4. The History of Single Sideband Modulation
Comparison of SSB with FM
Figure 4 shows the general relationship between received signal amplitude and signal-to-noise ratio for both an FM signal and an SSB signal. Good FM reception requires received signals that are strong enough to overcome the limiter threshold in the FM receiver. When this condition prevails, the s/n ratio for a given signal strength is better in the FM receiver than in the SSB receiver. However, when the signal strength falls below the FM limiter threshold, the s/n ratio in the FM receiver deteriorates rapidly, and the SSB receiver provides the best s/n ratio. The practical results are these: (1) With strong signals, FM can provide better s/n ratios than SSB. The advantage to be gained from this situation is dubious, since strong signals also provide good intelligibility in the SSB system, and a further increase in s/n ratio does not materially increase the readability. (2) With weak signals, the SSB system will provide an intelligible signal where the FM system fails. (3) The SSB system provides considerable savings in spectrum compared to the FM system.
Figure 4. Performance of SSB Versus FM
Single Sideband for VHF Propagation in the VHF range is marked by a great variety in the means by which signals get from one spot on the earth to another. In many cases, more than one mode of propagation is involved on a given contact thus causing fading and resultant distortion. Such degradation of signals can be substantially reduced through the use of SSB. Various atmospheric and man-made noise sources which must be considered in the h-f range are, for the most part, small factors at VHF. As a result, propagation modes can be used at VHF which are either non existent in the hf range or are obscured by other factors. Many of the propagation modes usable at VHF, such as the various forms of scatter, require effective weak-signal techniques. Single sideband is well suited for this type of communication. The use of scatter is a relatively new technique with a challenge for amateurs who enjoy being close to the state of the art. Although there are several mechanisms by which scatter occurs, the general effect is that signals are scattered by irregularities in the troposphere or ionosphere in a manner similar to the scattering of light from automobile headlights on a foggy night. In this way VHF signals are returned to the earth at distances considerably beyond the normal radio horizon. Since the transmitted signal is widely scattered, received signal levels are typically quite weak, however, signals propagated in this way also exhibit a high degree of reliability. To achieve maximum reliability, transmitter powers of several kilowatts are used for commercial and military circuits, but when the reliability requirements are relaxed to a level still quite acceptable by amateur standards, scatter contacts are possible within amateur power limitations. In either case, low-noise receivers and high-gain antennas are necessities. The possibilities for amateur phone communications via scatter are somewhat marginal, however, this serves only to make the problem more interesting and successful results more rewarding. Single sideband holds the greatest promise for success, since an SSB transmitter operating at the maximum legal power input produces considerably more sideband power output than a comparable AM transmitter. Auroral reflection, using CW, is regularly used for VHF DX work when aurora conditions occur. Voice contacts by this means, especially on the 144 MHz band, frequently are not too successful due to the doppler effect prevalent in this mode of propagation. Although it has yet to be fully exploited, SSB has demonstrated an ability to get through where AM fails. Much is still to be learned about certain aspects of VHF propagation. This fact alone makes VHF operation both interesting and challenging for many amateurs. By means such as SSB, the possibilities for increasing the normal working distances for phone signals are by no means exhausted. For those inclined to a more casual type of operation, SSB can provide a high percentage of enjoyable contacts in addition to making efficient use of available facilities.
The use of SSB for phone operation is particularly beneficial to the amateur. Because amateur operation is generally on random frequencies within a band, rather than on specific channels, suppression of the high-energy carrier eliminates the din of heterodynes common to AM operation. Since a great concentration of stations exists within many of the amateur phone bands, the spectrum economy of SSB is an important factor. It allows a greater number of satisfactory contacts to take place at the same time in comparison to other modes. The power economy of SSB permits a considerable reduction in the size of power supply equipment and reduces relative cost, particularly higher power levels. In situations where maximum power is required, such as in VHF scatter circuits or difficult HF paths, SSB provides the most sideband power output presently available within amateur power limitations . The benefits of SSB are greatest and most easily observed under poor propagating conditions. As a given transmission path deteriorates due to a combination of noise, severe selective fading, and narrow-band interference, the superiority of SSB over AM becomes increasingly evident. Studies have been made which give SSB a theoretical performance edge of several dB when conditions are marginal. Since the variables are sometimes difficult to relate, one of the most convincing methods of comparison from an amateur standpoint is to listen on the amateur DX phone bands. Signals from SSB stations are the first to become readable as the bands open and are the last to fade as the bands go out.
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