In very recent years, the revolution in digital signal processing technologies has been applied to SSB receivers and transmitters to produce levels of performance and flexibility (programmability) that are setting new standards in radio design. Also, developments in high-power, solid-state transistors and circuit design has revolutionized linear power amplifier (LPA) design. The development of ultralinear power amplifiers using feedforward techniques promises to reduce transmitter distortion products by two or three orders of magnitude. Improvements in high-power, fast-tuning vacuum tube amplifiers are described in this book. Advanced measurement techniques for high-performance receivers and transmitters have been developed.

This book chapter presents the high-frequency communication link design and establishment. The proceeding section provides a review of system design trade-off considerations and an overview of propagation and system performance predictions. Next, a design example showing how a radio link may be designed. And lastly, a new tool is introduced that provides automated assistance for HF skywave frequency selection and applications planning.

This chapter discusses the performance requirements of an exciter, its component parts are assembled in an architecture that meets the requirements and its necessary functions. A transceiver may be defined as an exciter and receiver combined in one functional unit.

The human voice does not utilize the SSB power amplifiers very well. The 15-dB peak-to-average ratio, or 24-dB dynamic range, means that a high power amplifier that handles the peaks well is just 'loafing' most of the time. Unless the amplifier and power supply are marginal, much greater power can be transmitted if the deficiencies of speech can be improved. The usual case is that amplifiers that will handle the peaks well are capable of higher average levels than natural speech provides. The central idea is that the weaker components which we will emphasize in this section contribute much to the intelligibility.

Although wideband receiver front ends are in increasing use, they cannot handle situations with very strong off-channel signals. Passive tuned circuit filters are still needed to protect against cross-modulation, IM, damage, and other interference effects. The trend is toward separately packaged preselector filter units using sophisticated tuning and control methods. Selectivity, noise figure degradation, and maximum interference voltage are important constraints. It is possible to design filters with an optimal trade-off of these parameters for given component limitations. With some forethought in the filter design, several preselectors can be driven by the same antenna in a multicoupling configuration. The same filters or similar circuits can be employed as postselectors to filter the transmitter signal before final power amplification, thus reducing its out-of-band noise and spurious emissions that could interfere with nearby receivers.

The application of precision frequency control has played an important role in establishing SSB as a reliable means of radio communications. The crystal oscillator thus provides one of the vital links required for the implementation of SSB. As will be seen subsequently, the crystal oscillators used for SSB are often required to maintain a higher degree of accuracy than those used in many other applications. The material covered in this chapter deals primarily with crystal oscillators in the frequency stability range required for SSB equipment operating at HF or VHF bands. The design of higher-precision frequency standards that might be used for microwave SSB links is beyond the scope of this book, and the reader is referred to the references at the end of this chapter. Thus, both temperature-compensated crystal oscillators and crystal ovens of the type used to achieve a frequency stability in the ppm range are addressed. Since the quartz crystal itself plays such a vital role in determining the oscillator performance, a section is included discussing many of the characteristics of the crystal itself. A brief sketch is also included presenting a historical review of frequency control in SSB equipment.

In applications where a high degree of linearity and low noise are required from the transmitter, external error correction may be added to improve performance. This chapter describes feedforward techniques for low transmitter noise and distortion.

The role of the antenna coupler is to provide an impedance matching function in either of two ways: narrowband or broadband. With narrowband matching a near perfect impedance match is achieved at a single frequency. With broadband matching a less than perfect impedance match is accepted in order to get an optimal match over a broad range of frequencies. This paper presents the current techniques used for narrowband antenna couplers. Included is the basic approach to be followed, some of the pitfalls, and general engineering advice. It is assumed that the reader has a computer at his/her disposal. Broadband matching of antennas in application where it is advantageous.

The disk that accompanies this book contains a collection of public domain programs for the IBM-compatible personal computer. These programs will be described in the following sections. They have all been tested over a period of time and are believed to be accurate and bug-free.