US2095774A - Method of modulation for radio transmission - Google Patents

Description

P. B. TAYLOR 2,095,? 74

METHOD OF MODULATION FOR RADIO TRANSMISSION Oct. 12, 1937.

Filed May 4, 1954 WITNESSES:

. Paul B. Taylor Patented Oct. 12, 1937 UNHED STATES rarest orricr IVIETHOD 0F MODULATION FOR RADIO TRANSMISSION Pennsylvania Application May 4, 1934, Serial No. 723,881

4. Claims.

My invention relates to systems for propagating electromagnetic waves, and particularly to systems for radiating signals and programs of music or speech by means of electromagnetic waves in the ether.

One object of my invention is to provide a system in which the shape of the field pattern about an antenna is varied from instant to instant while the total power radiated from the antenna may remain constant. In all or substantially all directions from the antenna the wave is modulated but by making the phase of the modulation different in different directions there may be little or no modulation of the total energy radiated from the antenna, so that the oscillator may work to full capacity at all times. In the system in use in the prior art the field pattern changes in size but not in shape with modulation. In the system of my invention the field pattern changes in shape but not in size with modulation.

One object of my invention is to provide a system in which electromagnetic waves will have one intensity in one direction at any particular instant and a different intensity in another direction, and in which the direction of maximum intensity may be made to change from one such direction to another periodically.

Another object of my invention is to provide a broadcasting system in which the maximum intensity of the radiated electromagnetic wave varies in direction periodically, so that although any given direction is traversed at one time or another at maximum intensity, this maximum intensity does not occur in every direction at the same instant of time. For many purposes the requirement for satisfactory operation is that the maximum instantaneous intensity of the broadcast wave must not fall below a certain value;

and this requirement can be fulfilled by my system with a lower expenditure of total power than with arrangements in which the maximum intensity of the electromagnetic field is the same in all directions.

Another object of my invention is to provide a broadcasting system in which a given maximum instantaneous intensity of magnetic field is attained, regardless of direction, but the instantaneous power output of the transmitting antenna system is reduced to a value less than that in conventional broadcasting systems of the prior art.

Other objects of my invention will become apparent upon reading the following description taken in connection with the drawing in which the single figure is a diagrammatic representation of a broadcasting system embodying the principles of my invention.

In broadcasting speech, music or telegraph signals the receiving instrument in general responds to periodic forces, usually of audible frequency, and the loudness of the indication or response is, in general, proportional to the maximum instantaneous energy of the electromagnetic field at the receiving system. Where receiving systems may be located in any direction in the area surrounding the broadcasting system, this requires that. maximum. intensity of the electromagnetic field should be substantially the same regardless of direction. In ordinary broadcasting systems this condition is met by transmitting systems in which the electromagnetic field varies, in accordance with the signal or sound being broadcast, periodically in respect to time but is fairly uniform and invariable at any particular instant as respects direction about the broadcasting station. In contrast to the foregoing I provide a transmitting system which produces an electromagnetic field which varies in time periodically at the signal frequency as respects direction relative to the transmitting station. It can be shown, as regards such an electromagnetic field, that while the maximum instantaneous intensity of the electromagnetic field in any particular direction is the same as the 3 tional type in which the electromagnetic field varies periodically with respect to time and not with respect to direction; and when this sinusoidal limitation does not exist, may be for less than 50 percent.

In general, a transmitter of the above described type can be produced by arranging two electromagnetic radiators or antennae separated from each in space by a substantial fraction of a wave length, supplying them respectively with periodic electric currents of proper frequency for radiation, and producing periodic variations of the phase of these currents relative to each other at a frequency corresponding to the sound or a signal which it is desired to transmit.

Referring particularly to the central figure of the drawing, I and 2 are antennae, preferably of the duplex or balanced type and of the same size, which are spaced apart from each other, this-spacing being preferably although not necessarily equalto a quarter of the wave length of p the radio frequency current supplied to these antennae. Ihe two antennae I and 2 are supplied With current of the same frequency from any desired source or sources; this may conveniently be done by supplying them through'lines 3l, 5+5 leading from the single source of radio frequency current source '5. Certain of the lines 3:l, may be provided with variable impedance elements 89, l@--I i by means of which the speed of propagation of alternating current waves from the source 1 to the antennae I and 2 may be adjusted at will. Since the phase of the radio frequency current in the antenna I relative.

to the phase of the radio frequency current in the antenna 2 will depend upon the speed of propagation along the respective lines 34, 56, this relative phase relationship may be adjusted to any desired value at any instant by making the value of the impedance 8ll diifer from the value of the impedance 9II by a suitable amount.

When the speed or propagation of the electric current over the line 3 i, 8l!3 is greater than the speed of propagation of the electric current lines 5, 9--i i, the currents in the antenna 5 will lead in phase the currents in the antenna 2; and vice versa. As an example, suppose the radio frequency current in the two antennae are equal in R. M. S. value and that this lead in phase of the currents in antenna I over the currents in antenna 2 is 90 electrical degrees. An electromagnetic wave radiated from antenna 2 and travelling in space in the direction of antenna I will reach the latter after an interval equal to where x is the distance between the antennae- I2, Z is the wave length of the radiated wave, and a is the time period of a radio frequency cycle. This means that the phase of this electromagnetic wave at antenna I is 360 electrical degrees behind the phase of the current in antenna 2. If the distance between antennae I and 2 is a quarter wave length, this radiated wave at antenna :1 is 90 electrical degrees behind the phase of the current in antenna 2. If, as in this example under discussion, the phase in the current of antenna 2 is 90 behind the phase 7 behind the phase of the current in antenna I; a

but in accordance with our example the'current in the antenna l is 90 electrical degreesahead of the current in the antenna 2 and accordingly At all points to the left of anthe phase difierence between the wave radiated from antenna I an'd'that radiated from antenna 2 at points to the right of antenna 2 in the drawing is zero; 1. e., the energies of the two waves are added to each other, with a resultant equal to substantially twice that of either one alone.

Summarizing the foregoing, radio frequency currents bearing the phase relation just described in the respective antennae I and 2 produce an electromagnetic field having a maximum intensity twice that due to either antennae alone at points to the right of antenna 2, and produce substantially zero electromagnetic intensity at points to the left of antenna I.

Now suppose that the respective phases of the currents of antennae I and '2 relative to each other are altered in any way; for example, by increasingthe impedances 8I 0 and decreasing the impedances I I to such a degree that the radio frequency current in antenna I lags behind the radio frequency current in antenna 2 by 90, their R. M. S. value still being equal however. Considerations similar to those just discussed will show that the electromagnetic field to the left of antenna I in the drawing is double the intensity of the field which would be due to antenna I alone; and at points to the right of antenna 2 the electromagnetic field intensity is substantially zero.

If the respective impedances 8Iil and 9II are varied periodically at the audio frequency of the sound to be transmitted, as by means of the microphone i2, an electromagnetic field, having an intensity double that due to either antenna alone will be displaced at the same periodicity and will sweep around in space about the antenna system I 2. The intensity of the response of any receiving instrument in the space swept over, being dependent upon the maximum instantaneous value of this electromagnetic field, will accordingly be proportional to twice the intensity of the field emanating from each antenna alone.

Where it is desired that the maximum intensity of the field, referred to in the preceding paragraph as sweeping around in space, shall be of constant magnitude, and that the field energy shall not be wasted partly in an ancillary beam of lower intensity than the main field, the range of variation of the phases of the currents in the respective antennae should not extend beyond plus or minus 90 electrical degrees from coincidence with each other.

It may be shown by considerations similar to those just applied that for antennae spaced a distance a: apart traversed by currents difiering in phase by 9 electrical degrees the two antenna radiations are V electrical degrees apart in phase beyond one antenna and electrical degrees apart beyond the other. 7

If, instead of antennae I-2, a single antenna were used, it would produce a maximum intensity of electromagnetic field equal to twice that of a single one of my antennae (i. e. it would produce the same maximum field intensity as was just described above) provided it carried twice the radio frequency current supplied to the latter. In accordance with well known principles of electromagnetic fields, the rate of energy radiation from an antenna of given dimension is proportional to the square of radio frequency current. The power required for this single antenna would accordingly be four times as great as that supplied to each antenna in my system, and correspondingly be twice as great as the combined power required for my antennae i and 2. It is accordingly seen that my system produces the same intensity of response in a distant receiver as a conventional signal antenna of the prior art with only half the power input required by the latter.

In order to permit the microphone E2 to control and periodically vary the impedance of reactors 8, these are provided with similar windings |3l4 supplied from a source of direct current i5. These reactor cores 8 and 9, also respectively provided with windings |6ll, are traversed by periodic current controlled by microphone l2; the winding it being arranged to magnetically oppose its associated winding 13 at the same time that the winding l? magnetically assists its associated winding it. With such an arrangement, the reactance of reactor 8 will be a maximum when the reactance of reactor 9 is a minimum, and this relationship will be reversed when the direction of the periodic current set up in these windings H3 and H by microphone I 2 reverses. In order that the current in each antenna shall not decrease when the reactance 8 increases, as is desirable though not essential for certain fields of use, I may provide a condenser it connected through a grid controlled high vacuum or gas-filled tube 59, in shunt to condenser l; and control the grid potential of the tube I9 so that its reactance to the radio frequency decreases to compensate the increase of reactance 8 and vice versa. This may be done by connecting the cathode of tube I9 to the mid-point of a resistor 26 in series with winding l6, and the grid of tube is to that end of this resistor 29 which is positive at the time winding i6 magnetically aids winding l3. A similar arrangement of a condenser and tube should shunt condenser l i, except that the grid of the latter tube should be connected to the other end of resistor 26.

The foregoing arrangement for periodically and oppositely varying the impedances 8, 9, l0, and I! is merely illustrative, and other known arrangements for varying impedances or for equalizing the antenna currents may be substituted therefor. For example, condensers lllil might be arranged to be altered in capacity in opposite senses by a microphone diaphragm governed by microphone 52; for example, condensers it, H might be the capacity between end plates on opposite sides of a diaphragm displaced in accordance with the vibrations of the diaphragm of microphone 52. Other types of variable condenser such as that illustrated in C. T. Allcutt Patent No. 1,716,161 might also be controlled by the voice frequency currents such as those generated by microphone I2; and in general substantially any known variable reactance or condenser capable of being made responsive at audio frequencies might be employed for controlling the rate of propagation in the lines 3 i, 5.

I have described a simple form of antenna arrangement adapted to make use of the principles of my invention; but it will be understood that this is only illustrative and that other antenna arrays capable of concentrating electromagnetic radiation in a desired direction, together with arrangements for periodically orientating this direction, may be employed and may be desirable where greater elaboration is commercially justified. Likewise it should be recognized that the particular phase relationships and space displacements of the antennae described in the foregoing example are illustrative only, and my invention is not limited thereto by such illustrations.

In accordance with the patent statutes I have described one particular embodiment of the principles of my invention, but this is intended to ac illustrative only and I accordingly desire that the following claims be given the broadest interpretation of which they are susceptible in view of the limitations imposed by the prior art.

I claim as my invention:

1. In combination, an antenna and means for supplying it with radio frequency current, a second antenna supplied with current of the same frequency and distant from said first antenna by a quarter wave length of said radio frequency current, and means for progressively varying the phase of the current in said first antenna from 9:) electrical degrees ahead to 90 electrical degrees behind that of the current in said second antenna.

2. In combination, an antenna and means for supplying it with radio frequency current, a second antenna supplied with current of the same frequency and distant from said first antenna by a quarter wave length of said current, and means for progressively varying the phase of the current in said first antenna relative to the current in said second antenna in accordance with a signal to be transmitted, whereby a beam of radiation is progressively swept around in space through approximately 360 degrees.

3. In combination, an antenna and means for supplying it with radio frequency current, a second antenna supplied with current of the same frequency and distant from said first antenna by a quarter Wave length of said radio frequency current, and means for progressively varying the phase of the current in said first antenna with a periodicity equal to a signal frequency from 90 degrees ahead to 90 electrical degrees behind that of the current in the second antenna.

i. In combination, an antenna and means for supplying it with radio frequency current, a second antenna supplied with current of the same frequency and distant from said first antenna by a quarter wave length of said current, and means for progressively varying the phase of the current in said first antenna relative to the current in said second antenna periodically at an audio frequency of the signal to be transmitted, whereby a beam of radiation is progressively swept around in space through approximately 360 degrees.

PAUL B. TAYLOR.

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