This invention has particular applicability in the field of highway or other roadway communications and in providing a restricted-range broadcast service in small communities where conventional broadcast transmitters cannot be used because of lack of availability of AM broadcast channels in the standard broadcast band, now almost fully occupied in many sections of the United States.
Many systems of the inductive-carrier type, including those of the applicant, have been employed in the past for highway, railroad and other uses. However, these have presented serious technical problems when operated at relatively-high carrier frequencies, such as those in the AM broadcast band. Radiation of electrical wave energy, which is an inherent characteristic of inductive-carrier systems when operated at radio frequencies, often extends over distances far in excess of the permissible limit specified by the Federal Communications Commission for low-power radio devices of restricted range type. While it has been possible, by careful adjustment of the radio frequency (r.f.) carrier level to comply with the Commission's rules in certain localized applications, such as the highway radio system installed by the applicant on the George Washington Bridge in 1940, experience in most cases has demonstrated that it is extremely difficult, and in some instances impossible, to comply with the FCC rules over any substantial period when unattended transmitters are employed and, at the same time, to maintain a sufficiently strong induction field at broadcast frequencies to enable good reception in radio-equipped cars traveling over lengths of highway served by the system.
Experience with roadside conductors of various types, including single and dual-conductor transmission lines has indicated that the strength of the induction field about these conductors is subject to substantial variation along their length. Near the transmitter source, for example, the field strength may be too high to comply with FCC rules at broadcast frequencies if a useful, noise-free signal is to be provided in cars on all lanes of the highway served by the system. In addition, if the cable is ground-laid or is in the surface of the right-of-way, as required on turnpikes and thruways where above-surface installations are not desired, variations in the inductive-signaling field due to changes in soil conductivity under different weather conditions and other irregularities in environmental conditions have been found to present difficulties over a substantial period of time in maintaining a reasonably-constant field strength and restriction of radiation within limits set by the FCC.
Moreover, experience with conventional forms of cables, or wires, when employed along the roadside as r.f. signal conductors for the purpose of producing an induction-signaling field as a means of impressing carrier-signal energy on the vertical whip antenna system of radio broadcast receivers carried by motor vehicles indicates that the coupling loss between the vertically disposed vehicle antenna and the horizontally-polarized signals from the roadside cable system, whether in the form of a single longitudinally-extending transmission line or in horizontal loop configuration, encompassing the roadway area, is unnecessarily high. This results in requirement of substantially more r.f. power in the roadside cable system than would be required if a vertically-polarized or convolutive field, having vertical and horizontal polarization characteristics, were provided. The present system incorporates as an important element what are believed to be unusual and novel means for developing such a convolutive field to produce a signal of maximum strength in receiving systems of motor vehicles carrying conventional antennas of vertical whip type. This, in turn assists in meeting the requirements of the FCC with respect to restricted-range radio devices.
An additional, and serious problem, is presented in applying inductive-carrier methods at AM broadcast frequencies in the vicinity of large metropolitan areas, such as New York City and environs, where the AM broadcast band is fully occupied. This is of primary importance insofar as applications of inductive-carrier methods in the field of highway communications is concerned since one of the most valuable functions in these urban areas is in providing information to drivers on such matters as traffic congestion, hazardous or unusual road conditions on the route ahead, routing instructions and other intelligence that will assist motorists on major, and often overcrowded, traffic arteries in the vicinity of large cities.
To illustrate the latter problem and to indicate the nature of the difficulty that is involved, it is pointed out that in the New York City area the lower frequencies in the AM broadcast band, where inductive-carrier systems at broadcast frequencies may most effectively be applied in highway communication services, are fully occupied. For example, 540 kilocycles, a preferred frequency for operation of inductive-carrier systems in areas where this channel is available, is used by a suburban station, employing a 250-watt transmitter in daytime service. The next channel that can be employed for conventional broadcast service in the New York City area under the Commission's allocation plan is 570 kilocycles, occupied by a 50-kilowatt metropolitan-class station. Signals from both stations can be heard throughout the area. If conventional AM broadcast equipment were to be used for the highway service on the frequency of 555 kilocycles, midway between the 540 KC and 570 KC channels assigned to local stations, mutual interference would be produced, assuming that as in standard broadcast operation modulation sidebands would extend to 10 kilocycles above and below the carrier frequency, since sideband areas would overlap. An additional communications problem is presented on parkways, turnpikes and new interstate highways with respect to hazards presented by disabled cars and inability of drivers to quickly summon aid, since conventional wayside telephones often are widely spaced and not locally available. Also, many turnpikes have no wayside telephone circuits to permit installation of telephones at reasonably spaced intervals, within easy walking distance from disabled cars.
Practicable solutions to the problems as set forth above are incorporated in the present invention. These solutions also produce a substantial improvement in the quality and intelligibility of received signals as reproduced by typical AM broadcast receivers now in general use in the majority of motor vehicles; relative uniformity and stability of operation of unattended roadside transmitters is provided; minimization of radiation of wave energy to areas remote from the roadway is attained while maximum intensity and uniformity of the induction field may be maintained over long distances on a common carrier frequency; unwanted transfer of signal energy to roadside electric-power or telephone lines, with the interference potential that such coupling may produce, is minimized; heterodyne beats between adjacent roadside transmitting zones is avoided; and in preferred emvodiments of the invention relaying of signals to vehicles traveling throughout the length of a highway is accomplished without demodulation and remodulation of carrier signals, thus greatly simplifying equipment, minimizing distortion and eliminating over-modulation difficulties that otherwise would exist at remote, unattended highway transmitting points along the roadway system. By use of self-powered carrier telephones that may be located at half-mile intervals along the roadside cable and coupled thereto, together with use of multiple carriers, a distresscalling system of value to motorists is provided. These and other improvements presented by the system of the invention are described in subsequent pages.