Zig-zag slotted coaxial cable for radio frequency signal coupling

A coaxial cable for radio frequency signal coupling comprises elongated, coaxial, central and external conductors. A continuous slot is provided in the external conductor in the form of a regularly alternately undulating or zig-zag pattern.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved coaxial cable for radio 
frequency signal coupling. 
An open type coaxial cable of the prior art is shown in the side view and 
sectional view of FIGS. 1 (a) and (b), respectively. The cable comprises 
coaxially arranged inner and outer conductors 1 and 2, respectively. A 
continuous slot 3 is provided in the outer conductor along the length of 
the cable. This type of cable makes use of the outwardly protruding 
portion of the TEM mode electromagnetic field which propagates in the 
space between the inner and outer conductors for purposes of communicating 
with an object adjacent the cable. Such object may be a train, an 
automobile, an industrial crane, or the like, may be movable or 
stationary, and would typically mount a signal pickup device on the side 
of the object facing the coaxial cable slot. The communication between the 
coaxial cable and moving object may be to implement remote control, or 
simply to convey information. The degree of radio-wave coupling between 
the cable and such a moving object travelling therealong often cannot be 
made sufficiently large, because the coaxial cable shown FIG. 1 has no 
mechanism for positively radiating leakage waves from the slot. Further, 
the protruding external electromagnetic field coupling the cable and the 
moving object may adversely affect the signal transmission of the cable 
due to the influence of the outer environment surrounding the cable, e.g., 
pollution, water, etc. Consequently, the quality of signal transmission 
may become fatally deteriorated. The greater the amount of radio-wave 
coupling required to be taken out, the more the width of the slot must be 
increased. This widening accentuates the effects of outer environmental 
changes, whereby stabilized radio-wave coupling and transmission is 
increasingly degraded. 
SUMMARY OF THE INVENTION 
The present invention eliminates the above-mentioned disadvantages by 
providing an open type coaxial cable for radio frequency signal coupling 
provided with an elongated slot having an alternately notched, undulating, 
or zig-zag configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of a coaxial cable according to the present invention is 
shown in FIGS. 2(a) and 2(b), wherein inner and outer conductors are 
designated 1 and 2, respectively. A continuous slot 4 is provided in the 
outer conductor along the length of the cable in the form of an 
alternately undulating of zig-zag pattern. The essence of the invention 
resides in this slot configuration in the outer conductor of the cable. 
The function of the slot will be explained with reference to FIG. 3. The 
principal transmission mode in a coaxial cable is the TEM mode, i.e., 
electric current flows along the conductor in the longitudinal direction 
of the cable. Accordingly, as shown in FIG. 3, a part of the electric 
current flowing in the external conductor, i.e., the 5.sub.1 component, is 
cut or interrupted by the slot since the latter is slanted with respect to 
the cable axis. Therefore, the slot provides an electromagnetic-wave 
exciting function whereby preferred radio frequency signal coupling 
characteristics can be obtained. 
In the conventional open type coaxial cable shown in FIG. 1, no 
electric-wave exciting function is provided because the slot is parallel 
to the cable axis. Such a cable simply makes use of the protruding portion 
of the inner electromagnetic field of the cable. As a result, the amount 
of radio frequency signal coupling between the cable and a moving station 
travelling therealong is considerably small and unstable due to outer 
environmental changes surrounding the cable which affect the transmitting 
characteristics thereof. 
The cable according to the present invention, however, provides an 
electromagnetic-wave exciting function over substantially the entire 
region of the continuous slot. Thus, it is easily possible to increase the 
amount of radio frequency signal coupling, and the coupling 
characteristics become more stable as compared with those of the prior 
art. 
In further accordance with the principles of the invention, the period P of 
the slot pattern should be appropriately determined. Specifically, if the 
period P is established to be approximately the same or slightly longer 
than the wave-length of the applied signal frequency, the lines of 
electric force may be represented by vectors 6.sub.1, 6.sub.2, 6.sub.3, 
etc. in FIG. 3. These lines of electric force, produced by the electric 
current, are cut by the slot, as a result of which leakage waves are 
produced which radiate or propagate outwardly in a direction perpendicular 
to the cable. If the period P is shorter than the wave-length of the 
applied signal frequency, such a radiating mode is not produced, but 
instead only a surface-wave mode appears around the cable, as in the prior 
art. 
The present invention is not limited to only the above described zig-zag 
slot pattern, but is equally applicable to any continuous slot 
configuration which alternates or corrugates with a constant period with 
respect to the cable axis. Other embodiments according to the present 
invention are shown in FIGS. 4(a), 4(b) and 4(c), for example. In FIG. 
4(a) the slot 4 has a rectangular configuration. In FIGS. 4(b) and 4(c) 
the slot 4 has a straight central portion and alternating or staggered 
notches in the opposite edges; the notches are rectangular in FIG. 4(b) 
and triangular in FIG. 4(c).