A Data Communication Historical Series
By Bob Pollard
Microwave can be used for either short and/or long haul communications. The system sends analog signals from point to point, using frequencies typically between 890 megahertz (Hz) and 20 gigahertz. Microwave communications came into widespread use to connect television-broadcast stations with network studios and to connect parts of the nationwide long-distance telephone network. Both applications demanded extremely high reliability.
Microwave frequencies are also used to communicate to and from satellites, which eliminates the need for repeater stations since the satellite can be positioned, stationary or synchronous, 22,500 miles above the earth. This allows a signal to be transmitted from the East Coast to the West Coast and anywhere in between, or reverse, via one satellite.
One major advantage of microwave signals is that they do not require cable, but they also have the distinct disadvantage because a line-of-sight from transmitter to receiver is required. For instance, depending on the terrain, between a network headquarters on the East Coast and a television broadcast station in the mid-west, there may be 30 to 40 microwave repeater stations since the microwave towers (repeater stations) must be placed so they are in sight of each other.
NOTE: In reference to satellite dishes used for television reception that receive the T V signal from a satellite. The signal from the satellite is a very low wattage signal, 4 to 5 watts unless it has changed in last few years, which requires a concave dish of some type to act as a signal collector. The TV signal in simple terms is reflected (bounces) off the face of the dish and is fed to a signal amplifier that is centrally positioned in front of the dish. The dish must be positioned to face the satellite, line of sight, without obstructions such as trees and buildings, in order to receive an adequate TV signal. Because of the low signal strength, weather conditions, such as rain, hail and fog, especially where there are many trees close by will have an adverse effect on signal reception. Microwave systems also require this direct line of sight and no major obstructions. The difference is that land based microwave systems operate at a much higher transmission power.
The following Microwave system describes one of the early Microwave systems installed across the country, which is part of our communication history. Systems existing today would be more modern and automated, but would basically provide the same type of service.
Western Union Microwave System (1960’s):
The W. U. Microwave system (MW) went from New York City to Los Angeles, New York City to Chicago and Chicago to Dallas. These were all main trunk routes using RCA tube type analog microwave equipment. The individual branch lines to local sites were extended as necessary. The multiplexing (Multiplexer / Mux) equipment, manufactured by GE, utilized solid state electronics, and divided the full base bandwidth into to 600 voice grade bands (channels).
The W. U. System was the first coast-to-coast microwave system implemented. The routing was called ‘Avoidance Routing’, which means all of the backbone routing was kept away from target sites such as the military installations and switching centers. For this reason many of the towers (repeater stations) ended up in the mountains. A note of interest is that when W. U. was trying to justify the MW System to the FCC they had to come up with a need for 40 voice grade bands and couldn't have done it without the U. S. A. F. and their switching centers. The FCC was very conservative and was limiting the allotment of assigned frequencies when this system was being developed.
The MW system was made up of several types of stations. Which were terminals, junction stations (major & minor) and individual MW stations. Terminals always broke down the base band (broadband) to the individual voice band (voice grade) level (four wires) and terminated into the Wire & Repeater room or a switching center Technical Control facility. The multiplexer's (Mux) at the terminals built up or reduced down the individual voice grade frequencies into pre-groups, groups, super groups and base bands. Initially pre-groups were a W. U. invention that was used to go into smaller terminals, which helped to completely load up the system. The pre-groups eventually became obsolete and were eliminated. After that the multiplexing scheme was pretty standard and most W. U. / telephone technicians could recognize the multiplexing criteria and frequency distribution. A pre-group was made up of 4 voice grade frequencies, 3 pre-groups were assigned to a group, 5 groups composed a super group and 10 super groups were assigned to a full base band. The base band is the total modulating bandwidth that is delivered from the multiplexer to the radio equipment (MW). The smaller terminals were fed from a junction station usually by L. D. (medium long distance) MW equipment that was designed by W. U. and manufactured by Raytheon.
The R F (Radio Frequencies) frequency bandwidth utilized was in the 4 to 6 Gigihertz (Hz) range. The system used a klystron tube amplifier as the power amplifier and was very hard to setup and maintain. The equipment was designed by W. U. and was ahead of it's time when new but later it should have been discarded and newer designs used. The klystrons are the same klystrons used in a kitchen microwave, and the ones W. U. used were just as reliable.
The designation for the main radio equipment was WLD-6 (wide band long distant - 6 GHz), the short haul was MLD-4 (medium band long distance - 4 GHz) and the Mux was TCS-600 (trans-continental system 600 channels). The radio system was broken down into Modular (Mod) sections. The MLD- 4 mod sections usually had from 1 to 4 microwave repeater stations and the WLD-6 were usually 10 to 12 stations. At the terminals the Mux would feed a frequency splitter that would send the base band to two transmitters. The two transmitters operated at different Radio Frequencies (RF). This provided frequency diversity as well equipment backup.
At the MW repeater site there was a receiver for each frequency (band) and they fed their own transmitter. The Radio Frequencies were fed to separate antennas (usually an 8-foot parabolic dish) through wave-guides. If there was line of sight from the rooftop of the microwave site the dishes were pointed directly. If there wasn't direct line of sight a tower was used with reflectors. The antenna on the rooftop would be pointed upward and the reflector at the top of the tower would act as a mirror to send the signal on to the next station. In the West there were very few towers because the stations were mostly on mountain peaks. In the Mid-west very high towers, usually around a couple hundred feet were used. The length of the wave-guide run, and the signal loses, determined whether or not a reflector was required. If a transmitter or receiver failed the other receiver would feed both transmitters. Multiple failures could occur in a modular section and still not cause signal interruption. The WLD-6 used a traveling wave tube for power amplification. These were very broadband, high powered and reliable Power Amplifiers.
At the end of a modular section was a Radio Frequency (RF) combiner. The combiner would recombine the R F signal and demodulate it to base band frequencies and give a 3 db signal to noise (s-n) ratio improvement. The signal was combined voltage wise and the noise was combined power wise. When the voltage doubles it goes up 6 db and when the power doubles it goes up 3 db, therefore, resulting in the 3db s-n improvement. The combiners provided a no-break R F-system. The combiner would also filter out all interruptions along the R F path. This was a very reliable and maintainable system. It was a rare situation when a communications failure resulted because of the microwave equipment.
At each modular section there was either a terminal or a Junction station ("J" station}. Some J stations were simple feed through points where they could junction (be routed) in another direction if they wanted to expand the system. Most J stations were three-way-points where the Multiplex interconnected, usually at the super group (60 voice bands) and group (12 voice bands) levels. There were two sets of multiplex, regular and standby. The regular multiplex had all active components and the standby multiplex backed up the regular multiplex in case of a failure. The standby multiplex had to be switched in remotely by command from the manned J station, which remotely monitored faults. The regular J stations did not have full time technician coverage. Usually a technician had several repeater stations to cover as well as a J station. There were 5 manned J stations in the country. They were McGraw, NY; Romney, WV; Carlinville, Ill; Berwick, Kan and Mt Aukum, Calif. These stations received continuous real time fault information from all the stations under their control. They monitored the condition of the commercial power, diesels, individual microwave transmitters, and receivers and multiplex equipment.
With a couple exceptions all stations (repeaters, towers) were connected to commercial power. The incoming power ran two motor/generators (mg). Each mg was coupled to a diesel engine through a large 400-pound flywheel. It was called a no break system because when commercial power failed a clutch would engage the diesel and the diesel would then drive the generator without the load (system) seeing a break in power. The stations were wired so that the power to each system was separated and each mg carried half the station load. Therefore, one mg could be down with no overall effect. All the radio, Mux and power equipment were wired to a fault system that reported into a central control station where they were monitored 24 hours a day.
Most repeater and remote junction stations were not normally manned. The Microwave Technicians quartered at five stations, as mentioned above in the J stations, usually covered 4 to 5 unmanned stations on a routine basis during normal working hours. These five Manned Junction stations monitored the entire system. The technicians were dispatched to trouble locations when a station failure alarm signal occurred in the manned junction station. All stations had a party line voice channel into the control station. There was a VHF (Very High Frequency) mobile system so each technician could be reached in his truck when he was on the road. All of the stations throughout the system were named after nearby geographical locations. This was a FCC requirement. Usually, looking at a map and knowing the stations name, a Technician could locate the station. In some cases a technician had to have specific instructions because many times access was through private property, rancher's gates, etc. Some of the stations in the West were so remote that it took 3 to 4 hours to reach them from a main road. Many of the Technicians had snow cats assigned to them, including the first station outside Los Angeles. It was located high in the mountains, about 8000 ft, and access would regularly be hampered by snow. To get to the Squaw Ridge site a technician had to take several ski lifts out of Squaw Valley and then hike up to the station at 8888 ft above sea level. Once the ski lift operators shut down the lifts at the end of the day and stranded one of the technicians.
A few of the sites had no commercial power available to them and therefore continuous running diesels provided power to the local system. Many terminals did not have standby power because they were shut down a portion of each day. The reason being it would not help to have the microwave working if the terminal was shut down. Some terminals had automatic start diesels.
Below is a recent picture (9/17/2000) of the Microwave Junction office (station) near Romney, WV.