US2312824A - Communication system - Google Patents

Description

March 2, 1943.

F. H. KROGER COMMUNICATION SYSTEM Fild Aug. 17, 1940 INVENTOR FRED H. KROGER BY ATTORNEY Fred H. Kroger, Les Radio Corporation of Delaware Application August 17, 1940, Serial No. 353,004

6 Claims.

This invention relates to a wireless communication system for use over very short distances. The system of the invention finds particular usefulness in establishing communication between points spaced about one thousand feet apart and less, although it should be understood that its range of usefulness is not. limited by this particular distance. 7

Some of the objects of the present invention are:

(1) To provide a wireless communication sys-- tem for use over short distances wherein for all points within the range of satisfactory communication the rate of decrease .with respect to distance of the signal power approximates the inverse sixth power of the distance;

(2) To provide a wireless communication system employing audio carrier waves (for example 9 kc.) and functioning by virtue of the induction fleld, and without depending on radiation.

FFICE Angela, Calif assignmto of America, a corporation (ca. its-e2) (3) To provide a wireless communication sys- I ter whose magnetic field falls of! as the inverse cube of the distance and whose usefulness-is not'interfered with by objects in the transmission path.

(4) To provide equipment of the foregoing type which is compact, of light weight, portable, inexpensive to manufacture, and consumes only a small amount of power, and does not cause interference to other communication systems not using wires.

The invention in its preferred form comprises a battery-operated and portable transceiver which requires only a simple switching operation to change the system from a transmitting con-' dition to a receiving condition, and vice versa. For this purpose, the equipment need use only standard small vacuum tubes of the batteryoperated type. The term "transceiver is here used to designate a single piece of apparatus which can be signal waves.

An essential aspect of the invention resides in the fact that it employs frequencies which are so low that throughout the distances over which satisfactory communication is provided, substantially all of the voltage received oy"'a"receiver in accordance with the invention is caused by the induction held of the transmitter, and the radiated field is so small as to be entirely negligible and ir.capable of producing interi'erence beyond the satisfactory range of communication. low frequencies I By means of such relatively am able to make the induction used both to transmit and receive field greatly exceed tances which while important uses.

The invention finds use in small are sufilcient for many establishing com- I munication between automobiles traveling in the diiferent transmitted frequencies to inform the driver of traffic conditions. The invention may also be employed at fires so that the firemen can freely advise each other and the fire chief of conditions and receive instructions. Other uses are in fabricating shops and in laboratories and the like, where the center of activity moves about and the layout is continually varying, and also in the erection of major size structures for the purpose of correlating activities within the range of some 400 it. The invention may also be used for military purposes and for communication between two houses separated a relatively short distance apart.

Other objects and aspects of the invention p r in the following description taken in conjunction with a drawing whose single figure shows, by way of example only, one embodiment of the present invention.

In the drawing there is shown a combined transmitting and receiving apparatus (sometimes called a transceiver) which requires only a simple switching operation to change the system from a transmitting condition to a receiving condition, and vice versa. The transceiver of the invention includes a suitable induction pickup device I, here shown by way of illustration as a large coil, 9. carrier frequency vacuum tube amplifier 2, a vacuum tube oscillator-detector 3, a signal amplifier 5, a loudspeaker or head-phone set, '5, and a microphone or telephone transmitter 6. The switches l, 8, 9, l0,- H, i2 and 23 are unicontrclled and when in the full line posltion condition connect the apparatus for receiving purposes, while in the dotted tion condition connect the apparatus for transmitting purposes. Condensers 24,- 24 are blocking condensers, and coils 25, 25 are choke coils. The 1 ii i, in combination with variable condenser Is, provides a circuit tuned to the carrier frequency. The plate circuit of the tube 3 includes a parallel tuned circuit it which is tuned line posithe radiation field up to dis- 1 2 to the carrier of the waves to be transmitted or received. Connected to one terminal of the tuned circuit I4 is a suitable network comprising a high resistance l5 and a by-pass condenser 23 in parallel relation. This resistance provides a highimpedance to audio frequencies, and if desired may be replaced by a suitable choke coil arrangement. The condenser 26 provides a bypass (that is, path of very low impedance) for currents of the carrier frequency.

The oscillator-detector 3 functions as a detector for the carrier frequency when used for receiving purposes and as an oscillator when used for transmitting purposes. In the receiving position and with the switches Ill, II and I2 in the full line positions indicated in the drawing, the incoming carrier waves are impressed on the grid of the vacuum tube 3 over lead 16. The output circuit l4 tuned to the carrier frequency will be short-circuited by switch II and hence only the demodulated audio frequency components of the received message waves will flow over lead II to the grid'of the signal vacuum tube amplifier 4. The signals amplified by tube 4 will pass over lead l8 to the loudspeaker or head-phones 5. In the transmitting condition, the network of resistance l5 and by-pass condenser 26 is short-circuited by switch ll now in the dotted position, and the carrier frequency tuned circuit l4 functions with the inductance coil l9 coupled thereto to provide a feed-back circuit for the vacuum tube 3 thus producing oscillations of the carrier frequency. Grid modulation of the oscillator is effected by means of the waves impressed on the grid of tube 3 over a path including microphone 5, switch 1, signal amplifier 4, lead l3, swich 23, audio frequency transformer 20, coil l9, switch l and lead 2 l.

A brief description of the operation of the system of the invention illustrated in the drawing will now be given. Assuming that the carrier frequency to be employed for transmission is 8000 cycles (8 kc.) and it is desired to transmit message waves, the switches 1,. 8, 9, l0, ll, 12 and 23 will be in the dotted line position as shown. The voice currents impressed on the microphone or telephone transmitter will be transmitted through the audio frequency transformer 22 and position to the grid of the vacuum tube signal amplifier 4 where they will be amplified and the amplified voice currents passed over lead l8 and switch 23 to the transformer 20 from which they will flow through the inductance coil I! to the grid of the vacuum tube 3, thus modulating the oscillations produced by the oscillator 3. Since the switch H is in the transmitting position the network of resistance be shorted and thus have no effect whatever, while the parallel tuned circuit I4 tuned to the carrier frequency of 8 kc, will have full eflect. The modulated carrier frequency oscillations from the oscillator 3 will flow over lead l1 and through blocking condenser 24 and switch 8 to the grid of the carrier amplifier 2 where they will be amplified and the output of this carrier amplifier applied to the carrier tuned circult composed of the variable condenser l3 and the coil I. The coil I may be of the loop type as shown, or a solenoid 01f a capacity antenna, and will produce an induction field which is solely relied upon for purposes of communication. This induction field, in the present case a magnetic field, falls off as the inverse cube of the will flow over switch I in closed I I5 and condenser 26 will distance from the apparatus of the invention and consequently is effective for only relatively short distances. Putting it another way, it may be said that the signal power produced at the transducer decreases with distance at a rate approximating the inverse sixth power of the distance. Within the relatively short distance within which communication is to be effected with a receiver of the type also shown in the drawing, let us say within a thousand feet more or less, the induction field greatly exceeds the radiation field. For distances, however, which are beyond the distance over which satisfactory communication is intended, the radiated field is so small as to be entirely negligible and incapable of producing interference with other systems of communication, such as radio frequency systems. Because of the characteristics of the induction field, the usual objects in the environment, such as buildings in the transmission path between.

the transmitter and receiver, will not cause shadows; that is, the signals from the transmitter of the invention will fiow around these objects and in this way the usefulness of the communication system is not interfered with.

For receiving purposes, the switches I, 8, 9, l3, ll, l2 and 23 will be in the full line position. The induction field produced by a transmitter of the type shown in the drawing will set up in the receiver tuned circuit I, I3 a carrier frequency which will be applied to the grid of the carrier amplifier 2. The output or amplified carrier frequency from tube 2 will pass through blocking condenser 24 and over lead l6 and switch 12 to the grid of the vacuum tube 3 now functioning as a detector. Since the carrier frequency tuned circuit l4 associated with a vacuum tube 3 will now be short-circuited, the network of resistance l5 and condenser 26 will be fully effective and the demodulated signal frequency components in the output of the detector will fiow over lead l1 and through blocking condenser 24 to the grid of the signal amplifier 4 from which the amplified components will be applied to the loudspeaker 5 over a path comprising lead l3 and switch 23.

Although the illustrated embodiment of the invention has been described for use with a carrier frequency of 8 kc., it should be distinctly understood that this particular carrier frequency is given by way of example only, and thatthe invention is not limited thereto since other relatively low frequency carrier frequencies can also be employed, so long as the carrier frequency is low enough to provide an inductive field which exceeds the radiation field over the distance of communication.

In one experimental embodiment tried out in practice, a carrier frequency of 11.5 kc. was employed and communication effected at a distance of 600 ft. In this experimental model a coil one .meter square with 40 turns of wire was employed for the coil. No carrier frequency amplifier was used and energy from the loop was fed directly into the oscillator-detector tube 3. It will be obvious that where a loop is employed it may be used either in the vertical or horizontal plane.

What I claim is: A

1. A transceiver for effecting wireless communication between stations spaced apart a very short distance, comprising an inductor, an electron discharge device circuit for generating and impressing upon said inductor a sinusoidal lowfrequency carrier currentysaid low frequency being ofsuch order that at distances from the eluding means for tance wireless prising an impedance device having an induction 'tinuous wave ciated withits transceiver where the induction field is smaller than the radiated held the intensity oi said radiated field is too small to cause interference with other signalling systems, a switch for convetting said electron discharge device circuit from a generator of low frequency carrier current to a detectorof low frequency carrier current, a loudspeaker, a microphone, an additional switch for operatively associating either said loudspeaker or said microphone with said device, whereby said loudspeaker is connected to the output of said device when said circuit functions as a detector and said microphone is connected to mid device when said circuit functions as'ar oscillator.

2. A system in accordance with claim 1, in-- simultaneously operating both of said switching means. 7 1 e i 3. A transceiver for use in every short distance wireless system or communication comcoil of which is associated with its input electrode, a microphone coupled to said generator for modulating the carrier frequency current,

quency current, a

prising an inductor, an electron discharge device circuit for generating and impressing upon said inductor a sinusoidal audio frequency carrier current, a switc for, converting said electron discharge device circuit from a generator of audio frequency carrier current'to a detector of audio frequency carrier current, a loudspeaker, a microphone, an additional switch for operatively associating either said loudspeaker or said microphone with said device, and means,

linked to 50th said switches for controlling the same simultaneously whereby said loudspeaker said device when said circuit functions as a detector and said is connected to the output 0! microphone is connected to the input of said device when said circuit functions asan se nlater a. transceiver system 0! communication, comtube oscillation generator im- Drwing on said device an audio frequency concarrier current, said generator in cluding steed-back circuit having a pair of inductively coupled coils one coil oi which is asaooutput electrode and the other for use in a very short dis-.

an electro-acoustic transducer, and switching means for disconnectingsaid vacuum tube from said microphone and operatively associating said tube with said transducer to serve as a detector of audio frequency carrier current received on said device.

5. A transceiver for use in a very short distance wireless system 01'. communication, comprising an impedance device having an induction field, a vacuum tube oscillation generator impressing on said device an audio frequency continuous wave carrier current, said generator including a feed-back circuit, said generator for modulating the carrier i'resignal amplifier having an input electrode coupled to said means and an output electrode coupled to said generator, an

audio frequency translating device. switches associated with the input electrode oi said ampliher for disconnecting said input electrode from said modulating means and {or operatively associating it with the output of said vacuum tube generator, and additional switches for disconnecting the output electrode of said amplifier operatively assofrom said generator and for ciating it withsaid translating device-and also for converting said generator 6.'A transceiver for use tance wireless system of communication, comprising an impedance device having an induction held, a vacuum tube oscillation generator impressing on said device an audio frequency continuous wave-carrier current, said generator ineluding an external feed-back circuit, a microphone coupied to said generator for modulating the carrier frequency current, an electro-acoustic transducer, and ing said vacuum operatively ducer to serve as a detector of switching means for disconnecttube from said microphone and associating said tube with said transaudio frequency on said device.

carrier current received means coupled to to a detector 0: ,audio frequency carrier current.

in a very short dic-

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