US2106806A - Relay system - Google Patents

Description

Feb. 1, 1938. v c, w, LATlMER ET AL 2,106,806

RELAY 'SYSTEM Filed July 8, 1936 4 Sheets-Sheet l A/EWBRUNSW/CK, MI I /V 14/ YORK kmmmr/o/v Rs;

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PH/l A DEZ PH/A INVENTOR cw. LATIMER J.L. FINCH A"; H. H. svERAeE ATTORNEY Feb. 1, 1938.

c. w. LATIMER ET AL RELAY SYSTEM Filed July 8, 1936 4 Sheets-Sheet 2 INVENTR c.w. LATIMER; .1. L. FiNCH ERAGE BY AND H H BEV c. w. LATIMER ET AL 2,106,806

RELAY SYSTEM Patented Feb. 1, 1938 UNITED STATES PATENT- OFFICE RELAY SYSTEM ware Application July 8, 1936, Serial No. 89,608

3 Claims.

This invention deals with the relaying of ultra short radio waves.

For many years, experiments have been in progress to apply facsimile methods to long dis- 5 tance radio circuits. It has been found that facsimile may be applied to these circuits very successfully for handling photographs, sketches, and similar material at relatively low speeds. However, since the signals on long distance radio circuits are propagated by reflections from the conducting layers of the ionosphere, the signals arlive over multiple paths differing slightly in length. The result is that the signals traveling by the different paths arrive at the receiver at slightly different times, thereby producing an echo efiect that smudges the received facsimile copy if the speed of transmission is too high. For average transmission conditions on long distance radio circuits, as normally operated, the multipath g0 echoes limit the speed of facsimile transmission to about 15 or words per minute, which is considerably lower than the limiting speed set by the manual operation inherent with the telegraph and printer.

Frequencies above about megacycles exhibit different characteristics than those below 30 megacycles. The very high frequencies are not turned back to earth by the ionosphere, so sky waves are absent. Propagation takes place by 30 ground wave only, so there is fundamentally only one path for the signals to follow and there are no echo eflects due to multiple path transmission. For this reason, the very high frequencies may be used for such services as television and high speed facsimile. However, the ground wave is attenuated rapidly beyond the horizon and also fading sets in at distances appreciably greater than the optical path distance. The attenuation beyond the horizon increases as the frequency is increased. Consequently, for the very high frequencies, the distance for high quality service is practically limited to the optical range. obvious that the range can be increased by placing the transmitting and receiving antennas at the highest possible elevations. In fact, the distance of transmission is generally proportional to the square root of the respective antenna elevations, and the field intensity at a given point within the optical distance is directly proportional to the product of the transmitting and receiving antenna elevations.

One object of this'invention is to provide a two way wide band automatic relay system particularly adapted for ultra short wave facsimile signailing. 1

Preferably this invention utilizes frequencies above mc. Since these high frequencies attenuate very rapidly beyond the horizon, relays are required for longer distance signalling. Moreover, relays near the city ends of the system may 5 be found desirable in order to lay down a strong enough signal to ride well above the high noise level experienced in urban areas.

Further detailed-description of the present ultra short radio relay system will be given in connec- 10 tion with the accompanying drawings.

Figure 1 diagrammatically illustrates a relaying system employing the principles of the present invention which has been set up for transmitting facsimile programs by ultra high frequency waves 15 between New York city and Philadelphia. The system illustrated in Figure 1 employs two way multi-channel transmission.

Figure 2 is a frequency allocation plan for a relay system such as described herein which, 20 among other things, will avoid cross talk and interference. I

Figure 3 is a diagrammatic illustration of traffic office terminal equipment for one direction; and 25 Figure 4 diagrammatically illustrates contro equipment at a repeater station.

Referring to Figure 1, it will be noted that at each relay station or point there are two receivers and two transmitters, one for the north bound 30 relay and the other forthe south bound relay. The radio transmitters may be of the type described by F. H. Kroger in his application Serial No. 38,707, filed August 31, 1935. The antennas may be of the type described by 'P. S. Carter in 35 his United States Patent No.'1,974,387, or in his United States application Serial No. 342, filed J anuary 4, 1935, and receivers may be of the type described by Bertram Trevor, et al. in application Serial No. 53,136, filed December 6, 1935. economyg'it is desirable to locate the north and south bound equipment, both transmitters and receivers, in the same building. It is also desirable to be able to locate the receiving and transmitting antennas in close proximity. This re- 45 quirement may introduce some special problems to avoid cross talk between the transmitters and to keep either transmitter from interfering witheither receiver. To avoid any possibility of cross talk between the transmitters, without excessive 50 precautions, frequency differences on the order of 10% should be employed. To avoid interference in the receivers, the received frequencies should be about 5% from either transmitter frequency. It was also desirable to lay out the fre- 55.

For 40 quency allocation plan in such a way as to provide for future extension of the relaying system in several directions from a given city.

A typical allocation plan is shown in Fig. 2. It will be noted that the received frequencies at a given relay point are separated by 0.5 mc. One transmitter is 4.5 me. higher in frequency than the nearest received frequency, while the second transmitter is 4.5 mc. lower than the nearest received frequency. The adjacent transmitter frequencies differ by 9.5 mc. This meets the requirement set up above for avoiding cross talk and interference, and makes it possible to mount transmitting antennas and receiving antennas on the same tower. By assigning every half megacycle in a 20 megacycle band, four two-way networks may be operated from any one point, plus two to four one-way circuits (depending upon whether these same frequencies are used for one-way service at an adjacent relay point). In any given network, with relays spaced 30 miles apart, on the average, any one frequency is not duplicated until a distance of 570 miles is reached. If all four networks are used out of a single given point, the frequencies will not be duplicated closer than 120 miles, which is believed to be sufficient to-avoid interference when frequencies above 85 mo. are used. The arrows with circles, shown in Fig. 2, correspond to the frequency assignments actually used on the New York- Philadelphia circuit diagrammatically illustrated in Fig.v 1, as may be noted by comparing Figs. 1 and 2, bearing in mind that relay point A of Fig, 2 corresponds to New York (Fig. 1), B to New Brunswick, C to Arneys Mount, and D to Philadelphia.

The receivers, preferably of the type described by Trevor et al. in their aforementioned appli' cation Serial No. 53,136, may be used with a first intermediate frequency of 30 mo, and a second intermediate frequency of 5.5 mc. These particular frequencies avoid spurious response at the frequencies appearing in the allocation plan. These spurious response points might be expected to occur at 4.5, 5 and 10 mc. below the received frequency, and at 4.5, 5, 10, 14 and 14.5 mc. above the received frequency. Triple detection may be used, as described in the said Trevor, et al. application in order to provide high discrimination against image response while at the same time providing adequate adjacent channel selectivity with reasonable provision for frequency drift.

The frequency of the transmitters is held to close limits by resonant line circuits of the temperature compensated type (4) as described by Kroger in his aforementioned application Serial No. 38,707, or as describedby Hansell and Carterin their article which was published April 1936 in Proceedings of the Institute of Radio Engineers.

Directive transmitting and receiving antennas are used. These antennas may be of the type described by Carter in his aforementioned application Serial No. 342, or may be of the type described by Carter in United States Patent No. 1,974,387.

The transmitters and receivers at the relay points are preferably located close together in the same building. The transmitter rooms are shielded from the receiver rooms in order to keep radio frequency potentials out of the control circuits and the receivers. All control lines passing from the receiver rooms to the transmitter rooms should be carefully filteredagainst radio fre q ency Po en ia s Fig, 3 is a schematic representation of the traffic office terminal equipment for one direction. Identical equipment is used for the opposite direction. The system is laid out to transmit, in each direction, two page facsimile channels, two printer channels, one telegraph channel, one one channel, and a remote start-stop channel. The latter channel is for the purpose of controlling the transmitters at the remote relay points, as will be explained later. The cue channel is for the purpose of providing direct communication between the operators of the facsimile machines.

The transmitting terminal equipment at the left, in Fig. 3, controls separate sources of audio frequency by means of the converter circuits. The modulated tones from the converters which contain suitable apparatus whereby the tones are suitably modulated or keyed by the terminal equipment pass through the filters to a common line. indicated on its respective filter. The lower five filters are standard voice frequency carrier filters with an effective band width of about cycles. These filters are wide enough to handle the printer, telegraph, and control channels. The two upper filters are special high pass and low pass types which are combined to pass an effective band of about 7500 cycles for each facsimile channel. The combined tones, occupying a total band width of about 20,000 cycles, are passed to the transmitter T over a wire line WLI. The transmitter is modulated about 80 percent by the combined tones. At the transmitter, a small portion of the radio frequency output is rectified by radio frequency pickup and rectifier R. and passed back to the central ofiice over another wire line WL2. Indicators are provided to show transmitter output and the percentage of modulation.

The modulations are passed on to the distant terminal-via the repeater stations RSI and RS2. The output of the terminal receiver is passed through corresponding filters which separate the modulated tones into their respective channels.

These tones are then amplified and rectified and,

the rectified currents operate the corresponding terminal receiving equipment.

The control equipment at the repeater stations is shown diagrammatically in detail in Fig. 4. The receivers at all points operate continuously. Let us assume that New York wishes to start up the south bound relay. The New York transmitter is started by the local start-stop d. c. control, as indicated in Fig. 3. As soon as the New York transmitter is on the air, this fact is indi- The mid frequency of each filter band is cated to the control point by the radiation inditurn, operates a large a. c. relay ACRI that applies 60 cycle power to the transmitter and its modulator. Automatic time delay relays turn on the plate power after the filaments have warmed up sufficiently.

As-soon as the New Brunswick transmitter is on the air, it, in turn, transmits the 595 cycle tone on to Arneys Mount (Fig. 1). As soon as the Arneys Mount transmitter is on the air, the tone is heard atPhiladelphia. If desired, the tone can be tied back over the circuit at Phila delphia so that the north bound circuit can be automatically started in the same way. However, to save time, it is preferable to close the contacts marked cross tie in Fig. 4, which causes the north and south bound transmitters at a given point to start up together. With this arrangement, either New York or Philadelphia can start the entire circuit with a minimum of delay. The circuit is shut down by removing the 595 cycle tone by opening switch S! (Fig. 3).

Although the circuit is unattended, it is desirable to provide communication at the relay points in order to assist the service man in checking up the circuit when he inspects the relay stations or is called out to correct some dimculty. Fig. 4 shows the communication arrangements. The general telegraph channel uses a carrier frequency of 425 cycles. Each of the relay points are provided with a 425 cycle oscillator O and a keying converter KC. The output of the con verter is connected to both the north and south bound circuits through 425 cycle band pass filters BF so that the 425 cycle tone passes in both directions to the other relay points as well as to both terminal ofiices. If any other point on the circuit wishes to communicate with the service man, they can key their local 425 cycle source, which passes over the circuit, is amplified at the relay station, and is heard on the loudspeaker. Any communication in either direction is heard on the loudspeaker since it is associated with amplifiers on both sides of the circuit. Telegraph sounders are used at the terminal offices rather than loudspeakers.

Preferably, means are provided for observing the modulation and checking the signal to noise ratios at the relay points as well as at the terminals.

As shown in Fig. 1, the frequency allocation at the terminal and relay stations follows the plan described in connection with Fig. 2 in order to minimize interference and cross talk. At each relay receiver, the received radio frequency waves are demodulated to reproduce the tones transmitted from the terminal stations or the tone or tones injected at other relay stations. The tones reproduced at the relay stations are then utilized to modulate the waves generated thereat by one of the relay station transmitters.

The receivers at each relay station are operative continuously; that is, tube voltages are continuously applied so that the receivers are in an operative radio receiving and translating condition. The transmitters, however, are inoperative until the remote start-stop tone is on the air.

As explained, this tone when reproduced at each relay station operates to cause application of plate, grid and filament voltage in proper sequence and value to the tubes of the relay transmitters.

What is claimed is:

1. In combination, a pair of terminal stations, an intermediate station, said intermediate station comprising a receiver normally operative and a transmitter normally inoperative, means including a carrier wave system for establishing communication between each station and another station, said system being characterized by different carrier frequencies for each radio path and each direction of transmission between stations, means including tone modulators for sub-dividing said system into a plurality of communications channels for each carrier wave frequency, and means including a device responsive to signals of a predetermined tone modulation transmitted from either terminal station for starting up the transmitter at the intermediate station.

2. The combination as set forth in claim 1 and including a concatenated carrier wave system having two-way communications channels between the terminal stations and a plurality of intermediate stations, the means for starting up the transmitter at at least one intermediate station being operable in response to signals transmitted by either one of the terminal stations.

3. A concatenated carrier Wave communications system comprising a plurality of intermediate relay stations intervening between two terminal stations, each of said intermediate stations including at least one normally operative receiver and at least one normally inoperative transmitter, means at each terminal station for transmitting an appropriate carrier wave to an adjacent intermediate station, means at each intermediate station for transmitting different carrier waves in different directions toward respectively different adjacent stations, means including tone modulators for obtaining a multiple channel band sub-division of each of said carrier waves into a plurality of communications channels and means for utilizing one of said communications channels as a monitoring tone frequency whereby signals may be transmitted from one station to another for successively starting up different transmitters at different stations.

CHESTER W. LATIMLER. JAMES L. FINCH. HAROLD H. BEVERAGE.

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