US 3268903 A
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Description (OCR text may contain errors)
3, 1966 J. A. KU-ECKEN ETAL 3268,9693
EXTENDIBLE DIPOLE ANTENNA 3 mm Cu m m mm w P u TU 4 i m D 3 7. 2 u A 4 G M 2 4 F m m U A m CHARLES G. COLONY INPUT q BY 44% A ORNEYS United States Patent 3,268,903 EXTENDIBLE DHPULE ANTENNA John A. Kueclren and Charles Gordon Colony, Cincinnati, Ohio, assignors to Avco Corporation, Cincinnati, Ohio, a corporation of Delaware Filed Dec. 12, 1962, Ser. No. 244,184 3 Claims. (Cl. 343-791) The present invention relates to extendible dipole antennas wherein the physical length of a dipole antenna can be adjusted for optimum operation at the transmission frequencies over a wide range of frequency and wherein the adjustment may be done in a simple manner without undue mechanical complications.
It is well known that optimum performance of a dipole antenna at a particular frequency is obtained by dimensioning the antenna to be tuned to the transmitted or received frequency. It -will be understood that antennas according to the invention will be useful either as transmitting or receiving antennas or both in accordance with the principle of reciprocity. Dipole antennas have of course been provided heretofore which were physically adjustable in length for optimum performance at various frequencies. Such arrangements have generally not been Widely used and have been disadvantageous either from the point of view of having poorer electrical characteristics than a fixed length dipole antenna or from the point of view of being unduly mechanically complicated and hence expensive and unreliable.
The present invention provides an extendible dipole antenna of novel construction which provides mechanical simplicity and electrical efiiciency. The antenna is not limited to any particular frequency range but is particularly useful in the range of frequencies from about 1 to 100 megacycles where a half-wave or full-wave dipole antenna would be of appreciable dimensions compared to the transmitter or receiver apparatus with which it might be utilized.
It is an object of the present invention to provide a dipole antenna tunable by adjustment of the effective length of its arms together with mechanism for simply and expeditiously adjusting the effective length of the arms simultaneously to substantially equal effective lengths.
It is a further object of the invention to provide an antenna of the foregoing type wherein the effective feed point at the center of the dipole antenna is the termination of a mulitiple conductor transmission line, 'a part of which also forms one arm of the dipole antenna.
It is a still further object of the present invention to provide an antenna of the foregoing type in which a choke is provided for isolating the dipole arm formed by the transmission line from ground potential where one of the conductors of the transmission line is grounded.
It is a still further object of the present invention to provide an antenna of the foregoing type in which the dipole arm formed by all or a part of a multi-couductor transmission line is a coaxial transmission line.
It is a still further object of the present invention to provide an antenna of the foregoing type in which the dipole arms are flexible conductors supported by a rigid dielectric element and retractable for storage on reels, with the reels coupled for retraction of the flexible elements at different rates bearing a predetermined ratio.
It is still another object of the invention to provide an antenna of the foregoing type in which one of the dipole arms of the antenna is formed by a flexible tape supported in a semi-cylindrical for-m by a dielectric support element.
Other objects and advantages will be apparent from a consideration of the following description in conjunction with the appended drawings in which:
FIGURE 1 is an elevational view of an antenna according to the present invention;
FIGURE 2 is an elevational view partially in section of an alternative form of antenna according to the present invention;
FIGURE 3 is an elevational view partially in section of a second alternative form of antenna according to the present invention; and
'FIGURE 4 is a sectional view of the antenna of FIG- URE 3 taken along the line l tin FIGURE 3.
Referring now to FIGURE 1 an antenna installation is shown comprising a mast 11 preferably of a non-con ductive material, and which may comprise a wood pole, a unitary or a sectionalized reenforced plastic tube, or the like.
Mast 11 is vertically supported upon a base 12 as illustrated in FIGURE 1. It should be appreciated, however, that the present invention is not limited to dipoles of vertical polarization. Thus the mast 11 could be replaced by a horizontal boom or in fact the antenna could be oriented in any desired direction to achieve the intended radiation pattern in accordance with techniques well known in the art.
Brackets 13 and 14 are provided on .mast 1 1 for supporting the antenna elements. Bracket 1d has affixed thereto a pulley 15 over which runs a halyard 16, which may be a nylon rope, for example, or any other form of: non-conductive cord. Secured to halyard 16 is a conductive flexible strand -17, which may be formed of stranded copper wire, for example. The conductive strand 17 is connected to bracket 13 and runs over a pulley 18. Pulley 18 is conductive and is conductively attached to the center conductor 19 of a flexible coaxial cable 21. As customary, the coaxial cable 21 comprises the center conductor 19 and an outer conductor 22 which may or may not be provided with an insulating jacket.
Coaxial cable 211 is reeled on a reel 23 which is rotatably mounted on mounting structure 26 by means of axial shaft 25. The outer conductor 22 is conductively coupled to flange 28 of reel 23, and a further conductive connection is provided through axial shaft 25 and mounting structure 26 to electrical ground.
. In FIGURE 1 the drum 27 of reel 23 is non-conductive and is provided with grooves or other guides to maintain separation between turns of coaxial cable 21 so that the turns of cable 21 form an inductive coil as respects the outer conductor 22. If an insulating jacket is provided for cable 21 it will then be unnecessary to maintain physical separation between the turns of cable 21. The inductive properties of the turns of cable 21 may be enhanced by providing a magnetic core in drum 27 which may, for example, be a ferrite material. It will be noted that several turns of cable 21 remain on reel 23 even when the cable is fully extended thus maintaining the inductive termination for cable 21 for all positions of the extendible antenna.
A contact 29 is provided which is electrically connected to the center conductor of cable 21. Contact 29 slides on slip ring 30 which in turn is electrically connected to lead 31, all of which are insulated from electrical ground. Thus a continuous electrical connection is provided to the coaxial cable 21 regardless of rotation of reel 23. Any suitable transmission line may be utilized for connecting the antenna of FIGURE 1 to a transmitter or receiver which will frequently be located some distance from the antenna.
FIGURE 1 shows the antenna condition for minimum wavelength. The dipole antenna has two arms comprising respectively the conductive strand 17 and the outer conductor of coaxial cable 21 along its length from pulley 18 to reel 23. Electrical grounding of the end of the latter dipole arm is prevented by the inductive reactance of the turns of cable on reel 23 which form, in effect, a radio frequency choke for the radiation signal propagating on the outer surface of conductor 22. The antenna is effectively center fed from the end of coaxial cable 21.
Drawing on halyard 16 will cause the free end of conductive strand 17 to be moved upward thus increasing the effective length of this arm of the dipole. At the same time the effective length of the dipole arm formed by coaxial cable 21 will be lengthened by an equivalent amount due to the block and tackle action of the pulley arrangement. The electrical operation of the antenna remains the same until in its maximum extension the conductive strand 17 extends from bracket 13 to bracket 14 and the coaxial cable 21 is fully extended from reel 23 to bracket 13. A ratio of antenna lengths of 2 to 1 and thus a ratio of perfectly tuned frequencies of 2 to 1 is provided by the arrangement of FIGURE 1. A hand powered or mechanically powered winch may be arranged to aid in extending and taking in halyard 16.
It may be noted that for minimum wavelength the arm of the dipole consisting of strand 17 is formed by a doubled length of strand 17 whereas in full extension a single length strand forms the upper arm of the dipole. This change in effective radiating area may be rendered negligible by making pulley 18 of small diameter or otherwise arranging that the doubled portion of strand 17 lies close upon itself and has little greater effective radiating area than the single strand.
The size of the drum 27 of reel 23 is not critical but may readily be chosen to accommodate the particular type cable employed; for example, a six inch diameter drum is satisfactory for a 59A/ U radio frequency cable. Reel 23 may be spring-loaded or otherwise arranged to reel in cable 21 as halyard 16 is slacked off and cable 21 is lowered.
In accordance with known techniques in the art the coaxial cable 21 should be selected to have a characteristic impedance matching the input impedance of the antenna, which for a half-wave dipole is approximately 73 ohms.
The antenna is not limited to half-wave operation and may, for example, be operated as a full-wave antenna. The impedance of a full-wave antenna is more dependent upon the dipole arm thickness or cross section and is generally higher than that of a half-wave antenna. Those skilled in the art may readily determined the proper impedance relationships for a full-wave antenna if such is desired. Furthermore, a single antenna may be operated both as a half-wave and a full-wave antenna, in which case the impedance relationships may be compromised or an impedance transformer may be switched in and out of the antenna circuit to change from full-wave to halfwave operation. It should be appreciated that the antenna need not be operated at resonance with the operating frequency in every case but that advantages relating to impedance match, directivity pattern, or the like, may be obtained by operation off the resonant frequency.
FIGURE 2 shows an alternative form of the antenna comprising a dielectric or insulating mast 41 having extending therethrough channels 42 and 43 adapted to accommodate semi-rigid conductors 44 and 45.
Conductor 44 is wound for storage on a reel 48 while conductor 45 is wound for storage on a reel 47. The reels 47 and 48 are coupled for simultaneous rotation as by gears 49 and 51. The gear ratio and reel diameters are selected so that the travel of conductor 44 is substantially twice that of 45 and the extended length of conductor 44 is accordingly substantially twice of that of conductor 45.
Suitable sliding contacts 55 and 56 are provided for conductors 44 and 45. Contacts 55 and 56 are connected respectively to the center and outer conductor of a coaxial line 57. Coaxial line 57 is provided with a choke 58 which serves to electrically isolate contact 56 and conductor 45 form electrical ground. The outer conductor of cable 57 is grounded by lead 59. Choke 58 may be a conventional ferrite-loaded coaxial cable choke.
As illustrated in FIGURE 2, remote control of the tun ing of the antenna according to the present invention may readily be provided by a servo motor 52 coupled by a suitable gear train 53-54 to drive reels 47 and 48.
The control signal for servo motor 52 is supplied over electrical leads 60, for example from a servo transmitter coupled to the receiver or transmitter tuning control.
The operation of the antenna of FIGURE 2 is analogous to that of the antenna of FIGURE 1. However, in the antenna of FIGURE 2 the unitary coaxial transmission line 21 is replaced by a two wire transmission line formed by segments of conductor 44 and conductor 45. The signal is transmitted to the center feed point of the antenna over the two wire transmission line formed by the extended portion of conductor and the coextensive portion of conductor 44. The remaining portion of conductor 44 acts as the upper arm of the dipole antenna while the two coextensive portions of conductors 44 and 45 operate as the lower arm of the dipole antenna. The input signal is propagated to the feed point in an antiphasal mode; the signal propagated on the dipole arm comprising conductors 44 and 45 is propagated in a cophasal mode.
It is an important feature of the antenna of FIGURE 2 that the two wire transmission line formed by portions of conductor 44 and conductor 45 may also be utilized as a quarter-wave matching transformer. Such a transformer is desirable in the event that the antenna impedance is different from the characteristic impedance of the transmission line to the antenna.
A specific example of impedance matching arrangement is illustrated in FIGURE 2 wherein the antenna is assumed to have an impedance of approximately 73 ohms, typical for a half-wave dipole antenna. The coaxial transmission line 57 is assumed to have a ohm characteristic impedance. As is well known in the art, a quarter-Wave length of transmission line having an impedance substantially equal to the geometric mean of the two impedances to be matched will provide a matching transformer. Accordingly, the separation between conductors 44 and 45 is arranged to provide (taking into account the dielectric constant of the material from which the mast is formed and the diameter of the conductors) a transmission line having a characteristic impedance of 60 ohms. It should particularly be noted that the adjustment of the antenna to different frequencies automatically maintains the transmission line matching section at one quarter-wavelength thereby optimizing its performance over a wide range of frequencies. This assumes that the antenna is operated as a half-wave dipole, as will usually be the case.
The basic concept of the antenna of FIGURE 2 is subject to numerous variations, one of which is illustrated in FIGURE 3. In FIGURE 3 the short conductor 45 is replaced by a flexible pre-stressed tape 66. The tape 66 is pre-stressed to assume a tubular or near tubular shape. The channel 63 for conductor 64 will then prefer-ably be centered in the mast 61. Tape 66 supports itself by mapping itself around mast 61. At the bottom of mast 61 is a slot 60 which straightens out tape 61 so that it may readily be wound on a reel 67. Reel 67 may be supported in a housing 62 along with a reel for conductor 64.
As in the apparatus of FIGURE 2, the reels 67 and 65 are coupled together by gears such as 69 and 70 so that tape 66 is always extended only half as far as conductor 64. Suitable connections are made between reels 65 and 67 to conductors 72 and 71 coupling the antenna of FIGURE 3 to a transmitter or receiver. A suitable choke or balun may be used to provide a transition from antenna to transmission line in FIGURE 3, in accordance with the embodiments of FIGURE 1 or FIGURE 2. A servo motor 73 is provided to drive reels 65 and 67.
The operation of the antenna of FIGURE 3 is substantially the same as that of the antenna of FIGURE 2 except that the transmission line formed by the coextensive portion of conductors 66 and 64 in FIGURE 3 is a coaxial or semi-coaxial transmission line. The prestressed tape 65, as illustrated in FIGURE 3, does not completely surround the mast 61 but it will be understood that it may extend around more or less of the mast periphery, it being preferred that the tape 66 extend more than half Way around mast 61 in order that it be supported thereby. It may be noted in the case of both FIG- URE 2 and FIGURE 3 that the range of adjustment of the antenna is not limited to a ratio of lengths of two to one but rather the antenna length may be reduced theoretically Without limit and in practice a ratio of 30 to 1 appears readily achievable.
It will be apparent to those skilled in the art that the tape or ribbon of FIGURE 3 may be additionally guided by a surrounding dielectric tube and that other configurations of ribbons or tapes may be guided to provide various forms of transmission lines other than coaxial. Furthermore the radiators may be self supporting telescoping elements of rigid form. The movement of the respective conductors in 2 to 1, or other desired relationship, may be accomplished by other convenient means such as pneumatically or hydraulically.
In FIGURE 2 the conductors could be tensioned and extended With the aid of halyards attached to the ends thereof and wound on spring Wound reels at the upper extremity of the antenna mast.
The present invention may be useful and desirable in all tunable communications systems capable of using resonant half-Wave dipole antennas to advantage. Furthermore the idea may be extended to include other types of antennas such as the end fed whip antenna or the 300 ohm folded dipole antenna. The invention may also be extended to Yagi antennas wherein the parasitic elements may or may not be adjusted in length and spacing in synchronism with the adjustment of the active dipole element. A plurality of dipoles according to the invention may be arranged in a multi-element array.
In addition to those embodiments of the invention described or suggested there Will be numerous variations and modifications apparent to those of skill in the art. Accordingly, it is desired that the scope of the invention not be limited to the embodiments illustrated or suggested but rather that it be determined by reference to the appended claims.
What is claimed is:
1. An extendible dipole antenna comprising an extendible section of coaxial radio frequency transmission line having an outer conductor and a center conductor, an elongated conductor coupled electrically to the center conductor of said transmission line, said elongated conductor being extendible with respect to said transmission line, means for supporting said transmission line and said elongated conductor to form a dipole antenna fed at the junction of said elongated conductor and said transmission line, means for simultaneously extending said transmission line and extending said elongated conductor with respect to said transmission line while maintaining their extended lengths in a predetermined ratio, a radio frequency input to said antenna electrically coupled to said transmission line, and a storage reel having an axis extending in a direction normal to said dipole antenna, said coaxial line being at least in part Wound around said reel so that the turns on said central conductor provide a radio frequency choke isolating the radiation signal propagated on the outer conductor of said radio frequency transmission line from said radio frequency input.
2. An extendible dipole antenna comprising an extendible multi-conductor transmission line, an extendible elongated conductor electrically coupled to one conductor of said transmission line and extendible therefrom, mechanical advantage means for supporting said transmission line and said conductor to form a dipole antenna, means for simultaneously extending said conductor and said transmission line While maintaining their extended lengths in a predetermined ratio, and a radio frequency input to said antenna electrically coupled to said transmission line.
3. In a dipole antenna the combination of a. storage reel, an extendible section of coaxial radio frequency transmission line adapted to be at least partially wound on said reel and having an inner conductor and an outer conductor, a block and pulley device forming an electrical connection and comprising a block element and a pulley element, said block element being connected to said central conductor, a flexible conductor looped around said pulley element of said device and a halyard connected to said flexible conductor for controlling the effective length of said flexible conductor by displacing said block, thereby simultaneously utilizing the mechanical advantage provided by said block and pulley to extend the effective length of said coaxial line.
References Cited by the Examiner UNITED STATES PATENTS 2,474,242 6/1949 Geiringer 343-823 X 2,485,457 10/1949 Potter 343-79l 2,702,345 2/ 1955 Walter 343823 X 2,834,012 5/1958 Allen 343-723 ELI LIEBERMAN, Acting Primary Examiner.