1. The History of Single Sideband Modulation
The History of Single Sideband
In the early days of radio, little was known about sidebands. The concept of an amplitude-modulated signal being a composite rather than an indivisible whole was not well defined and was a subject open to vigorous controversy. In 1914 it was established mathematically that an amplitude-modulated wave consists of a carrier and two identical sidebands which are spaced above and below the carrier by an amount equal to the modulating frequency. The following year experiments were conducted at the US Naval Radio Station in Arlington, Virginia, using very low frequencies with an antenna which was tuned to pass one sideband and attenuate the other. This tended to substantiate the concept of a composite wave and, in addition, indicated that one sideband contained all the elements necessary for voice transmission. Another investigator had found that injection of the carrier frequency at the receiver improved detection of the received signal. These discoveries paved the way for development of the concept of single-sideband transmission and reception.
1915 John R. Carson applied for a patent on his idea to suppress the carrier and one sideband. After much litigation the patent was granted in 1923. In that year the first trans-Atlantic radio telephone demonstration used SSB with pilot carrier on a frequency of 52 kc. Single sideband was used because of limited power capacity of the equipment and the narrow bandwidths of efficient antennas for those frequencies. By 1927 trans-Atlantic SSB radiotelephony was open for public service. In subsequent years the use of SSB was limited mainly to low-frequency and wire applications. This may have been due in part to a general lack of interest in spectrum-conserving techniques as increasing knowledge opened up new portions of the spectrum. In addition, early developments in FM transmission stimulated a belief that this mode might prove to be the ultimate in voice communication. The resulting slow development of SSB technology precluded practical SSB transmission and reception at high frequencies. Amateur SSB activity followed very much the same pattern. Although amateurs have been responsible in part for much of the communications pioneering done in the past, early developments in single sideband are an exception. Some activity took place about 1933, but it was nearly fifteen years later when the use of SSB began in earnest on the amateur bands .
The advent of World War II brought with it a heretofore unparalleled need for communication facilities. From necessity, advances in electronic technology progressed at a high rate. There were major breakthroughs, not only in basic knowledge, but also in manufacturing techniques. These thrusts forward, and those in the years following the war, were important factors in the development of h-f SSB communication. Developments such as highly stable variable oscillators (VFO) and the mechanical filter made SSB not only practical but economical. Continued advances in technology have refined techniques to the point where SSB has become a dominant mode of radio communication.
The Conservation of RF Spectrum
The radio-frequency spectrum, once thought to be adequate for all needs, is becoming crowded. As the world's technical sophistication progresses, the requirements for rapid and dependable radio communications increase. The competition for available space in the spectrum is intense. Services which have enjoyed use of the spectrum for many years are being asked to present cogent reasons why their frequency allocations should not be changed, reduced, or even eliminated. Low-priority services and those which are wasteful of allocated spectrum space are in precarious positions. The communications needs resulting from the increasing speed and volume of air traffic, both military and commercial, are examples of the heavy demand for more voice channels .
The HF spectrum, from 2 to 30 mc, has been particularly hard pressed to provide more voice channels. Some relief has been obtained by advances in microwave technology and VHF and UHF scatter techniques. Expansions in these areas actually have improved communications for many types of services. The fact remains that an increasing number of services need the long-distance communications links obtainable only by propagation in the HF-range. During years of low sunspot activity, the problem is compounded by lowered MUF which effectively shrinks the h-f portion of the spectrum. Single sideband has demonstrated the capacity to double the number of available voice channels within a given frequency range as compared to conventional AM. At the same time it has proved more dependable. Continuing refinements in SSB technology, such as increased stability of oscillators, further reductions in voice-channel width, and improved linearity in amplifiers, have widened the performance gap. There is good reason to believe that the advantages of SSB will continue to be needed as the frontiers of VHF and UHF propagation are rolled back. Present developments in these portions of the spectrum have attracted the attention of great numbers of users and potential users of radio communications.
The amateur service in particular has felt the pressure of increasingly crowded conditions on the h-f amateur bands. New licenses are being granted at a rate far in excess of that experienced in the past. Although organized amateurs have presented a strong case for the retention of amateur frequencies, future changes could result in reduced rather than expanded allocations. Therefore, continued overcrowding appears inevitable. The use of SSB has done much to alleviate congestion in the phone bands, but there is room for further improvement. The exclusive use of SSB is desirable. This, coupled with intelligent operating practices to minimize distortion products, will result in a greater number of satisfactory contacts within the limited spectrum available.
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