For the Record: Scatter Communications |
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It seems Amateur radio operators in every country have perpetually been in a battle against government bureaucrats. Early on, Hams were allocated small segments of available "usable" spectrum and a fairly wide swath of the less desirable spectrum. That was especially true of the microwave bands when there was not a lot of components and equipment available that could operate at the higher frequencies. Those familiar with history are familiar with the situation where Amateurs developed a lot of the technology for operating in the upper MHz and into the GHz realm, and then the government snatched it from them for commercial and military use. Hams pretty much pioneered atmospheric scatter as a means for long distance (DX), over-the-horizon communications using those upper frequencies. This editorial from a 1955 issue of Radio & Television News magazine reports on a relatively newer variation dubbed "forward-scatter," which works in 40-80 MHz band (6-meters and VHF) when the transmitted power is sufficiently high. The military, which discovered the phenomenon and had recently declassified it, was considering allowing Amateurs to operate there, the hesitation being primarily the amount of radiated power required, which could cause troublesome interference. An article entitled, "Scatter Communication," by Maurice P. Johnson, appeared in the August 1958 issue of Radio & TV News. For the Record: Scatter Communications
"Forward scatter" is the phenomenon of putting a v.h.f. or u.h.f. signal in an area, where under the classical definition, it does not belong. Waves between 40 and 80 megacycles will normally pass through the ionosphere without sufficient refraction to permit reception much beyond the immediate horizon. The military have found, however, that given a high e.r.p. these waves will return to earth between 700 and 1200 miles from the transmitter. The returned signal is garbled and not suitable for voice communication, but can be used for frequency shift keying or multiplexing. Signal strengths are weak and according to information available at this writing are received with a 99.9 % reliability - something which cannot be done on any other channel out of the line-of-sight. The military has taken advantage of this new system and currently has a link working between Maine and Thule, Greenland. Indications are available pointing to a v.h.f. link from Maine into Scotland and England. Should such a link be installed it will be the first time that propagation conditions have been better and shown greater reliability than the terminal transmitting and receiving equipment. Such v.h.f. "forward scatter" is caused by ionospheric discontinuities in the upper D and lower E regions (50 to 65 miles above the surface). The u.h.f. "forward scatter," on the other hand, is caused by tropospheric discontinuities below 30,000 feet in altitude. The latter mechanism, which is theoretically related to ionospheric scattering, appears capable of traversing 200 to 250 miles. Unlike the garbled scatter from the ionosphere, the scatter from the lower atmosphere retains excellent fidelity. Recent announcements by the Bell Labs indicate that excellent TV signals have been consistently propagated over 200-mile hops. Initial experiments by the military with v.h.f. scatter took place on 49.8 megacycles between Cedar Rapids, Iowa and Sterling, Virginia. Numerous radio amateurs operating in the 6-meter band will recall the "big signals" on 49.8, 49.7, and 49.6 megacycles from 1952 to 1954. Analyses of the data obtained from these transmissions indicate that v.h.f. scatter is most effective between 40 and 80 megacycles. The u.h.f. scatter appears to be effective over a tremendously wide range of frequencies - probably extending from at least 800 to 7500 megacycles. Some Bell Labs experiments have been made around 3700 megacycles and others at 5050 megacycles, both of which have proven to be equally efficient. At the present time the only limitation toward the greater use of u.h.f. scatter is terminal transmitting and receiving equipment. Development and greater use of u.h.f. scattering will alleviate the burden of providing numerous microwave relay towers for transcontinental TV and telephone traffic. Development of "forward scattering" in the v.h.f. bands will solve many of the problems concerning circuit reliability. Thus both of these phenomena deserve the attention of our research personnel, but outside of the immediate use made of v.h.f. scatter by the military there appears to be little likelihood of its acceptance by the commercials operating medium range point-to-point circuits. Obviously the military does not want to keep these developments to itself. Present frequency allocations, even as late as Atlantic City 1947, do not provide for the use of the v.h.f. by point-to-point services - especially by a then unheard of mode of propagation! Without this increased frequency utilization for 700 to 1200 mile point-to-point circuits the commercials will continue to suffer interruptions due to ionospheric storms and severe crowding, or even shortages. The problem is being very seriously studied. Shifting of certain point-to-point services to the very-high-frequencies would give more channels to the aeronautical and maritime mobiles which are now in desperate need of additional frequencies between 5 and 20 megacycles. However, there are no channels set aside in the v.h.f. range of 40 to 80 megacycles for the "forward scatterers." The question then arises, is this valuable means of communication to be of advantage only to the military - or will the commercials eventually force the abandonment of the amateur radio 6-meter band (50 to 54 megacycles), or even the low-band TV channels 2 to 6 between 54 and 88 megacycles?
Posted October 20, 2020 |
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A new term, familiar to the military, but almost
unknown to readers of the technical press is fast reaching a prominence that may
equal that of television or radar. It is "forward-scatter," a name applied to the
mechanism encountered in radio wave propagation which now affords a remarkable new
means of communication. 