US1688692A - Wired radio on power lines - Google Patents

Description

Oct. 23, 1928.

R. D. DUNCAN, JR

WIRED RADIO ON POWER LINES Filed March l2, 1925 INVENTOR effa/fa v BY M? 0%M1WM ATTORNEY` Patented Oct. 23, 1928.

UNITEDI STATES PATENT OFFICE.

ROBERT D. DUNCAN, J' R., 0E EAST ORANGE, NEW' JERSEY, ASSIGNOR TO WIRED RADIO, -IN C., OF NEW YORK, N. Y., A CORPORATION 0F DELAWARE.

winni) RADIO oN POWER LINES.

Application led March 12 1925. Serial No. 14,856.4

This invention has to do with the transmission of high frequency carrier current for signaling purposes over electric power lines-especially wired radio broadcastingand has vfor its object to modify the'charac- -teristics, at high frequencies, of powervdistribution transformers which, under some conditions, are found to be deficient in their ability to transmit high frequency currents, and thereby to obtain more uniform distribution of the carrier current through the entire power network.

Experience has shown that in electric power distribution systems, on which high frequency currents are superimposed as in wired radio broadcasting, that as between subscribers about equally distant from the central power station, at which the carrier current is impressed on the system, there is often a large difference in the strength of signals received. This condition has been found to be due, in some cases, to the fact that certain power transformers,4 not being designed with a view to passing high frequencies, have such high input impedance as viewed from the transmission line that very little high frequency current can pass.

through the turns of the primary windings, and, accordingly, there is failure to induce suflicient high frequency current into the secondary windings. This condition may be cured by the use of high frequency by-passp ing circuits between the primary and secondary windings but to do so it is necessary to employ expensive condensers,` properly housed and mounted. Ordinarily two condensers would be required for each trans- .inductance is preferably so chosen that the effective overall reactance at the high frequency dealt with, of the combined primary radio operation over electric lighting svsi tems, with the consequent absence of coinpreliensive experimental data, the high frequency deficiencies of various types and sizes of distribution transformers have had to be determined by actual receiving tests at subscribers stations served by different transformers. Having thus determined the existence of deficiencies, a subsequent study and determination of the high frequency characteristics of the transformers-has disclosed the reasons for such deficiencies .and yielded the present method of correcting therefor.

For a more comprehensive explanation and description of the invention reference will now be had to the accompanying drawing, in which, f

Fig. l is a diagrammatic illustration of a power transmission line having a traiisformer bridged across it and on which itis assumed there is superimposed a high frequency carrier current. Figs. 2, 3 4and 4; are reactance and resistance curves,

Fig. 5 is an equivalent circuit diagram illustrating the character of the high frequency impedance of the primary winding of `the transformer of Fig. 1,

In Fig. 1, 1 and 2 are the two conductors,

respectively, of a power transmission line carrying low frequency power current generally at a relatively high voltage and high frequency signaling current or carrier current. 3 is the primary winding of a power transformer 4, and is bridged across the line. The secondar winding 5 is connected to local distribution ines 6, 7 and 8 and at its mid point is usually grounded as indicatedat 9. Across the lines 6, 7 and 8 are shown lamps or other power consuming devices 10 and wired radio receiving apparatus 11.

The high frequency ,current` I, which'will flow through the primary winding 3, due to the high frequency voltage E. across the terminals of the primary winding is given by (1) :i--ZE where Z is the input impedance of the transformer eective at the particular high frequency dealt with. The lower the value of Z,

the greater that of I, for a constant E.

. relatively low positive value, that is, inductive reactance predominates-increasing with increase of frequency to a positive maximum value, and drops through zero to a maximum negative value, that is, capacitive reactance which with further increase of frequency approaches zero asymptotically.

The frequency fo may be of the order of several thousand cycles, its exact value depending upon the size of the transformer, its loadingand other factors.

The resistance component of the impedance as a function ofthe frequency is given by curve 13 of Fig. 3 and, as indicated, is always positiveattaining a high value at fo.

At any frequency f, the impedance, is expressed by and since a2 is negative, that'is, capacitive,

ordinarily dealt with: in a ,certain one, kilowatt transformer, at a frequency of 40,000 cycles r=7,000 ohms; m=53,000 ohms; Z= 53,500 ohms.

From Equation 1) it is obvious that an increase in current I may be had by a decrease in the value of Z. With a constant frequency, the impedance Z may be decreased by decreasing either of its components r or The latter offers vthe most immediate possibilities. By adding, in series with the primary winding 3, an auxiliary inductance as shown in Fig. 6, the capacitive reactance is either wholly or in part offset. In Fig. 6 the auxiliary inductance is represented by reference numeral 16.

The manner of obtaining a reduction in the capacitive reactance is illustrated by curves 12 and 14 of Fig. 2 and curve 15 of Fig. 4.

The reactance of a pure inductance is given by (4) 1E" 2 'lrfL where f is the frequency and L the inductance. This is the equation, of a straight line with w and f as the variables and is represented by curve 14 of Fig. 2. The effective reactance of the circuit 16, 3 of Fig. 6 is the algebraic sum of the ordinates of curves 12 and 14 which yield curve 15 of Fig. 4. Then in Fig. 4,

Again, viewed from the standpoint of lines 1, 2, the circuit of Fig. 6 has the characteristics o f and may be represented by the equivalent circuit of Fig. 7 The impedance of this circuit is Utilizing the values previously given wherein m=53,000 ohms and lassuming that it is desired to reduce this by it is obvious that m =26,500 ohms.

From (4) L= 0.1055 henrys Translated into simple language, this means, in the case of the transformer previously referred to, that the reactance of the primary winding at 40,000 cycles may be reduced one-half by the addition, in Vseries therewith, of an auxiliary inductance of .1055 henrys. Assuming that the high frequency voltage drop across the primary winding and auxiliary inductance in series remains unchanged, the high frequency currents passing through the transformer would be approximately doubled.

In most alternating current power distribution networks, the lines extending from the power stations or sub-stations each have a considerable number of transformers, some of which are much more efficient than others in passing high frequency carrier currents. It has been found, however, that the majority of such transformers are capable of passing sufficient high frequency current to require no modification, that is'- to say, they do not require the addition of auxiliary inductance. In fact, it is found that usually only a small percentage of the transformers require such treatment. As a rule it is not desirable to modify the high frequencyimpedance of a deficient transformer' so that it will pass the maximum possible amount of highI frequency current, but only to such an extent that it will pass whatever is regarded as the necessary amount to effect satisfactory reception at every subscribers station served.

The auxiliary inductance should, of course, be constructed to carry safely the power current which may be in the neighborhood of amperes.

It is believed that in no case would the reactance of an auxiliary inductance, such as would be required, be suiiiciently high to materially affect the power current.

I claim:

1. A wired radio system comprising a power transmission line for the delivery of power and high frequency signaling energy to a plurality of subscriber stations, apower transformer lat each of said subscriber stations, said power transformer having primary and secondary windings for delivery of said power to a load circuit, an auxiliary inductance device interposed in series with said primary winding and connected across said transmission line, the combined impedance of said primary winding and said auxiliary inductance at the frequency of said signaling energy being less than the impedance of said primary winding alone at the same frequency said auxiliary inductance being designed to pass said power to said'load simultaneously with the delivery of said high frequency signaling energy to said load through said transformer.

2. A wired radio system comprising a transmission line for simultaneously conveying power and high frequency signaling energy to a plurality of subscriber stations, aV plurality of power transformers each having primary and secondary windings, with the primary windings thereof connected in bridge to said line, a load circuit connected to said secondary windings including power .consuming devices and wired radio receivmg apparatus, and means for substantially equalizing the supply of high frequency signaling energy to said wired radio receiving apparatus at each of said subscriber stations said means comprising an inductance device connected in series with the primary winding of selected power transformers and in bridge to said line, said inductance devices each having such value that the combined impedance of associatedprimary windings and said inductance devices at the frequency of said signaling energy is substantially less than the impedance of said primary winding alone at sa1d frequency whereby said signaling energy may be transferred to said wired radio receiving apparatus atlrelatively large amplitude simultaneously with the supply of power to said load circuit through said transformers.

In testimony whereof I aiix my signature.

r ROBERT D. DNCAN, JR.

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