Wire antenna for circularly polarized wave
This is a continuation of application Ser No. 08/035,705, filed Mar. 23, 1993, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wire antenna for circularly polarized wave, particularly to a wire antenna for circularly polarized wave that has a planar and thin shape as a whole. 
     2. Description of the Related Art 
     Conventionally known antennas for circularly polarized wave are a helical antenna, a cross dipole antenna, a patch antenna, etc. In recent years, the antenna for circularly polarized wave has been being required in mobile communications. There is a need for the antenna to be a shape without projections on its outside. 
     A patch antenna utilizing a dielectric substrate is known as an antenna serving for the above use. However, an expensive polytetrafluoroethylene (Teflon, Registered Trademark) substrate or the like is necessary in order to make the antenna thin, small and light. Therefore, the antenna satisfies requirements but is very costly. The helical antenna is a wire antenna, but is hard to be practised into a thin type. The cross dipole antenna needs two feed points, so that a power distributor and a phase shifter are indispensable, thus making its costs high. 
     Japanese Laid Open Patent Publication No. 61-252701 shows an art related to such antennas for circularly polarized wave. 
     This publication discloses an antenna in which a branch conductor of a length of about one fourth λ (lambda) is arranged parallel to a loop conductor of an overall length of about one λ (lambda). It generates circularly polarized waves by adjusting a gap between the conductors. 
     Japanese Laid Open Patent Publication No. 2-214304 shows another art related to such antennas for circularly polarized wave. 
     This publication discloses an antenna in which an earth plate is disposed on a rear surface of a dielectric substrate and in which a circular loop conductor is provided on a surface of the dielectric substrate. Particularly, a central conductor of a feeding coaxial line is linearly connected with an L-shaped element fitted on the loop conductor, and an outer housing conductor is linearly connected with the earth plate. 
     The antennas for curcularly polarized wave shown in the publications 61-252701 and 2-214304 are three-dimensional. 
     According to an experiment, in the antenna of 61-252701, a gap between the loop conductor and the branch conductor as well as a height from a reflecting plate are important. Unless the above dimensional accuracy is strictly kept, the antenna cannot maintain a good characteristic. Moreover, there is a practical problem in consideration of mechanical strength of a structure for standing the conductors on the reflecting plate, specially in view of vibration when mounted on a car. 
     On the other hand, in the antenna of 2-214304, the dielectric substrate is indispensable for its structure. Generally, the dielectric substrate of good high-frequency characteristic is expensive, so that it is difficult to decrease costs. If the antenna is constructed in a wire shape, a perturbation element must be added on a point of the loop, thus needing a soldering step or the like. 
     SUMMARY OF THE INVENTION 
     In view of the above, taking notice of low prices of materials of a wire antenna, it is an object of the invention to provide a wire antenna for circularly polarized wave that is made of inexpensive components and has a shape suitable for mobiles. 
     In accordance with one preferred mode of the invention, there is provided a wire antenna for circularly polarized wave that has one turn element and a three-fourths turn element. The antenna is composed of a loop conductor of an overall length of seven-fourths (lambda) and a loop of seven-fourths turn. The loop conductor has a rectangular shape with opposite ends separated. A feeding point is positioned at one turn point of the loop conductor. 
     Further objects and advantages of the invention will be apparent from the following description, reference being had to the accompanying drawings, wherein preferred embodiments of the invention are clearly shown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view showing one embodiment of a wire antenna for circularly polarized wave of the invention. 
     FIG. 2 is a graph showing a VSWR characteristic of a wire antenna for circularly polarized wave of the invention. 
     FIG. 3 is a graph showing an axial ratio characteristic of a wire antenna for circularly polarized wave of the invention. 
     FIG. 4 is a perspective view showing a measurement coordinate system of a wire antenna for circularly polarized wave of the invention. 
     FIG. 5 is a graph showing a radiation pattern of X-Z plane of a wire antenna for circularly polarized wave of the invention. 
     FIG. 6 is a graph showing a radiation pattern of Y-Z plane of a wire antenna for circularly polarized wave of the invention. 
     FIG. 7 is a perspective view showing an example of a way of fitting of one embodiment of a wire antenna for circularly polarized wave of the invention 
     FIG. 8(a) is a sectional view showing an example of a section of a wire antenna for circularly polarized wave taken along the line VIII---VIII of FIG. 7. 
     FIG. 8(b) is a sectional view showing another example of a section of a wire antenna for circularly polarized wave taken along the line VIII--VIII of FIG. 7. 
     FIG. 9(a) is a front view showing a basic shape of one embodiment of a wire antenna for circularly polarized wave of the invention. 
     FIG. 9(b) is a front view showing a balanced system of one embodiment of a wire antenna for circularly polarized wave of the invention. 
     FIG. 9(c) is a front view showing an unbalanced system of one embodiment of a wire antenna for circularly polarized wave of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, a preferred mode of a wire antenna for circularly polarized wave of the invention will be described hereafter. 
     Referring to FIG. 1, the present embodiment of the antenna is structured into a loop of seven-fourths turn and has an overall length of seven-fourths wave length λ0 at free space. A feeding point thereof is positioned at a point of one turn. Namely, when feeding terminals 2a and 2b are supplied parallel with power, elements 3a and 3b constituting a loop conductor radiate circularly polarized waves. The outer element 3a is a rectangular loop of one turn, while the inner element 3b is a rectangular loop of three-fourths turn. There is held a constant distance d between parallel sides of the elements 3a and 3b. There is provided a fixed distance d1 between a three-fourths point (leading end) of the inner element 3b and the outer element 3a. The present embodiment uses a piano wire of a loop wire diameter of 0.3 mm, while setting a length L=0.27λ0, the distance d=0.016λ0, the distance d1=0.026λ0. Here, the λ0 shows a free space wave length. 
     Characteristics of the above constructed antenna are shown in FIGS. 2 to 6. FIG. 2 shows a characteristic of a voltage standing wave ratio (VSWR) of the antenna. FIG. 3 shows its axial ratio characteristic. FIG. 4 illustrates a measurement coordinate system of the antenna. FIG. 5 illustrates a radiation pattern of X-Z plane. FIG. 6 shows its radiation pattern of Y-Z plane. 
     These data show what degree of circularly polarized waves the antenna radiate. For example, it is understood that the antenna radiates good circularly polarized waves of the axial ratio of 6 dB, a bandwidth of 6 per cent or more, and a half power of 120 degrees. It is also understood that an electric field is maximum at a bore site direction of the antenna 1 in view of the radiation directivity. 
     Next, an example of way of fitting the present embodiment of the antenna is described referring to FIG. 7 and FIG. 8. 
     As shown in the figures, parallel feed line 4 is connected with the feeding terminals 2a and 2b. The feed line 4 is electrically connected with a connector 5. A predetermined impedance matching is made between them. The feed line 4 is disposed as shown in FIG. 8(a) and FIG. 8(b), for example. Namely, FIG. 8(a) shows the feed line 4 welded to a film 6. FIG. 8(b) shows the feed line 4 sandwitched between the film 6 and an adhesive film 7. In the present embodiment, the feed line 4 is arranged planarly on the film 6 as well as the elements 3a and 3b and the feeding terminals 2a and 2b. 
     A principle of operation of the antenna 1 is explained referring to FIG. 9. The antenna 1 shown in FIG. 9(a) is decomposed into a balanced system shown in FIG. 9(b) and an unbalanced system shown in FIG. 9(c). 
     A characteristic impedance Z0 of a line of the balanced system of the antenna 1 is: 
     
         Z0=120ln (d/a)                                             [Ω] 
    
     Here, the symbol a shows a radius of the element, and the symbol d shows the distance between the outer element 3a and the inner element 3b. 
     On the other hand, a coefficient k multiplied by voltages of the unbalanced system is a voltage distribution factor. Since a left-handed circularly polarized wave can be generated by giving equal amplitude and 90 degrees of phase difference to the voltages of FIG. 9(c): 
     
         v.sub.1 /V0=-j 
    
     This can be realized via a transmission line of FIG. 9(b) as follows: 
     
         L≅λ0/4 
    
     
         Z0≅R/k.sup.2 
    
     Here, the symbol R shows an input resistance of a single loop element. 
     As described above, the present embodiment of the antenna is composed of the outer element 3a of one turn and the inner element 3b of three-fourths turn. Namely, the antenna 1 is defined by the loop conductor of the rectangular shape both ends of which are separated and which has an overall length of about 7/4λ0 and is 7/4 turn loop. Moreover, the feeding terminals 2a and 2b as the feed point is positioned at the one turn point of the loop conductor. 
     In resume, the feeding terminals 2a and 2b are arranged at one turn point of the rectangular and open shaped elements 3a and 3b or the loop conductor that has the entire length of 7/4λ0 and is 7/4 turn loop. 
     Accordingly, all the parts, namely the elements 3a and 3b (loop conductor), feeding terminals 2a and 2b and the feed line 4 can be disposed in the same plane on the film 6, as shown in FIG. 7. Thus, the antenna 1 can be planar shape and thin, that is suitable for mobiles like cars. Moreover, it is unnecessary for the antenna 1 to add a perturbation element on the way of the loop, so that a soldering step can be omitted and that there is no need for a dielectric substrate of high frequency characteristic that is required in prior art and very expensive. Namely, the antenna 1 is a cheap wire antenna of a simple construction and appropriate to mass production. 
     While the elements 3a and 3b of the loop conductor have a rectangular shape, they may be alternately be another shape, as long as the element 3a is one turn and the element 3b is 3/4 turn. 
     The preferred embodiments described herein are therefore illustrative and not restrictive, the scope of the invention being indicated in the appended claims and all variations which come within the meaning of the claims are intended to be embraced therein.