New Link in U.S. Defense - Pacific Scatter
November 1961 Radio-Electronics

November 1961 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

The November 1961 issue of Radio-Electronics magazine contained a couple articles on long-distance wireless communications by exploiting the reflective properties of the Earth's upper atmosphere. "Nature's Invisible Radio Mirror" is one of the others. This "Pacific Scatter Communications System" (PSCS) article describes a series of transmit / receive stations located on eight Pacific islands used to pass voice and teleprinter messages from areas around the Philippines and Okinawa to Hawaii. At the time, it was the longest of such systems, with expansion sites being constructed to Taiwan and beyond. Both ionospheric (34-55 MHz, 40-60 kW) and tropospheric (800 MHz, 1 kW) scatter / reflection were exploited. Prior to the PSCS, communications were often broken and/or filled with heavy interference.

New Link in US Defense - Pacific Scatter

By Jordan McQuay

Long-range multichannel trans-Pacific teleprinter and voice system is 7,400 miles long; uses ionoscatter and troposcatter with other advanced techniques for high reliability.

Longest and largest of its kind in the world, the new Pacific Scatter Communication System* presently consists of eight radio relay links connecting Hawaii, Midway, Wake, Guam and other island with the Philippines and Okinawa (Fig. 1), All use ionoscatter propagation - except the short troposcatter link within Hawaii. Additional relay links, now under construction, will extend the Pacific System to Taiwan (Formosa) and several other points in Asia.

Until the Pacific System became operational, high-frequency (hf) radio communication between many of the sites - particularly the long haul from Hawaii to the Philippines and Okinawa - was unreliable, unpredictable and often impossible. This was due largely to propagational effects and disturbances, some general in nature, and some peculiar to the Pacific area. Most of these problems were solved by introducing vhf and uhf scatter techniques of propagation.

Stations of the system continuously transmit and receive 16 multiplexed teleprinter channels plus a party-line 2.2-kc voice channel. Unless destined for a specific station, teleprinter traffic is not delayed by decoding and encoding procedures, but passes through relay stations as electronic data.

Exceptional system reliability - greater than 99.9% - results from critical design, extensive automation and standardization operation and maintenance.

All major equipment at each station is in duplicate: antennas, transmitters, exciters, receivers, multiplex terminals, power supplies and other critical elements. All duplicate or reserve equipment is also on, and is ready to use. If any piece of equipment fails or gives trouble, the standby is switched in automatically. Interference, whether accidental or intentional, can be circumvented electronically.

Each station has a central monitor and supervisory console equipped with indicators, meters and graph recorders that show continuously the quality of teleprinter traffic and the operating status of all primary and reserve equipment and facilities.

Each station is self-supporting and self-sufficient, with Diesel power supplies and other plant equipment - plus housekeeping facilities (Fig. 2).

Conventional long-range hf communication requires many changes in operating frequency to meet varying atmospheric conditions during every 24-hour period. Relay links of the Pacific System use a fixed frequency at all times. This extends traffic time to maximum, simplifies operation and maintenance and helps prevent clutter of the frequency spectrum.

lonoscatter Technique

To make use of ionospheric scatter, vhf signals are transmitted upward at such an angle that they are splashed against this layer above the earth. Some of these waves penetrate the ionosphere and are lost in outer space. Some of them bounce back and forth, between earth and ionosphere, in all directions - attenuating quickly. A few of these waves, however, return directly to earth as forward-scattered fragments of the original signal and can be detected by highly sensitive receivers located 600 to 1,200 miles from the transmitting station. When the received signal is instantly transmitted again, it can be received once more in the same manner - thus covering as much as several thousand miles. Two relay links illustrate the principle of scatter transmission and reception in Fig. 3.

This ionoscatter technique is effective at an operating frequency between 35 and 55 mc. At lower frequencies, the signal would be adversely affected by various propagational effects, and communication would not be reliable unless the operating frequency was changed several times a day. At higher frequencies, other propagational effects and tropospheric influences would contribute to low signal strength and uncertain reliability.

Any of the ionoscatter links of the Pacific System may operate within either of two frequency bands: a low band between 34 and 37 mc, and a high band between 49 and 55 mc. Both the transmitter and distant receiving group or any link are tuned to the identical frequency, within either of these two bands.

An average power output of 40-60 kw is great enough for transmitters of the system. Greater power would be dissipated in the ionosphere.

At the receiving site, fragments of the forward-scattered signal may return to earth at any number of nearby points. For this reason, four vhf receivers operate continuously to provide quadruple-diversity reception. The strongest signal from any of the four receivers is selected automatically. This is the desired signal at a receiving site - the forward-scattered signal of Fig. 3.

There is an undesired signal occasionally: the back-scattered signal. This is also composed of scattered fragments of the signal originally transmitted. But these fragments have literally bounced back and forth, several times, between earth and ionosphere, so that they arrive later than the desired forward-scattered signal. This kind of reception is known as multipath delay and introduces serious interference and distortion affecting the mutiplexed teleprinter channels.

Nature overcomes part of the problem of multipath delay - with the occasional enhancement of forward-scattered signals by reflection from ionized meteor trails in space. To take advantage of this natural phenomenon, each receiver has a dynamic range of more than 100 db, which permits reception of a wide range of signal intensities. Coupled with this is the automatic ability of each receiving group to select the signal of greatest intensity - invariably the forward-scattered signal - from the quadruple-diversity configuration of the receiving group.

A typical ionoscatter relay station of the Pacific System consists essentially of a transmitting group with a regular and reserve transmitter, a receiving group with four quadruple-diversity receivers, and a regular and reserve antenna array. A simplified block diagram of a typical ionoscatter station is shown in Fig. 4.

lonoscatter Arrays

Essential to the operational efficiency of ionoscatter relay links of the Pacific System are the high-gain duplexed antenna arrays (Fig. 5).

Although large rhombics could be used to achieve high gain and high directivity, they require a great deal of ground area. This is not practicable on many of the islands of the Pacific.

At the other extreme are Yagi arrays, which need little ground area but require delicate installation and sensitive tuning. Because of high-velocity winds encountered in the Pacific region, these arrays are unsuitable.

The optimum is a corner-reflector array of entirely new design. Although costly to erect, it requires much less ground area than a rhombic and is not sensitive in adjustment like a Yagi.

The new type of corner-reflector array consists essentially of two 60° corner-reflector assemblies stacked vertically. Since the system must operate within either a high band or a low band of frequencies, the dual stacked array is doubled - the low-band portion stacked atop the high-band portion of the array - and the whole mounted on a three-tower steel structure.

Since a relay station must operate in two almost-opposite directions, there are two complete dual-band corner-reflector arrays at such stations. The dual-band arrays are separated normal to the path azimuth, and provide quadruple space-diversity reception.

Radiating-receiving elements of each array are full-wave center-fed dipoles. There are 12 dipoles each for the low-and high-band portions of each array. The dipoles are supported about 0.55 wavelength from the apex of the reflecting curtain. The curtain apex angle is 60°.

At the center feed point of each dipole, the impedance is about 200 ohms because of the relatively large diameter of these elements plus the use of internal coaxial matching sections. Balanced feed lines are shielded single conductors constructed of Styroflex coaxial cable.

The reflecting curtain is composed of 10-gauge copper-welded wires. These are spaced 7 inches apart, arranged horizontally and evenly spring-tensioned to achieve an essentially plane reflective surface with low wind resistance. For details of array construction, see Figs. 6 and 7.

Each complete dual-band corner-reflector array is mounted on a structural support consisting of three vertical galvanized-steel towers which are heavily guyed. (Note the use of sea anchors in Fig. 5.)

These towers vary in height from 66 to as much as 400 feet, depending upon topographical as well as propagational factors at each island site. All metal surfaces are specially treated and all guy wires are copper-clad-to resist the rapid corrosion characteristic of salty-humid climate.

Each radiating-receiving section of an array has a duplexed feed system, using branching filters, which permits simultaneous transmission and reception for either low- or high-band portions of the array (Fig. 4). This system consists of two notch filters which reject transmitter-generated thermal noise at the receiving frequency, plus four cavities which pass the receiving-frequency signals but prevent transmitter power from reaching the quadruple-diversity receivers. As a result, rf energy from the transmitter is negligible at the output terminals of the receiver cavities. Matching networks between the cavity outputs and the preamplifier input terminals provide a 50-ohm source for the various inputs of the receiving system. There are three resonant cavity sections and a high-level shorted-line section in a complete branching filter, each associated with a low-band or high-band portion of an array. There are two sets of branching filters at the input of each of the four preamplifiers for space-diversity operation.

lonoscatter Equipment

Transmitting equipment for each ionoscatter relay link consists of a 60-kw power amplifier (in duplicate), two frequency converters (in duplicate), two FSK (frequency-shift keying) modulators (in duplicate) and switching and auxiliary equipment (Fig. 4). This transmitting group accepts a composite signal from the multiplex terminal, converts the signal into a broad frequency-shifted rf signal, multiplies its frequency to that of the operating or carrier frequency, and finally amplifies this signal and feeds it to either the low- or high-band portions of the antenna array. The transmitter delivers an average power of 60 kw class-C, about 40 kw class-B, with eight air-cooled type 3X2500A3 triodes in a grounded-grid configuration and a symmetrical tank circuit composed of shorted sections of coaxial lines.

The receiving group for each ionoscatter relay link includes two rf pre-amplifiers (in duplicate), an FSK receiver (in duplicate) and switching and auxiliary equipment (Fig. 4). This group accepts the outputs of the rf preamplifiers, combines them for diversity reception, separates and demodulates them into teleprinter (dc) components and audio (voice) frequencies, and then feeds them to the multiplex terminal and distribution panel. Each FSK receiver has two front ends - for low-band and high-band operation. Either produces a first if of 2.2 mc. The second if is 50 kc. The mark and space frequencies are 6 kc apart, with half-power bandwidths of 700 cycles each. Any frequency-selective fading that results from the 6-kc mark-space separation affects a decision threshold computer, which separately stores the mark and space data and derives therefrom the optimum detector threshold level.

Troposcatter Facilities

To utilize tropospheric scatter, uhf signals are transmitted upward at such an angle that forward-scattered fragments of the original signal return to earth in a manner similar to the ionoscatter technique (Fig. 3). In the case of ionoscatter, the haphazard return of signals is caused by clouds of ionized particles. In the case of troposcatter, the same effect is supposed to be produced by clouds of water vapor within the troposphere, a layer of atmosphere just above the earth.

Only one relay link of the Pacific System uses troposcatter. This is the link between Oahu and Kauai (Hawaiian Islands), which provides wide-band multichannel communication for about 115 miles with an operating or carrier frequency of about 800 mc.

Each terminal of the link uses two 19-foot paraboloid antennas, duplexed for simultaneous transmission and reception. One antenna has horizontal, the other vertical polarization.

Transmitters at each station are 1-kw klystron amplifiers with FM exciters. The equipment provides 12 full-duplex voice-frequency channels plus one narrow-band voice order-wire channel.

Quadruple-diversity reception is provided by four conventional uhf receivers equipped with a base-band combiner to select the receiver having the signal of greatest intensity.

All major equipment at each troposcatter station is in duplicate. Automatic alarm and control devices initiate switching electronically in event of trouble or failure.

Other Facilities

Multiplexing equipment, which provides duplex teleprinter channels, is a prime component of every station of the Pacific System. This equipment with switching and test equipment constitutes the multiplex terminal (Fig. 8).

Depending on the crystal selected for timing purposes, the multiplex equipment will function with teleprinter signals at speeds of 60, 75 or 100 wpm.

Individual teleprinter channels are sampled in time sequence to generate or develop the composite signal.

For relay purposes, receiver and transmitter are synchronized automatically, with frame synchronism assuring proper agreement at all times. The multiplex equipment initiates re-framing operations after any three successive synchronizing groups are lost, or within 45 seconds of first loss of synchronism.

Modular plug-in units are used extensively in the multiplex equipment. Transistorized magnetic storage, shift and read-out circuits are used exclusively, all operating at lower power and low heat levels for maximum reliability. All timing circuits are controlled by stable oscillators.

A valuable facility of the Pacific System is the single-circuit party-line 2.2-kc voice-frequency channel that connects all stations of the system. Selective signaling with coded in-band binary tone groups makes it possible to dial any station without participation by other stations of the system.

Each station is equipped with a monitor booth containing a central alarm, meters and recorders, which collectively indicate and record the current operational status of all major equipment and facilities as determined at critical check points.

Each station is also provided with facilities for measuring and recording long-term median as well as instantaneous signal levels. Four graph recorders continuously measure variations in signal and noise intensity as well as any multiplex distortion. Also recorded are the depth, duration and frequency of any signal fading.

A wide variety of test and monitoring equipment is also available for maintaining the high reliability of all equipment and facilities at each station of the Pacific Scatter Communication System.

The system was designed, developed and installed by Page Communications Engineers, Inc., of Washington, D. C., a subsidiary of Northrop Corp. Technical operation and maintenance of many of the stations are also being provided by the Page organization.

* See Radio-Electronics, August 1960, page 6.

Posted July 25, 2025