Description:
The present invention relates to a high frequency cable and conductor system of the type in which a partial field is developed external to the cable and wherein a conductor or the conductors are enveloped by dielectric material which, in turn, is enveloped by a metallic shield.
High frequency signals are sometimes to be transmitted from a stationary transmitter to a mobile receiver, or vice versa, and often under special operating conditions. For example, rail vehicles are to remain in contact with a station ahead, even while passing through a tunnel, wherein regular h-f transmission would be interrupted. For purposes of providing continued transmission, a conductive system is required which, on one hand, permits propagation of h-f energy at low losses, but, on the other hand, the conductive system is to radiate such h-f energy at a particular intensity along its extension. Such a conductive system, e.g., a radiating cable, will be installed along the track and the vehicle receiver may pick up signals anywhere along the cable.
For purposes of this transmission it is known to provide symmetrical but unshielded h-f conductors; however, it was found that the transmission characteristics of such conductor system depends to a considerable extent on the condition of the environment; weather, climate, etc., provide controlling parameters for the intensity of radiation, as the overall impedance of the cable varies accordingly. This is particularly true if the cable is attached to ground.
Another type of radiating conductor is known which, to some extent, can be regarded as approximation of a coaxial conductor system. The system has inner and outer conductors, but the outer conductor has an axial slot along the entire extension of the cable. The cable radiates through that slot. The known radiating cables of that type have the disadvantage that the electric field and signal strength drops rapidly with (radial) distance from the cable. This is particular due to the following. The field itself has a significant stray component so that there are high dielectric losses and the overall signal attenuation in such a system is therefore high. Moreover, these dielectric losses are significantly dependent on atmospheric conditions which is the essential contributing factor for a change of the radiation characteristics with weather.
In order to attain a sufficiently strong input for the receiver under these conditions, the power input for the cable has to be rather high. However, the dimensions of the cable limit the amount of energy that can be transmitted. Of course, the dimensions of the cable could be increased up to a point where sufficient signal strength is available under all conditions. However, the cable itself becomes more expensive and heavier, i.e., cumbersome, and is therefore, more difficult to install.
It is an object of the present invention to provide a radiating h-f cable in which the losses are by themselves low and independent from weather or climate. In accordance with the present invention, it was discovered, that the conductor system should include a particularly constructed outer conductor, serving either as shield or as return conductor. This outer conductor is to be, on one hand, a physically, closed tube, but of such configuration that current can develop and propagate therein in and along a helical path only, extending around a center line of the conductor system. In particular, the outer conductor is constructed from electrically serially interconnected, axially juxtaposed conductor loops providing a closed tube as well as helical conduction path around the center axis of the system.
An h-f cable constructed in accordance with the invention has the advantage that the electric field cannot emanate from this physically closed return conductor or screen. On the other hand, the helical, coil-like contour of the current path causes development of an inductive field external to the cable that is useful for signal extraction and low loss pick-up by a receiver. As the electric field component does not contribute to the radiation, dielectric losses are eliminated, and the weather dependency of the radiating characteristics of the h-f cable is eliminated accordingly.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:
FIG. 1 illustrates a cross section through a radiating cable and serving as transmitting antenna and signal propagation medium in accordance with the first example of the preferred embodiment of the present invention;
FIG. 2 illustrates a two-conductor cable in a cross section likewise improved in accordance with the preferred embodiments of the invention;
FIG. 3 is a side view of the cable of FIG. 1, partially broken open for showing the several layers within the cable construction; however, the figure is also subject to interpretation as side view of the conductor system of FIG. 2;
FIG. 4 is a section view of a modified construction for the outer conductor of a radiating conductor system, as another example of the preferred embodiment of the invention;
FIG. 5 shows in plan view a sheet from which a still different outer conductor can be constructed; and
FIG. 6 is a section of an almost completed outer conductor, as made from the sheet of FIG. 5.
Proceeding now to the detailed description of the drawings, in FIGS. 1 and 3 thereof is illustrated a first cable construction having an inner conductor or core 1 which is coaxially enveloped by a return conductor 2 of tubular configuration. There is no slot in this tube. The two conductors 1 and 2 are separated from each other and maintained in concentrical and coaxial position by dielectric material 3. The center line of this conductor system is the center axis of core 1. The tubular, outer conductor 2 is additionally enveloped by a jacket 4 of insulating and dielectric material.
The particular construction of the outer conductor tube 2 can be seen best from FIG. 3. In particular, it can be seen that a completely physically closed (radially) tube has been constructed by winding and looping wire onto the tubular dielectric layer 3. The wire or wires 8 are wound on the dielectric tube 3 so that neighboring loops actually engage, without leaving any axial gap of helical contour. The wire should be insulated at least to the extent that upon closely winding the wire in loops, there will be little or no current flow axially from loop to loop. Through the gapless winding of one or several wires a physically closed tube is constructed which serves, in this case, as return conductor as well as radiator. Jacket 4 may be extruded or otherwise deposited on tube 2 and may serve to maintain the loops, establishing tube 2, in position.
Turning now to FIG. 2, two conductors 5 and 6 are disposed in parallel relation to each other along a center line 10 and embedded in that position in a somewhat ovally contoured dielectric layer 3'. That configuration is enveloped by a tube with corresponding cross section, 7. In this case, tube 7 serves only as shield. The tubular configuration of shield 7 is again obtained through the winding of wire or wires in helical configuration around the dielectric body 3. A dielectric jacket 9 is disposed on tube 7. The contour and construction described is analogous to the one before, and it is readily apparent, that in such view the cable of FIG. 2 will appear as shown in FIG. 3. It was found that by using a closed tube made from closely positioned looping wires as shield or as return conductor, the inductive field component as between inner and outer conductors can, in fact, be coupled out of the cable at sufficient signal strength.
Either kind of conductor system, FIG. 1 or FIG. 2, may have its outer conductor tube constructed differently, as will be explained now with reference to FIG. 4. Strip or tape 11 comprised of a metal layer or strip 12 with insulative, dielectric backing or substrate 13 is wound on the dielectric tube 3 (or 3') with the metal tape 12 facing the body 3 (or 3') and the dielectric backing facing outwardly. The tape is wound so that adjacent loops overlap, to make sure there is no gap in between adjacent loops, but the metal layer in one loop engages only the insulative backing of adjacent loop. The overlap is not essential in principle but a convenient measure to avoid gaps between adjacent loops. The insulative backing serves as outer insulators; however, this does not preclude the possibility of providing an additional jacket. It should be noted, that the avoidance of gaps between adjacent loops is not a principle requirement of operativeness, but any gap introduces lossiness into the cable and should be avoided for that reason.
The invention can be practiced somewhat differently but the difference relates in principle only to the manufacturing of the cable to obviate the need for helical winding of tape or wire. The basic construction of core 1 with dielectric cover, 3 or 3', as the case may be, remains the same; but the shield or return conductor is made somewhat differently. The construction begins with a metal strip 15 having oblique, parallel slots 16. The stripping is held together along both edges, as the slots do not traverse the strip completely. That tape, or strip 16, is now longitudinally bent and wrapped around the core 3 or 3'. However, the strip is wider than the circumference of the tube to be made, so that the strip will abut along the lines 161 and 162. The edge portions are then bent up, with lines 161 and 162 serving as fold lines, to form a tap as shown in FIG. 6. The abutting edge portions are then tap-welded to obtain a closed tube. The tape is then cut off, e.g., along the dash-dot lines so that no straight, axially continuous conductor path remains. As for example, portion 163 is welded to 164, a continuous helix has been formed, so that current flows in this conductor only along a helical path.
The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.