inductance; the electrically long antenna may be shortened by adding series capacitance. Although the feed impedance or radiation resistance of a quarter-wave length vertical grounded antenna is 36 ohms, higher impedance is available by advancing upwards from ground potential along the length of the antenna. Thus, one point might be a feed point for a 36-ohm line, while a point somewhat higher up along the antenna would serve as a feedpoint for a 52-ohm line. Of course, ground is one of the connections for the transmission line in both instances. 
     With the conventional antennas utilized heretofore, available space may preclude their use. Vertical grounded antennas have been &#34;shortened&#34; by adding an electrically conductive sphere at the upper end thereof to provide additional capacitance. The use of such a &#34;capacitance ball&#34; does not always provide a satisfactory alternate. 
     Although in some instances, it is important that antennas, such as radar antennas, have directivity, i.e., an antenna that radiates more energy in one direction than another, other applications require that the antenna be non-directional, isotropic or omni-directional. Accordingly, it is an object of the present invention to provide a new and novel omni-directional antenna. 
     A further object of the present invention is to provide an antenna, including a plurality of cylindrical antenna sections which are selectively coupled in series circuit relation to provide a variable length antenna having a selected length which is a multiple or sub-multiple of a quarter-wave length at a selected operational frequency. 
     Still another object of the present invention is to provide an antenna, including a plurality of folded antenna sections which can be selectively coupled to provide a total length of antenna which is a multiple or sub-multiple of a quarter-wave length of the operating frequency. 
     A still further object of the present invention is to provide an antenna of the type described, including mechanism for mechanically controlling the length of the antenna by means of telescoping antenna sections. 
     Still another object of the present invention is to provide an antenna, including a plurality of variable length antenna folds which can be tuned to different frequencies by changing the physical lengths of selected folds. 
     A further object of the present invention is to provide an antenna, including a plurality of coaxial, radially spaced, axially extending, current conducting, different length cylinders which are selectively coupled together. 
     A still further object of the present invention is to provide an antenna, including a plurality of radially spaced antenna elements each including a pair of radially spaced antenna segments each of which includes telescoping cylinders which can be adjusted to tune the antenna to different wavelengths. 
     Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds. 
     SUMMARY OF THE INVENTION 
     An antenna comprising a plurality of axially extending, radially spaced, elongated electrical conductors each having a circumferential extent different than the circumferential extent of the adjacent conductors, and mechanism electrically coupling opposite ends of each conductor to the adjacent opposite ends of alternate adjacent conductors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may more readily be understood by reference to the accompanying drawings, in which: 
     FIG. 1 is an axial sectional view, taken along the line 1--1 of FIG. 2; 
     FIG. 2 is a top plan sectional view, taken along the line 2--2 of FIG. 1; 
     FIG. 3 is a schematic electrical diagram of the antenna illustrated in FIGS. 1 and 2; and 
     FIG. 4 is a schematic electrical diagram of a slightly modified arrangement of the antenna illustrated in FIGS. 1-3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now more particularly to the drawing, an antenna constructed according to the present invention, generally designated 10, includes a plurality of radially spaced, axially extending, co-axial antenna elements, generally designated 12, 14, 16 and 18. The radially outermost antenna element 12 of antenna 10 is mounted on an insulated plate or base 11. Each of the antenna elements 12, 14, 16 and 18, include a pair of radially spaced, variable length, antenna segments designated by the numerals 12a, 12b; 14a, 14b; 16a, 16b; 18a, 18b, respectively. 
     It should be noted that the axial lengths of antenna segments 12a and 12b are equal; the axial lengths of antenna segments 14a and 14b are equal; and the axial lengths of antenna segments 16a and 16b are equal; and the axial lengths of antenna segments 18a and 18b are equal. 
     The antenna segment 12a includes a pair of radially outer, electrically conductive, hollow, right circular tubular cylinders 20 and 22 telescopically mounted in sliding engagement. The cylinder 20 is mounted on the insulated base 11. The radially inner antenna segment 12b includes a pair of electrically conductive, hollow, axially extending reduced diameter right circular tubular cylinders 24 and 26 telescopically mounted in sliding engagement. 
     The antenna segment 14a includes similar telescopically mounted tubular cylinders 28 and 30 which are substantially identical to the cylinders 24 and 26 respectively, with the exception that the diameters of cylinders 27 and 30 are less than the diameters of cylinders 24 and 26 respectively and the axial length of cylinder 28 is greater than the axial length of the cylinder 24. The antenna segment 14b includes telescoping cylinders 32 and 34 which, except for a reduced diameter, are identical to the cylinders 28 and 30 respectively. 
     The antenna segment 16a includes similar tubular telescoping cylinders 36 and 38 which are identical to the cylinders 32 and 34, with the exception that the diameters of cylinders 36 and 38 are less than the diameters of cylinders 32 and 34 respectively and the axial length of the tubular cylinder 36 is greater than the axial length of the cylinder 32. The antenna segment 16b includes a similar further reduced diameter tubular cylinders 40 and 42 which are otherwise identical to the tubular elements 36 and 38 respectively. 
     The antenna segment 18a includes tubular cylinders 44 and 46 which are identical to the tubular cylinders 40 and 42, with the exception that the diameters of cylinders 44 and 46 are less than the diameters of tubular cylinders 40 and 42 respectively and tubular cylinder 44 is longer than the tubular cylinder 40. 
     The antenna segment 18b includes tubular cylinders 48 and 50 which with the exception of a still further reduced diameter are identical to the tubular cylinders 44 and 46. 
     The cylinders of the respective antenna elements 12, 14, 16 and 18 are coaxially mounted on the base 11 about a longitudinal axis a. The material for the antenna elements which can suitably comprise copper should be chosen from such stock that the base impedance should be close to 52 ohms at the resonant frequency. 
     The lowermost (as illustrated in FIG. 1) axial end 52 of the radially outermost cylinder 20 is coupled to a source of high frequency signals 54 via a conductor 56. 
     The lower axial end 58 of the tubular cylinder 24 is coupled to the lower end 60 of the radially inner, adjacent tubular cylinder 28 via an annular, electrically conductive ring 62. The lower axial end 64 of the tubular cylinder 32 is coupled to the lower end 66 of the tubular cylinder 36 via an electrically conductive reduced diameter annular web or ring 68. The lower axial end 70 of the tubular cylinder 40 is coupled to the lower axial end 72 of the cylinder 44 via an annular, electrically conductive web 74. 
     The upper (as illustrated in FIG. 1) axial ends of the coaxial cylinder 22 and 26 are electrically coupled via an electrically conductive annular ring 76. The upper axial ends of the coaxial cylinders 30 and 34 are coupled via a reduced diameter annular, electrically conductive ring 78. The upper axial ends of the cylinders 38 and 42 are electrically coupled via an electrically conductive, still further reduced diameter annular ring 80. The axially upper ends of the cylinders 46 and 50 are coupled via a still further reduced diameter electrically conductive annular ring 82. 
     The coupling rings 62, 68, 74, which may suitably comprise electrically conductive material such as copper, serially couple the antenna elements 12, 14, 16 and 18, which may also be manufactured from electrically conductive material such as copper, in series circuit relation. The coupling rings 62, 68, 74, 76, 78, 80 and 82 are so arranged that the opposite ends of the coaxial, radially spaced antenna segments 12b, 14b, 16b and 18b are connected to the adjacent opposite ends of the next adjacent antenna segment. For example, the coupling ring 76 couples the upper end of antenna segment 12b to the upper end of the antenna segment 12a, whereas the coupling ring 62 couples the lower axial end of the antenna segment 12b to the lower axial end of the antenna segment 14a. This arrangement serially couples the antenna segments 12a, 12b, . . . 18a, 18b to provide a continuous antenna element. 
     The axial lengths of the antenna segments 12a, 12b, . . . 18a, 18b are such that when coupled in circuit as illustrated in FIG. 1, the current will flow in the axial paths represented by the arrows 84-92 and the total axial, current conducting length of all of the antenna segments 12a, 12b, . . . 18a, 18b may be substantially equal to one-eighth of the wave length of the lowest operating frequency provided by the signal source 54. The lowermost end 52 of the copper cylinder 20 will be the high current point and the lowermost end 94 of the innermost cylinder 48 will be the high voltage point in the circuit. If desired, a small condensor 95, illustrated in chain lines in FIG. 1, may be coupled between the high voltage point 94 and the ground plane. 
     The effective electrical length of the antenna may be mechanically adjusted by merely axially sliding any selected ones of the pairs of cylinders 22, 26; 30, 34; 38, 42; and 46, 50. 
     Referring now more particularly to FIG. 3, the effective length of the antenna 10 may also be shortened by use of any ones of a plurality of electrically conductive, annular shorting rings 96, 97 and 98 which are selectively inserted into current connecting relation between the lower (as illustrated in FIGS. 1 &amp; 3) axial ends of the cylinders 28 and 32; cylinders 36 and 40, cylinders 44 and 48 respectively. In FIG. 3, all of the shorting rings 96, 97 and 98 are illustrated as being installed between the lower ends of cylinders 30, 34; 38 and 42; and 46, 50 respectively so that the current travels along the paths represented by the arrows 84, 85, 86, 99, to the terminal end 94. When shorting rings 96, 97 and 98 are coupled in circuit as illustrated in FIG. 3 to short circuit the antenna elements 14, 16 and 18, only the antenna element 12 is operative to radiate a signal. When the circuit elements are connected as illustrated in FIG. 1, the current conducting antenna segments 12a and 12b are of such axial length that the total axial length of the current conducting path represented by the arrows 84 and 85 is substantially equal to one-half of the wave length of the shortest operating frequency of signal source 54. If, only the shorting ring 98 is utilized, the total axial length of the antenna segments 12a, 12b, 14b and 16b is such that the radiating element will be substantially equal to one-half of an intermediate operating frequency of the signal source 54. 
     The antenna 10 thus provides a radiating element which has a selectively variable wavelength and can be utilized in a much smaller &#34;window&#34; than could a conventional, grounded quarter-wave length antenna. 
     Since the cylindrical segments 12a, 12b, 14a, 14b, 16a, 16b, 18a and 18b are coaxial, the antenna will be omni-directional or will radiate equally in all directions. Accordingly, the antenna constructed according to the present invention is particularly well adapted for use where space is a prime factor, such as in automobiles. The antenna constructed according to the present invention provides an antenna which operates over a broad frequency range and is yet relatively inexpensive to manufacture. Tuning and loading of the antenna is effected by changing the physical length of selected antenna segments or by progressively shorting selected antenna segments, or both, depending on the frequency or application. 
     FIG. 4 illustrates a dipole arrangement including two antenna halves each identical to the antenna section illustrated in FIGS. 1-3, the innermost high voltage, low current conducting point 94 is coupled to the high voltage point 94 of the opposite identical half of the dipole via a condensor 95. 
     THE OPERATION 
     It will be assumed that the source 54 is operating at the lowest frequency. The circuit elements will be positioned as illustrated in FIG. 1 so that the total axial length of the current conducting path through the serially coupled antenna elements is substantially equal to a quarter wavelength of the operating frequency. 
     If the operating frequency is slightly increased, one or more of the pairs of cylinders 22, 26; 30, 34; 38, 42; and 46, 50 may be moved downwardly (as illustrated in FIG. 1) to reduce the total axial, current conducting length of the antenna 10. 
     If the operating frequency is still further increased, the shorting ring 98 may be installed between the lower ends of cylinders 44 and 48, as illustrated in FIG. 3, to short circuit the antenna elements 18. 
     If the source 54 is operating at the highest operative frequency, the shorting rings 96, 97 and 98 are positioned as illustrated in FIG. 3 to short circuit antenna elements 14, 16 and 18 and the cylinders 22 and 26 are axially moved downwardly to the position illustrated in chain lines in FIG. 1 so that the current flows through the shortest axial path along only antenna segments 12a and 12b. 
     It is to be understood that the drawings and descriptive matters are in all cases to be interpreted as merely illustrative of the principles of the invention, rather than as limiting the same in any way, since it is contemplated that various changes may be made in various elements to achieve like results without departing from the spirit of the invention or the scope of the appended claims.