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The Microwave Era Begins
October 1950 Radio & Television News

October 1950 Radio & TV News
October 1950 Radio & Television News Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

"The year 1950 will be recorded historically as the year the microwave relay made its impact felt." That was the closing sentence by Philco Sales Engineer Leo Sands in his 1950 Radio & Television News magazine article entitled, "The Microwave Era Begins." Mr. Sands was not suggesting that it was the start of the widespread use of microwaves in general, but specifically the use of microwaves for long distance, broadband transmission of telecommunications signals. 1950 is about the time microwave relay stations began appearing on hilltops and rooftops of tall buildings all across the land, with the goal of replacing coaxial lines which needed to be strung or buried from end to end. Great cost is associated with a hardline approach for acquisition of land rights, installation, and maintenance. Yes, those kinds of expenses are required for microwave relay stations, too, but in the long run they tend to be much lower, and the service much more reliable and "upgradable." Lots of people opposed the installation of the unsightly, behemoth towers, and many people expressed concern over exposure to microwave energy. If only they knew then how miniscule their worries were compared to today's situation with cell towers within eyeshot of just about ever locations on earth!

The Microwave Era Begins

The Microwave Era Begins, October 1950 Radio & Televsion News - RF Cafe

Western Union's microwave tower, Neshanic, N. J. Upper antennas (truncated) are part of Philco's 6000 mc. microwave relay system. Lower parabolic antennas are part of RCA's 4000 mc. microwave relay system. Diversity receiving antenna may be seen lower left.

Top section of "H" fixture microwave antenna-supporting structure. 100 ft. wood poles are used. Plane reflectors, 4 by 6 feet, are mounted on top and reflect microwave beam from 4 ft. parabolic antennas below. This is part of the Rock Island's microwave communications system from Norton to Goodland.

A.T.&T. microwave tower at repeater station located between Richmond and Norfolk, Va. Philco 10 ft. parabolic antenna is on top of tower. Waveguide is used to feed units.

By Leo G. Sands

Sales Engineer Philco Corporation

Microwave relays will someday replace cross-country overhead telephone lines. Equipment described will be displayed at the 27th annual meeting of the Communications Section of the Assn. of American Railroads meeting at French Lick, October 17-19.

All over the country, towers topped by queer looking mirror-like reflectors or parabolic antennas are arising. These towers, spaced from 15 to 50 miles apart, are spelling the doom of the overhead telephone wires that follow almost every highway and railroad track. The complete elimination of the pole line is still far off, but construction of new open wire pole lines, except for local distribution, seems unlikely.

For several years, there has been talk of the day when beamed radio would start taking the place of wires for point-to-point overland communications. That day has arrived. Although the microwave art is not new, inexpensive equipment was not available until 1949 and very few systems had been ordered prior to 1950.

In the prewar year 1940, a radio relay system was installed by Philco engineers to bring television programs to New York and Philadelphia. This relay link operated in the vicinity of 200 megacycles. In 1947 this pioneer radio relay was replaced with a 1400 megacycle microwave system. During the war, the armed services made considerable use of microwave and u.h.f. equipment for point-to-point communications. Today, television broadcasters make use of microwave links to transmit television signals from their studios to the television transmitters.

Although it is comparatively easy to build microwave transmitters and receivers, the biggest problem was the design of repeater equipment for long haul relay systems. A long microwave relay system consists of two terminals and a large number of intermediate repeaters. These repeaters must be capable of receiving and re-transmitting microwave signals with a minimum of distortion to provide high quality circuits and very little crosstalk.

The ideal repeater would consist of a microwave amplifier in which de-modulation and re-modulation does not take place. However, such a repeater has not yet been developed. Currently available repeaters fall into three classifications: (1) a transmitter and receiver connected back-to-back; (2) a heterodyne repeater; and (3) a feedback repeater. All three types have merit and can be used with multiplexing systems compatible with the specific type of repeater.

The first commercial application of negative feedback at microwave frequencies is incorporated in the feedback type microwave repeater developed by Philco. In this repeater, the output of the receiver is fed back to the receiver's own local oscillator klystron, causing it to track the incoming frequency modulated signal. The output of the klystron is divided so that a small percentage of this output is injected into the mixer of the superheterodyne type receiver and the major portion of the output is fed into the antenna system as the outgoing signal. To provide duplex operation, two repeater units, one for each direction of transmission, are multiplexed into common antennas. Only a single klystron-type tube is required for both transmission and reception in a single direction.

A back-to-back type of microwave repeater consists of a receiver whose output is fed to a transmitter. For duplex operation, two transmitters and two receivers are required at each repeater station. In this type of repeater at least two microwave oscillator tubes are usually required for each direction, one as the receiver local oscillator and the other in the transmitter.

Block diagram show simplified circuitry of two types of microwave repeater units - RF Cafe

Block diagram show simplified circuitry of two types of microwave repeater units.

4-section r.f. filter for a Philco microwave receiver - RF Cafe

A four-section r.f. filter for a Philco microwave receiver. This is part of the Philco CLR-5 repeater for use in 6575-6875 mc. band. Tuning slugs are pretuned and sealed at factory.

The heterodyne type repeater is not commonly used in communications relay systems due to its cost and complexity. However, it has found wide use in television relays. Here a comparatively low frequency signal produced by beating the incoming modulated microwave signal with the output of the receiver local oscillator is amplified. This low frequency signal (v.h.f.) is used to beat against a microwave oscillator to produce a microwave signal at the sum or difference of the two frequencies.

Several types of tubes are used as microwave signal sources and include lighthouse tubes, planer triodes, magnetrons, and klystrons. Power outputs of these tubes vary from a few milliwatts to several watts.

Several groups of radio frequencies in the microwave region have been allocated by the Federal Communications Commission for point-to-point use by various industries, transportation services, public safety organizations, broadcasters, and communications common carriers. The 6000 megacycle band offers many advantages, such as high antenna gain, adaptability to simple circuitry, availability of reliable long-life tubes, and excellent propagation characteristics.

Antennas with parabolic reflectors are generally employed. The effective power gain, for example, of such an antenna with a dish of four foot diameter is in the order of 34 db. at 6500 megacycles. This means that a one watt transmitter will effectively radiate the equivalent of a 2500 watt signal. A waveguide is often used for connecting the antenna to the microwave transmitter, receiver, or repeater. Alternatively, passive reflectors are used in lieu of long waveguide runs. A plane reflector, rectangular in shape, is mounted atop a tower or other suitable supporting structure and the parabolic antenna is mounted near the ground aimed at the reflector. The signal is bounced off the mirror-like reflector in the desired direction in the same manner as a beam of light is reflected by a mirror. In practice the parabolic antenna dishes are usually mounted on brackets on the roof of the equipment shelter. As the antennas are exposed to the elements, heating facilities are provided for feed horns and antenna dishes when used in cold climates. With the antennas mounted outdoors, there is the possibility that the dishes may be eventually filled with leaves, dirt, or snow. Furthermore, the end of the feed horn makes an attractive target for the hunter. To provide greater protection for the paraboloids and feed horns, Philco engineers have recently designed a new type of microwave equipment shelter in which the antennas are mounted indoors under the roof. Windows made of a special type of pressed Fiberglas, virtually transparent to microwaves but opaque to light, are installed in the slanting roof of the shelter. The antennas are aimed through these windows at the plane reflectors on the tower. To prevent frosting or the accumulation of snow, thermostatically controlled infrared lamps are used to heat the Fiberglas windows.

One antenna with or without a plane reflector is used at terminals for simultaneous transmission and reception. Two antennas are required at repeater stations in combination with waveguide feeds or plane reflectors, each of which is used for transmitting and receiving simultaneously in one direction to and from the adjacent repeater or terminal.

The popular term for a single link, the space between adjacent repeaters or terminals is a "hop." For example, a three hop system consists of two terminals and two repeaters. The signal originating at a terminal on frequency f1 is retransmitted by the first repeater on frequency f2 and is retransmitted again on frequency f1 by the second repeater. The signal arrives at the far terminal in three hops. For duplex operation, another signal travels simultaneously in the reverse direction.

Philco CLR-5 microwave repeater for 6575-6875 mc. band - RF Cafe

Philco CLR-5 microwave repeater for 6575-6875 mc. band. Two complete one-way feedback repeaters and common power supply are housed in a single cabinet. The microwave carrier is frequency modulated. It may be used with frequency-division or time-division multiplex terminals. Only one klystron is required for each direction. The same klystron serves both as the receiver local oscillator and the FM transmitter tube.

Philco TLR-2 microwave repeater for television service as used by Western Union - RF Cafe

Philco TLR-2 microwave repeater for television service as used by Western Union. A similar unit is used by American Telephone & Telegraph Company in its microwave relay system.

The practical length of a hop is determined by the heights of the antenna supports, terrain, transmitter power, receiver sensitivity, and antenna gain, coupled with good engineering practice. Line-of-sight conditions are not good enough except for very short hops. At least 50 feet of clearance above trees is considered desirable. Hops varying in length from 15 to 50 miles are common. Longer hops where sufficient terrain clearance is available could be considered, however, long hops are more apt to suffer from fading.

A properly designed microwave re-lay system makes allowances for fading. When frequency modulation of the microwave signal is employed, shallow fades go unnoticed. A microwave system with a 30 db. fading margin provides a continuous signal without serious degradation of circuit quality even during deep fades. In the 2000 megacycle band, deep fades are not as frequent as in the 6000 megacycle band. However, the higher antenna gain available at 6000 megacycles permits designing a system with a greater fading margin, with the result that operation in either band is almost identical. The hop length at 2000 megacycles or 6000 megacycles can be the same, as here again the much greater antenna gain at the higher frequency more than compensates for the slight difference in propagation characteristics.

The width of the transmit-fed beam from a four foot parabolic antenna at 6000 megacycles is three degrees. Although this appears to be a very narrow beam, 25 miles out it is over a mile wide at the half power points. Much emphasis has been placed on tower twist, but an analysis of the facts reveals that tower rigidity is not as important as has been popularly supposed. As fading seldom occurs at the same time as high wind velocity, the fading margin also compensates for tower twist.

The basic microwave system is capable of a modulated intelligence bandwidth of considerable proportions. To transmit several simultaneous voice conversations, musical programs, telegraph messages, etc., the modulation acceptance band of the microwave system is subdivided by means of multiplex channelizing equipment. These fall into two general classifications, frequency-division and time-division multiplex systems.

The most common form of frequency-division multiplexing device is the standard telephone wire line carrier terminal employing AM with single sideband transmission and with the carrier suppressed. This type of channelizing equipment which is very economical with bands pace may be used with single hop systems or with multiple hop microwave relay systems employing repeaters which introduce very little distortion. As the carrier is suppressed, this type of multiplex equipment lends itself to party-line service on a bridging basis. Telephone carrier terminals of the single side-band type but without suppression of the carrier may be used for deriving through circuits but not bridged party-line channels.

Another form of frequency-division multiplex system is the FM subcarrier which lends itself well to microwave applications where economy of band-space is not important. The FM subcarrier is not as critical of repeater distortion as is the single sideband suppressed carrier, hence it may be effectively used with the back-to-back type of repeater. However, its extravagant use of bandspace does limit the number of channels that can be derived.

Several types of time-division multiplexing systems have been developed, making use of pulse amplitude modulation, pulse time modulation, pulse position modulation, pulse width modulation and pulse code modulation. Pulse amplitude modulation, popularly referred to as P.A.M. provides high quality voice circuits with a minimum of crosstalk and with economical use of bandspace. For example, a 32 voice channel P.A.M. multiplex terminal requires less than 300 kilocycles of bandspace. With P.A.M. it is possible to provide party-line circuits as well as through trunk circuits. Individual voice channels may be dropped off and injected at intermediate microwave repeaters without degradation of the channel.

A cost analysis reveals that for systems requiring four or fewer channels, particularly when many drop-off's are required, frequency-division multiplexing is less expensive, whereas in systems with a greater number of voice channels, time-division multiplexing can be provided at less cost. Unless otherwise specified a channel is a voice channel 300 to 3300 cycles wide. Telegraph, teleprinter, telemeter, and supervisory control channels require much less bandspace than a voice channel. When a frequency-division multiplexing system is used for deriving voice channels, narrow-band carriers for on-off transmission, such as telegraph carriers, are generally applied directly to the microwave equipment modulation input. In time-division systems, a single voice band may be subdivided to handle from 8 to 16 AM or frequency-shift telegraph or telemeter carriers.

How microwave signals are relayed - RF Cafe

How microwave signals are relayed.

Block diagram of carrier amplifier repeater and back-to-back receiver-transmitter - RF Cafe

Block diagram of carrier amplifier repeater and back-to-back receiver-transmitter.

Oscilloscope patterns obtained - RF Cafe

Fig. 4 - Oscilloscope patterns obtained.

With the exception of communications common carriers, the channel requirements of most potential users of microwave communications systems does not approach 32 voice channels. Manufacturers are offering multiplex systems with 3, 4, 5, 8, 10, 12, 16, 20, 24, or 32 voice channels as required by the user. Some of the equipment being offered is expandable in steps of 1 or 4 voice channels.

To provide compatibility with telephone systems the multiplex terminals must be provided with suitable termination equipment. A two-way voice channel normally terminates on a four-wire basis, two to the transmitter input and two from the receiver output, so that a hybrid network is required to provide two-wire termination which will permit connection into a switchboard or a conventional telephone instrument. In addition, provision must be made for ringdown or dial signaling.

Although the microwave industry is just starting to grow, great progress has already been made in the past year. The first railroad-operated microwave communications system to be established on a permanent basis has been installed by the Chicago, Rock Island and Pacific Railroad along its Denver to Chicago main line. This 5 hop pilot system between Norton and Goodland, Kansas will supplement and perhaps eventually replace wire line communications facilities in an area where snow, wind, sleet, and dust raise havoc with pole lines. Initially, the Rock Island's 6000 megacycle microwave link, 106 miles in length, will provide facilities for a train dispatcher's party-line telephone channel, a party-line message telephone channel, a local party-line telegraph circuit, and four through telegraph circuits.

 - RF Cafe

Lower section of "H" fixture antenna support and microwave equipment shelter. This Philco Type CLR-5 microwave repeater and emergency gasoline engine-driven power generator are housed within the prefabricated concrete slab building. A pair of 4 ft. parabolic antennas are mounted on the- roof.

 - RF Cafe

Four foot parabolic antenna for the 6575-6875 mc. band. The unit has a three degree beam and an effective power gain of 34 db.

The Santa Fe Railway System is installing a 6000 megacycle microwave communications system to provide additional communications facilities between Beaumont and Galveston, Texas. Eight voice channels derived by pulse amplitude modulation will be provided. Three unattended repeater stations are to be installed on the Bolivar Peninsula to make up a four hop relay system. To assure uninterrupted communication, standby microwave equipment will be provided at both terminals as well as at the repeaters. An automatic fault-alarm system will advise maintenance personnel at Galveston of equipment or primary power failures as well as identification of the station requiring attention.

In the petroleum industry, a number of pipe line companies are installing microwave relay systems to provide direct communication between pumping stations, regional offices, and for remote control of system-wide v.h.f. mobile radio systems. The Humble Pipe Line Company is to install a 6000 megacycle microwave communications system along 400 miles of pipe line between Houston and Kemper, Texas. Two terminals and eighteen intermediate repeater stations will make up the Humble relay system. Pulse amplitude modulated multiplex equipment will provide eight voice channels of which one to four will be dropped off at intermediate repeaters.

The Bonneville Power Administration of the United States Department of the Interior has awarded contracts for equipment for a vast microwave relay system which will blanket the State of Washington. In circuit miles, this will be the largest microwave system ordered to date for non-common carrier service. The Bonneville communications network will also make use of pulse amplitude modulation for deriving up to 24 voice channels. In addition to telephone facilities, the microwave system will also be used for power line relaying, remote control, and the locating of faults along power transmission lines.

For relaying of television programs, a number of microwave relay systems have already been installed and many more are projected. A 6000 megacycle microwave link installed by the Western Union Telegraph Company has been in continuous operation for over two years. This link extends from the Chrysler Building in New York to the P.S.F.S: Building in Philadelphia via two intermediate repeaters located at Neshanic and Mt. Laurel, N. J. Paralleling this link is the Philco-owned television relay which links the Empire State Building in New York with the WPTZ transmitter at Philadelphia. One intermediate repeater at Mt. Rose, N. J., joins the two terminals.

The American Telephone & Telegraph Company has a number of microwave relay systems in operation and according to the newspapers many more are planned. In April of 1950, a 6000 megacycle microwave relay system was placed in service between Richmond and Norfolk, Virginia, by the Bell System to feed network TV programs to the Hampton Roads area.

It is obvious that the microwave relay art has emerged from the experimental stage to become a vital part of the nation's communications system. In times of national emergency, microwave relay systems can be used to augment existing wire line facilities and they can be installed in much less time. Economically, wire lines cannot compete, as the cost of a microwave communications system runs from only $400 to $800 per mile. The current estimated cost of building a two-wire pole line runs from $800 to $1500 per mile. A pair of wires will provide one telephone circuit unless multiplexed. The number of channels that can be derived from a single pair of wires is limited by electrical losses. Multi-channel carrier equipment for wire line telephony is more expensive and more complex than microwave multiplex equipment.

Performance-wise, the microwave relay, being less vulnerable to storms, should provide greater reliability than overhead wire lines. Furthermore, it will carry more types of intelligence and can be expanded more readily and at lower cost. Microwave systems have provided uninterrupted service during snow, sleet, and wind storms which have prostrated wire lines. With well designed equipment and adequate preventive maintenance, reliability approaching 100% can be attained.

The year 1950 will be recorded historically as the year the microwave relay made its impact felt.



Posted April 7, 2022

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