Abstract:
A railroad communication system includes a radio transmitter for generating radio communications signals and a length of railroad rail coupled to the radio transmitter. The length of rail is disposed on a set of nonconductive railroad ties to form a transmission line for radiating the radio communications signals to a radio receiver in a vicinity of the length of railroad rail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/983,769, filed Apr. 24, 2014, which is incorporated herein by reference for all purposes. 
    
    
     FIELD OF INVENTION 
     The present invention relates in general to the wireless transmission of communications signals, and in particular to systems and methods for using a railroad rail as a radiating element for transmitting wireless communications signals. 
     BACKGROUND OF INVENTION 
     Railroads use a number of different wireless communications systems, including radios, in their operations. For example, radio communications between locomotives and waysides is an important component of the Positive Train Control (PTC) system being implemented in the United States. In addition, railroads rely on radios to communicate with personnel out in the field, including those working in the proximity of active railroad tracks. Hence improving railroad radio communications capabilities is an important factor in ensuring safe and efficient railroad operations. 
     SUMMARY OF INVENTION 
     The principles of the present invention are generally embodied in systems and methods in which a conventional railroad rail is used to carry and radiate radio frequency (RF) signals at one or more frequencies to nearby radio receivers. Among other things, these systems and methods support the transmission of messages to alert rail side workers of an approaching train, transmit positive train control (PTC) messages between locomotives and wayside radio units, as well as provide a radio frequency transmission structure suitable for other railway radio communications applications. 
     One particular representative embodiment of the principles of the present invention is a railroad communication system, which includes a radio transmitter for generating radio communications signals and a length of railroad rail coupled to the radio transmitter. The length of rail is disposed on a set of nonconductive railroad ties to form a transmission line for radiating the radio communications signals to a radio receiver in a vicinity. 
     Among other things, the present principles take advantage of the existing railroad infrastructure as a component in an extensive communications system that is critical for maintaining efficient railroad operations and safety. Advantageously, these principles can be applied to rail blocks having rails separated by insulators for maintaining DC communications or for continuous rail systems. Existing radios, such as those used in the PTC system, can suitably be used to generate the transmit signals, as well as receive signals radiated from the rail. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a conceptual diagram of a small section of a microstrip structure commonly used as a transmission line for carrying electrical signals; 
         FIG. 2  is a perspective view of a small section of conventional railroad track, including a portion of one of a pair of parallel rails and their associated ties; 
         FIG. 3  is a cross-sectional view of a section of conventional railroad rail; 
         FIG. 4  is a perspective view illustrating the insulators between a pair of conventional rails of a small section of a conventional railroad track; 
         FIG. 5  illustrates the radiated signal strength along a representative section of railroad track operating as a radiator according to the principles of the present invention; 
         FIG. 6  illustrates a representative application of the present inventive principles in which a radio transmits a wireless warning signal using a railroad rail as a radiating element to another radio carried by a worker working trackside in the vicinity of the railroad rail; 
         FIG. 7  illustrates another representative application of the inventive principles in which a wayside radio transmits wireless signals using a railroad track as a radiating element to another radio on a locomotive on the railroad rail; and 
         FIG. 8  shows exemplary interconnection between an transmitting radio and a railroad rail being used as a radiating element for transmitting wireless signals. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in  FIGS. 1-8  of the drawings, in which like numbers designate like parts. 
     The structure formed by a conventional railroad sitting on a conventional railroad tie is similar to that of a microstrip transmission line, although the relative dimensions of the railroad rail are much larger than that of the typical microstrip line used in small-scale electrical systems, such as printed circuit boards. As a result, a rail can be used as a transmission line for carrying and radiating radio frequency signals at several different frequencies. These signals could, for example, carry warning messages to alert rail side workers of an approaching train, transmit positive train control (PTC) messages from wayside radio units to nearby locomotives, and carry similar signals needed for implementing various other railway communications. 
     More specifically,  FIG. 1  illustrates a conventional microstrip structure  100  used as a transmission line for radio frequency (RF) and microwave signals. In exemplary microstrip structure  100 , a microstrip  101 , which a strip of conductive material having a width W, a length l, and a thickness t, is separated from a ground plane  102  by a layer of dielectric  103  of thickness h. 
     For comparison, a small section of conventional railroad rail  200  is shown in  FIG. 2 , along with its cross-section in  FIG. 3 . Rail  200  includes a head  300 , a base  301 , and a web  302 . A typical heavy freight rail is about 2 23/32″ wide across head  300  (i.e., W=2 23/32″) and about 6⅝″ tall, as measured from the bottom of base  301  to the top of head  300  (i.e., t=6⅝″). As shown in  FIG. 2 , the typical heavy freight rail is suspended over the ground by 7″ tall ties  201  (i.e., h=7″). Using these figures for W, t, and h respectively, the characteristic impedance of a rail as microstrip is approximately 180 Ohms. 
     A simulation was performed in which these rail dimensions were entered into an Method of Moments electromagnetic simulation tool and driven with a source signal at 220 MHz, which is the nominal communications frequency used in the PTC system. Included in the simulation was a ⅛″ gap with a Kevlar insulator  401  ( FIG. 4 ), typically used for electrically isolating adjacent track blocks when the rail is used for DC signaling. (The principles of the present invention are equally applicable to continuously welded tracks, which use audio signaling detectors, which are not affected by RF signals.) 
       FIG. 5  shows the simulated radiated signal strength along a length of the track and demonstrates that a electric field (c) of −6 dBV/m can be consistently achieved, which is well above the minimum signal level requirements of current radio receivers. Under the simulated conditions, the electrical field was found to be sufficient to support communications with the handheld radios carried by railroad workers within a nominal 1500 foot radius along a nominal 1000 foot radiating length of track  200 . (While the −6 dBV/m value for the electric field was determined through simulation using the exemplary dimensions described above for the rail and ties, the actual value for the electrical field strength may vary in actual implementations, depending on such factors as differences in rail head width, rail height, tie height, transmitter power, and so on. Given the physical dimensions of the track and ties, the transmitter power may accordingly be varied depending on the desired size of the communications area surrounding the radiating track. For example, depending on the transmitter, the radial coverage of the electrical field could be extended beyond the simulated 1500 foot nominal radius and/or the length of the radiating section of track extended beyond the simulated 1000 feet to a mile or more.) 
     This ability of the rail to radiate signals therefore advantageously allows for the implementation of numerous communication applications between devices in close proximity of the rails. In other words, the rail becomes part of the communications link between radios located near the rail and a wireless aggregation radio located at wayside. Two exemplary implementations are shown in  FIGS. 6 and 7 . 
     In  FIG. 6 , a wayside PTC radio  600  and an optional track radio  601  transmit messages to the radio receivers  602   a  and  602   b  carried railroad workers in the vicinity of rail  200 . These messages could carry, for example, warnings about the approach of a train on the track. PCT radio  600  and track radio  601 , as well as the required modulation and messaging protocols, could be, for example, those described in U.S. Pat. No. 8,279,796, U.S. Pat. No. 8,340,056. U.S. Pat. No. 8,374,291, and U.S. Pat. No. 8,605,754, which are incorporated herein for all purposes. Optional track radio  601  is preferably used when a different frequency, modulation, or messaging protocol from that used by PTC radio  600  is desired. 
     In  FIG. 7 , a similar PTC radio  600  at a wayside is shown transmitting PTC messages to a corresponding radio on a train locomotive  700  using one of the rails  200  of the track as a radiator. An electric field of −6 dBV/m advantageously provides sufficient signal strength at the height of the locomotive  700  PTC antenna for reliable message transmission. 
     A preferred interconnection between the PCT and/or track radios  600  and  601  shown in  FIGS. 6 and 7  and the rail being used as a radiator is shown in  FIG. 8 . In the embodiment shown in  FIG. 8 , a coaxial cable  800  carries the RF signal transmitted by PTC radio  600 , for the system shown in  FIG. 7 , or by track radio  601 , for the system shown in  FIG. 6 , to rail  200 . The center conductor of coaxial cable  800  couples to rail  200  through a bolt  801 , which preferably extends through an existing hole in web  302 . In alternate embodiments, conductive tape or conductive epoxy may be used to couple the center conductor of coaxial cable  800  to rail web  305  in lieu of bolt  801 . The shield of coaxial cable  800  is grounded through a ground rod  802  and a ground lead  803 . In alternate embodiments, different radio-to-rail interconnection techniques may be used. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     It is therefore contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.