Description:
BACKGROUND OF THE INVENTION
1. THE FIELD OF THE INVENTION
The present invention relates to a leaky coaxial cable having broad band characteristics particularly suitable for use in vehicular communication systems.
2. The Description of the Prior Art
In recent times great progress has been made in the art of vehicular communication systems. In such communication systems, open type transmission lines installed along the vehicle track which transmit electromagnetic waves while radiating a part of the transmitted wave into the space around the line, play an important role in vehicular communication.
Among various kinds of open type transmission lines available, a leaky coaxial cable is considerable to be the most suitable for this purpose.
Conventional leaky coaxial cables as shown in FIG. 1, however, have generally an array of slots of identical shape arranged with a simple periodicity. The radiation property frequency band of a leaky wave of such a leaky coaxial cable is so narrow that broad band communication would not be possible. Recently, however, as considerably more information for transmission is required than ever in vehicular communication, the broader band characteristics of the leaky coaxial cable became of greater necessity
SUMMARY OF THE INVENTION
The leaky coaxial cable of the present invention is characterized by a slot array cut in the outer conductor, and the energy of the electromagnetic wave transmitted inside the cable is partially leaked through this slot array to produce uniform electromagnetic fields in the surrounding outer space when the slot configuration of the slots is properly made.
The object of the present invention is to give broad band characteristics in the radiation properties of a leaky coaxial cable.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is a perspective view showing a leaky coaxial cable of the prior art.
FIG. 2 is a cylindrical coordinate diagram used in the explanation of the invention.
FIG. 3 is a graph which is used in the explanation of the invention.
FIGS. 4,5,6,7,8 and 9 are perspective views showing different embodiments of the leaky coaxial cable of the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION.
As shown in FIG. 1 of a prior art, the typical slot layout in leaky coaxial cables heretofore in use is one having a simple periodicity. In FIG. 1, (1) denotes the inner conductor of the coaxial cable, (2) the outer conductor, and (31), (32), - - - the slots, respectively. These slots (31), (32), - - - are located periodically, so that the electromagnetic field in the external space may be given by Equation (1). ##SPC1##
Where r, φ and z are the cylindrical coordinates defined in FIG. 2, Hm(2) (γn. r) the m-th order Hankel function of the second kind, γn the propagation constant of the n-th harmonic wave, exp the exponential function, p the pitch of slots (the repetition interval), and Amn, and Bmn are certain constants.
As is well known, there is also the following relationship in regard to the n-th propagation constant γn in such a periodic structure of slots as shown in FIG. 1;
γn2 = k2 - (β+ 2πn/ p)2 = (2π/λ)2 -(2π/λg + 2πn/p)2 (2)
where k and β are the propagation constants in the free space and in the waveguide, respectively, and λ and λg are the wavelength in free space and in the waveguide, respectively.
When γn2 is positive in Equation (2), the n-th harmonic wave can radiate out superiorly. Conversely when γn2 is negative, it becomes a surface wave which exists only in the neighborhood of the cable. A surface wave, in other words, does not play an important role in coupling with a vehicular antenna. Therefore, it is necessary to make γn2 positive, at least, so as to obtain good coupling between the leaky coaxial cable and the vehicular antenna. Furthermore, not only must γn2 >0, but basically it is also required that only one such leaky wave satisfying the condition γn2 >0 be transmitted at a time. The reason for this is that if two component waves satisfy the condition γn2 >0 are transmitted at the same time, beating occurs, because their phase constants in the z-direction (βz = β + 2πn/ p) are different with each other (since n is different), so that a great fluctuation occurs in the coupling level which greatly obstructs communication.
When the required conditions for good coupling as discussed above are satisfied, the pitch of slots p must become subject to the following restriction in view to Equation (2).
(1/ν + 1)/2 < λ/p < 1/ν + 1 (3) (ν > 1/3) where ν represents the ratio of the waveguide wavelength to the free space wavelength. The equation (3) shows the relationship wherein only the component wave for n = -1 becomes the leaky wave. Since ν is usually very near 1, ν/p falls within the range of 1 - 2. If λ/ p is in the neighbourhood of 1, γ- 2 (the next component of n = -1) comes near to a real number, so that the required conditions may not be satisfied. In consequence, the ratio of λ/ p having a value 1.5 - 2 may actually be effective. In other words, the effective frequency range is limited as follows;
C/2 p < C/λ < C/1.5p (4)
C = light velocity
The present invention is concerned with the basic concepts for extending the limitation of the effective frequency band for communication by a leaky coaxial cable. The leaky coaxial cable of the present invention is characterized by its double periodical slot array configuration. That is, the wave source intensity and phase of the l-th slot are made so as to satisfy the following equation.
Sl=[A. cos 2πze/p + B ] exp (-J β z e) (5)
where ze = l. po (po denotes the slot interval), and A and B are arbitrary constants. Equation (5) means that the radiation intensity of each slot changes sinusoidally along the axis of the cable.
In this case, the n-th propagation constant γn can be deformed as shown below.
γn2 + = (2π/λ)2 - (2π/λg + 2π/ p + 2πn/po)2 }
γn2 - = (2π/λ)2 - (2π/λg - 2π/ p + 2πn/ po)2 } (6)
γn2 = (2 π/ λ)2 - (2 π/ λg + 2 πn/ po)2 }
where p denotes the repetition period of slot array, po the interval between slot centers of the adjacent slots, and γn2 + , γn2 - and γn2 the square of the propagation constant owing to the small periodicity po.
When the required conditions promising the transmission of one and only one leaky component wave are sought for in Eq. (6), this is shown by the shaded part in FIG. 3. That is to say, λ/p must satisfy the following relations for good coupling in such a double periodical configulation of slots as indicated in the relation (5).
1/ν - 1 < λ/ p <1/ν + 1 (7) λ/ po > 1/ν + 1 + λ/p (8)
According to the condition (7), the permissible range of λ/ p is enlarged as compared with the condition (3). That is to say, λ/ p can take an arbitrary value within the range of 0 - 2, because of ν ≉ 1. When po is suitably small as compared with p, the condition (8) is satisfied. Accordingly, the present invention is able to obtain a broad band radiation property, over a wide frequency band.
FIG. 4 shows an example of an embodiment of the present invention. In the figure, (41), (42), - - - denote slots. All of these straight line slots have the same slot length, and have the small periodicity po and the large periodicity p. In such a double periodical slot structure, every slot inclination angle with respect to the cable axis is changed discretely one after another in order to realize the sinusoidal distribution of the wave source intensity as shown in Eq. (5). It is well known that wave source intensity of a slot increases according to slot inclination angle with respect to the cable axis, and a slot in parallel to the cable axis will not radiate. In the figure, (42), (43) and (44) corresponds to a positive half cycle of the sinusoidal change in the wave source intensity distribution, (46), (47) and (48) a negative half cycle of the sinusoidal change in said intensity distribution, and (41), (45) and (49) to zero points of the sinusoidal change in said distribution. The slots (41), (45) and (49) corresponding to the zero amplitude points may be eliminated because they do not contribute to the required sinusoidal distribution of radiation intensity indicated in Eq. (5). FIG. 5 shows another example of a slot array whose slots corresponding to the zero amplitude of the sinusoidal change in the radiation intensity distribution are eliminated.
FIG. 6 shows another example of an embodiment of the present invention. In the figure, all of the slots (42), (43), - - - in each group have the same slot inclination angle with respect to the cable axis, but every slot length is changed discretely one after another for realizing the sinusoidal distribution of the wave source intensity as shown in Eq. (5). It is well known that wave source intensity of a slot increases according to slot length. In FIG. 6, slots corresponding to the zero amplitude points of the sinusoidal change in the wave source intensity distribution indicated by Eq. (5) are eliminated.
FIG. 7 shows another example of an embodiment of the present invention which mixes the above mentioned two example embodiments. That is, the required conditions in the wave source intensity distribution indicated by Eq. (5) is accomplished by changing the slot length and slot inclination angle simultaneously. Similarly, slots corresponding to the zero amplitude points of the sinusoidal change in the wave source intensity distribution may be eliminated.
FIGS. 8 and 9 shows other example embodiments of the present invention using another slot shape. Exemplarily shown in the figure, the shape of the slot is a bent line, whose straight part (5) is inclined in relation to the cable axis to effect the radiation, and bent parts (6 ) at both ends, which are parallel with the cable axis, are subsidiary to the radiation effect. These two examples utilize the basic concept of the present invention to provide a sinusoidal distribution in the radiation intensity at each slot along the cable axis. In FIGS. 8 and 9, this concept is accomplished by changing slot length and the slot inclination angle one after another along the cable axis, respectively. This concept characterizing the present invention can be realized in any radiation structure using slot elements in any leaky coaxial cable.