Patent Publication Number: US-4220956-A

Title: Collinear series-fed radio frequency antenna array

Description:
This invention is generally directed to radio frequency antenna arrays and, in particular, to a collinear, series-fed array of radio frequency transmitting or receiving elements. 
     Collinear arrays of dipole radiators are well known in the art. They are typically used for communications applications where it is desired to transmit or receive equally well in all azimuthal directions but only toward the horizon in elevation. Furthermore, a series-fed collinear array of microstrip radiator elements is disclosed and claimed in my copending application, Ser. No. 683,203, filed May 4, 1976. However, there are communications applications where both technical and economic considerations make it difficult to employ either standard collinear arrays of dipole radiators or the series-fed collinear microstrip array. One example of such applications is an antenna array designed for deployment from an object suspended high above the earth and intended for communication with the earth. Such an array is preferably capable of deformation for compact storage prior to deployment. In addition, some applications of this type (i.e., deployment from a balloon) involve but brief single usages of the whole structure before it is discarded, thereby requiring a minimum cost for manufacture and usage. 
     If a series-fed microstrip array is used, a ground plane must be located at least about 0.01 wavelength behind the radiator patches. If such separation is significantly reduced beyond 0.01 wavelength, the antenna&#39;s performance, especially efficiency, is significantly degraded. However, for most frequencies, 0.01 wavelength implies a fairly thick structure comprising a dielectric with conductive layers laminated on both sides (one side being etched to form the microstrip radiator patches). Such a thick structure is not conveniently formed (i.e., rolled) into a compact package for storage purposes. A conventional collinear array of dipole radiators is, of course, even less adapted to such applications. 
     Now, however, it has been discovered that a collinear, series-fed array of radiators may be fabricated by simply etching a copper laminate adhered to one side of a very thin dielectric sheet. Both the copper laminate and the dielectric sheet may be made very thin and flexible since the only purpose of the dielectric is to physically support the shaped conductors etched thereon. Such a structure can easily be manufactured with a total thickness of less than 0.005 inch and can be manufactured relatively inexpensively. In other words, the antenna of this invention may be very simply manufactured as a thin, flexible (rollable), lightweight array having a directional radiation pattern in everyplane that includes the length of the array and an omnidirectional pattern in planes disposed orthogonal to the length of the array. 
     The array may be connected at one end to a mounting bracket and at the other end to a weight for automatically extending and/or maintaining the array in an elongated operating position after deployment from a compact storage location. In the presently preferred exemplary embodiment, the array is formed by two spaced apart shaped electrical conductors which extend substantially parallel to one another in a longitudinal direction and which have regularly spaced complementary changes in their transverse dimensions along the longitudinal direction. These shaped conductors are substantially isolated from electromagnetic interaction with other unconnected electrical conductors in the immediate vicinity. That is, there is no underlying ground plane or the like associated directly with the shaped conductors as might be expected, for example, in the case of the traditional microstrip radiator structures. The result is a substantially omnidirectional radiation pattern in planes transverse to the longitudinal direction of the array elements. The changes in transverse dimensions may comprise a series of projections directed outwardly away from the other shaped conductor or a series of projections which are directed inwardly towards the other shaped conductor. 
    
    
     These as well as other advantages and objects of this invention will be more completely understood by reading the following detailed description of the presently preferred exemplary embodiments in conjunction with the accompanying drawings, of which: 
     FIG. 1 is a partial cutaway plan view of the presently preferred exemplary embodiment of this invention; 
     FIG. 2 is a cutaway plan view of the exemplary embodiment shown in FIG. 1 where the elements are dimensioned and spaced by one-half wavelength and where exemplary electric field patterns are depicted schematically to help explain the operation of the antenna; 
     FIG. 3 is a cutaway plan view of another exemplary embodiment of this invention constituting a variation of the structure shown in FIG. 1; and 
     FIG. 4 is a perspective view of the antenna array deployed from a mounting bracket by a weight attached to the opposite end of the array. 
    
    
     Some radiation can be obtained from a simple microstrip transmission line by merely providing regularly spaced variations in the dimensions of one of the conductors forming the transmission line. In effect, the impedance of the transmission line is varied at regular intervals along the line related to the wavelength of energy propagating therealong. Since such a line is unshielded, these variations in impedance will cause radiation to occur in a relatively uncontrolled manner. 
     However, it has been discovered that a more controlled radiation pattern may be achieved by also varying the dimensions of the other half of the microstrip line in a complementary fashion as shown in FIG. 1. The array shown in FIG. 1 operates much like a balanced antenna but does not require a balun. 
     As shown in FIG. 1, the antenna array comprises two spaced-apart shaped electrical conductors 10 and 12. These conductors extend parallel to one another in a longitudinal direction as indicated by arrows 14 and 16. Conductors 10 and 12 also have regularly spaced complementary changes in their transverse dimensions along the longitudinal direction. For example, as shown in FIG. 1, these changes comprise a series of projections 18 on conductor 10 directed outwardly or away from conductor 12. At the same time, conductor 12 has projections 20 which are also directed outwardly or away from conducttor 10. The projections 18 and 20 are complementary, that is, their occurrences alternate on conductors 10 and 12 as can be seen in FIG. 1. 
     In the exemplary embodiment of FIG. 1, the projections 18 and 20 are rectangularly shaped and connected in series along the longitudinal direction by strips 24 and 26, respectively, disposed toward the inboard sides of the rectangularly-shaped projections so as to define a substantially constant separation gap 22 along a straight line separating the conductors 10 and 12. The radio frequency input and/or output to the array may be through a conventional transmission line such as, for example, a coaxial cable 28 having its inner conductor connected to one of the shaped antenna conductors (10 as shown in FIG. 1) and its outer shield conductor connected to the other shaped conductor (12 as shown in FIG. 1). 
     For ease of construction and to also conveniently maintain the desired orientation of the conductors 10 and 12, they are formed by selectively etching a copper or other conductive laminate adhered to one side of a dielectric sheet 30. The etching process employed is substantially the same as that used for forming printed circuit boards, microstrip antennas, and the like. Suitable dielectric and conductive materials and dimensions will be apparent to those in the art for particular antenna applications requiring a desired degree of flexibility for the array structure. Since the dielectric 30 forms no necessary electical function, it may be made as thin as mechanically feasible and of a material which best serves the mechanical function it is to perform so long as it continues to perform as a good electrical dielectric at the intended frequencies of antenna operation. 
     When the radiating transitions are spaced substantially one-half wavelength apart as shown in FIG. 2, the electric fields in the vicinity of each transition are as schematically shown in FIG. 2. As is apparent from this sketch, the horizontal field components cancel in the far field, whereas vertical components add. Accordingly, the resulting array is polarized along its longitudinal length similar to the polarization expected from a collinear array of dipole radiators. The result is a directional pattern in every plane that includes the length of the array (i.e., the radiation pattern is directed away from the end of the array) which is yet omnidirectional in planes disposed orthogonal to the array length. Accordingly, if the array is suspended above the earth with its longitudinal axis directed toward the center of the earth, its radiation pattern will be properly directed to a desired portion of the earth&#39;s surface. 
     A convenient technique for so directing the longitudinal axis of the antenna array is shown in FIG. 4. Here, the antenna array 40 is attached at one end to a mounting bracket 42 which also includes a coaxial cable connector 44 used in connecting the antenna to a coaxial transmission line, as will be appreciated. The other end of the array 40 is connected to a weight 46 which, through gravity forces, will maintain the longitudinal axis of the antenna directed to the earth&#39;s center. The weight 46 may be connected to the array 40 through an elastic or shock-absorbing member 48 as shown in FIG. 4. Furthermore, the entire array 40 and the connected weight 46 may be conveniently folded or rolled for storage purposes near the mounting bracket 42 prior to deployment. For a typical UHF application, the array 40 may have a length of several feet (i.e., 13-20 feet). 
     Because of the complementary variations in transverse dimension of the two spaced-apart conductors 10 and 12, better control is achieved over the radiation pattern provided by this antenna. For example, the fraction of total available power radiated from each transition is substantially determined by the width of each projection 18, 20. The relative phases of radiation which occur from each transitional location is substantially determined by their location along the array axis or, in other words, by the length of the projections 18, 20 and the interconnecting strips 24, 26. With control of these two variables, the radiation pattern can be shaped according to conventional antenna array design techniques substantially as desired. 
     Somewhat surprisingly, it has been discovered that even where the length of projections 18, 20 is reduced to approximately one-fourth wavelength, efficient radiation still results and produces an end-fire radiation pattern similar to that expected from a Yagi array. 
     A variation of the preferred exemplary embodiment shown in FIG. 1 is depicted in FIG. 3. Here, the projections 18 and 20 of conductors 10 and 12 are directed inwardly toward one another to define a separation gap 22 which extends along a non-straight path. In other words, the projections 18 and 20 are alternately extended toward the separation gap. Of course, the shaped conductors 10 and 12 in the embodiment of FIG. 3 would still preferably be formed by selectively etching the separation gap 22 along the serpentine-like path in a conductor laminated to one side of a very thin dielectric material. It is believed that the variation shown in FIG. 3 should operate very much the same as that shown in FIG. 1. 
     While only a few exemplary embodiments of this invention have been described in detail, those in the art will appreciate that such exemplary embodiments may be varied and/or modified in many different ways without departing from the novel and advantageous features of this invention. Accordingly, all such variations and modifications are intended to be included within the scope of the following claims.