Patent GB-216992-A (GB)
Title: Improvements in loaded telephone & the like transmission systems
IPC: H01B11/16
Filed: 1923-04-06
Abstract: 216,992. Western Electric Co., Ltd., (Western Electric Co., Inc.). April 6, 1923. Wired wireless systems. - In a loaded telephone &c. transmission system including a phantomed group of conductors, the phantom 'circuit is loaded to transmit one range of frequencies and the physical side-circuits are loaded to transmit another range of frequencies. The invention is described as applied to a system in which voice frequencies are transmitted over the phantom circuit, whilst carrier frequencies of the order of 30,000 cycles per second are transmitted over the two physical circuits. Fig. 1 shows the loading employed in a section of cable circuit intervening between a terminal office and two phantomed open-wire circuts S<1>1, S'2. Phantom-terminating transformers A1, A2 are inserted in the side-circuits S1 S2 from the terminal office, tape being taken from the mid-points of the secondary windings of these transformers to form the phantom circuit P<x>. In view of the different transmission requirements of the carrier currents to be superposed on the side-circuits and of the voice currents sent over the phantom circuit, independent sets of loading coils are used for the side-circuits and for the phantom circuit. In the arrangement shown, a phantom loading coil Ipx is provided for each three equally-spaced side-circuit loading coils Lc, but in practice the ratio of one to six is used. The phantom loading coils Lpx are inserted at side-circuit loading points. The resistance, inductance, and capacity effects which the phantom coils add to the side-circuits tend to cause impedance irregularities which are reduced or neutralized in the following manner. The resistance effect is minimized by so designing the coils that they have the least possible resistance. The inductance effect is neutralize by designing the side-circuit loading coils at the phantom loading point to have an inductance less than the other side-circuit coils by an amount equal to the inductance added to the side-circuits by the phantom coil. The capacity effect is made as low as possible by providing more insulation between the core and the inner windings and between the inner and outer windings of the phantom coil, and by increasing the air-space between the outer windings and the casing. The minimum capacity is then compensated for by reducing the capacity of the side-circuit coils at the phantom loading points by this amount, by shortening the loading section, or by selecting for this loading section a cable circuit having a correspondingly lower capacity. If condensers are used for equalizing the capacity of the loading sections the added capacity may be used to constitute a part of such a condenser. The winding layout of a side-circuit loading coil at a phantom leading point is shown In Fig. 8. As the core A, B is formed of wood or other non-conducting material no capacity effect is present between the inner windings and the core, but a large capacity Cw exists between the inner and outer windings Ai. Bi and Ao, Bo. and a smaller capacity Cp between the outer windings Ao, Bo and the casing : the capacity distribution with respect to the windings is illustrated in Fig. 9. In order to obtain a capacity distribution in the side-circuit of the phantom loading unit corresponding to that produced in the side-circuit by the ordinary side-circuit loading coils, a capacity distribution in the phantom loading coil similar to that shown in Fig. 9 is necessary. This is achieved by winding the phantom coil as illustrated in Fig. 5. The core C comprises four quadrants a, b, c, d, upon each of which an inner winding ai, bi, ci, di and an outer winding ao, bo, co, do is wound. Of the four conductors 1 - - 4 shown, the conductors 1, 2 are used for one sidecircuit and the conductors 3, 4 for the other, an outer winding on one quadrant and an inner winding on the opposite quadrant being included in each conductor. The condensers Kc, Kw, Kp respectively represent the capacity effects between the core and the inner windings, the inner and outer windings, and the outer windings and the casing P. The distribution of these capacities, which is shown in Fig. 6, closely approximates to that of the side.-coils, shown in Fig. 9. The direction of the windings on the phantom coil is evident from Fig. 11, from which it will be seen that for parallel phantom currents flowing over the conductors 1, 2 and returning over conductors 3,4, the resultant fluxes due to the currents in all the windings are aiding, whilst the fluxes due to side-circuit currents over single conductors oppose in inner and outer windings. To secure symmetrical capacity distribution in the loading unit comprising the phantom coil and the two side coils at the same point, the side coil windings are inserted at the electrical centre of the fine windings on the phantom coil, as shown in Fig. 11. The ratios between the resistance and inductance per unit length in the cable and openwire lines are made approximately the same by designing the side-circuit and the phantom coils to have definite resistances dependent upon the gauge of the cable conductors. In designing the phantom loading coils the leakage inductance added to the phantom cable circuit by the sidecircuit loading coils is considered as part of the distributed inductance of the phantom cable circuit and correspondingly allowed for.